arash11/wikipedia-physics-corpus · Datasets at Hugging Face

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or, integrating:	16	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,000	26	2,024	8	10	31	2
where h (0) is the height of the water at r = 0. In other words, the surface of the water is parabolic in its dependence upon the radius.	137	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,001	27	2,024	8	10	31	2
The shape of the water's surface can be found in a different, very intuitive way using the interesting idea of the potential energy associated with the centrifugal force in the co-rotating frame. In a reference frame uniformly rotating at angular rate Ω, the fictitious centrifugal force is conservative and has a potential energy of the form:	343	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,002	28	2,024	8	10	31	2
where r is the radius from the axis of rotation. This result can be verified by taking the gradient of the potential to obtain the radially outward force:	154	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,003	29	2,024	8	10	31	2
The meaning of the potential energy (stored work) is that movement of a test body from a larger radius to a smaller radius involves doing work against the centrifugal force and thus gaining potential energy. But this test body at the smaller radius where its elevation is lower has now lost equivalent gravitational potential energy.	333	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,004	30	2,024	8	10	31	2
Potential energy therefore explains the concavity of the water surface in a rotating bucket. Notice that at equilibrium the surface adopts a shape such that an element of volume at any location on its surface has the same potential energy as at any other. That being so, no element of water on the surface has any incentive to move position, because all positions are equivalent in energy. That is, equilibrium is attained. On the other hand, were surface regions with lower energy available, the water occupying surface locations of higher potential energy would move to occupy these positions of lower energy, inasmuch as there is no barrier to lateral movement in an ideal liquid.	683	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,005	31	2,024	8	10	31	2
We might imagine deliberately upsetting this equilibrium situation by somehow momentarily altering the surface shape of the water to make it different from an equal-energy surface. This change in shape would not be stable, and the water would not stay in our artificially contrived shape, but engage in a transient exploration of many shapes until non-ideal frictional forces introduced by sloshing, either against the sides of the bucket or by the non-ideal nature of the liquid, killed the oscillations and the water settled down to the equilibrium shape.	557	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,006	32	2,024	8	10	31	2
To see the principle of an equal-energy surface at work, imagine gradually increasing the rate of rotation of the bucket from zero. The water surface is flat at first, and clearly a surface of equal potential energy because all points on the surface are at the same height in the gravitational field acting upon the water. At some small angular rate of rotation, however, an element of surface water can achieve lower potential energy by moving outward under the influence of the centrifugal force; think of an object moving with the force of gravity closer to the Earth's center: the object lowers its potential energy by complying with a force. Because water is incompressible and must remain within the confines of the bucket, this outward movement increases the depth of water at the larger radius, increasing the height of the surface at larger radius, and lowering it at smaller radius. The surface of the water becomes slightly concave, with the consequence that the potential energy of the water at the greater radius is increased by the work done against gravity to achieve the greater height. As the height of water increases, movement toward the periphery becomes no longer advantageous, because the reduction in potential energy from working with the centrifugal force is balanced against the increase in energy working against gravity. Thus, at a given angular rate of rotation, a concave surface represents the stable situation, and the more rapid the rotation, the more concave this surface. If rotation is arrested, the energy stored in fashioning the concave surface must be dissipated, for example through friction, before an equilibrium flat surface is restored.	1,681	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,007	33	2,024	8	10	31	2
To implement a surface of constant potential energy quantitatively, let the height of the water be h ( r ) : then the potential energy per unit mass contributed by gravity is g h ( r ) and the total potential energy per unit mass on the surface is	247	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,008	34	2,024	8	10	31	2
with U 0 the background energy level independent of r. In a static situation (no motion of the fluid in the rotating frame), this energy is constant independent of position r. Requiring the energy to be constant, we obtain the parabolic form:	242	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,009	35	2,024	8	10	31	2
where h (0) is the height at r = 0 (the axis). See Figures 1 and 2.	67	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,010	36	2,024	8	10	31	2
The principle of operation of the centrifuge also can be simply understood in terms of this expression for the potential energy, which shows that it is favorable energetically when the volume far from the axis of rotation is occupied by the heavier substance.	259	Bucket_argument	https://en.wikipedia.org/wiki/Bucket_argument	1,011	37	2,024	8	10	31	2
Four views of the constellation Orion :	39	Constellation	https://en.wikipedia.org/wiki/Constellation	1,012	0	2,024	8	10	31	2
A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.	199	Constellation	https://en.wikipedia.org/wiki/Constellation	1,013	1	2,024	8	10	31	2
The first constellations likely go back to prehistory. People used them to relate stories of their beliefs, experiences, creation, mythology. Different cultures and countries invented their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. The recognition of constellations has changed significantly over time. Many changed in size or shape. Some became popular, only to drop into obscurity. Some were limited to a single culture or nation. Naming constellations also helped astronomers and navigators identify stars more easily.	611	Constellation	https://en.wikipedia.org/wiki/Constellation	1,014	2	2,024	8	10	31	2
Twelve (or thirteen) ancient constellations belong to the zodiac (straddling the ecliptic, which the Sun, Moon, and planets all traverse). The origins of the zodiac remain historically uncertain; its astrological divisions became prominent c. 400 BC in Babylonian or Chaldean astronomy. Constellations appear in Western culture via Greece and are mentioned in the works of Hesiod, Eudoxus and Aratus. The traditional 48 constellations, consisting of the zodiac and 36 more (now 38, following the division of Argo Navis into three constellations) are listed by Ptolemy, a Greco-Roman astronomer from Alexandria, Egypt, in his Almagest. The formation of constellations was the subject of extensive mythology, most notably in the Metamorphoses of the Latin poet Ovid. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Due to Roman and European transmission, each constellation has a Latin name.	1,008	Constellation	https://en.wikipedia.org/wiki/Constellation	1,015	3	2,024	8	10	31	2
In 1922, the International Astronomical Union (IAU) formally accepted the modern list of 88 constellations, and in 1928 adopted official constellation boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations. Some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation's name.	569	Constellation	https://en.wikipedia.org/wiki/Constellation	1,016	4	2,024	8	10	31	2
Other star patterns or groups called asterisms are not constellations under the formal definition, but are also used by observers to navigate the night sky. Asterisms may be several stars within a constellation, or they may share stars with more than one constellation. Examples of asterisms include the teapot within the constellation Sagittarius, or the big dipper in the constellation of Ursa Major.	402	Constellation	https://en.wikipedia.org/wiki/Constellation	1,017	5	2,024	8	10	31	2
The word constellation comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars"; it came into use in Middle English during the 14th century. The Ancient Greek word for constellation is ἄστρον (astron). These terms historically referred to any recognisable pattern of stars whose appearance was associated with mythological characters or creatures, earthbound animals, or objects. Over time, among European astronomers, the constellations became clearly defined and widely recognised. In the 20th century, the International Astronomical Union (IAU) recognized 88 constellations.	607	Constellation	https://en.wikipedia.org/wiki/Constellation	1,018	6	2,024	8	10	31	2
A constellation or star that never sets below the horizon when viewed from a particular latitude on Earth is termed circumpolar. From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic (or zodiac) ranging between 23.5° north and 23.5° south.	489	Constellation	https://en.wikipedia.org/wiki/Constellation	1,019	7	2,024	8	10	31	2
Stars in constellations can appear near each other in the sky, but they usually lie at a variety of distances away from the Earth. Since each star has its own independent motion, all constellations will change slowly over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring common proper motions of individual stars by accurate astrometry and their radial velocities by astronomical spectroscopy.	521	Constellation	https://en.wikipedia.org/wiki/Constellation	1,020	8	2,024	8	10	31	2
The 88 constellations recognized by the IAU as well as those by cultures throughout history are imagined figures and shapes derived from the patterns of stars in the observable sky. Many officially recognized constellations are based on the imaginations of ancient, Near Eastern and Mediterranean mythologies. Some of these stories seem to relate to the appearance of the constellations, e.g. the assassination of Orion by Scorpius, their constellations appearing at opposite times of year.	490	Constellation	https://en.wikipedia.org/wiki/Constellation	1,021	9	2,024	8	10	31	2
Constellation positions change throughout the year due to night on Earth occurring at gradually different portions of its orbit around the Sun. As Earth rotates toward the east, the celestial sphere appears to rotate west, with stars circling counterclockwise around the northern pole star and clockwise around the southern pole star.	334	Constellation	https://en.wikipedia.org/wiki/Constellation	1,022	10	2,024	8	10	31	2
Because of Earth's 23.5° axial tilt, the zodiac is distributed equally across hemispheres (along the ecliptic), approximating a great circle. Zodiacal constellations of the northern sky are Pisces, Aries, Taurus, Gemini, Cancer, and Leo. In the southern sky are Virgo, Libra, Scorpius, Sagittarius, Capricornus, and Aquarius. The zodiac appears directly overhead from latitudes of 23.5° north to 23.5° south, depending on the time of year. In summer, the ecliptic appears higher up in the daytime and lower at night, while in winter the reverse is true, for both hemispheres.	575	Constellation	https://en.wikipedia.org/wiki/Constellation	1,023	11	2,024	8	10	31	2
