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132-417: Alpha Sagittarii ( α Sagittarii , abbreviated Alpha Sgr , α Sgr ), also named Rukbat / ˈ r ʌ k b æ t / , is a star in the constellation of Sagittarius . Alpha Sagittarii is a blue, class B dwarf star . It does not appear particularly bright in the sky to the naked eye, with a visual apparent magnitude of +3.97. The star has an effective temperature about twice that of

264-456: 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

396-611: A rogue planet into a highly eccentric orbit which passes both through the Oort cloud and through the inner solar system on a 250 year orbit. 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 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

528-467: 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

660-478: A Sun-centered universe, this being known as the heliocentric system. The tradition of thought which appears in all of these systems of the universe, even with their divergent mechanisms, is the presence of the sphere of fixed stars. In the sixteenth century, a number of writers inspired by Copernicus, such as Thomas Digges , Giordano Bruno and William Gilbert argued for an indefinitely extended or even infinite universe, with other stars as distant suns, paving

792-484: 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

924-491: 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

1056-540: A circular motion . His cosmos was geocentric, with the Earth at the center, surrounded by a layer of water and air, which was in turn surrounded by a layer of fire which filled the space until reaching the Moon. Aristotle also proposed a fifth element called "aether," which is purported to make up the Sun, the planets, and the stars. However, Aristotle believed that while the planets rotate,

1188-410: 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

1320-491: A correct explanation of eclipses . As far as the Sun and the Moon were conceived as spherical bodies, and as they do not collide at solar eclipses , this implies than the outer space should have some certain, indeterminate, depth. Eudoxus of Cnidus , in around 380 BC, devised a geometric-mathematical model for the movements of the planets based on (conceptual) concentric spheres centered on Earth, and by 360 BC Plato claimed in his Timaeus that circles and spheres were

1452-440: 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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1584-446: A giant celestial sphere , or firmament , which revolves daily around Earth. Hence it was known as the "sphere of fixed stars", which acted as the supposed limit of the whole universe . For many centuries, the term fixed stars was a synonym for that celestial sphere. Many ancient cultures observed new stars now called novas , which provided some clue the heavens were not completely unchanging, but as novae fade in few weeks or months,

1716-509: 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"

1848-539: 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

1980-438: A quantity either of air or of fire...the noise which they created would inevitably be tremendous, and this being so, it would reach and shatter things here on earth". His theory that the stars may be carried but were fixed and do not autonomously move or rotate was widely accepted for a time. Aristarchus (3rd cent. BC), proposed an early heliocentric universe , which would later inspire the work of Copernicus . In his model ,

2112-463: 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

2244-601: 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,

2376-499: 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

2508-434: 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

2640-710: A star of this spectral class. The most likely explanation is that the companion is an active pre-main sequence star or else a star that has just reached the main sequence. α Sagittarii ( Latinised to Alpha Sagittarii ) is the star's Bayer designation . It is unclear why Bayer designated this star as the alpha in his Uranometria star atlas (placing it in the second magnitude class), rather than Kaus Australis or Nunki , which are at least five times brighter than α Sgr. This led some old star charts to occasionally depict Alpha and Beta Sagittarii as much brighter than they are in reality, as they are invisible from northern Europe, being too far south to see there. The star bore

2772-712: A steady light. However, fixed stars show parallax. It can be used to find the distance to nearby stars. This motion is only apparent; it is the Earth that moves. This effect was small enough not to be accurately measured until the 19th century, but from about 1670 and onward, astronomers such as Jean Picard , Robert Hooke , John Flamsteed , and others began detecting motion from the stars and attempting measurements. These movements amounted to significant, if almost imperceptibly small, fractions. The first successful stellar parallax measurements were done by Thomas Henderson in Cape Town , South Africa from 1832 to 1833, where he measured

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2904-413: A student of Plato, was born around 400 BC. A mathematician and an astronomer, he generated one of the earliest sphere-centric models of the planet systems, based on his background as a mathematician. Eudoxus's model was geocentric, with the Earth being a stationary sphere at the center of the system, surrounded by 27 rotating spheres. The farthest sphere carried stars, which he declared to be fixed within

3036-523: A table of the first two batches of names approved by the WGSN; which included Rukbat for this star (Delta Cassiopeiae was later given the name Ruchbah ). In Chinese , 天淵 ( Tiān Yuān ), meaning Celestial Spring , refers to an asterism consisting of Alpha Sagittarii, Beta¹ Sagittarii and Beta² Sagittarii . Consequently, the Chinese name for Alpha Sagittarii itself is 天淵三 ( Tiān Yuān sān , English:

