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89-430: A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction . A large group of stars bound by gravitation is generally called a star cluster or galaxy , although, broadly speaking, they are also star systems. Star systems are not to be confused with planetary systems , which include planets and similar bodies (such as comets ). A star system of two stars

178-456: A protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 million years for a star like the sun, up to 100 million years for a red dwarf. Early stars of less than 2  M ☉ are called T Tauri stars , while those with greater mass are Herbig Ae/Be stars . These newly formed stars emit jets of gas along their axis of rotation, which may reduce

267-467: A stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10   M ☉ every year, or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10 to 10   M ☉ each year, significantly affecting their evolution. Stars that begin with more than 50  M ☉ can lose over half their total mass while on

356-425: A 0.03  AU separation. This pair share an orbit with EZ Aquarii B that has an 823-day period. The A and B components of Luyten 789-6 together emit X-rays. This star is a red dwarf of type M5V which has a mass of 0.1187 ± 0.0011 solar masses. It has a parallax of 293.6 ± 0.9 mas . Its period in days around EZ Aquarii C is 3.786 516 ± 0.000 005  d with an eccentricity of 0 which together make up

445-487: A brief period of carbon fusion before the core becomes degenerate. During the AGB phase, stars undergo thermal pulses due to instabilities in the core of the star. In these thermal pulses, the luminosity of the star varies and matter is ejected from the star's atmosphere, ultimately forming a planetary nebula. As much as 50 to 70% of a star's mass can be ejected in this mass loss process. Because energy transport in an AGB star

534-496: A burst of electron capture and inverse beta decay . The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before. A supernova explosion blows away

623-409: A close binary with a distant companion, with the other star(s) previously in the system ejected into interstellar space at high velocities. This dynamic may explain the runaway stars that might have been ejected during a collision of two binary star groups or a multiple system. This event is credited with ejecting AE Aurigae , Mu Columbae and 53 Arietis at above 200 km·s and has been traced to

712-410: A continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence. Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae ), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like

801-411: A designation system, identifying the hierarchy within the system has the advantage that it makes identifying subsystems and computing their properties easier. However, it causes problems when new components are discovered at a level above or intermediate to the existing hierarchy. In this case, part of the hierarchy will shift inwards. Components which are found to be nonexistent, or are later reassigned to

890-875: A diagram multiplex if there is a node with more than two children , i.e. if the decomposition of some subsystem involves two or more orbits with comparable size. Because, as we have already seen for triple stars, this may be unstable, multiple stars are expected to be simplex , meaning that at each level there are exactly two children . Evans calls the number of levels in the diagram its hierarchy . Higher hierarchies are also possible. Most of these higher hierarchies either are stable or suffer from internal perturbations . Others consider complex multiple stars will in time theoretically disintegrate into less complex multiple stars, like more common observed triples or quadruples are possible. Trapezia are usually very young, unstable systems. These are thought to form in stellar nurseries, and quickly fragment into stable multiple stars, which in

979-440: A difference between " fixed stars ", whose position on the celestial sphere does not change, and "wandering stars" ( planets ), which move noticeably relative to the fixed stars over days or weeks. Many ancient astronomers believed that the stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped prominent stars into asterisms and constellations and used them to track

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1068-536: A different subsystem, also cause problems. During the 24th General Assembly of the International Astronomical Union in 2000, the WMC scheme was endorsed and it was resolved by Commissions 5, 8, 26, 42, and 45 that it should be expanded into a usable uniform designation scheme. A sample of a catalog using the WMC scheme, covering half an hour of right ascension , was later prepared. The issue

1157-518: A much larger gravitationally bound structure, such as a star cluster or a galaxy. The word "star" ultimately derives from the Proto-Indo-European root "h₂stḗr" also meaning star, but further analyzable as h₂eh₁s- ("to burn", also the source of the word "ash") + -tēr (agentive suffix). Compare Latin stella , Greek aster , German Stern . Some scholars believe the word is a borrowing from Akkadian " istar " ( Venus ). "Star"

1246-546: A net release of energy. Some massive stars, particularly luminous blue variables , are very unstable to the extent that they violently shed their mass into space in events supernova impostors , becoming significantly brighter in the process. Eta Carinae is known for having underwent a supernova impostor event, the Great Eruption, in the 19th century. As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on

