A star (from the Latin : stella ) is a spheroid light of plasma that holds its shape due to its own gravity . The nearest star to Earth is the sun . [ 1 ] Other stars are visible to the naked eye from Earth at night, appearing as a variety of fixed points of light in the sky due to their immense distance from it. [ 2 ] Historically, the most prominent stars were grouped into constellations andasterisms , and the brightest ones were called with proper names. Astronomers have compiled an extensive catalog , providing stars with standardized designations . However, most of the stars in the Universe , including all those outside our galaxy , the Milky Way , are invisible to the naked eye from Earth. In fact, most are invisible from our planet even through high-powered telescopes .
For at least part of its life, a star shines due to the thermonuclear fusion of hydrogen into helium in its core, releasing energy which passes through the interior of the star and then radiates into outer space . Almost all the natural elements heavier than helium are created by stellar nucleosynthesis during the life of the star and, in some of them, by supernova nucleosynthesis when they explode. Near the end of its life a star can also contain degenerate matter . Astronomers can determine mass, age, metallicity(chemical composition) and many other properties of stars by observing their motion through space, their luminosity and spectrum , respectively. The total mass of a star is the main determinant of its evolution and final destiny. Other characteristics of stars, including diameter and temperature, change throughout their lives, while a star's environment affects its rotation and motion. A scatter plot of many stars that refers to their luminosity , absolute magnitude , surface temperature , and spectral type , known as theHertzsprung-Russell diagram (HR Diagram), allows to determine the age and evolutionary state of a star.
The life of a star begins with the gravitational collapse of a gaseous nebula made up mainly of hydrogen, along with helium and traces of heavier elements. When the stellar nucleus is dense enough, hydrogen begins to turn into helium through nuclear fusion , releasing energy during the process. [ 3 ] The remains inside the star carry energy outside the core through a combination of process heat transfer by radiation and convection. The internal pressure of the star prevents it from collapsing further under its own gravity. When the hydrogen fuel in the core is depleted, a star with at least 0.4 times the mass of the Sun will expand to become a red giant , [ 4 ] in some cases fusing heavier elements in the core or their layers around the core (like carbon or oxygen ). The star then evolves into a degenerate form, expelling a portion of its matter into the interstellar medium, where it will contribute to the formation of a new generation of stars. [ 5 ] Meanwhile, the core becomes a stellar remnant.: a white dwarf , a neutron star , or (if it's massive enough) a black hole .
The binary system and multiestelares consist of two or more stars that are gravitationally bound to each other, and generally move around each other in orbits stable. When two stars are in relatively close orbit, their gravitational interaction can have a significant impact on their evolution. [ 6 ] Stars bound gravitationally to each other can be part of much larger structures, such as star clusters or galaxies .
Historically, stars have been important to civilizations around the world, they have been part of religious practices and were used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently attached to a celestial sphere and were immutable. By convention astronomers grouped stars into constellations and used them to track the movements of the planets and the inferred position of the Sun. [ 7 ] The Sun's motion against the background stars (and the horizon) was used to create calendars , that could be used to regulate agricultural practices. [ 9] TheGregorian calendar, currently used almost all over the world, is asolar calendarbased on the angle of the Earth's axis of rotation with respect to its local star, the Sun.
The oldest accurately dated star chart was an achievement of ancient Egyptian astronomy in 1534 BC. C. [ 10 ] The earliest known star catalogs were compiled by ancient Babylonian astronomers in Mesopotamia in the late second millennium BC, during the Casita period ( ca 1531-1155 BC). [ 11 ]
The first catalog of stars in Greek astronomy was created by Aristilus around 300 BC, with the help of Timocharis . [ 12 ] Hipparchus' catalog of stars (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's catalog of stars . [ 13 ] Hipparchus is known for the discovery of the first nova (new star) registered. [ 14 ] Many of the constellations and star names in use today are derived from Greek astronomy.