3168-653: A theory formerly put forth by Aristotle. He continued to examine the skies and constellations and soon knew that the "fixed stars" which had been studied and mapped were only a tiny portion of the massive universe that lay beyond the reach of the naked eye. When in 1610 he aimed his telescope to the faint strip of the Milky Way , he found it resolves into countless white star-like spots, presumably farther stars themselves. The development of Isaac Newton 's laws, published in his work Philosophiæ Naturalis Principia Mathematica in 1687, raised further questions among theorists about

3300-452: A total about 386,400 km (240,100 mi). This was around 24,500,000 times lower than Archimedes' computation. Around 130 AD, Ptolemy adopted Apollonius' epicycles in his geocentric model . Epicycles are described as an orbit within an orbit. For example, looking at Venus, Ptolemy claimed that it orbits the Earth, and as it orbits the Earth, it also orbits the original orbit riding a second, minor local sphere. (Ptolemy emphasised that

3432-407: 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

3564-420: 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

3696-418: Is one of several cases which treated stars as being fixed to a sphere beyond the earth. Later scientific literature shows astronomical thought which kept a version of this idea until the seventeenth century. Western astronomical knowledge was based on the traditional thoughts from philosophical and observational inquiries of Greek Antiquity . Other cultures contributed to thought about the fixed stars including

3828-409: 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,

3960-555: 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

4092-485: 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

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4224-741: The Algol paradox , where the most-evolved star in a system is the least massive. Fixed stars The term fixed stars is a misnomer because those celestial objects are not actually fixed with respect to one another or to Earth. Due to their immense distance from Earth, these objects appear to move so slowly in the sky that the change in their relative positions is nearly imperceptible on human timescales, except under careful examination with modern instruments , such as telescopes , that can reveal their proper motions . Hence, they can be considered to be "fixed" for many purposes, such as navigation , charting of stars , astrometry , and timekeeping . Due to

4356-470: The Babylonians . A typical method to determine proper motion is to measure the position of a star relative to a limited, selected set of very distant objects that exhibit no mutual movement, and that, because of their distance, are assumed to have very small proper motion. Another approach is to compare photographs of a star at different times against a large background of more distant objects. The star with

4488-685: 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

4620-513: 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

4752-407: The Sun and is nearly three times as massive, with a luminosity in visible wavelengths about 117 times that of the Sun. Based on an excess emission of infrared radiation , it may have a debris disk , much like Vega . It is a single-lined spectroscopic binary system. The ROSAT All Sky Survey discovered that Alpha Sagittarii is emitting an excess flux of X-rays, which is not expected to originate from

4884-456: 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

5016-632: 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

5148-449: 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

5280-555: The thermonuclear fusion of hydrogen into helium in its core. This process releases energy that traverses 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

5412-524: 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

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5544-424: 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,

5676-513: The Babylonians, who from the eighteenth to the sixth century BC constructed constellation maps. Maps of the stars and the idea of mythological stories to explain them were largely being acquired all over the world and in several cultures. One similarity between them all was the preliminary understanding that the stars were fixed and immobile in the universe. This understanding was incorporated into theorized models and mathematical representations of

5808-525: The Copernican system, still pictures an image labelling the outmost celestial sphere as Sphaera Stellar Fixar , Latin for sphere of fixed stars, following the long-held belief in such sphere. This view was later superseded in his book Astronomia nova (1609), where he established his laws of planetary motion , the mathematical basis for his own Rudolphine Tables , which are working tables from which planetary positions could be shown. Kepler's laws were

5940-427: The Earth being the nearest object to it. In this system, the stars are contained in the furthest sphere, which also rotates, but too slowly for motion to be observed. The motion of the stars is instead explained by the motion of the Earth about the central fire. Another Pythagorean, Ecphantos of Syracuse (c. 400 BC) proposed a system quite similar to that of Philolaos, but without a central fire. Instead, this cosmos

6072-413: 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

6204-413: 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,

6336-541: The Marriage of Philology and Mercury"), also called De septem disciplinis ("On the seven disciplines") was read, taught, and commented upon throughout the early Middle Ages and shaped European education during the early medieval period and the Carolingian Renaissance . Nicolaus Copernicus (1473-1543) created a heliocentric system composed of orbs carrying each of the heavenly bodies. The final orb in his model

