A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth; the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way. For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and radiates into outer space. All occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, for some stars by supernova nucleosynthesis when it explodes.
Near the end of its life, a star can contain degenerate matter. Astronomers can determine the mass, age and many other properties of a star by observing its motion through space, its luminosity, spectrum respectively; the total mass of a star is the main factor. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined. A star's life begins with the gravitational collapse of a gaseous nebula of material composed of hydrogen, along with helium and trace amounts of heavier elements; when the stellar core is sufficiently dense, hydrogen becomes converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes.
The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements in shells around the core; as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole. Binary and multi-star systems consist of two or more stars that are gravitationally bound and move around each other in stable orbits; when two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. Stars have been important to civilizations throughout the world, they have used for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun; the motion of the Sun against the background stars was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun; the oldest dated star chart was the result of ancient Egyptian astronomy in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period; the first star catalogue in Greek astronomy was created by Aristillus in 300 BC, with the help of Timocharis. The star catalog of Hipparchus included 1020 stars, was used to assemble Ptolemy's star catalogue.
Hipparchus is known for the discovery of the first recorded nova. Many of the constellations and star names in use today derive from Greek astronomy. In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185; the brightest stellar event in recorded history was the SN 1006 supernova, observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. The SN 1054 supernova, which gave birth to the Crab Nebula, was observed by Chinese and Islamic astronomers. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars, they built the first large observatory research institutes for the purpose of producing Zij star catalogues. Among these, the Book of Fixed Stars was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters and galaxies.
According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky
Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface; the rotation of a star produces an equatorial bulge due to centrifugal force. As stars are not solid bodies, they can undergo differential rotation, thus the equator of the star can rotate at a different angular velocity than the higher latitudes. These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field; the magnetic field of a star interacts with the stellar wind. As the wind moves away from the star its rate of angular velocity slows; the magnetic field of the star interacts with the wind, which applies a drag to the stellar rotation. As a result, angular momentum is transferred from the star to the wind, over time this slows the star's rate of rotation. Unless a star is being observed from the direction of its pole, sections of the surface have some amount of movement toward or away from the observer.
The component of movement, in the direction of the observer is called the radial velocity. For the portion of the surface with a radial velocity component toward the observer, the radiation is shifted to a higher frequency because of Doppler shift; the region that has a component moving away from the observer is shifted to a lower frequency. When the absorption lines of a star are observed, this shift at each end of the spectrum causes the line to broaden. However, this broadening must be separated from other effects that can increase the line width; the component of the radial velocity observed through line broadening depends on the inclination of the star's pole to the line of sight. The derived value is given as v e ⋅ sin i, where ve is the rotational velocity at the equator and i is the inclination. However, i is not always known, so the result gives a minimum value for the star's rotational velocity; that is, if i is not a right angle the actual velocity is greater than v e ⋅ sin i. This is sometimes referred to as the projected rotational velocity.
In fast rotating stars polarimetry offers a method of recovering the actual velocity rather than just the rotational velocity. For giant stars, the atmospheric microturbulence can result in line broadening, much larger than effects of rotational drowning out the signal. However, an alternate approach can be employed; these occur when a massive object passes in front of the more distant star and functions like a lens magnifying the image. The more detailed information gathered by this means allows the effects of microturbulence to be distinguished from rotation. If a star displays magnetic surface activity such as starspots these features can be tracked to estimate the rotation rate. However, such features can form at locations other than equator and can migrate across latitudes over the course of their life span, so differential rotation of a star can produce varying measurements. Stellar magnetic activity is associated with rapid rotation, so this technique can be used for measurement of such stars.