Despite the apparent immutability of the skies, Chinese astronomers were aware that new stars could appear. [ 15 ] In 185 d. C., were the first to observe and write about a supernova, now known as SN 185 . [ 16 ] The brightest recorded stellar event in history was supernova SN 1006 , which was observed in 1006 and described by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. [ 17 ] Supernova SN 1054 , which gave rise to the Crab Nebula, was also observed by Chinese and Islamic astronomers. [ 18 ] [ 19 ] [ 20 ]
The medieval Islamic astronomers gave Arabic names to many stars that are still used today and invented numerous astronomical instruments with which to calculate the positions of the stars. They also built the first large research institutes and observatories, mainly for the purpose of producing Zij catalogs of stars. [ 21 ] Among them, the Persian astronomer Abd Al-Rahman Al Sufi wrote the Book of Fixed Stars (964), which observed various stars, star clusters (including the Omicron Velorum and the Brocchi clusters) and galaxies (including the Andromeda Galaxy ). [ 22 ] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments that had the properties of nebulous stars and in 1019 he also gave the latitudes of several stars during a lunar eclipse . [ 23 ]
According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched each other and appeared to be a continuous image due to the effect of the refraction of the sublunar material, citing his observation of the conjunction of Jupiter and Mars in 500 AH (AD 1106/1107) as evidence. [ 24 ] Early European astronomers, such as Tycho Brahe , identified new stars in the night sky (later called novae ), suggesting that the heavens were not immutable. In 1584, Giordano Brunosuggested that the stars were like the Sun and could have other planets , possibly Earth-like, orbiting them, [ 25 ] an idea that had already been suggested previously by the ancient Greek philosophers , Democritus and Epicurus , [ 26 ] And by medieval Islamic cosmologists [ 27 ] such as Fakhr al-Din al-Razi . [ 28 ]In the following century the idea that stars were equal to the Sun was reaching a consensus among astronomers. To explain why these stars did not exert any net gravitational force on the solar system, Isaac Newton suggested that the stars were equally distributed in all directions, an idea pioneered by theologian Richard Bentley . [ 29 ]
In 1667 the Italian astronomer Geminiano Montanari recorded observed variations in the luminosity of the star Algol . Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, showing that they had changed their positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus. [ 25 ]
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 markers in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars was constantly rising to one side of the sky, towards the core of the Milky Way. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction. [ 30 ] In addition to his other accomplishments, William Herschel is also notable for his discovery that some stars are not simply found along the same line of sight,
The science of astronomical spectroscopy was started by Joseph von Fraunhofer and Angelo Secchi . Comparing the spectra of stars like Sirius to the Sun, they found differences in the strength and number of their absorption lines - the dark lines in a stellar spectrum caused by absorption from the atmosphere of specific frequencies. In 1865 Secchi began to classify stars by spectral types . [ 31 ] However, the modern version of the star classification scheme was developed by Annie J. Cannon during the 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 . The parallax measurements demonstrated the great separation of the stars in the heavens. [ 25 ] The observation of double stars gained increasing importance during the 19th century. In 1834 Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. In 1899, Edward Pickering discovered the first spectroscopic binary when he observed the periodic division of the spectral lines of the starMizar in a period of 104 days. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg, Wilhelm von Struve, and SW Burnham , which allowed the masses of the stars to be determined from the computation of the orbital elements . In 1827 Felix Savary gave the first solution to the problem of deriving an orbit of binary stars from telescopic observations. [ 32 ] The 20th century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschilddiscovered that the color of a star, and therefore its temperature, could be determined by comparing the visual magnitude with the photographic magnitude . The development of the photoelectric photometer enabled precise magnitude measurements at multiple wavelength ranges. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope of the Mount Wilson Observatory . [ 33 ]
During the first decades of the 20th century, important theoretical work was produced on the physical structure of stars. In 1913, the Hertzsprung-Russell diagram was developed , which prompted the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. In 1925 Cecilia Payne-Gaposchkin proposed for the first time in her doctoral thesis that stars are made primarily of hydrogen and helium. [ 34 ] The spectra of stars were further understood through advances in quantum physics . This made it possible to determine the chemical composition of the stellar atmosphere. [ 35]
With the exception of supernovae , individual stars have been observed mainly in the Local Group , [ 36 ] and especially in the visible part of the Milky Way (as evidenced by the detailed star catalogs available for our galaxy). [ 37 ] But some stars have been observed in the M100 galaxy of the Virgo cluster , about 100 million light years from Earth. [ 38 ] In the Local Supercluster it is possible to see cumulus cloudsof stars, and current telescopes could, in principle, observe faint individual stars in the Local Group [ 39 ] (see Cepheids ). However, outside the local Supercluster of galaxies, neither stars nor star clusters have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of a trillion light years, [ 40 ] ten times as far as the most distant group of stars previously observed.