6468-491: 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

6600-439: 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

6732-445: The Sun, all surrounded by the sphere of fixed stars. His model was not widely accepted, despite his authority; he was one of the earliest developers of the system of the seven liberal arts , the trivium ( grammar , logic , and rhetoric ) and the quadrivium ( arithmetic , geometry , music , astronomy ), that structured early medieval education. Nonetheless, his single encyclopedic work, De nuptiis Philologiae et Mercurii ("On

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6864-449: 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

6996-468: The Sun, entirely stationary, laid at the center, and all planets revolved around it. Beyond the planets was the sphere of fixed stars, also motionless. This system presented two more unique ideas in addition to being heliocentric: the Earth rotated daily to create day, night, and the perceived motions of the other heavenly bodies, and the sphere of fixed stars at its boundary were immensely distant from its center. This massive distance had to be assumed due to

7128-531: The Third Star of Celestial Spring .) This star, together with Beta¹ Sagittarii and Beta² Sagittarii , were Al Ṣuradain (ألسردين), the two Surad, desert birds. A fictionalized version of the Rukbat system is the setting for Anne McCaffrey 's Dragonriders of Pern series of novels. In the canon of this series, the system has five planets in standard orbits, two asteroid belts, an Oort cloud , and has also captured

7260-495: 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

7392-616: 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 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

7524-496: 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

7656-408: 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

7788-612: 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

7920-409: The component across the line of sight. In 1718 Edmund Halley announced his discovery that the fixed stars actually have proper motion. Proper motion was not noticed by ancient cultures because it requires precise measurements over long periods of time to notice. In fact, the night sky today looks very much as it did thousands of years ago, so much so that some modern constellations were first named by

8052-400: 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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8184-562: The cosmos by philosophers like Anaximander and Aristotle from the Ancient Greeks. Anaximander proposed this original (and erroneous) order of the celestial objects above the Earth: first a nearest layer with the fixed stars plus planets, then another layer with the Moon, and finally an outer one with the Sun. To him, the stars , as well as the Sun and Moon, were apertures of "wheel-like condensations filled with fire". All other later models of

8316-417: 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

8448-465: The distance to the Moon is roughly 380,000 km (236,100 mi), nearly matching Aristarchus. This imposed a minimum radius for the sphere of fixed stars at center-to-center Earth to Moon distance plus the Moon's radius (approx. 1/3 Earth radius), plus the width of the Sun (it being, at least, the same that the Moon), plus the indeterminate thickness of the planets' spheres (believed to be thin, anyway), for

8580-430: The distance to the Sun, but sufficient to assert that the Sun is much bigger than Earth and it is much further away than the Moon. So the minor body, the Earth, must orbit the major one, the Sun, and not the opposite. This reasoning led him to assert that, as stars do not show evident parallax viewed from Earth along a single year, they must be very, very far away from the terrestrial surface and, assuming they were all at

8712-405: 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

8844-437: The epicycle motion does not apply to the Sun.) This device necessarily enlarges each of the celestial spheres , thus making the outer sphere of the fixed stars yet larger. When scholars applied Ptolemy's epicycles, they presumed that each planetary sphere was exactly thick enough to accommodate them. By combining this nested sphere model with astronomical observations, scholars calculated what became generally accepted values at

8976-448: The equivalence of two descriptions of the apparent retrograde motions of planets (assuming the geocentric model): one using eccentrics and another deferent and epicycles . In the following century, measures of the sizes and distances of the Earth and the Moon improved. Around 200 BC Eratosthenes determined that the radius of the Earth is roughly 6,400 km (4,000 mi). Circa 150 BC Hipparchus used parallax to determine that

9108-454: The fact that stars were observed to have no parallax , which can only be explained by geocentricity or immense distances which create a parallax too small to be measured. Ptolemy , 100-175 AD, summarized ideas about the cosmos through his mathematical models and his book Mathematical Syntaxis , much more commonly known as the Almagest . It was written around 150 AD, and Ptolemy declared that

9240-517: 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

9372-460: The fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface. Around 210 BC, Apollonius of Perga shows

9504-408: The fixed stars were, at least, a bit farther than the Moon, the Sun and the rest of the planets . Meanwhile, circa 450 BC Anaxagoras was the first philosopher to consider the Sun as a huge object (larger than the land of Peloponnesus ), and consequently, to realize how far from Earth it might be. He had suggested that the Moon is rocky , thus opaque , and closer to the Earth than the Sun, giving