Observation of starspots has shown that these features can vary the rotation rate of a star, as the magnetic fields modify the flow of gases in the star. Gravity tends to contract celestial bodies into a perfect sphere, the shape where all the mass is as close to the center of gravity as possible, but a rotating star is not spherical in shape, it has an equatorial bulge. As a rotating proto-stellar disk contracts to form a star its shape becomes more and more spherical, but the contraction doesn't proceed all the way to a perfect sphere. At the poles all of the gravity acts to increase the contraction, but at the equator the effective gravity is diminished by the centrifugal force; the final shape of the star after star formation is an equilibrium shape, in the sense that the effective gravity in the equatorial region cannot pull the star to a more spherical shape. The rotation gives rise to gravity darkening at the equator, as described by the von Zeipel theorem. An extreme example of an equatorial bulge is found on the star Regulus A.
The equator of this star has a measured rotational velocity of 317 ± 3 km/s. This corresponds to a rotation period of 15.9 hours, 86% of the velocity at which the star would break apart. The equatorial radius of this star is 32% larger than polar radius. Other rotating stars include Alpha Arae, Pleione and Achernar; the break-up velocity of a star is an expression, used to describe the case where the centrifugal force at the equator is equal to the gravitational force. For a star to be stable the rotational velocity must be below this value. Surface differential rotation is observed on stars such as the Sun when the angular velocity varies with latitude; the angular velocity decreases with increasing latitude. However the reverse has been observed, such as on the star designated HD 31993; the first such star, other than the Sun, to have its differential rotation mapped in detail is AB Doradus. The underlying mechanism that causes differential rotation is turbulent convection inside a star. Convective motion carries energy toward the surface through the mass movement of plasma.
This mass of plasma carries a portion of the angular velocity of the star. When turbulence occurs through shear and rotation, the angular momentum can become redistributed to different latitudes thro
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process, it is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Three quarters of the Sun's mass consists of hydrogen; the Sun is a G-type main-sequence star based on its spectral class. As such, it is informally and not accurately referred to as a yellow dwarf, it formed 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System; the central mass became so hot and dense that it initiated nuclear fusion in its core. It is thought that all stars form by this process.
The Sun is middle-aged. It fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result; this energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to become a red giant, it is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, no longer produce energy by fusion, but still glow and give off heat from its previous fusion; the enormous effect of the Sun on Earth has been recognized since prehistoric times, the Sun has been regarded by some cultures as a deity.
The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of, the predominant calendar in use today. The English proper name Sun may be related to south. Cognates to English sun appear in other Germanic languages, including Old Frisian sunne, Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, Gothic sunnō. All Germanic terms for the Sun stem from Proto-Germanic *sunnōn; the Latin name for the Sun, Sol, is not used in everyday English. Sol is used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars; the related word solar is the usual adjectival term used for the Sun, in terms such as solar day, solar eclipse, Solar System. A mean Earth solar day is 24 hours, whereas a mean Martian'sol' is 24 hours, 39 minutes, 35.244 seconds. The English weekday name Sunday stems from Old English and is a result of a Germanic interpretation of Latin dies solis, itself a translation of the Greek ἡμέρα ἡλίου.
The Sun is a G-type main-sequence star. The Sun has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The Sun is heavy-element-rich, star; the formation of the Sun may have been triggered by shockwaves from more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars; the heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is 1 astronomical unit, though the distance varies as Earth moves from perihelion in January to aphelion in July.
At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports all life on Earth by photosynthesis, drives Earth's climate and weather; the Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometres; the tidal effect of the planets is weak and does not affect the shape of the Sun. The Sun rotates faster at its equator than at its poles; this differential rotation is caused by convective motion
Hipparcos was a scientific satellite of the European Space Agency, launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky; this permitted the accurate determination of proper motions and parallaxes of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars; the resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos' follow-up mission, was launched in 2013; the word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and a reference to the ancient Greek astronomer Hipparchus of Nicaea, noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.