Due to the Solar System 's 60° tilt, the galactic plane of the Milky Way is inclined 60° from the ecliptic, between Taurus and Gemini (north) and Scorpius and Sagittarius (south and near which the Galactic Center can be found). The galaxy appears to pass through Aquila (near the celestial equator) and northern constellations Cygnus, Cassiopeia, Perseus, Auriga, and Orion (near Betelgeuse), as well as Monoceros (near the celestial equator), and southern constellations Puppis, Vela, Carina, Crux, Centaurus, Triangulum Australe, and Ara.	540	Constellation	https://en.wikipedia.org/wiki/Constellation	1,024	12	2,024	8	10	31	2
Polaris, being the North Star, is the approximate center of the northern celestial hemisphere. It is part of Ursa Minor, constituting the end of the Little Dipper's handle.	172	Constellation	https://en.wikipedia.org/wiki/Constellation	1,025	13	2,024	8	10	31	2
From latitudes of around 35° north, in January, Ursa Major (containing the Big Dipper) appears to the northeast, while Cassiopeia is the northwest. To the west are Pisces (above the horizon) and Aries. To the southwest Cetus is near the horizon. Up high and to the south are Orion and Taurus. To the southeast above the horizon is Canis Major. Appearing above and to the east of Orion is Gemini : also in the east (and progressively closer to the horizon) are Cancer and Leo. In addition to Taurus, Perseus and Auriga appear overhead.	534	Constellation	https://en.wikipedia.org/wiki/Constellation	1,026	14	2,024	8	10	31	2
From the same latitude, in July, Cassiopeia (low in the sky) and Cepheus appear to the northeast. Ursa Major is now in the northwest. Boötes is high up in the west. Virgo is to the west, with Libra southwest and Scorpius south. Sagitarrius and Capricorn are southeast. Cygnus (containing the Northern Cross) is to the east. Hercules is high in the sky along with Corona Borealis.	379	Constellation	https://en.wikipedia.org/wiki/Constellation	1,027	15	2,024	8	10	31	2
January constellations include Pictor and Reticulum (near Hydrus and Mensa, respectively).	90	Constellation	https://en.wikipedia.org/wiki/Constellation	1,028	16	2,024	8	10	31	2
In July, Ara (adjacent to Triangulum Australe) and Scorpius can be seen.	72	Constellation	https://en.wikipedia.org/wiki/Constellation	1,029	17	2,024	8	10	31	2
Constellations near the pole star include Chamaeleon, Apus and Triangulum Australe (near Centaurus), Pavo, Hydrus, and Mensa.	125	Constellation	https://en.wikipedia.org/wiki/Constellation	1,030	18	2,024	8	10	31	2
Sigma Octantis is the closest star approximating a southern pole star, but is faint in the night sky. Thus, the pole can be triangulated using the constellation Crux as well as the stars Alpha and Beta Centauri (about 30° counterclockwise from Crux) of the constellation Centaurus (arching over Crux).	301	Constellation	https://en.wikipedia.org/wiki/Constellation	1,031	19	2,024	8	10	31	2
It has been suggested that the 17,000-year-old cave paintings in Lascaux, southern France, depict star constellations such as Taurus, Orion's Belt, and the Pleiades. However, this view is not generally accepted among scientists.	228	Constellation	https://en.wikipedia.org/wiki/Constellation	1,032	20	2,024	8	10	31	2
Inscribed stones and clay writing tablets from Mesopotamia (in modern Iraq) dating to 3000 BC provide the earliest generally accepted evidence for humankind's identification of constellations. It seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared later in many of the classical Greek constellations.	417	Constellation	https://en.wikipedia.org/wiki/Constellation	1,033	21	2,024	8	10	31	2
The oldest Babylonian catalogues of stars and constellations date back to the beginning of the Middle Bronze Age, most notably the Three Stars Each texts and the MUL.APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age.	421	Constellation	https://en.wikipedia.org/wiki/Constellation	1,034	22	2,024	8	10	31	2
The classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names.	281	Constellation	https://en.wikipedia.org/wiki/Constellation	1,035	23	2,024	8	10	31	2
Biblical scholar E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four-quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius, and the Eagle standing in for Scorpio. The biblical Book of Job also makes reference to a number of constellations, including עיש ‘Ayish "bier", כסיל chesil "fool" and כימה chimah "heap" (Job 9:9, 38:31–32), rendered as "Arcturus, Orion and Pleiades" by the KJV, but ‘Ayish "the bier" actually corresponding to Ursa Major. The term Mazzaroth מַזָּרוֹת, translated as a garland of crowns, is a hapax legomenon in Job 38:32, and it might refer to the zodiacal constellations.	718	Constellation	https://en.wikipedia.org/wiki/Constellation	1,036	24	2,024	8	10	31	2
There is only limited information on ancient Greek constellations, with some fragmentary evidence being found in the Works and Days of the Greek poet Hesiod, who mentioned the "heavenly bodies". Greek astronomy essentially adopted the older Babylonian system in the Hellenistic era, first introduced to Greece by Eudoxus of Cnidus in the 4th century BC. The original work of Eudoxus is lost, but it survives as a versification by Aratus, dating to the 3rd century BC. The most complete existing works dealing with the mythical origins of the constellations are by the Hellenistic writer termed pseudo-Eratosthenes and an early Roman writer styled pseudo- Hyginus. The basis of Western astronomy as taught during Late Antiquity and until the Early Modern period is the Almagest by Ptolemy, written in the 2nd century.	816	Constellation	https://en.wikipedia.org/wiki/Constellation	1,037	25	2,024	8	10	31	2
In the Ptolemaic Kingdom, native Egyptian tradition of anthropomorphic figures represented the planets, stars, and various constellations. Some of these were combined with Greek and Babylonian astronomical systems culminating in the Zodiac of Dendera ; it remains unclear when this occurred, but most were placed during the Roman period between 2nd to 4th centuries AD. The oldest known depiction of the zodiac showing all the now familiar constellations, along with some original Egyptian constellations, decans, and planets. Ptolemy's Almagest remained the standard definition of constellations in the medieval period both in Europe and in Islamic astronomy.	660	Constellation	https://en.wikipedia.org/wiki/Constellation	1,038	26	2,024	8	10	31	2
Ancient China had a long tradition of observing celestial phenomena. Nonspecific Chinese star names, later categorized in the twenty-eight mansions, have been found on oracle bones from Anyang, dating back to the middle Shang dynasty. These constellations are some of the most important observations of Chinese sky, attested from the 5th century BC. Parallels to the earliest Babylonian (Sumerian) star catalogues suggest that the ancient Chinese system did not arise independently.	482	Constellation	https://en.wikipedia.org/wiki/Constellation	1,039	27	2,024	8	10	31	2
Three schools of classical Chinese astronomy in the Han period are attributed to astronomers of the earlier Warring States period. The constellations of the three schools were conflated into a single system by Chen Zhuo, an astronomer of the 3rd century (Three Kingdoms period). Chen Zhuo's work has been lost, but information on his system of constellations survives in Tang period records, notably by Qutan Xida. The oldest extant Chinese star chart dates to that period and was preserved as part of the Dunhuang Manuscripts. Native Chinese astronomy flourished during the Song dynasty, and during the Yuan dynasty became increasingly influenced by medieval Islamic astronomy (see Treatise on Astrology of the Kaiyuan Era). As maps were prepared during this period on more scientific lines, they were considered as more reliable.	831	Constellation	https://en.wikipedia.org/wiki/Constellation	1,040	28	2,024	8	10	31	2
A well-known map from the Song period is the Suzhou Astronomical Chart, which was prepared with carvings of stars on the planisphere of the Chinese sky on a stone plate; it is done accurately based on observations, and it shows the supernova of the year of 1054 in Taurus.	272	Constellation	https://en.wikipedia.org/wiki/Constellation	1,041	29	2,024	8	10	31	2
Influenced by European astronomy during the late Ming dynasty, charts depicted more stars but retained the traditional constellations. Newly observed stars were incorporated as supplementary to old constellations in the southern sky, which did not depict the traditional stars recorded by ancient Chinese astronomers. Further improvements were made during the later part of the Ming dynasty by Xu Guangqi and Johann Adam Schall von Bell, the German Jesuit and was recorded in Chongzhen Lishu (Calendrical Treatise of Chongzhen period, 1628). Traditional Chinese star maps incorporated 23 new constellations with 125 stars of the southern hemisphere of the sky based on the knowledge of Western star charts; with this improvement, the Chinese Sky was integrated with the World astronomy.	786	Constellation	https://en.wikipedia.org/wiki/Constellation	1,042	30	2,024	8	10	31	2
Historically, the origins of the constellations of the northern and southern skies are distinctly different. Most northern constellations date to antiquity, with names based mostly on Classical Greek legends. Evidence of these constellations has survived in the form of star charts, whose oldest representation appears on the statue known as the Farnese Atlas, based perhaps on the star catalogue of the Greek astronomer Hipparchus. Southern constellations are more modern inventions, sometimes as substitutes for ancient constellations (e.g. Argo Navis). Some southern constellations had long names that were shortened to more usable forms; e.g. Musca Australis became simply Musca.	683	Constellation	https://en.wikipedia.org/wiki/Constellation	1,043	31	2,024	8	10	31	2
Some of the early constellations were never universally adopted. Stars were often grouped into constellations differently by different observers, and the arbitrary constellation boundaries often led to confusion as to which constellation a celestial object belonged. Before astronomers delineated precise boundaries (starting in the 19th century), constellations generally appeared as ill-defined regions of the sky. Today they now follow officially accepted designated lines of right ascension and declination based on those defined by Benjamin Gould in epoch 1875.0 in his star catalogue Uranometria Argentina.	612	Constellation	https://en.wikipedia.org/wiki/Constellation	1,044	32	2,024	8	10	31	2
The 1603 star atlas " Uranometria " of Johann Bayer assigned stars to individual constellations and formalized the division by assigning a series of Greek and Latin letters to the stars within each constellation. These are known today as Bayer designations. Subsequent star atlases led to the development of today's accepted modern constellations.	347	Constellation	https://en.wikipedia.org/wiki/Constellation	1,045	33	2,024	8	10	31	2
The southern sky, below about −65° declination, was only partially catalogued by ancient Babylonians, Egyptians, Greeks, Chinese, and Persian astronomers of the north. The knowledge that northern and southern star patterns differed goes back to Classical writers, who describe, for example, the African circumnavigation expedition commissioned by Egyptian Pharaoh Necho II in c. 600 BC and those of Hanno the Navigator in c. 500 BC.	432	Constellation	https://en.wikipedia.org/wiki/Constellation	1,046	34	2,024	8	10	31	2
The history of southern constellations is not straightforward. Different groupings and different names were proposed by various observers, some reflecting national traditions or designed to promote various sponsors. Southern constellations were important from the 14th to 16th centuries, when sailors used the stars for celestial navigation. Italian explorers who recorded new southern constellations include Andrea Corsali, Antonio Pigafetta, and Amerigo Vespucci.	465	Constellation	https://en.wikipedia.org/wiki/Constellation	1,047	35	2,024	8	10	31	2