9636-460: The idea of fixed stars found within the teleology of the tale. Padaric Colum has written a book, The Children of Odin , which in much detail reiterates the story of how the Aesir gods brought the giant named Ymir to his demise and created the world from his body, affixing sparks from the fiery Muspelheim , or the fixed stars, to the dome of the sky, which was the skull of Ymir. The Norse creation myth

9768-405: The large distances of astronomical objects , human vision is unable to perceive the three-dimensional depth of outer space , giving the impression that all stars and other extrasolar objects are equidistant from the observer. In the astronomical tradition of Aristotelian physics which spanned from ancient Greece to early scientific Europe, the fixed stars were believed to exist attached on

9900-507: The largest known proper motion is Barnard's Star . Radial velocity of stars, and other deep-space objects, can be revealed spectroscopically thru the Doppler-Fizeau effect , by which the frequency of the received light decreases for objects that were receding ( redshift ) and increases for objects that were approaching ( blueshift ), when compared to the light emitted by a stationary object. William Huggins ventured in 1868 to estimate

10032-437: 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

10164-677: 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;

10296-412: 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

10428-646: The mechanisms of the heavens: the universal force of gravity suggested that stars could not simply be fixed or at rest, as their gravitational pulls cause "mutual attraction" and therefore cause them to move in relation to each other. The term " Solar System " entered the English language by 1704, when John Locke used it to refer to the Sun, planets, and comets as a whole. By then it had been established beyond doubt, thanks to increased telescopic observations plus Keplerian and Newtonian celestial mechanics , that planets are other worlds, and stars are other distant suns, so

10560-456: 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

10692-445: 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

10824-453: The movement could be caused by a vacuum, because then the objects would move much too fast and without sensible directions. He stated that everything was moved by something and started exploring a concept similar to gravity. He was one of the first to argue (and prove) that the Earth was round, drawing on observations of eclipses and the movements of the other planets relative to the Earth. He proceeded to conclude that most planets navigated in

10956-479: 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

11088-411: The night sky by both professional and amateur astronomers . Pythagorean philosophers held a number of different views on the structure of the universe, but each included a sphere of fixed stars as its boundary. Philolaos (c. 5th cent. BC) proposed a universe which had at its center a central fire , invisible to man. All of the planets, the Moon, Sun, and stars rotated about this central fire, with

11220-403: 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

11352-511: The orbital motion of the Earth. The other group contained the naked eye planets , which they called wandering stars . (The Sun and Moon were sometimes called stars and planets as well.) The planets seem to move forward and back , changing their position over short periods of time (weeks or months). They always seem to move within the band of stars called the zodiac by Westerners. The planets can also be distinguished from fixed stars because stars tend to twinkle, while planets appear to shine with

11484-417: 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

11616-404: 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

11748-406: The parallax of and distance to one of the closest stars ― Alpha Centauri . Henderson did not publish these observations until 1839, after Friedrich Wilhelm Bessel published his parallax observations and distance estimate to 61 Cygni in 1838. The fixed stars exhibit real motion as well, however. This motion may be viewed as having components that consist in part of motion of the galaxy to which

11880-494: The peculiarity of stars that moved. These "wandering stars", planets, moved across the background of fixed stars which were spread along a sphere surrounding encompassing the universe. This geocentric view was held through the Middle Ages, and was later countered by subsequent astronomers and mathematicians alike, such as Nicolaus Copernicus and Johannes Kepler , who challenged the long-standing view of geocentrism and constructed

12012-478: The phenomenon was not understood then, as well as of comets. In European scientific astronomy, evidence that disproved the firmament was gathered gradually. The Copernican Revolution of the 1540s fueled the idea held by some philosophers in ancient Greece and the Islamic world that stars were actually other suns, possibly with their own planets. The definitive discovery of proper motion was announced in 1718, and parallax

12144-472: The planetary system show a celestial sphere containing fixed stars on the outermost part of the universe, its edge, within it lie all the rest of the moving luminaires. Plato , Aristotle and other like Greek thinkers of antiquity, and later the Ptolemaic model of the cosmos showed an Earth-centered universe. Ptolemy was influential with his heavily mathematical work, the Almagest , which attempts to explain

12276-489: The planets (which according to Plato, included the Moon and Sun). The outermost portion of this sphere was the location of the stars. This sphere of fire rotated about the Earth, carrying the stars with it. The belief that the stars were fixed in their place in the sphere of fire was of great importance to all of Plato's system. The stars' position was used as a reference for all celestial motions and used to create Plato's ideas of planets possessing multiple motions. Eudoxus ,