By the second half of the 20th century, the accurate measurement of star positions from the ground was running into insurmountable barriers to improvements in accuracy for large-angle measurements and systematic terms. Problems were dominated by the effects of the Earth's atmosphere, but were compounded by complex optical terms and gravitational instrument flexures, the absence of all-sky visibility. A formal proposal to make these exacting observations from space was first put forward in 1967. Although proposed to the French space agency CNES, it was considered too complex and expensive for a single national programme, its acceptance within the European Space Agency's scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The underlying scientific motivation was to determine the physical properties of the stars through the measurement of their distances and space motions, thus to place theoretical studies of stellar structure and evolution, studies of galactic structure and kinematics, on a more secure empirical basis.
Observationally, the objective was to provide the positions and annual proper motions for some 100,000 stars with an unprecedented accuracy of 0.002 arcseconds, a target in practice surpassed by a factor of two. The name of the space telescope, "Hipparcos" was an acronym for High Precision Parallax Collecting Satellite, it reflected the name of the ancient Greek astronomer Hipparchus, considered the founder of trigonometry and the discoverer of the precession of the equinoxes; the spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of 29 cm. A special beam-combining mirror superimposed two fields of view, 58 degrees apart, into the common focal plane; this complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, providing an unvignetted field of view of about 1°×1°. The telescope used a system of grids, at the focal surface, composed of 2688 alternate opaque and transparent bands, with a period of 1.208 arc-sec.
Behind this grid system, an image dissector tube with a sensitive field of view of about 38-arc-sec diameter converted the modulated light into a sequence of photon counts from which the phase of the entire pulse train from a star could be derived. The apparent angle between two stars in the combined fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. Targeting the observation of some 100,000 stars, with an astrometric accuracy of about 0.002 arc-sec, the final Hipparcos Catalogue comprised nearly 120,000 stars with a median accuracy of better than 0.001 arc-sec. An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper, its purpose was to monitor and determine the satellite attitude, in the process, to gather photometric and astrometric data of all stars down to about 11th magnitude. These measurements were made in two broad bands corresponding to B and V in the UBV photometric system.
The positions of these latter stars were to be determined to a precision of 0.03 arc-sec, a factor of 25 less than the main mission stars. Targeting the observation of around 400,000 stars, the resulting Tycho Catalogue comprised just over 1 million stars, with a subsequent analysis extending this to the Tycho-2 Catalogue of about 2.5 million stars. The attitude of the spacecraft about its center of gravity was controlled to scan the celestial sphere in a regular precessional motion maintaining a constant inclination between the spin axis and the direction to the Sun; the spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day at an angle of 43° to the Sun. The Z-axis rotated about the sun-satellite line at 6.4 revolutions/year. The spacecraft consisted of two platforms and six vertical panels, all made of aluminum honeycomb; the solar array consisted of three deployable sections. Two S-band antennas were located on the top and bottom of the spacecraft, providing an omni-directional downlink data rate of 24 kbit/s.
An attitude and orbit-control subsystem ensured correct dynamic attitude control and determination during the operational lifetim
Canis Minor is a small constellation in the northern celestial hemisphere. In the second century, it was included as an asterism, or pattern, of two stars in Ptolemy's 48 constellations, it is counted among the 88 modern constellations, its name is Latin for "lesser dog", in contrast to Canis Major, the "greater dog". Canis Minor contains only two stars brighter than the fourth magnitude, with a magnitude of 0.34, Gomeisa, with a magnitude of 2.9. The constellation's dimmer stars were noted by Johann Bayer, who named eight stars including Alpha and Beta, John Flamsteed, who numbered fourteen. Procyon is the seventh-brightest star in the night sky, as well as one of the closest. A yellow-white main sequence star, it has a white dwarf companion. Gomeisa is a blue-white main sequence star. Luyten's Star is a ninth-magnitude red dwarf and the Solar System's next closest stellar neighbour in the constellation after Procyon; the fourth-magnitude HD 66141, which has evolved into an orange giant towards the end of its life cycle, was discovered to have a planet in 2012.