The concept of constellation was already known during the Babylonian period . Ancient sky watchers imagined that the arrangement of the prominent stars formed pictures, and associated them with particular aspects of nature or their myths. Twelve of these formations were located along the plane of the ecliptic and became the basis of astrology . [ 41 ] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations .
Like certain constellations and the Sun itself, individual stars have their own myths . [ 42 ] For the ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs, meaning "wanderer"), represented several important deities, from which the names of the planets Mercury , Venus , Mars were taken , Jupiter and Saturn . [ 42 ] ( Uranus and Neptune were also Greek and Roman gods, but neither was known in ancient times due to their low brightness and their names were assigned by later astronomers).
Around 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 a series of star maps and applied Greek letters as designations for the stars in each constellation. Later a numbering system based on the right ascension of the star was invented and added to John Flamsteed's catalog of stars in his book Historia coelestis Britannica (the 1712 edition), which is why this numbering system came to be called a naming Flamsteed or Flamsteed numbering . [ 43 ] 
The only internationally recognized authority for designating celestial bodies is the International Astronomical Union (IAU). [ 45 ] This association maintains the Working Group on Star Names (WGSN) [ 46 ] which catalogs and standardizes the proper names of stars. Various private companies sell star names, what the British Library calls an unregulated commercial company . [ 47 ] [ 48 ]The AIU has disassociated itself from this commercial practice and these names are not recognized by the IAU, professional astronomers, or the amateur astronomer community. [ 49 ] One such firm is the International Star Registry , which during the 1980s was accused of deceptive practices for making the assigned name appear official . This practice of ISR, now interrupted, was informally labeled racquet and fraud [ 50 ] [ 51 ] [ 52 ] [ 53 ]And the New York City Department of Consumer Affairs issued a warning against ISR for engaging in a deceptive business practice. [ 54 ] [ 55 ]
Units of measure
Although stellar parameters can be expressed in SI units or CGS units , it is often more convenient to express mass , luminosity and radius in solar units, based on the characteristics of the Sun. In 2015 the UAI defined a set of Solar nominal values (defined as SI constants, without uncertainties) that can be used to quote stellar parameters:
The solar mass M ⊙ was not explicitly defined by the UAI due to the large relative uncertainty (10 −4 ) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and the joint solar mass (GM ⊙ ) has been determined with much greater precision, the IAU defined the nominal solar mass parameter as:
However, the nominal solar mass parameter can be combined with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to obtain a solar mass of approximately 1.9885 × 10 30 kg. Although the exact values of luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the IAU nominal constants from 2015 will remain the same SI values, as they are still useful for quote 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 the AIU defined the astronomical constant as an exact length in meters: 149 597 870 700 m. [ 56 ]
Star formation and evolution
Stars condense in the densest regions of space , although those regions are less dense than the interior of a vacuum chamber . These regions, known as molecular clouds , consist primarily of hydrogen, with about 23 to 28 percent helium and some heavier elements. An example of these star-forming regions is the Orion Nebula . [ 57 ] Most stars form in groups of tens to hundreds of thousands of stars. [ 58 ]
The massive stars in these clusters can powerfully illuminate those clouds, ionize hydrogen, and create H II regions . Such feedback effects, from star formation, can eventually disrupt the cloud and prevent additional star formation.