1335-919: A non-hierarchical system by this method, the same subsystem number will be used more than once; for example, a system with three visual components, A, B, and C, no two of which can be grouped into a subsystem, would have two subsystems numbered 1 denoting the two binaries AB and AC. In this case, if B and C were subsequently resolved into binaries, they would be given the subsystem numbers 12 and 13. The current nomenclature for double and multiple stars can cause confusion as binary stars discovered in different ways are given different designations (for example, discoverer designations for visual binary stars and variable star designations for eclipsing binary stars), and, worse, component letters may be assigned differently by different authors, so that, for example, one person's A can be another's C . Discussion starting in 1999 resulted in four proposed schemes to address this problem: For

1424-406: A number of more complicated arrangements. These arrangements can be organized by what Evans (1968) called mobile diagrams , which look similar to ornamental mobiles hung from the ceiling. Examples of hierarchical systems are given in the figure to the right ( Mobile diagrams ). Each level of the diagram illustrates the decomposition of the system into two or more systems with smaller size. Evans calls

1513-429: A physical hierarchical triple system, which has an outer star orbiting an inner physical binary composed of two more red dwarf stars. Triple stars that are not all gravitationally bound might comprise a physical binary and an optical companion (such as Beta Cephei ) or, in rare cases, a purely optical triple star (such as Gamma Serpentis ). Hierarchical multiple star systems with more than three stars can produce

1602-463: A series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed 's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering . The internationally recognized authority for naming celestial bodies

1691-614: A set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters: The solar mass M ☉ was not explicitly defined by the IAU due to the large relative uncertainty ( 10 ) of the Newtonian constant of gravitation G . Since the product of the Newtonian constant of gravitation and solar mass together ( G M ☉ ) has been determined to much greater precision,

1780-413: A single star. In these systems there is little interaction between the orbits and the stars' motion will continue to approximate stable Keplerian orbits around the system's center of mass, unlike the unstable trapezia systems or the even more complex dynamics of the large number of stars in star clusters and galaxies . In a physical triple star system, each star orbits the center of mass of

1869-499: A star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy ). When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability , it begins to collapse under its own gravitational force. As

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1958-434: A star of more than 9 solar masses expands to form first a blue supergiant and then a red supergiant . Particularly massive stars (exceeding 40 solar masses, like Alnilam , the central blue supergiant of Orion's Belt ) do not become red supergiants due to high mass loss. These may instead evolve to a Wolf–Rayet star , characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached

2047-465: A system in which each subsystem in a mobile diagram is encoded by a sequence of digits. In the mobile diagram (d) above, for example, the widest system would be given the number 1, while the subsystem containing its primary component would be numbered 11 and the subsystem containing its secondary component would be numbered 12. Subsystems which would appear below this in the mobile diagram will be given numbers with three, four, or more digits. When describing

2136-407: A white dwarf is no longer a plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time. In massive stars, fusion continues until the iron core has grown so large (more than 1.4  M ☉ ) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos , and gamma rays in

2225-772: Is an optical multiple star Physical multiple stars are also commonly called multiple stars or multiple star systems . Most multiple star systems are triple stars . Systems with four or more components are less likely to occur. Multiple-star systems are called triple , ternary , or trinary if they contain 3 stars; quadruple or quaternary if they contain 4 stars; quintuple or quintenary with 5 stars; sextuple or sextenary with 6 stars; septuple or septenary with 7 stars; octuple or octenary with 8 stars. These systems are smaller than open star clusters , which have more complex dynamics and typically have from 100 to 1,000 stars. Most multiple star systems known are triple; for higher multiplicities,

2314-524: Is approaching the Solar System and, in about 32,300 years, will be at its minimal distance of about 8.2 ly (2.5 pc) from the Sun. The ChView simulation shows that currently its nearest neighbouring star is Lacaille 9352 at about 4.1 ly (1.3 pc) from EZ Aquarii. All three components are M-type red dwarfs . The pair EZ Aquarii AC form a spectroscopic binary with a 3.8-day orbit and

2403-450: Is called a hierarchical system : the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system's center of mass . Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on. Each level of the hierarchy can be treated as a two-body problem by considering close pairs as if they were

2492-420: Is cognate (shares the same root) with the following words: asterisk , asteroid , astral , constellation , Esther . Historically, stars have been important to civilizations throughout the world. They have been part of religious practices, divination rituals, mythology , used for celestial navigation and orientation, to mark the passage of seasons, and to define calendars. Early astronomers recognized