There are two faint deep-sky objects within the constellation's borders. The 11 Canis-Minorids are a meteor shower. Though associated with the Classical Greek uranographic tradition, Canis Minor originates from ancient Mesopotamia. Procyon and Gomeisa were called MASH. TAB. BA or "twins" in the Three Stars Each tablets, dating to around 1100 BC. In the MUL. APIN, this name was applied to the pairs of Pi3 and Pi4 Orionis and Zeta and Xi Orionis; the meaning of MASH. TAB. BA evolved as well, becoming the twin deities Lulal and Latarak, who are on the opposite side of the sky from Papsukal, the True Shepherd of Heaven in Babylonian mythology. Canis Minor was given the name DAR. LUGAL, its position defined as "the star which stands behind it ", in the MUL. APIN; this name may have referred to the constellation Lepus. DAR. LUGAL was denoted DAR. MUŠEN and DAR. LUGAL. MUŠEN in Babylonia. Canis Minor was called tarlugallu in Akkadian astronomy. Canis Minor was one of the original 48 constellations formulated by Ptolemy in his second-century Almagest, in which it was defined as a specific pattern of stars.
The Ancient Greeks called the constellation προκυων/Procyon, "coming before the dog", transliterated into Latin as Antecanis, Praecanis, or variations thereof, by Cicero and others. Roman writers appended the descriptors parvus, minor or minusculus, primus or sinister to its name Canis. In Greek mythology, Canis Minor was sometimes connected with the Teumessian Fox, a beast turned into stone with its hunter, Laelaps, by Zeus, who placed them in heaven as Canis Major and Canis Minor. Eratosthenes accompanied the Little Dog with Orion, while Hyginus linked the constellation with Maera, a dog owned by Icarius of Athens. On discovering the latter's death, the dog and Icarius' daughter Erigone took their lives and all three were placed in the sky—Erigone as Virgo and Icarius as Boötes; as a reward for his faithfulness, the dog was placed along the "banks" of the Milky Way, which the ancients believed to be a heavenly river, where he would never suffer from thirst. The medieval Arabic astronomers maintained the depiction of Canis Minor as a dog.
There was one slight difference between the Ptolemaic vision of the Arabic. The Arabic names for both Procyon and Gomeisa alluded to their proximity and resemblance to Sirius, though they were not direct translations of the Greek. Among the Merazig of Tunisia, shepherds note six constellations that mark the passage of the dry, hot season. One of them, called Merzem, includes the stars of Canis Minor and Canis Major and is the herald of two weeks of hot weather; the ancient Egyptians thought of this constellation as the jackal god. Alternative names have been proposed: Johann Bayer in the early 17th century termed the constellation Fovea "The Pit", Morus "Sycamine Tree". Seventeenth-century German poet and author Philippus Caesius linked it to the dog of Tobias from the Apocrypha. Richard A. Proctor gave the constellation the name Felis "the Cat" in 1870, explaining that he sought to shorten the constellation names to make them more manageable on celestial charts. Canis Minor is confused with Canis Major and given the name Canis Orionis.
In Chinese astronomy, the stars corresponding to Canis Minor lie in the Vermilion Bird of the South. Procyon and Eta Canis Minoris form an asterism known as Nánhé, the Southern River. With its counterpart, the Northern River Beihe, Nánhé was associated with a gate or sentry. Along with Zeta and 8 Cancri, 6 Canis Minoris and 11 Canis Minoris formed the asterism Shuiwei, which means "water level". Combined with additional stars in Gemini, Shuiwei represented an official who managed floodwaters or a marker of the water level. Neighboring Kore
A star catalogue or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, this article covers only some of the more quoted ones. Star catalogues were compiled by many different ancient people, including the Babylonians, Chinese and Arabs, they were sometimes accompanied by a star chart for illustration. Most modern catalogues are available in electronic format and can be downloaded from space agencies data centres. Completeness and accuracy is described by the weakest apparent magnitude V and the accuracy of the positions. From their existing records, it is known that the ancient Egyptians recorded the names of only a few identifiable constellations and a list of thirty-six decans that were used as a star clock; the Egyptians called the circumpolar star "the star that cannot perish" and, although they made no known formal star catalogues, they nonetheless created extensive star charts of the night sky which adorn the coffins and ceilings of tomb chambers.