All stars spend most of their existence as main sequence stars , fueled primarily by the nuclear fusion of hydrogen into helium within their nuclei. However stars of different masses have markedly different properties at various stages of their development. The final destination of the most massive stars differs from that of less massive stars, to equal their luminosities and the impact they have on their environment, so that astronomers usually group the stars by mass: [ 59 ]
- Very low mass stars , with masses below 0.5 M ☉ , are completely convective and distribute helium evenly throughout the star while in the main sequence. Hence, they never undergo cladding burn or become red giants but stop merging and become helium white dwarfs , slowly cooling after depleting their hydrogen. [ 60 ] However, as the life span of 0.5 M ☉ stars is longer than the age of the universe , none of these stars has reached the white dwarf stage.
- Low-mass stars (including the Sun), with a mass between 0.5 M ☉ and 1.8-2.5 M ☉ depending on composition, turn into red giants as their hydrogen core it runs out and they start to burn helium in the core in a helium flash ; they develop a carbon-oxygen nucleus, later degenerated into the giant asymptotic branch ; they eventually shed their outer shell as a planetary nebula and leave behind their core in the form of a white dwarf.
- Intermediate mass stars , between 1.8-2.5 M ☉ and 5-10 M ☉ , go through evolutionary stages similar to low-mass stars, but after a relatively short period in the red bunch the helium without flash and go through an extended period in red bunching before forming a degenerate carbon-oxygen core.
- Massive stars, generally have a minimum mass of 7-10 M ☉ (possibly as low as 5-6 M ☉ ). After exhausting the hydrogen in the nucleus, these stars become supergiants and fuse elements heavier than helium. They end their life when their nuclei collapse and explode as supernovae.
Star formation begins with gravitational instability within a molecular cloud caused by regions of higher density - often triggered by the compression of the clouds by radiation from massive stars, by expanding bubbles in the interstellar medium, by collision from different molecular clouds or by the collision of galaxies (as in a starburst galaxy ) -. [ 61 ] [ 62 ] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability begins to collapse under its own gravitational force. [ 63 ]
As the cloud collapses, individual conglomerates of dense dust and gas form a " Bok globule ." When it collapses and the density increases, the gravitational energy is converted into heat and the temperature increases. When the protostar cloud has reached approximately the stable condition of hydrostatic equilibrium , a protostar forms in the core. [ 64 ]
Generally these pre-main sequence stars are surrounded by a protoplanetary disk and powered primarily by gravitational energy conversion. Its period of gravitational contraction lasts around 10 to 15 million years.
The early stars less than 2 M ☉ are called T Tauri stars , while those with the highest mass are the Herbig Ae / Be stars . These newly formed stars emit jets of gas along their axis of rotation, which can reduce the angular momentum of the collapsing star and give rise to tiny patches of nebulosity known as Herbig-Haro objects . [ 65 ] [ 66 ] These jets in combination with radiation of nearby massive stars, can help remove the surrounding cloud from which the star was formed. [ 67 ]
Early in their development, T Tauri stars follow Hayashi's trajectory : they contract and diminish in luminosity while remaining at approximately the same temperature.
It is observed that most stars are part of binary star systems and the properties of these systems are the result of the conditions in which they were formed. [ 68 ]
A gas cloud must lose its angular momentum to collapse and form a star. The fragmentation of the cloud into multiple stars distributes part of that angular momentum. These interactions tend to further split separate (soft) binary systems, while also causing hard systems to become more closely linked. This produces the separation of the binary systems into their two observed population distributions.