Many of the 88 IAU-recognized constellations in this region first appeared on celestial globes developed in the late 16th century by Petrus Plancius, based mainly on observations of the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman. These became widely known through Johann Bayer 's star atlas Uranometria of 1603. Fourteen more were created in 1763 by the French astronomer Nicolas Louis de Lacaille, who also split the ancient constellation Argo Navis into three; these new figures appeared in his star catalogue, published in 1756.	554	Constellation	https://en.wikipedia.org/wiki/Constellation	1,048	36	2,024	8	10	31	2
Several modern proposals have not survived. The French astronomers Pierre Lemonnier and Joseph Lalande, for example, proposed constellations that were once popular but have since been dropped. The northern constellation Quadrans Muralis survived into the 19th century (when its name was attached to the Quadrantid meteor shower), but is now divided between Boötes and Draco.	374	Constellation	https://en.wikipedia.org/wiki/Constellation	1,049	37	2,024	8	10	31	2
A list of 88 constellations was produced for the IAU in 1922. It is roughly based on the traditional Greek constellations listed by Ptolemy in his Almagest in the 2nd century and Aratus ' work Phenomena, with early modern modifications and additions (most importantly introducing constellations covering the parts of the southern sky unknown to Ptolemy) by Petrus Plancius (1592, 1597/98 and 1613), Johannes Hevelius (1690) and Nicolas Louis de Lacaille (1763), who introduced fourteen new constellations. Lacaille studied the stars of the southern hemisphere from 1751 until 1752 from the Cape of Good Hope, when he was said to have observed more than 10,000 stars using a refracting telescope with an aperture of 0.5 inches (13 mm).	734	Constellation	https://en.wikipedia.org/wiki/Constellation	1,050	38	2,024	8	10	31	2
In 1922, Henry Norris Russell produced a list of 88 constellations with three-letter abbreviations for them. However, these constellations did not have clear borders between them. In 1928, the IAU formally accepted the 88 modern constellations, with contiguous boundaries along vertical and horizontal lines of right ascension and declination developed by Eugene Delporte that, together, cover the entire celestial sphere; this list was finally published in 1930. Where possible, these modern constellations usually share the names of their Graeco-Roman predecessors, such as Orion, Leo, or Scorpius. The aim of this system is area-mapping, i.e. the division of the celestial sphere into contiguous fields. Out of the 88 modern constellations, 36 lie predominantly in the northern sky, and the other 52 predominantly in the southern.	833	Constellation	https://en.wikipedia.org/wiki/Constellation	1,051	39	2,024	8	10	31	2
The boundaries developed by Delporte used data that originated back to epoch B1875.0, which was when Benjamin A. Gould first made his proposal to designate boundaries for the celestial sphere, a suggestion on which Delporte based his work. The consequence of this early date is that because of the precession of the equinoxes, the borders on a modern star map, such as epoch J2000, are already somewhat skewed and no longer perfectly vertical or horizontal. This effect will increase over the years and centuries to come.	521	Constellation	https://en.wikipedia.org/wiki/Constellation	1,052	40	2,024	8	10	31	2
The constellations have no official symbols, though those of the ecliptic may take the signs of the zodiac. Symbols for the other modern constellations, as well as older ones that still occur in modern nomenclature, have occasionally been published.	249	Constellation	https://en.wikipedia.org/wiki/Constellation	1,053	41	2,024	8	10	31	2
The Great Rift, a series of dark patches in the Milky Way, is most visible in the southern sky. Some cultures have discerned shapes in these patches. Members of the Inca civilization identified various dark areas or dark nebulae in the Milky Way as animals and associated their appearance with the seasonal rains. Australian Aboriginal astronomy also describes dark cloud constellations, the most famous being the "emu in the sky" whose head is formed by the Coalsack, a dark nebula, instead of the stars.	505	Constellation	https://en.wikipedia.org/wiki/Constellation	1,054	42	2,024	8	10	31	2
Footnotes	9	Constellation	https://en.wikipedia.org/wiki/Constellation	1,055	43	2,024	8	10	31	2
Citations	9	Constellation	https://en.wikipedia.org/wiki/Constellation	1,056	44	2,024	8	10	31	2
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture (although a narrower sense of the word suspension is distinguished from colloids by larger particle size). A colloid has a dispersed phase (the suspended particles) and a continuous phase (the medium of suspension). The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.	686	Colloid	https://en.wikipedia.org/wiki/Colloid	1,057	0	2,024	8	10	31	2
Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.	175	Colloid	https://en.wikipedia.org/wiki/Colloid	1,058	1	2,024	8	10	31	2
Colloidal suspensions are the subject of interface and colloid science. This field of study began in 1845 by Francesco Selmi, who called them pseudosolutions, and expanded by Michael Faraday and Thomas Graham, who coined the term colloid in 1861.	246	Colloid	https://en.wikipedia.org/wiki/Colloid	1,059	2	2,024	8	10	31	2
Colloid : Short synonym for colloidal system.	45	Colloid	https://en.wikipedia.org/wiki/Colloid	1,060	3	2,024	8	10	31	2
Colloidal : State of subdivision such that the molecules or polymolecular particles dispersed in a medium have at least one dimension between approximately 1 nm and 1 μm, or that in a system discontinuities are found at distances of that order.	244	Colloid	https://en.wikipedia.org/wiki/Colloid	1,061	4	2,024	8	10	31	2
Colloids can be classified as follows:	38	Colloid	https://en.wikipedia.org/wiki/Colloid	1,062	5	2,024	8	10	31	2
Homogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal suspensions, colloidal foams, colloidal dispersions, or hydrosols.	194	Colloid	https://en.wikipedia.org/wiki/Colloid	1,063	6	2,024	8	10	31	2
Hydrocolloids describe certain chemicals (mostly polysaccharides and proteins) that are colloidally dispersible in water. Thus becoming effectively "soluble" they change the rheology of water by raising the viscosity and/or inducing gelation. They may provide other interactive effects with other chemicals, in some cases synergistic, in others antagonistic. Using these attributes hydrocolloids are very useful chemicals since in many areas of technology from foods through pharmaceuticals, personal care and industrial applications, they can provide stabilization, destabilization and separation, gelation, flow control, crystallization control and numerous other effects. Apart from uses of the soluble forms some of the hydrocolloids have additional useful functionality in a dry form if after solubilization they have the water removed - as in the formation of films for breath strips or sausage casings or indeed, wound dressing fibers, some being more compatible with skin than others. There are many different types of hydrocolloids each with differences in structure function and utility that generally are best suited to particular application areas in the control of rheology and the physical modification of form and texture. Some hydrocolloids like starch and casein are useful foods as well as rheology modifiers, others have limited nutritive value, usually providing a source of fiber.	1,401	Colloid	https://en.wikipedia.org/wiki/Colloid	1,064	7	2,024	8	10	31	2
The term hydrocolloids also refers to a type of dressing designed to lock moisture in the skin and help the natural healing process of skin to reduce scarring, itching and soreness.	181	Colloid	https://en.wikipedia.org/wiki/Colloid	1,065	8	2,024	8	10	31	2
Hydrocolloids contain some type of gel-forming agent, such as sodium carboxymethylcellulose (NaCMC) and gelatin. They are normally combined with some type of sealant, i.e. polyurethane to 'stick' to the skin.	208	Colloid	https://en.wikipedia.org/wiki/Colloid	1,066	9	2,024	8	10	31	2
A colloid has a dispersed phase and a continuous phase, whereas in a solution, the solute and solvent constitute only one phase. A solute in a solution are individual molecules or ions, whereas colloidal particles are bigger. For example, in a solution of salt in water, the sodium chloride (NaCl) crystal dissolves, and the Na and Cl ions are surrounded by water molecules. However, in a colloid such as milk, the colloidal particles are globules of fat, rather than individual fat molecules. Because colloid is multiple phases, it has very different properties compared to fully mixed, continuous solution.	608	Colloid	https://en.wikipedia.org/wiki/Colloid	1,067	10	2,024	8	10	31	2
The following forces play an important role in the interaction of colloid particles:	84	Colloid	https://en.wikipedia.org/wiki/Colloid	1,068	11	2,024	8	10	31	2
The Earth’s gravitational field acts upon colloidal particles. Therefore, if the colloidal particles are denser than the medium of suspension, they will sediment (fall to the bottom), or if they are less dense, they will cream (float to the top). Larger particles also have a greater tendency to sediment because they have smaller Brownian motion to counteract this movement.	375	Colloid	https://en.wikipedia.org/wiki/Colloid	1,069	12	2,024	8	10	31	2
The sedimentation or creaming velocity is found by equating the Stokes drag force with the gravitational force :	112	Colloid	https://en.wikipedia.org/wiki/Colloid	1,070	13	2,024	8	10	31	2
where	5	Colloid	https://en.wikipedia.org/wiki/Colloid	1,071	14	2,024	8	10	31	2
and v is the sedimentation or creaming velocity.	48	Colloid	https://en.wikipedia.org/wiki/Colloid	1,072	15	2,024	8	10	31	2
The mass of the colloidal particle is found using:	50	Colloid	https://en.wikipedia.org/wiki/Colloid	1,073	16	2,024	8	10	31	2
where	5	Colloid	https://en.wikipedia.org/wiki/Colloid	1,074	17	2,024	8	10	31	2
and ρ ρ 1 − − ρ ρ 2 is the difference in mass density between the colloidal particle and the suspension medium.	111	Colloid	https://en.wikipedia.org/wiki/Colloid	1,075	18	2,024	8	10	31	2
By rearranging, the sedimentation or creaming velocity is:	58	Colloid	https://en.wikipedia.org/wiki/Colloid	1,076	19	2,024	8	10	31	2
There is an upper size-limit for the diameter of colloidal particles because particles larger than 1 μm tend to sediment, and thus the substance would no longer be considered a colloidal suspension.	198	Colloid	https://en.wikipedia.org/wiki/Colloid	1,077	20	2,024	8	10	31	2
The colloidal particles are said to be in sedimentation equilibrium if the rate of sedimentation is equal to the rate of movement from Brownian motion.	151	Colloid	https://en.wikipedia.org/wiki/Colloid	1,078	21	2,024	8	10	31	2
There are two principal ways to prepare colloids:	49	Colloid	https://en.wikipedia.org/wiki/Colloid	1,079	22	2,024	8	10	31	2
The stability of a colloidal system is defined by particles remaining suspended in solution and depends on the interaction forces between the particles. These include electrostatic interactions and van der Waals forces, because they both contribute to the overall free energy of the system.	290	Colloid	https://en.wikipedia.org/wiki/Colloid	1,080	23	2,024	8	10	31	2
A colloid is stable if the interaction energy due to attractive forces between the colloidal particles is less than kT, where k is the Boltzmann constant and T is the absolute temperature. If this is the case, then the colloidal particles will repel or only weakly attract each other, and the substance will remain a suspension.	328	Colloid	https://en.wikipedia.org/wiki/Colloid	1,081	24	2,024	8	10	31	2