12408-436: The planets were star-like, single points, the sphere of the fixed stars should implicitly be farther than previously thought. Around 280 BC, Aristarchus of Samos offered the first definite discussion of the possibility of a heliocentric cosmos , and by geometrical means he estimated the Moon's orbital radius at 60 Earth radii , and its physical radius as one-third that of the Earth. He made an inaccurate attempt to measure

12540-640: 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

12672-519: The possibility that such a huge sphere could complete a single revolution of 360° around the Earth in only 24 hours was deemed improbable, and this point was one of the arguments of Nicholas Copernicus for leaving behind the centuries-old geocentric model. The highest upper bound ever given was by Jewish astronomer Levi ben Gershon (Gersonides) who, circa 1300, estimated the distance to the fixed stars to be no less than 159,651,513,380,944 Earth radii, or about 100,000 light-years in modern units. This

12804-500: The preferred shape of the universe, and that the Earth was at the centre and the stars forming the outermost shell, followed by planets, the Sun, and the Moon. Around 350 BC Aristotle modified Eudoxus' model by supposing the spheres were material and crystalline. He was able to articulate the spheres for most planets, however, the spheres for Jupiter and Saturn crossed each other. Aristotle solved this complication by introducing an unrolled sphere. By all these devices, and even assuming

12936-403: 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

13068-497: 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

13200-604: The questions of what they are seeing. Norse mythology originates from northern Europe, around the geographical location of modern-day region of Scandinavia and northern Germany . The Norse mythology consists of tales and myths derived from Old Norse , which was a Northern German language from the Middle Ages . There is a series of manuscript texts written in Old Norse which contain a collection of [35] poems written from oral tradition. Among historians there seems to be speculation of

13332-466: The same distance from us, he gave a relative estimation. Following the heliocentric ideas of Aristarcus (but not explicitly supporting them), around 250 BC Archimedes in his work The Sand Reckoner computes the diameter of the universe centered around the Sun to be about 10 × 10   stadia (in modern units, about 2 light years , 18.93 × 10   km , 11.76 × 10   mi ). In Archimedes' own words: His [Aristarchus'] hypotheses are that

13464-461: 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

13596-451: 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

13728-410: The specific dates of the poems written, however, the estimated record of the texts is around the beginning of the thirteenth century. Although the oral tradition of passing down tales existed long before the advent of text manuscripts and print versions. Among surviving texts there is mention of the mythological god, Odin . Scholars have recounted the tale of the Αesir Gods creation myth which includes

13860-608: The sphere. Thus, though the stars were moved around the Earth by the sphere which they occupied, they themselves did not move and were therefore considered fixed. Aristotle , who lived from 384 to 322 BC studied and published similar ideas to Plato, and based on the Eudoxus' system, but he improved on them through his books Metaphysics and On the Heavens written around 350 BC. He claimed that all things have some way of moving, (including "heavenly bodies," or planets,) but he denies that

13992-456: The star belongs, in part of rotation of that galaxy, and in part of motion peculiar to the star itself within its galaxy. In the case of star systems or star clusters , the individual components even move with respect to each other in a non-linear manner. Relative to the Solar System , this real motion of a star is divided into radial motion and proper motion , with "proper motion" being

14124-500: 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

14256-434: The stars are fixed within their celestial spheres, but the spheres themselves are not fixed. The rotations of these spheres thus explain the subtle movements of the constellations throughout the year. Martianus Capella (fl. c. 410–420) describes a modified geocentric model, in which the Earth is at rest in the center of the universe and circled by the Moon, the Sun, three planets and the stars, while Mercury and Venus circle

14388-407: The stars are represented as being contained in a fixed sphere at the boundary of the cosmos. Johannes Kepler (1571–1630) was a devoted Copernican, following Copernicus's models and ideas yet developing them. He was also an assistant of Tycho Brahe, and he could access his patron's accurate measurements in his observational database. Kepler's Mysterium cosmographicum (1596), a strong defense of

14520-446: The stars still remain fixed. His argument was that if such a massive body was moving, there must surely be evidence that is noticeable from the Earth. However, one cannot hear the stars moving, nor can they really see their progress, so Aristotle concludes that while they may be shifted by the planets, they do not move themselves. He writes in On the Heavens , "If the bodies of the stars moved in

14652-623: The stars' placement in relation to each other and distances apart remained unchanged by the rotation of the heavens. He utilized a method using eclipses to find the star distances and calculated the distance of the Moon based on parallax observations. Shortly after, he wrote a follow-up called Planetary Hypotheses. Ptolemy used and wrote about the geocentric system, drawing greatly on traditional Aristotelian physics, but using more complicated devices, known as deferent and epicycles he borrowed from previous works by geometer Apollonius of Perga and astronomer Hipparchus of Nicaea . He declared that