Although the ancient Sumerians were the first to record the names of constellations on clay tablets, the earliest known star catalogues were compiled by the ancient Babylonians of Mesopotamia in the late 2nd millennium BC, during the Kassite Period. They are better known by their Assyrian-era name'Three Stars Each'; these star catalogues, written on clay tablets, listed thirty-six stars: twelve for "Anu" along the celestial equator, twelve for "Ea" south of that, twelve for "Enlil" to the north. The Mul. Apin lists, dated to sometime before the Neo-Babylonian Empire, are direct textual descendants of the "Three Stars Each" lists and their constellation patterns show similarities to those of Greek civilization. In Ancient Greece, the astronomer and mathematician Eudoxus laid down a full set of the classical constellations around 370 BC, his catalogue Phaenomena, rewritten by Aratus of Soli between 275 and 250 BC as a didactic poem, became one of the most consulted astronomical texts in antiquity and beyond.
It contains descriptions of the positions of the stars, the shapes of the constellations and provided information on their relative times of rising and setting. In the 3rd century BC, the Greek astronomers Timocharis of Alexandria and Aristillus created another star catalogue. Hipparchus completed his star catalogue in 129 BC, which he compared to Timocharis' and discovered that the longitude of the stars had changed over time; this led him to determine the first value of the precession of the equinoxes. In the 2nd century, Ptolemy of Roman Egypt published a star catalogue as part of his Almagest, which listed 1,022 stars visible from Alexandria. Ptolemy's catalogue was based entirely on an earlier one by Hipparchus, it remained the standard star catalogue in the Arab worlds for over eight centuries. The Islamic astronomer al-Sufi updated it in 964, the star positions were redetermined by Ulugh Beg in 1437, but it was not superseded until the appearance of the thousand-star catalogue of Tycho Brahe in 1598.
Although the ancient Vedas of India specified how the ecliptic was to be divided into twenty-eight nakshatra, Indian constellation patterns were borrowed from Greek ones sometime after Alexander's conquests in Asia in the 4th century BC. The earliest known inscriptions for Chinese star names were written on oracle bones and date to the Shang Dynasty. Sources dating from the Zhou Dynasty which provide star names include the Zuo Zhuan, the Shi Jing, the "Canon of Yao" in the Book of Documents; the Lüshi Chunqiu written by the Qin statesman Lü Buwei provides most of the names for the twenty-eight mansions. An earlier lacquerware chest found in the Tomb of Marquis Yi of Zeng contains a complete list of the names of the twenty-eight mansions. Star catalogues are traditionally attributed to Shi Shen and Gan De, two rather obscure Chinese astronomers who may have been active in the 4th century BC of the Warring States period; the Shi Shen astronomy is attributed to Shi Shen, the Astronomic star observation to Gan De.
It was not until the Han Dynasty that astronomers started to observe and record names for all the stars that were apparent in the night sky, not just those around the ecliptic. A star catalogue is featured in one of the chapters of the late 2nd-century-BC history work Records of the Grand Historian by Sima Qian and contains the "schools" of Shi Shen and Gan De's work. Sima's catalogue—the Book of Celestial Offices —includes some 90 constellations, the stars therein named after temples, ideas in philosophy, locations such as markets and shops, different people such as farmers and soldiers. For his Spiritual Constitution of the Universe of 120 AD, the astronomer Zhang Heng compiled a star catalogue comprising 124 constellations. Chinese constellation names were adopted by the Koreans and Japanese. A large number of star catalogues were published by Muslim astronomers in the medieval Islamic world; these were Zij treatises, including Arzachel's Tables of Toledo, the Maragheh observatory's Zij-i Ilkhani and Ulugh Beg's Zij-i-Sultani.