If the interaction energy is greater than kT, the attractive forces will prevail, and the colloidal particles will begin to clump together. This process is referred to generally as aggregation, but is also referred to as flocculation, coagulation or precipitation. While these terms are often used interchangeably, for some definitions they have slightly different meanings. For example, coagulation can be used to describe irreversible, permanent aggregation where the forces holding the particles together are stronger than any external forces caused by stirring or mixing. Flocculation can be used to describe reversible aggregation involving weaker attractive forces, and the aggregate is usually called a floc. The term precipitation is normally reserved for describing a phase change from a colloid dispersion to a solid (precipitate) when it is subjected to a perturbation. Aggregation causes sedimentation or creaming, therefore the colloid is unstable: if either of these processes occur the colloid will no longer be a suspension.	1,040	Colloid	https://en.wikipedia.org/wiki/Colloid	1,082	25	2,024	8	10	31	2
Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation.	119	Colloid	https://en.wikipedia.org/wiki/Colloid	1,083	26	2,024	8	10	31	2
A combination of the two mechanisms is also possible (electrosteric stabilization).	83	Colloid	https://en.wikipedia.org/wiki/Colloid	1,084	27	2,024	8	10	31	2
A method called gel network stabilization represents the principal way to produce colloids stable to both aggregation and sedimentation. The method consists in adding to the colloidal suspension a polymer able to form a gel network. Particle settling is hindered by the stiffness of the polymeric matrix where particles are trapped, and the long polymeric chains can provide a steric or electrosteric stabilization to dispersed particles. Examples of such substances are xanthan and guar gum.	492	Colloid	https://en.wikipedia.org/wiki/Colloid	1,085	28	2,024	8	10	31	2
Destabilization can be accomplished by different methods:	57	Colloid	https://en.wikipedia.org/wiki/Colloid	1,086	29	2,024	8	10	31	2
Unstable colloidal suspensions of low-volume fraction form clustered liquid suspensions, wherein individual clusters of particles sediment if they are more dense than the suspension medium, or cream if they are less dense. However, colloidal suspensions of higher-volume fraction form colloidal gels with viscoelastic properties. Viscoelastic colloidal gels, such as bentonite and toothpaste, flow like liquids under shear, but maintain their shape when shear is removed. It is for this reason that toothpaste can be squeezed from a toothpaste tube, but stays on the toothbrush after it is applied.	598	Colloid	https://en.wikipedia.org/wiki/Colloid	1,087	30	2,024	8	10	31	2
The most widely used technique to monitor the dispersion state of a product, and to identify and quantify destabilization phenomena, is multiple light scattering coupled with vertical scanning. This method, known as turbidimetry, is based on measuring the fraction of light that, after being sent through the sample, it backscattered by the colloidal particles. The backscattering intensity is directly proportional to the average particle size and volume fraction of the dispersed phase. Therefore, local changes in concentration caused by sedimentation or creaming, and clumping together of particles caused by aggregation, are detected and monitored. These phenomena are associated with unstable colloids.	708	Colloid	https://en.wikipedia.org/wiki/Colloid	1,088	31	2,024	8	10	31	2
Dynamic light scattering can be used to detect the size of a colloidal particle by measuring how fast they diffuse. This method involves directing laser light towards a colloid. The scattered light will form an interference pattern, and the fluctuation in light intensity in this pattern is caused by the Brownian motion of the particles. If the apparent size of the particles increases due to them clumping together via aggregation, it will result in slower Brownian motion. This technique can confirm that aggregation has occurred if the apparent particle size is determined to be beyond the typical size range for colloidal particles.	637	Colloid	https://en.wikipedia.org/wiki/Colloid	1,089	32	2,024	8	10	31	2
The kinetic process of destabilisation can be rather long (up to several months or years for some products). Thus, it is often required for the formulator to use further accelerating methods to reach reasonable development time for new product design. Thermal methods are the most commonly used and consist of increasing temperature to accelerate destabilisation (below critical temperatures of phase inversion or chemical degradation). Temperature affects not only viscosity, but also interfacial tension in the case of non-ionic surfactants or more generally interactions forces inside the system. Storing a dispersion at high temperatures enables to simulate real life conditions for a product (e.g. tube of sunscreen cream in a car in the summer), but also to accelerate destabilisation processes up to 200 times. Mechanical acceleration including vibration, centrifugation and agitation are sometimes used. They subject the product to different forces that pushes the particles / droplets against one another, hence helping in the film drainage. Some emulsions would never coalesce in normal gravity, while they do under artificial gravity. Segregation of different populations of particles have been highlighted when using centrifugation and vibration.	1,258	Colloid	https://en.wikipedia.org/wiki/Colloid	1,090	33	2,024	8	10	31	2
In physics, colloids are an interesting model system for atoms. Micrometre-scale colloidal particles are large enough to be observed by optical techniques such as confocal microscopy. Many of the forces that govern the structure and behavior of matter, such as excluded volume interactions or electrostatic forces, govern the structure and behavior of colloidal suspensions. For example, the same techniques used to model ideal gases can be applied to model the behavior of a hard sphere colloidal suspension. Phase transitions in colloidal suspensions can be studied in real time using optical techniques, and are analogous to phase transitions in liquids. In many interesting cases optical fluidity is used to control colloid suspensions.	740	Colloid	https://en.wikipedia.org/wiki/Colloid	1,091	34	2,024	8	10	31	2
A colloidal crystal is a highly ordered array of particles that can be formed over a very long range (typically on the order of a few millimeters to one centimeter) and that appear analogous to their atomic or molecular counterparts. One of the finest natural examples of this ordering phenomenon can be found in precious opal, in which brilliant regions of pure spectral color result from close-packed domains of amorphous colloidal spheres of silicon dioxide (or silica, SiO 2). These spherical particles precipitate in highly siliceous pools in Australia and elsewhere, and form these highly ordered arrays after years of sedimentation and compression under hydrostatic and gravitational forces. The periodic arrays of submicrometre spherical particles provide similar arrays of interstitial voids, which act as a natural diffraction grating for visible light waves, particularly when the interstitial spacing is of the same order of magnitude as the incident lightwave.	973	Colloid	https://en.wikipedia.org/wiki/Colloid	1,092	35	2,024	8	10	31	2
Thus, it has been known for many years that, due to repulsive Coulombic interactions, electrically charged macromolecules in an aqueous environment can exhibit long-range crystal -like correlations with interparticle separation distances, often being considerably greater than the individual particle diameter. In all of these cases in nature, the same brilliant iridescence (or play of colors) can be attributed to the diffraction and constructive interference of visible lightwaves that satisfy Bragg’s law, in a matter analogous to the scattering of X-rays in crystalline solids.	582	Colloid	https://en.wikipedia.org/wiki/Colloid	1,093	36	2,024	8	10	31	2
The large number of experiments exploring the physics and chemistry of these so-called "colloidal crystals" has emerged as a result of the relatively simple methods that have evolved in the last 20 years for preparing synthetic monodisperse colloids (both polymer and mineral) and, through various mechanisms, implementing and preserving their long-range order formation.	371	Colloid	https://en.wikipedia.org/wiki/Colloid	1,094	37	2,024	8	10	31	2
Colloidal phase separation is an important organising principle for compartmentalisation of both the cytoplasm and nucleus of cells into biomolecular condensates —similar in importance to compartmentalisation via lipid bilayer membranes, a type of liquid crystal. The term biomolecular condensate has been used to refer to clusters of macromolecules that arise via liquid-liquid or liquid-solid phase separation within cells. Macromolecular crowding strongly enhances colloidal phase separation and formation of biomolecular condensates.	537	Colloid	https://en.wikipedia.org/wiki/Colloid	1,095	38	2,024	8	10	31	2
Colloidal particles can also serve as transport vector of diverse contaminants in the surface water (sea water, lakes, rivers, fresh water bodies) and in underground water circulating in fissured rocks (e.g. limestone, sandstone, granite). Radionuclides and heavy metals easily sorb onto colloids suspended in water. Various types of colloids are recognised: inorganic colloids (e.g. clay particles, silicates, iron oxy-hydroxides), organic colloids (humic and fulvic substances). When heavy metals or radionuclides form their own pure colloids, the term " eigencolloid " is used to designate pure phases, i.e., pure Tc(OH) 4, U(OH) 4, or Am(OH) 3. Colloids have been suspected for the long-range transport of plutonium on the Nevada Nuclear Test Site. They have been the subject of detailed studies for many years. However, the mobility of inorganic colloids is very low in compacted bentonites and in deep clay formations because of the process of ultrafiltration occurring in dense clay membrane. The question is less clear for small organic colloids often mixed in porewater with truly dissolved organic molecules.	1,118	Colloid	https://en.wikipedia.org/wiki/Colloid	1,096	39	2,024	8	10	31	2
In soil science, the colloidal fraction in soils consists of tiny clay and humus particles that are less than 1μm in diameter and carry either positive and/or negative electrostatic charges that vary depending on the chemical conditions of the soil sample, i.e. soil pH.	270	Colloid	https://en.wikipedia.org/wiki/Colloid	1,097	40	2,024	8	10	31	2
Colloid solutions used in intravenous therapy belong to a major group of volume expanders, and can be used for intravenous fluid replacement. Colloids preserve a high colloid osmotic pressure in the blood, and therefore, they should theoretically preferentially increase the intravascular volume, whereas other types of volume expanders called crystalloids also increase the interstitial volume and intracellular volume. However, there is still controversy to the actual difference in efficacy by this difference, and much of the research related to this use of colloids is based on fraudulent research by Joachim Boldt. Another difference is that crystalloids generally are much cheaper than colloids.	702	Colloid	https://en.wikipedia.org/wiki/Colloid	1,098	41	2,024	8	10	31	2
Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood.	552	Physical_cosmology	https://en.wikipedia.org/wiki/Physical_cosmology	1,099	0	2,024	8	10	31	3