14784-441: The stationary Earth, which is orbited by the Moon and Sun. The planets then revolve about the Sun while it revolves about the Earth. Beyond all of these heavenly bodies lies a sphere of fixed stars. This sphere rotates about the stationary Earth, creating the perceived motion of the stars in the sky. This system has an interesting feature in that the Sun and planets cannot be contained in solid orbs (their orbs would collide), but yet

14916-400: 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

15048-485: 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

15180-468: 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

15312-455: 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

15444-512: The time for the distances to the Sun: about 4 million kilometres (2.5 million miles), and to the edge of the universe: about 73 million kilometres (45 million miles), still around 130,000 times less than Archimedes. Ptolemy's methods, written in his Almagest , were accurate enough to keep them largely undisputed for more than 1,500 years. But by the European Renaissance ,

15576-400: 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

15708-419: The tipping point in finally disproving the old geocentric (or Ptolemaic) cosmic theories and models, what was backed by the first uses of telescope by his contemporary Galileo Galilei , also an advocate of Copernicus. First Greeks, as many other ancient cultures, thought of sky as it was a giant dome-like structure only a few meters above the highest mountains. The myth of Atlas tells that this Titan held

15840-541: The traditional names Rukbat and Alrami , derived from the Arabic rukbat al-rāmī 'the knee of the archer'. The star Delta Cassiopeiae also bore the traditional names Ruchbah or Rukbat , from the Arabic word ركبة rukbah meaning "knee". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included

15972-473: The way to deprecate the Aristotelian sphere of the fixed stars. (This was a revival of beliefs held by Democritus , Epicurus , and Fakhr al-Din al-Razi ). The studies of the heavens were revolutionized with the invention of the telescope . First developed in 1608, Galileo Galilei heard about it and made a telescope for himself. He immediately noticed that the planets were not, in fact, perfectly smooth,

16104-421: The whole Solar System is actually only a small part of an immensely large universe, and definitively something distinct. Astronomers and natural philosophers before divided the lights in the sky into two groups. One group contained the fixed stars , which appear to rise and set but keep the same relative arrangement over time, and show no evident stellar parallax , which is a change in apparent position caused by

16236-442: The whole heavens on his shoulders. Around 560 BC, Anaximander was the first to present a system where the celestial bodies turned at different distances. But erroneously, he thought the stars were closer to Earth (about 9 to 10 times the Earth's size) than the Moon (18-19 times) and the Sun (27-28 times). Nonetheless, later Pythagorians as Philolaus around 400 BC, also conceived a universe with orbiting bodies , thus assuming

16368-500: Was an overestimate; although in the actual universe there are stars farther than that distance, both in the Milky Way (about three times wider) and all the external galaxies , the closest star from Earth (other than the Sun) is Proxima Centauri at about 4.25 light-years only. The attempts to explain the universe stem from observations of the objects found in the sky. Different cultures historically have various stories to provide an answer to

16500-487: Was centered on the Earth, which remained stationary but rotated on an axis, while the Moon, Sun, and planets revolved about it. This system's final boundary was a fixed sphere of stars, and the perceived motion of the stars was thought to be caused by the rotation of the Earth. Plato 's (c. 429-347 BC) universe was centered on a completely stationary Earth, constructed with a series of concentric spheres. The outer sphere of this system consisted of fire and contained all of

16632-435: 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

16764-419: 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

16896-528: Was suspected in the 1670s but shown definitively in the 1830s. Other cultures (such as Chinese astronomy) either never had a belief in a sphere of fixed stars, or constructed it in different ways. (See Cosmology § Historical cosmologies .) People in many cultures have imagined that the brightest stars form constellations , which are apparent pictures in the sky seeming to be persistent, being deemed also as fixed . That way, constellations have been used for centuries, and still are today, to identify regions of

17028-468: Was that of the fixed stars. This final orb was the largest of his cosmos, in both diameter and thickness. This orb of stars is entirely fixed, as the stars are embedded in the sphere, and the sphere itself is immobile. The perceived motion of the stars, therefore, is created by the daily rotation of the Earth about its axis. Tycho Brahe 's (1546-1601) system of the universe has been called "geo-heliocentric" due to its twofold structure. At its center lies

17160-586: 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

17292-603: 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

17424-480: 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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