A corona is an aura of plasma that surrounds the Sun and other stars. The Sun's corona extends millions of kilometres into outer space and is most seen during a total solar eclipse, but it is observable with a coronagraph; the word corona is a Latin word meaning "crown", from the Ancient Greek κορώνη. Spectroscopy measurements indicate strong ionization in the corona and a plasma temperature in excess of 1,000,000 kelvins, much hotter than the surface of the Sun. Light from the corona comes from the same volume of space; the K-corona is created by sunlight scattering off free electrons. The F-corona is created by sunlight bouncing off dust particles, is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the E-corona is due to spectral emission lines produced by ions that are present in the coronal plasma. In 1724, French-Italian astronomer Giacomo F. Maraldi recognized that the aura visible during a solar eclipse belongs to the Sun not to the Moon.
In 1809, Spanish astronomer José Joaquín de Ferrer coined the term'corona'. Based in his own observations of the 1806 solar eclipse at Kinderhook, de Ferrer proposed that the corona was part of the Sun and not of the Moon. English astronomer Norman Lockyer identified the first element unknown on Earth in the Sun's chromosphere, called helium. French astronomer Jules Jenssen noted that the size and shape of the corona changes with the sunspot cycle. In 1930, Bernard Lyot invented the coronograph, which allows to see the corona without a total eclipse. In 1952, American astronomer Eugene Parker proposed that the solar corona might be heated by myriad tiny'nanoflares', miniature brightenings resembling solar flares that would occur all over the surface of the Sun; the high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a unknown element, "coronium". Instead, these spectral features have since been explained by ionized iron.
Bengt Edlén, following the work of Grotrian, first identified the coronal spectral lines in 1940 as transitions from low-lying metastable levels of the ground configuration of ionised metals. The sun's corona is much hotter than the visible surface of the Sun: the photosphere's average temperature is 5800 kelvins compared to the corona's one to three million kelvins; the corona is 10−12 times as dense as the photosphere, so produces about one-millionth as much visible light. The corona is separated from the photosphere by the shallow chromosphere; the exact mechanism by which the corona is heated is still the subject of some debate, but possibilities include induction by the Sun's magnetic field and magnetohydrodynamic waves from below. The outer edges of the Sun's corona are being transported away due to open magnetic flux and hence generating the solar wind; the corona is not always evenly distributed across the surface of the sun. During periods of quiet, the corona is more or less confined to the equatorial regions, with coronal holes covering the polar regions.
However, during the Sun's active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with sunspot activity. The solar cycle spans 11 years, from solar minimum to the following minimum. Since the solar magnetic field is continually wound up due to the faster rotation of mass at the sun's equator, sunspot activity will be more pronounced at solar maximum where the magnetic field is more twisted. Associated with sunspots are coronal loops, loops of magnetic flux, upwelling from the solar interior; the magnetic flux pushes the hotter photosphere aside, exposing the cooler plasma below, thus creating the dark sun spots. Since the corona has been photographed at high resolution in the X-ray range of the spectrum by the satellite Skylab in 1973, later by Yohkoh and the other following space instruments, it has been seen that the structure of the corona is quite varied and complex: different zones have been classified on the coronal disc.
The astronomers distinguish several regions, as described below. Active regions are ensembles of loop structures connecting points of opposite magnetic polarity in the photosphere, the so-called coronal loops, they distribute in two zones of activity, which are parallel to the solar equator. The average temperature is between two and four million kelvins, while the density goes from 109 to 1010 particle per cm3. Active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights above the Sun's surface: sunspots and faculae, occur in the photosphere, spicules, Hα filaments and plages in the chromosphere, prominences in the chromosphere and transition region, flares and coronal mass ejections happen in the corona and chromosphere. If flares are violent, they can perturb the photosphere and generate a Moreton wave. On the contrary, quiescent prom