or, integrating:

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where h (0) is the height of the water at r = 0. In other words, the surface of the water is parabolic in its dependence upon the radius.

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The shape of the water's surface can be found in a different, very intuitive way using the interesting idea of the potential energy associated with the centrifugal force in the co-rotating frame. In a reference frame uniformly rotating at angular rate Ω, the fictitious centrifugal force is conservative and has a potential energy of the form:

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Bucket_argument

https://en.wikipedia.org/wiki/Bucket_argument

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where r is the radius from the axis of rotation. This result can be verified by taking the gradient of the potential to obtain the radially outward force:

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The meaning of the potential energy (stored work) is that movement of a test body from a larger radius to a smaller radius involves doing work against the centrifugal force and thus gaining potential energy. But this test body at the smaller radius where its elevation is lower has now lost equivalent gravitational potential energy.

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Potential energy therefore explains the concavity of the water surface in a rotating bucket. Notice that at equilibrium the surface adopts a shape such that an element of volume at any location on its surface has the same potential energy as at any other. That being so, no element of water on the surface has any incentive to move position, because all positions are equivalent in energy. That is, equilibrium is attained. On the other hand, were surface regions with lower energy available, the water occupying surface locations of higher potential energy would move to occupy these positions of lower energy, inasmuch as there is no barrier to lateral movement in an ideal liquid.

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We might imagine deliberately upsetting this equilibrium situation by somehow momentarily altering the surface shape of the water to make it different from an equal-energy surface. This change in shape would not be stable, and the water would not stay in our artificially contrived shape, but engage in a transient exploration of many shapes until non-ideal frictional forces introduced by sloshing, either against the sides of the bucket or by the non-ideal nature of the liquid, killed the oscillations and the water settled down to the equilibrium shape.

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To see the principle of an equal-energy surface at work, imagine gradually increasing the rate of rotation of the bucket from zero. The water surface is flat at first, and clearly a surface of equal potential energy because all points on the surface are at the same height in the gravitational field acting upon the water. At some small angular rate of rotation, however, an element of surface water can achieve lower potential energy by moving outward under the influence of the centrifugal force; think of an object moving with the force of gravity closer to the Earth's center: the object lowers its potential energy by complying with a force. Because water is incompressible and must remain within the confines of the bucket, this outward movement increases the depth of water at the larger radius, increasing the height of the surface at larger radius, and lowering it at smaller radius. The surface of the water becomes slightly concave, with the consequence that the potential energy of the water at the greater radius is increased by the work done against gravity to achieve the greater height. As the height of water increases, movement toward the periphery becomes no longer advantageous, because the reduction in potential energy from working with the centrifugal force is balanced against the increase in energy working against gravity. Thus, at a given angular rate of rotation, a concave surface represents the stable situation, and the more rapid the rotation, the more concave this surface. If rotation is arrested, the energy stored in fashioning the concave surface must be dissipated, for example through friction, before an equilibrium flat surface is restored.

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https://en.wikipedia.org/wiki/Bucket_argument

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To implement a surface of constant potential energy quantitatively, let the height of the water be h ( r ) : then the potential energy per unit mass contributed by gravity is g h ( r ) and the total potential energy per unit mass on the surface is

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with U 0 the background energy level independent of r. In a static situation (no motion of the fluid in the rotating frame), this energy is constant independent of position r. Requiring the energy to be constant, we obtain the parabolic form:

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where h (0) is the height at r = 0 (the axis). See Figures 1 and 2.

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The principle of operation of the centrifuge also can be simply understood in terms of this expression for the potential energy, which shows that it is favorable energetically when the volume far from the axis of rotation is occupied by the heavier substance.

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Four views of the constellation Orion :

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A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.

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The first constellations likely go back to prehistory. People used them to relate stories of their beliefs, experiences, creation, mythology. Different cultures and countries invented their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. The recognition of constellations has changed significantly over time. Many changed in size or shape. Some became popular, only to drop into obscurity. Some were limited to a single culture or nation. Naming constellations also helped astronomers and navigators identify stars more easily.

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Twelve (or thirteen) ancient constellations belong to the zodiac (straddling the ecliptic, which the Sun, Moon, and planets all traverse). The origins of the zodiac remain historically uncertain; its astrological divisions became prominent c. 400 BC in Babylonian or Chaldean astronomy. Constellations appear in Western culture via Greece and are mentioned in the works of Hesiod, Eudoxus and Aratus. The traditional 48 constellations, consisting of the zodiac and 36 more (now 38, following the division of Argo Navis into three constellations) are listed by Ptolemy, a Greco-Roman astronomer from Alexandria, Egypt, in his Almagest. The formation of constellations was the subject of extensive mythology, most notably in the Metamorphoses of the Latin poet Ovid. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Due to Roman and European transmission, each constellation has a Latin name.

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In 1922, the International Astronomical Union (IAU) formally accepted the modern list of 88 constellations, and in 1928 adopted official constellation boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations. Some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation's name.

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Other star patterns or groups called asterisms are not constellations under the formal definition, but are also used by observers to navigate the night sky. Asterisms may be several stars within a constellation, or they may share stars with more than one constellation. Examples of asterisms include the teapot within the constellation Sagittarius, or the big dipper in the constellation of Ursa Major.

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The word constellation comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars"; it came into use in Middle English during the 14th century. The Ancient Greek word for constellation is ἄστρον (astron). These terms historically referred to any recognisable pattern of stars whose appearance was associated with mythological characters or creatures, earthbound animals, or objects. Over time, among European astronomers, the constellations became clearly defined and widely recognised. In the 20th century, the International Astronomical Union (IAU) recognized 88 constellations.

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A constellation or star that never sets below the horizon when viewed from a particular latitude on Earth is termed circumpolar. From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic (or zodiac) ranging between 23.5° north and 23.5° south.

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Stars in constellations can appear near each other in the sky, but they usually lie at a variety of distances away from the Earth. Since each star has its own independent motion, all constellations will change slowly over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring common proper motions of individual stars by accurate astrometry and their radial velocities by astronomical spectroscopy.

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The 88 constellations recognized by the IAU as well as those by cultures throughout history are imagined figures and shapes derived from the patterns of stars in the observable sky. Many officially recognized constellations are based on the imaginations of ancient, Near Eastern and Mediterranean mythologies. Some of these stories seem to relate to the appearance of the constellations, e.g. the assassination of Orion by Scorpius, their constellations appearing at opposite times of year.

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Constellation positions change throughout the year due to night on Earth occurring at gradually different portions of its orbit around the Sun. As Earth rotates toward the east, the celestial sphere appears to rotate west, with stars circling counterclockwise around the northern pole star and clockwise around the southern pole star.

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Because of Earth's 23.5° axial tilt, the zodiac is distributed equally across hemispheres (along the ecliptic), approximating a great circle. Zodiacal constellations of the northern sky are Pisces, Aries, Taurus, Gemini, Cancer, and Leo. In the southern sky are Virgo, Libra, Scorpius, Sagittarius, Capricornus, and Aquarius. The zodiac appears directly overhead from latitudes of 23.5° north to 23.5° south, depending on the time of year. In summer, the ecliptic appears higher up in the daytime and lower at night, while in winter the reverse is true, for both hemispheres.

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Due to the Solar System 's 60° tilt, the galactic plane of the Milky Way is inclined 60° from the ecliptic, between Taurus and Gemini (north) and Scorpius and Sagittarius (south and near which the Galactic Center can be found). The galaxy appears to pass through Aquila (near the celestial equator) and northern constellations Cygnus, Cassiopeia, Perseus, Auriga, and Orion (near Betelgeuse), as well as Monoceros (near the celestial equator), and southern constellations Puppis, Vela, Carina, Crux, Centaurus, Triangulum Australe, and Ara.

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Polaris, being the North Star, is the approximate center of the northern celestial hemisphere. It is part of Ursa Minor, constituting the end of the Little Dipper's handle.

172

Constellation

https://en.wikipedia.org/wiki/Constellation

1,025

13

2,024

8

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31

2

0

From latitudes of around 35° north, in January, Ursa Major (containing the Big Dipper) appears to the northeast, while Cassiopeia is the northwest. To the west are Pisces (above the horizon) and Aries. To the southwest Cetus is near the horizon. Up high and to the south are Orion and Taurus. To the southeast above the horizon is Canis Major. Appearing above and to the east of Orion is Gemini : also in the east (and progressively closer to the horizon) are Cancer and Leo. In addition to Taurus, Perseus and Auriga appear overhead.

534

Constellation

https://en.wikipedia.org/wiki/Constellation

1,026

14

2,024

8

10

0

31

2

0

From the same latitude, in July, Cassiopeia (low in the sky) and Cepheus appear to the northeast. Ursa Major is now in the northwest. Boötes is high up in the west. Virgo is to the west, with Libra southwest and Scorpius south. Sagitarrius and Capricorn are southeast. Cygnus (containing the Northern Cross) is to the east. Hercules is high in the sky along with Corona Borealis.