2581-428: Is known as a binary star , binary star system or physical double star . If there are no tidal effects, no perturbation from other forces, and no transfer of mass from one star to the other, such a system is stable, and both stars will trace out an elliptical orbit around the barycenter of the system indefinitely. (See Two-body problem ) . Examples of binary systems are Sirius , Procyon and Cygnus X-1 ,

2670-409: Is primarily by convection , this ejected material is enriched with the fusion products dredged up from the core. Therefore, the planetary nebula is enriched with elements like carbon and oxygen. Ultimately, the planetary nebula disperses, enriching the general interstellar medium. Therefore, future generations of stars are made of the "star stuff" from past stars. During their helium-burning phase,

2759-488: Is the International Astronomical Union (IAU). The International Astronomical Union maintains the Working Group on Star Names (WGSN) which catalogs and standardizes proper names for stars. A number of private companies sell names of stars which are not recognized by the IAU, professional astronomers, or the amateur astronomy community. The British Library calls this an unregulated commercial enterprise , and

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2848-491: Is the Orion Nebula . Most stars form in groups of dozens to hundreds of thousands of stars. Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions . Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation. All stars spend the majority of their existence as main sequence stars , fueled primarily by

2937-509: The Algol paradox , where the most-evolved star in a system is the least massive. EZ Aquarii William E. Kunkel announced that Luyten 789-6 was a flare star in 1972, after having observed seven flares. It was given its variable star designation , EZ Aquarii, in 1978. The configuration of the inner binary pair may permit a circumbinary planet to orbit near their habitable zone , however no exoplanets have yet been observed. EZ Aquarii

3026-701: The M87 and M100 galaxies of the Virgo Cluster , as well as luminous stars in some other relatively nearby galaxies. With the aid of gravitational lensing , a single star (named Icarus ) has been observed at 9 billion light-years away. The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along

3115-526: The New York City Department of Consumer and Worker Protection issued a violation against one such star-naming company for engaging in a deceptive trade practice. Although stellar parameters can be expressed in SI units or Gaussian units , it is often most convenient to express mass , luminosity , and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined

3204-605: The Trapezium cluster in the Orion Nebula some two million years ago. The components of multiple stars can be specified by appending the suffixes A , B , C , etc., to the system's designation. Suffixes such as AB may be used to denote the pair consisting of A and B . The sequence of letters B , C , etc. may be assigned in order of separation from the component A . Components discovered close to an already known component may be assigned suffixes such as Aa , Ba , and so forth. A. A. Tokovinin's Multiple Star Catalogue uses

3293-461: The angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects . These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed. Early in their development, T Tauri stars follow the Hayashi track —they contract and decrease in luminosity while remaining at roughly

3382-632: The interstellar medium . These elements are then recycled into new stars. Astronomers can determine stellar properties—including mass, age, metallicity (chemical composition), variability , distance , and motion through space —by carrying out observations of a star's apparent brightness , spectrum , and changes in its position in the sky over time. Stars can form orbital systems with other astronomical objects, as in planetary systems and star systems with two or more stars. When two such stars orbit closely, their gravitational interaction can significantly impact their evolution. Stars can form part of

3471-453: The photographic magnitude . The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory . Important theoretical work on the physical structure of stars occurred during

3560-591: The 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and gave the latitudes of various stars during a lunar eclipse in 1019. According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be

3649-476: The 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters. Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit —approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012,

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3738-413: The IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m. Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber . These regions—known as molecular clouds —consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region

3827-413: The IAU defined the nominal solar mass parameter to be: The nominal solar mass parameter can be combined with the most recent (2014) CODATA estimate of the Newtonian constant of gravitation G to derive the solar mass to be approximately 1.9885 × 10  kg . Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties,

3916-497: The Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley . The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since

4005-439: The Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass. As the hydrogen-burning shell produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25  M ☉ , the mass of the helium core becomes degenerate prior to helium fusion . Finally, when

4094-449: The Sun, and may have other planets , possibly even Earth-like, in orbit around them, an idea that had been suggested earlier by the ancient Greek philosophers , Democritus and Epicurus , and by medieval Islamic cosmologists such as Fakhr al-Din al-Razi . By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on

4183-502: The band of the ecliptic and these became the basis of astrology . Many of the more prominent individual stars were given names, particularly with Arabic or Latin designations. As well as certain constellations and the Sun itself, individual stars have their own myths . To the Ancient Greeks , some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which