379

Constellation

https://en.wikipedia.org/wiki/Constellation

1,027

15

2,024

8

10

0

31

2

0

January constellations include Pictor and Reticulum (near Hydrus and Mensa, respectively).

90

Constellation

https://en.wikipedia.org/wiki/Constellation

1,028

16

2,024

8

10

0

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2

0

In July, Ara (adjacent to Triangulum Australe) and Scorpius can be seen.

72

Constellation

https://en.wikipedia.org/wiki/Constellation

1,029

17

2,024

8

10

0

31

2

0

Constellations near the pole star include Chamaeleon, Apus and Triangulum Australe (near Centaurus), Pavo, Hydrus, and Mensa.

125

Constellation

https://en.wikipedia.org/wiki/Constellation

1,030

18

2,024

8

10

0

31

2

0

Sigma Octantis is the closest star approximating a southern pole star, but is faint in the night sky. Thus, the pole can be triangulated using the constellation Crux as well as the stars Alpha and Beta Centauri (about 30° counterclockwise from Crux) of the constellation Centaurus (arching over Crux).

301

Constellation

https://en.wikipedia.org/wiki/Constellation

1,031

19

2,024

8

10

0

31

2

0

It has been suggested that the 17,000-year-old cave paintings in Lascaux, southern France, depict star constellations such as Taurus, Orion's Belt, and the Pleiades. However, this view is not generally accepted among scientists.

228

Constellation

https://en.wikipedia.org/wiki/Constellation

1,032

20

2,024

8

10

0

31

2

0

Inscribed stones and clay writing tablets from Mesopotamia (in modern Iraq) dating to 3000 BC provide the earliest generally accepted evidence for humankind's identification of constellations. It seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared later in many of the classical Greek constellations.

417

Constellation

https://en.wikipedia.org/wiki/Constellation

1,033

21

2,024

8

10

0

31

2

0

The oldest Babylonian catalogues of stars and constellations date back to the beginning of the Middle Bronze Age, most notably the Three Stars Each texts and the MUL.APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age.

421

Constellation

https://en.wikipedia.org/wiki/Constellation

1,034

22

2,024

8

10

0

31

2

0

The classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names.

281

Constellation

https://en.wikipedia.org/wiki/Constellation

1,035

23

2,024

8

10

0

31

2

0

Biblical scholar E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four-quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius, and the Eagle standing in for Scorpio. The biblical Book of Job also makes reference to a number of constellations, including עיש ‘Ayish "bier", כסיל chesil "fool" and כימה chimah "heap" (Job 9:9, 38:31–32), rendered as "Arcturus, Orion and Pleiades" by the KJV, but ‘Ayish "the bier" actually corresponding to Ursa Major. The term Mazzaroth מַזָּרוֹת, translated as a garland of crowns, is a hapax legomenon in Job 38:32, and it might refer to the zodiacal constellations.

718

Constellation

https://en.wikipedia.org/wiki/Constellation

1,036

24

2,024

8

10

0

31

2

0

There is only limited information on ancient Greek constellations, with some fragmentary evidence being found in the Works and Days of the Greek poet Hesiod, who mentioned the "heavenly bodies". Greek astronomy essentially adopted the older Babylonian system in the Hellenistic era, first introduced to Greece by Eudoxus of Cnidus in the 4th century BC. The original work of Eudoxus is lost, but it survives as a versification by Aratus, dating to the 3rd century BC. The most complete existing works dealing with the mythical origins of the constellations are by the Hellenistic writer termed pseudo-Eratosthenes and an early Roman writer styled pseudo- Hyginus. The basis of Western astronomy as taught during Late Antiquity and until the Early Modern period is the Almagest by Ptolemy, written in the 2nd century.

816

Constellation

https://en.wikipedia.org/wiki/Constellation

1,037

25

2,024

8

10

0

31

2

0

In the Ptolemaic Kingdom, native Egyptian tradition of anthropomorphic figures represented the planets, stars, and various constellations. Some of these were combined with Greek and Babylonian astronomical systems culminating in the Zodiac of Dendera ; it remains unclear when this occurred, but most were placed during the Roman period between 2nd to 4th centuries AD. The oldest known depiction of the zodiac showing all the now familiar constellations, along with some original Egyptian constellations, decans, and planets. Ptolemy's Almagest remained the standard definition of constellations in the medieval period both in Europe and in Islamic astronomy.

660

Constellation

https://en.wikipedia.org/wiki/Constellation

1,038

26

2,024

8

10

0

31

2

0

Ancient China had a long tradition of observing celestial phenomena. Nonspecific Chinese star names, later categorized in the twenty-eight mansions, have been found on oracle bones from Anyang, dating back to the middle Shang dynasty. These constellations are some of the most important observations of Chinese sky, attested from the 5th century BC. Parallels to the earliest Babylonian (Sumerian) star catalogues suggest that the ancient Chinese system did not arise independently.

482

Constellation

https://en.wikipedia.org/wiki/Constellation

1,039

27

2,024

8

10

0

31

2

0

Three schools of classical Chinese astronomy in the Han period are attributed to astronomers of the earlier Warring States period. The constellations of the three schools were conflated into a single system by Chen Zhuo, an astronomer of the 3rd century (Three Kingdoms period). Chen Zhuo's work has been lost, but information on his system of constellations survives in Tang period records, notably by Qutan Xida. The oldest extant Chinese star chart dates to that period and was preserved as part of the Dunhuang Manuscripts. Native Chinese astronomy flourished during the Song dynasty, and during the Yuan dynasty became increasingly influenced by medieval Islamic astronomy (see Treatise on Astrology of the Kaiyuan Era). As maps were prepared during this period on more scientific lines, they were considered as more reliable.

831

Constellation

https://en.wikipedia.org/wiki/Constellation

1,040

28

2,024

8

10

0

31

2

0

A well-known map from the Song period is the Suzhou Astronomical Chart, which was prepared with carvings of stars on the planisphere of the Chinese sky on a stone plate; it is done accurately based on observations, and it shows the supernova of the year of 1054 in Taurus.

272

Constellation

https://en.wikipedia.org/wiki/Constellation

1,041

29

2,024

8

10

0

31

2

0

Influenced by European astronomy during the late Ming dynasty, charts depicted more stars but retained the traditional constellations. Newly observed stars were incorporated as supplementary to old constellations in the southern sky, which did not depict the traditional stars recorded by ancient Chinese astronomers. Further improvements were made during the later part of the Ming dynasty by Xu Guangqi and Johann Adam Schall von Bell, the German Jesuit and was recorded in Chongzhen Lishu (Calendrical Treatise of Chongzhen period, 1628). Traditional Chinese star maps incorporated 23 new constellations with 125 stars of the southern hemisphere of the sky based on the knowledge of Western star charts; with this improvement, the Chinese Sky was integrated with the World astronomy.

786

Constellation

https://en.wikipedia.org/wiki/Constellation

1,042

30

2,024

8

10

0

31

2

0

Historically, the origins of the constellations of the northern and southern skies are distinctly different. Most northern constellations date to antiquity, with names based mostly on Classical Greek legends. Evidence of these constellations has survived in the form of star charts, whose oldest representation appears on the statue known as the Farnese Atlas, based perhaps on the star catalogue of the Greek astronomer Hipparchus. Southern constellations are more modern inventions, sometimes as substitutes for ancient constellations (e.g. Argo Navis). Some southern constellations had long names that were shortened to more usable forms; e.g. Musca Australis became simply Musca.

683

Constellation

https://en.wikipedia.org/wiki/Constellation

1,043

31

2,024

8

10

0

31

2

0

Some of the early constellations were never universally adopted. Stars were often grouped into constellations differently by different observers, and the arbitrary constellation boundaries often led to confusion as to which constellation a celestial object belonged. Before astronomers delineated precise boundaries (starting in the 19th century), constellations generally appeared as ill-defined regions of the sky. Today they now follow officially accepted designated lines of right ascension and declination based on those defined by Benjamin Gould in epoch 1875.0 in his star catalogue Uranometria Argentina.

612

Constellation

https://en.wikipedia.org/wiki/Constellation

1,044

32

2,024

8

10

0

31

2

0

The 1603 star atlas " Uranometria " of Johann Bayer assigned stars to individual constellations and formalized the division by assigning a series of Greek and Latin letters to the stars within each constellation. These are known today as Bayer designations. Subsequent star atlases led to the development of today's accepted modern constellations.

347

Constellation

https://en.wikipedia.org/wiki/Constellation

1,045

33

2,024

8

10

0

31

2

0

The southern sky, below about −65° declination, was only partially catalogued by ancient Babylonians, Egyptians, Greeks, Chinese, and Persian astronomers of the north. The knowledge that northern and southern star patterns differed goes back to Classical writers, who describe, for example, the African circumnavigation expedition commissioned by Egyptian Pharaoh Necho II in c. 600 BC and those of Hanno the Navigator in c. 500 BC.

432

Constellation

https://en.wikipedia.org/wiki/Constellation

1,046

34

2,024

8

10

0

31

2

0

The history of southern constellations is not straightforward. Different groupings and different names were proposed by various observers, some reflecting national traditions or designed to promote various sponsors. Southern constellations were important from the 14th to 16th centuries, when sailors used the stars for celestial navigation. Italian explorers who recorded new southern constellations include Andrea Corsali, Antonio Pigafetta, and Amerigo Vespucci.

465

Constellation

https://en.wikipedia.org/wiki/Constellation

1,047

35

2,024

8

10

0

31

2

0

Many of the 88 IAU-recognized constellations in this region first appeared on celestial globes developed in the late 16th century by Petrus Plancius, based mainly on observations of the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman. These became widely known through Johann Bayer 's star atlas Uranometria of 1603. Fourteen more were created in 1763 by the French astronomer Nicolas Louis de Lacaille, who also split the ancient constellation Argo Navis into three; these new figures appeared in his star catalogue, published in 1756.

554

Constellation

https://en.wikipedia.org/wiki/Constellation

1,048

36

2,024

8

10

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31

2

0

Several modern proposals have not survived. The French astronomers Pierre Lemonnier and Joseph Lalande, for example, proposed constellations that were once popular but have since been dropped. The northern constellation Quadrans Muralis survived into the 19th century (when its name was attached to the Quadrantid meteor shower), but is now divided between Boötes and Draco.

374

Constellation

https://en.wikipedia.org/wiki/Constellation

1,049

37

2,024

8

10

0

31

2

0

A list of 88 constellations was produced for the IAU in 1922. It is roughly based on the traditional Greek constellations listed by Ptolemy in his Almagest in the 2nd century and Aratus ' work Phenomena, with early modern modifications and additions (most importantly introducing constellations covering the parts of the southern sky unknown to Ptolemy) by Petrus Plancius (1592, 1597/98 and 1613), Johannes Hevelius (1690) and Nicolas Louis de Lacaille (1763), who introduced fourteen new constellations. Lacaille studied the stars of the southern hemisphere from 1751 until 1752 from the Cape of Good Hope, when he was said to have observed more than 10,000 stars using a refracting telescope with an aperture of 0.5 inches (13 mm).

734

Constellation

https://en.wikipedia.org/wiki/Constellation

1,050

38

2,024

8

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31

2

0

In 1922, Henry Norris Russell produced a list of 88 constellations with three-letter abbreviations for them. However, these constellations did not have clear borders between them. In 1928, the IAU formally accepted the 88 modern constellations, with contiguous boundaries along vertical and horizontal lines of right ascension and declination developed by Eugene Delporte that, together, cover the entire celestial sphere; this list was finally published in 1930. Where possible, these modern constellations usually share the names of their Graeco-Roman predecessors, such as Orion, Leo, or Scorpius. The aim of this system is area-mapping, i.e. the division of the celestial sphere into contiguous fields. Out of the 88 modern constellations, 36 lie predominantly in the northern sky, and the other 52 predominantly in the southern.

833

Constellation

https://en.wikipedia.org/wiki/Constellation

1,051

39

2,024

8

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0

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2

0

The boundaries developed by Delporte used data that originated back to epoch B1875.0, which was when Benjamin A. Gould first made his proposal to designate boundaries for the celestial sphere, a suggestion on which Delporte based his work. The consequence of this early date is that because of the precession of the equinoxes, the borders on a modern star map, such as epoch J2000, are already somewhat skewed and no longer perfectly vertical or horizontal. This effect will increase over the years and centuries to come.

521

Constellation

https://en.wikipedia.org/wiki/Constellation

1,052

40

2,024

8

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31

2

0

The constellations have no official symbols, though those of the ecliptic may take the signs of the zodiac. Symbols for the other modern constellations, as well as older ones that still occur in modern nomenclature, have occasionally been published.

249

Constellation

https://en.wikipedia.org/wiki/Constellation

1,053

41

2,024

8

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2

0

The Great Rift, a series of dark patches in the Milky Way, is most visible in the southern sky. Some cultures have discerned shapes in these patches. Members of the Inca civilization identified various dark areas or dark nebulae in the Milky Way as animals and associated their appearance with the seasonal rains. Australian Aboriginal astronomy also describes dark cloud constellations, the most famous being the "emu in the sky" whose head is formed by the Coalsack, a dark nebula, instead of the stars.

505

Constellation

https://en.wikipedia.org/wiki/Constellation

1,054

42

2,024

8

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31

2

0

Footnotes

9

Constellation

https://en.wikipedia.org/wiki/Constellation

1,055

43

2,024

8

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31

2

0

Citations

9

Constellation

https://en.wikipedia.org/wiki/Constellation

1,056

44

2,024

8

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31

2

0

A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture (although a narrower sense of the word suspension is distinguished from colloids by larger particle size). A colloid has a dispersed phase (the suspended particles) and a continuous phase (the medium of suspension). The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.

686

Colloid

https://en.wikipedia.org/wiki/Colloid

1,057

0

2,024

8

10

0

31

2

0

Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.

175

Colloid

https://en.wikipedia.org/wiki/Colloid

1,058

1

2,024

8

10

0

31

2

0

Colloidal suspensions are the subject of interface and colloid science. This field of study began in 1845 by Francesco Selmi, who called them pseudosolutions, and expanded by Michael Faraday and Thomas Graham, who coined the term colloid in 1861.

246

Colloid

https://en.wikipedia.org/wiki/Colloid

1,059

2

2,024

8

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31

2

0

Colloid : Short synonym for colloidal system.

45

Colloid

https://en.wikipedia.org/wiki/Colloid

1,060

3

2,024

8

10

0

31

2

0

Colloidal : State of subdivision such that the molecules or polymolecular particles dispersed in a medium have at least one dimension between approximately 1 nm and 1 μm, or that in a system discontinuities are found at distances of that order.

244

Colloid

https://en.wikipedia.org/wiki/Colloid

1,061

4

2,024

8

10

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31

2

0

Colloids can be classified as follows:

38

Colloid

https://en.wikipedia.org/wiki/Colloid

1,062

5

2,024

8

10

0

31

2

0

Homogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal suspensions, colloidal foams, colloidal dispersions, or hydrosols.

194

Colloid

https://en.wikipedia.org/wiki/Colloid

1,063

6

2,024

8

10

0

31

2

0

Hydrocolloids describe certain chemicals (mostly polysaccharides and proteins) that are colloidally dispersible in water. Thus becoming effectively "soluble" they change the rheology of water by raising the viscosity and/or inducing gelation. They may provide other interactive effects with other chemicals, in some cases synergistic, in others antagonistic. Using these attributes hydrocolloids are very useful chemicals since in many areas of technology from foods through pharmaceuticals, personal care and industrial applications, they can provide stabilization, destabilization and separation, gelation, flow control, crystallization control and numerous other effects. Apart from uses of the soluble forms some of the hydrocolloids have additional useful functionality in a dry form if after solubilization they have the water removed - as in the formation of films for breath strips or sausage casings or indeed, wound dressing fibers, some being more compatible with skin than others. There are many different types of hydrocolloids each with differences in structure function and utility that generally are best suited to particular application areas in the control of rheology and the physical modification of form and texture. Some hydrocolloids like starch and casein are useful foods as well as rheology modifiers, others have limited nutritive value, usually providing a source of fiber.

1,401

Colloid

https://en.wikipedia.org/wiki/Colloid

1,064

7

2,024

8

10

0

31

2

0

The term hydrocolloids also refers to a type of dressing designed to lock moisture in the skin and help the natural healing process of skin to reduce scarring, itching and soreness.

181

Colloid

https://en.wikipedia.org/wiki/Colloid

1,065

8

2,024

8

10

0

31

2

0

Hydrocolloids contain some type of gel-forming agent, such as sodium carboxymethylcellulose (NaCMC) and gelatin. They are normally combined with some type of sealant, i.e. polyurethane to 'stick' to the skin.

208

Colloid

https://en.wikipedia.org/wiki/Colloid

1,066

9

2,024

8

10

0

31

2

0

A colloid has a dispersed phase and a continuous phase, whereas in a solution, the solute and solvent constitute only one phase. A solute in a solution are individual molecules or ions, whereas colloidal particles are bigger. For example, in a solution of salt in water, the sodium chloride (NaCl) crystal dissolves, and the Na and Cl ions are surrounded by water molecules. However, in a colloid such as milk, the colloidal particles are globules of fat, rather than individual fat molecules. Because colloid is multiple phases, it has very different properties compared to fully mixed, continuous solution.

608

Colloid

https://en.wikipedia.org/wiki/Colloid

1,067

10

2,024

8

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2

0

The following forces play an important role in the interaction of colloid particles:

84

Colloid

https://en.wikipedia.org/wiki/Colloid

1,068

11

2,024

8

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0

31

2

0

The Earth’s gravitational field acts upon colloidal particles. Therefore, if the colloidal particles are denser than the medium of suspension, they will sediment (fall to the bottom), or if they are less dense, they will cream (float to the top). Larger particles also have a greater tendency to sediment because they have smaller Brownian motion to counteract this movement.

375

Colloid

https://en.wikipedia.org/wiki/Colloid

1,069

12

2,024

8

10

0

31

2

0

The sedimentation or creaming velocity is found by equating the Stokes drag force with the gravitational force :

112

Colloid

https://en.wikipedia.org/wiki/Colloid

1,070

13

2,024

8

10

0

31

2

0

where

5

Colloid

https://en.wikipedia.org/wiki/Colloid

1,071

14

2,024

8

10

0

31

2

0

and v is the sedimentation or creaming velocity.

48

Colloid

https://en.wikipedia.org/wiki/Colloid

1,072

15

2,024

8

10

0

31

2

0

The mass of the colloidal particle is found using:

50

Colloid

https://en.wikipedia.org/wiki/Colloid

1,073

16

2,024

8

10

0

31

2

0

where

5

Colloid

https://en.wikipedia.org/wiki/Colloid

1,074

17

2,024

8

10

0

31

2

0

and ρ ρ 1 − − ρ ρ 2 is the difference in mass density between the colloidal particle and the suspension medium.

111

Colloid

https://en.wikipedia.org/wiki/Colloid

1,075

18

2,024

8

10

0

31

2

0

By rearranging, the sedimentation or creaming velocity is:

58

Colloid

https://en.wikipedia.org/wiki/Colloid

1,076

19

2,024

8

10

0

31

2

0

There is an upper size-limit for the diameter of colloidal particles because particles larger than 1 μm tend to sediment, and thus the substance would no longer be considered a colloidal suspension.

198

Colloid

https://en.wikipedia.org/wiki/Colloid

1,077

20

2,024

8

10

0

31

2

0

The colloidal particles are said to be in sedimentation equilibrium if the rate of sedimentation is equal to the rate of movement from Brownian motion.