4272-502: The chemical composition of the stellar atmosphere to be determined. With the exception of rare events such as supernovae and supernova impostors , individual stars have primarily been observed in the Local Group , and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for the Milky Way galaxy) and its satellites. Individual stars such as Cepheid variables have been observed in

4361-408: The cloud collapses, individual conglomerations of dense dust and gas form " Bok globules ". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium , a protostar forms at the core. These pre-main-sequence stars are often surrounded by

4450-612: The cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions. Stars spend about 90% of their lifetimes fusing hydrogen into helium in high-temperature-and-pressure reactions in their cores. Such stars are said to be on

4539-400: The core. The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium. Binary stars ' evolution may significantly differ from that of single stars of

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4628-417: The direction of the Milky Way core . His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction. In addition to his other accomplishments, William Herschel is noted for his discovery that some stars do not merely lie along the same line of sight, but are physical companions that form binary star systems. The science of stellar spectroscopy

4717-405: The end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth. The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce

4806-526: The first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis. The spectra of stars were further understood through advances in quantum physics . This allowed

4895-466: The first level of the hierarchy, lower-case letters (a, b, ...) for the second level, and numbers (1, 2, ...) for the third. Subsequent levels would use alternating lower-case letters and numbers, but no examples of this were found in the sample. Star A star is a luminous spheroid of plasma held together by self-gravity . The nearest star to Earth is the Sun . Many other stars are visible to

4984-401: The last of which probably consists of a star and a black hole . A multiple star system consists of two or more stars that appear from Earth to be close to one another in the sky. This may result from the stars actually being physically close and gravitationally bound to each other, in which case it is a physical multiple star, or this closeness may be merely apparent, in which case it

5073-437: The main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity. The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion ( 4.6 × 10 ) years ago. Every star generates

5162-677: The main sequence. The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion ( 10 ) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25  M ☉ , called red dwarfs , are able to fuse nearly all of their mass while stars of about 1  M ☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion ( 10 × 10 ) years;

5251-412: The main sequence. Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity . A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields, which affects

5340-456: The most extreme of 0.08  M ☉ will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature. Since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85  M ☉ are expected to have moved off

5429-445: The motions of the planets and the inferred position of the Sun. The motion of the Sun against the background stars (and the horizon) was used to create calendars , which could be used to regulate agricultural practices. The Gregorian calendar , currently 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 accurately dated star chart

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5518-487: The naked eye at night ; their immense distances from Earth make them appear as fixed points of light. The most prominent stars have been categorised into constellations and asterisms , and many of the brightest stars have proper names . Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations . The observable universe contains an estimated 10 to 10 stars. Only about 4,000 of these stars are visible to

5607-442: The naked eye—all within the Milky Way galaxy . A star's life begins with the gravitational collapse of a gaseous nebula of material largely comprising hydrogen , helium, and trace heavier elements. Its total mass mainly determines its evolution and eventual fate. A star shines for most of its active life due to the thermonuclear fusion of hydrogen into helium in its core. This process releases energy that traverses

5696-484: The names of the planets Mercury , Venus , Mars , Jupiter and Saturn were taken. ( Uranus and Neptune were Greek and Roman gods , but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.) Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created

5785-403: The nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass: The formation of

5874-478: The number of known systems with a given multiplicity decreases exponentially with multiplicity. For example, in the 1999 revision of Tokovinin's catalog of physical multiple stars, 551 out of the 728 systems described are triple. However, because of suspected selection effects , the ability to interpret these statistics is very limited. Multiple-star systems can be divided into two main dynamical classes: or Most multiple-star systems are organized in what

5963-417: The outer convective envelope collapses and the star then moves to the horizontal branch. After a star has fused the helium of its core, it begins fusing helium along a shell surrounding the hot carbon core. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red-giant phase, but with a higher luminosity. The more massive AGB stars may undergo

6052-404: The outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly 1.4  M ☉ , it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf . White dwarfs lack the mass for further gravitational compression to take place. The electron-degenerate matter inside

6141-664: The positions of the stars. They built the first large observatory research institutes, mainly to produce Zij star catalogues. Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi , who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters ) and galaxies (including the Andromeda Galaxy ). According to A. Zahoor, in

6230-662: The primary of the system. It has an absolute magnitude at wavelengths centered at 5500 Angstroms of 15.33 making it the brightest of the three. Some alternate designations for it are EZ Aqr, GL 866A, L 789-6 A and LHS 68. There is less known about this star compared to A. Its type is likely a type MV with a mass of 0.1145 ± 0.0012 solar masses. It orbits the AC system with a period of 822.6 ± 0.2 d at an eccentricity of 0.439 ± 0.001 . It has an absolute magnitude of 15.58, making it dimmer than A but brighter than C. Some alternate designations for it are GL 866B and L 789-6 B. Like

6319-403: The problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827. The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against

6408-498: The process may eject components as galactic high-velocity stars . They are named after the multiple star system known as the Trapezium Cluster in the heart of the Orion Nebula . Such systems are not rare, and commonly appear close to or within bright nebulae . These stars have no standard hierarchical arrangements, but compete for stable orbits. This relationship is called interplay . Such stars eventually settle down to

6497-497: The proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham , allowing the masses of stars to be determined from computation of orbital elements . The first solution to

6586-461: The same mass. For example, when any star expands to become a red giant, it may overflow its Roche lobe , the surrounding region where material is gravitationally bound to it; if stars in a binary system are close enough, some of that material may overflow to the other star, yielding phenomena including contact binaries , common-envelope binaries, cataclysmic variables , blue stragglers , and type Ia supernovae . Mass transfer leads to cases such as

6675-455: The same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track . Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed. A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of

6764-445: The star's interior and radiates into outer space . At the end of a star's lifetime as a fusor , its core becomes a stellar remnant : a white dwarf , a neutron star , or—if it is sufficiently massive—a black hole . Stellar nucleosynthesis in stars or their remnants creates almost all naturally occurring chemical elements heavier than lithium . Stellar mass loss or supernova explosions return chemically enriched material to

6853-506: The star's outer layers, leaving a remnant such as the Crab Nebula. The core is compressed into a neutron star , which sometimes manifests itself as a pulsar or X-ray burster . In the case of the largest stars, the remnant is a black hole greater than 4  M ☉ . In a neutron star the matter is in a state known as neutron-degenerate matter , with a more exotic form of degenerate matter, QCD matter , possibly present in

6942-400: The strength of its stellar wind. Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres . As stars of at least 0.4  M ☉ exhaust

7031-485: The supply of hydrogen at their core, they start to fuse hydrogen in a shell surrounding the helium core. The outer layers of the star expand and cool greatly as they transition into a red giant . In some cases, they will fuse heavier elements at the core or in shells around the core. As the stars expand, they throw part of their mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. In about 5 billion years, when

7120-468: The surface due to strong convection and intense mass loss, or from stripping of the outer layers. When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process ). This process continues, with the successive stages being fueled by neon (see neon-burning process ), oxygen (see oxygen-burning process ), and silicon (see silicon-burning process ). Near

7209-415: The system. Usually, two of the stars form a close binary system , and the third orbits this pair at a distance much larger than that of the binary orbit. This arrangement is called hierarchical . The reason for this arrangement is that if the inner and outer orbits are comparable in size, the system may become dynamically unstable, leading to a star being ejected from the system. EZ Aquarii is an example of

7298-458: The temperature increases sufficiently, core helium fusion begins explosively in what is called a helium flash , and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump , slowly burning helium, before

7387-400: The time of the ancient Greek astronomers Ptolemy and Hipparchus. William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this, he deduced that the number of stars steadily increased toward one side of the sky, in

7476-435: Was developed by Annie J. Cannon during the early 1900s. The first direct measurement of the distance to a star ( 61 Cygni at 11.4 light-years ) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens. Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in

7565-580: Was discussed again at the 25th General Assembly in 2003, and it was again resolved by commissions 5, 8, 26, 42, and 45, as well as the Working Group on Interferometry, that the WMC scheme should be expanded and further developed. The sample WMC is hierarchically organized; the hierarchy used is based on observed orbital periods or separations. Since it contains many visual double stars , which may be optical rather than physical, this hierarchy may be only apparent. It uses upper-case letters (A, B, ...) for

7654-419: Was pioneered by Joseph von Fraunhofer and Angelo Secchi . By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines —the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types . The modern version of the stellar classification scheme

7743-600: Was the SN 1006 supernova, which was 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 also 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

7832-614: 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 ( c.  1531 BC  – c.  1155 BC ). The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis . The star catalog of Hipparchus (2nd century BC) included 1,020 stars, and

7921-480: Was used to assemble Ptolemy 's star catalogue. Hipparchus is known for the discovery of the first recorded nova (new star). Many of the constellations and star names in use today derive from Greek astronomy. Despite 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 SN 185 . The brightest stellar event in recorded history

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