151

Colloid

https://en.wikipedia.org/wiki/Colloid

1,078

21

2,024

8

10

0

31

2

0

There are two principal ways to prepare colloids:

49

Colloid

https://en.wikipedia.org/wiki/Colloid

1,079

22

2,024

8

10

0

31

2

0

The stability of a colloidal system is defined by particles remaining suspended in solution and depends on the interaction forces between the particles. These include electrostatic interactions and van der Waals forces, because they both contribute to the overall free energy of the system.

290

Colloid

https://en.wikipedia.org/wiki/Colloid

1,080

23

2,024

8

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2

0

A colloid is stable if the interaction energy due to attractive forces between the colloidal particles is less than kT, where k is the Boltzmann constant and T is the absolute temperature. If this is the case, then the colloidal particles will repel or only weakly attract each other, and the substance will remain a suspension.

328

Colloid

https://en.wikipedia.org/wiki/Colloid

1,081

24

2,024

8

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2

0

If the interaction energy is greater than kT, the attractive forces will prevail, and the colloidal particles will begin to clump together. This process is referred to generally as aggregation, but is also referred to as flocculation, coagulation or precipitation. While these terms are often used interchangeably, for some definitions they have slightly different meanings. For example, coagulation can be used to describe irreversible, permanent aggregation where the forces holding the particles together are stronger than any external forces caused by stirring or mixing. Flocculation can be used to describe reversible aggregation involving weaker attractive forces, and the aggregate is usually called a floc. The term precipitation is normally reserved for describing a phase change from a colloid dispersion to a solid (precipitate) when it is subjected to a perturbation. Aggregation causes sedimentation or creaming, therefore the colloid is unstable: if either of these processes occur the colloid will no longer be a suspension.

1,040

Colloid

https://en.wikipedia.org/wiki/Colloid

1,082

25

2,024

8

10

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31

2

0

Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation.

119

Colloid

https://en.wikipedia.org/wiki/Colloid

1,083

26

2,024

8

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0

31

2

0

A combination of the two mechanisms is also possible (electrosteric stabilization).

83

Colloid

https://en.wikipedia.org/wiki/Colloid

1,084

27

2,024

8

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0

31

2

0

A method called gel network stabilization represents the principal way to produce colloids stable to both aggregation and sedimentation. The method consists in adding to the colloidal suspension a polymer able to form a gel network. Particle settling is hindered by the stiffness of the polymeric matrix where particles are trapped, and the long polymeric chains can provide a steric or electrosteric stabilization to dispersed particles. Examples of such substances are xanthan and guar gum.

492

Colloid

https://en.wikipedia.org/wiki/Colloid

1,085

28

2,024

8

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0

31

2

0

Destabilization can be accomplished by different methods:

57

Colloid

https://en.wikipedia.org/wiki/Colloid

1,086

29

2,024

8

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31

2

0

Unstable colloidal suspensions of low-volume fraction form clustered liquid suspensions, wherein individual clusters of particles sediment if they are more dense than the suspension medium, or cream if they are less dense. However, colloidal suspensions of higher-volume fraction form colloidal gels with viscoelastic properties. Viscoelastic colloidal gels, such as bentonite and toothpaste, flow like liquids under shear, but maintain their shape when shear is removed. It is for this reason that toothpaste can be squeezed from a toothpaste tube, but stays on the toothbrush after it is applied.

598

Colloid

https://en.wikipedia.org/wiki/Colloid

1,087

30

2,024

8

10

0

31

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The most widely used technique to monitor the dispersion state of a product, and to identify and quantify destabilization phenomena, is multiple light scattering coupled with vertical scanning. This method, known as turbidimetry, is based on measuring the fraction of light that, after being sent through the sample, it backscattered by the colloidal particles. The backscattering intensity is directly proportional to the average particle size and volume fraction of the dispersed phase. Therefore, local changes in concentration caused by sedimentation or creaming, and clumping together of particles caused by aggregation, are detected and monitored. These phenomena are associated with unstable colloids.

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https://en.wikipedia.org/wiki/Colloid

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Dynamic light scattering can be used to detect the size of a colloidal particle by measuring how fast they diffuse. This method involves directing laser light towards a colloid. The scattered light will form an interference pattern, and the fluctuation in light intensity in this pattern is caused by the Brownian motion of the particles. If the apparent size of the particles increases due to them clumping together via aggregation, it will result in slower Brownian motion. This technique can confirm that aggregation has occurred if the apparent particle size is determined to be beyond the typical size range for colloidal particles.

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https://en.wikipedia.org/wiki/Colloid

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The kinetic process of destabilisation can be rather long (up to several months or years for some products). Thus, it is often required for the formulator to use further accelerating methods to reach reasonable development time for new product design. Thermal methods are the most commonly used and consist of increasing temperature to accelerate destabilisation (below critical temperatures of phase inversion or chemical degradation). Temperature affects not only viscosity, but also interfacial tension in the case of non-ionic surfactants or more generally interactions forces inside the system. Storing a dispersion at high temperatures enables to simulate real life conditions for a product (e.g. tube of sunscreen cream in a car in the summer), but also to accelerate destabilisation processes up to 200 times. Mechanical acceleration including vibration, centrifugation and agitation are sometimes used. They subject the product to different forces that pushes the particles / droplets against one another, hence helping in the film drainage. Some emulsions would never coalesce in normal gravity, while they do under artificial gravity. Segregation of different populations of particles have been highlighted when using centrifugation and vibration.

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https://en.wikipedia.org/wiki/Colloid

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In physics, colloids are an interesting model system for atoms. Micrometre-scale colloidal particles are large enough to be observed by optical techniques such as confocal microscopy. Many of the forces that govern the structure and behavior of matter, such as excluded volume interactions or electrostatic forces, govern the structure and behavior of colloidal suspensions. For example, the same techniques used to model ideal gases can be applied to model the behavior of a hard sphere colloidal suspension. Phase transitions in colloidal suspensions can be studied in real time using optical techniques, and are analogous to phase transitions in liquids. In many interesting cases optical fluidity is used to control colloid suspensions.

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Colloid

https://en.wikipedia.org/wiki/Colloid

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A colloidal crystal is a highly ordered array of particles that can be formed over a very long range (typically on the order of a few millimeters to one centimeter) and that appear analogous to their atomic or molecular counterparts. One of the finest natural examples of this ordering phenomenon can be found in precious opal, in which brilliant regions of pure spectral color result from close-packed domains of amorphous colloidal spheres of silicon dioxide (or silica, SiO 2). These spherical particles precipitate in highly siliceous pools in Australia and elsewhere, and form these highly ordered arrays after years of sedimentation and compression under hydrostatic and gravitational forces. The periodic arrays of submicrometre spherical particles provide similar arrays of interstitial voids, which act as a natural diffraction grating for visible light waves, particularly when the interstitial spacing is of the same order of magnitude as the incident lightwave.

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https://en.wikipedia.org/wiki/Colloid

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Thus, it has been known for many years that, due to repulsive Coulombic interactions, electrically charged macromolecules in an aqueous environment can exhibit long-range crystal -like correlations with interparticle separation distances, often being considerably greater than the individual particle diameter. In all of these cases in nature, the same brilliant iridescence (or play of colors) can be attributed to the diffraction and constructive interference of visible lightwaves that satisfy Bragg’s law, in a matter analogous to the scattering of X-rays in crystalline solids.

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https://en.wikipedia.org/wiki/Colloid

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The large number of experiments exploring the physics and chemistry of these so-called "colloidal crystals" has emerged as a result of the relatively simple methods that have evolved in the last 20 years for preparing synthetic monodisperse colloids (both polymer and mineral) and, through various mechanisms, implementing and preserving their long-range order formation.

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https://en.wikipedia.org/wiki/Colloid

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Colloidal phase separation is an important organising principle for compartmentalisation of both the cytoplasm and nucleus of cells into biomolecular condensates —similar in importance to compartmentalisation via lipid bilayer membranes, a type of liquid crystal. The term biomolecular condensate has been used to refer to clusters of macromolecules that arise via liquid-liquid or liquid-solid phase separation within cells. Macromolecular crowding strongly enhances colloidal phase separation and formation of biomolecular condensates.

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https://en.wikipedia.org/wiki/Colloid

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Colloidal particles can also serve as transport vector of diverse contaminants in the surface water (sea water, lakes, rivers, fresh water bodies) and in underground water circulating in fissured rocks (e.g. limestone, sandstone, granite). Radionuclides and heavy metals easily sorb onto colloids suspended in water. Various types of colloids are recognised: inorganic colloids (e.g. clay particles, silicates, iron oxy-hydroxides), organic colloids (humic and fulvic substances). When heavy metals or radionuclides form their own pure colloids, the term " eigencolloid " is used to designate pure phases, i.e., pure Tc(OH) 4, U(OH) 4, or Am(OH) 3. Colloids have been suspected for the long-range transport of plutonium on the Nevada Nuclear Test Site. They have been the subject of detailed studies for many years. However, the mobility of inorganic colloids is very low in compacted bentonites and in deep clay formations because of the process of ultrafiltration occurring in dense clay membrane. The question is less clear for small organic colloids often mixed in porewater with truly dissolved organic molecules.

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https://en.wikipedia.org/wiki/Colloid

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In soil science, the colloidal fraction in soils consists of tiny clay and humus particles that are less than 1μm in diameter and carry either positive and/or negative electrostatic charges that vary depending on the chemical conditions of the soil sample, i.e. soil pH.

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https://en.wikipedia.org/wiki/Colloid

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Colloid solutions used in intravenous therapy belong to a major group of volume expanders, and can be used for intravenous fluid replacement. Colloids preserve a high colloid osmotic pressure in the blood, and therefore, they should theoretically preferentially increase the intravascular volume, whereas other types of volume expanders called crystalloids also increase the interstitial volume and intracellular volume. However, there is still controversy to the actual difference in efficacy by this difference, and much of the research related to this use of colloids is based on fraudulent research by Joachim Boldt. Another difference is that crystalloids generally are much cheaper than colloids.

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https://en.wikipedia.org/wiki/Colloid

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Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood.

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https://en.wikipedia.org/wiki/Physical_cosmology

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