Stars consume about 90% of their existence by fusing hydrogen into helium at high temperatures and in high-pressure reactions near the nucleus. These stars are said to be in the main sequence , and are called dwarf stars. Starting from the age zero main sequence, the proportion of helium in a star's core will increase steadily, as well as the rate of nuclear fusion in the core will also slowly increase, as will the star's temperature and luminosity. [ 69 ] The Sun, for example, is estimated to have increased in luminosity by 40% since it reached the main sequence 4.6 billion (4.6 × 10 9 ) years ago. [ 70 ]
Each star generates a stellar wind of particles that causes a continuous flow of gas into space. For most stars, the lost mass is negligible. The Sun loses 10 −14 M ☉ each year, [ 71 ] or about 0.01% of its total mass over its lifetime. However, very massive stars can lose 10-7 to 10-5 M☉ each year, which significantly affects their evolution. [ 72 ] Stars that start with more than 50 M ☉ can lose more than half of their total mass while in the main sequence. [ 73 ]
The time a star consumes in the main sequence depends mainly on the amount of fuel it has and the speed at which it fuses it. The Sun is expected to live 10 billion (10 10 ) years. Massive stars consume their fuel very quickly and are short-lived. Low-mass stars consume their fuel very slowly. Stars smaller than 0.25 M ☉ , called red dwarfs , are able to fuse almost all of their mass, while stars around 1 M ☉they can only fuse about 10% of their mass. The combination of their slow fuel consumption and their relatively large supply of usable fuel allows low-mass stars to last for about one billion (10 12 ) years; those over 0.08 M ☉ will last about 12 billion years. Red dwarfs get hotter and brighter when they accumulate helium. When they finally run out of hydrogen, they contract into a white dwarf and their temperature drops. [ 60 ] However, since the lifespan of these stars is greater than the current age of the universe (13.8 billion years), stars smaller than about 0.85 M ☉ are not expected [74 ] have been moved from the main sequence.
In addition to mass, elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call metallicity the chemical concentration of these elements in a star. The metallicity of a star can influence the time it takes for the star to burn its fuel and controls the formation of its magnetic fields, [ 75 ] which affects the strength of its stellar wind. [ 76 ] The oldest stars in the population IIthey have substantially less metallicity than the younger stars in Population I 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 .
Main post sequence
As stars of at least 0.4 M ☉ [ 4 ] deplete their supply of hydrogen in their nucleus, they begin to fuse hydrogen in an area outside the helium nucleus. Their outer layers are greatly expanded and cooled as they form a red giant . In about 5 billion years, when the Sun enters the helium-burning phase, it will expand to a maximum radius of about 1 astronomical unit (150 million kilometers), 250 times its current size, and lose 30% of its mass. current. [ 77 ] [ 78 ]
As the combustion of the hydrogen 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 nucleus degenerates before the fusion of helium . Finally, when the temperature rises sufficiently, the fusion of helium begins explosively in what is called a helium flash, and the star rapidly contracts in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. . For the most massive stars, the fusion of the helium nucleus begins before the nucleus degenerates, and the star spends some time in the red bunch, burning helium slowly before the outer convective envelope collapses and the star moves to the horizontal branch. [ 6 ]
After the star has fused the helium in its core, the carbon product fuses, producing a hot core with an outer shell of fusion helium. Then the star follows an evolutionary path called the giant asymptotic branch (AGB) that is parallel to the other red giant phase described, but with a higher luminosity. The most massive AGB stars can undergo a brief period of carbon fusion before the nucleus degenerates.
During its helium-burning phase, a star with more than nine solar masses expands to form first a blue supergiant and then a red one . Particularly massive stars can evolve into a Wolf-Rayet star , characterized by spectra dominated by emission lines of elements heavier than hydrogen that have reached the surface due to strong convection and intense mass loss.
When helium is depleted in the core of a massive star, the core contracts and the temperature and pressure rise enough to fuse the carbon (see carbon combustion process ). This process continues, with successive stages powered by neon (see neon combustion process ), oxygen (see oxygen combustion process) and silicon (see silicon combustion process). Near the end of the star's life, the fusion continues along a series of consecutive layers within a massive star. Each layer merges a different element; the outermost layer fuses hydrogen, the next one fuses helium, and so on. [ 79 ]
The final stage occurs when a massive star begins to produce iron . Since iron nuclei are more closely linked than any heavier nucleus, any fusion beyond iron does not produce a net release of energy. Such a process continues to a very limited extent, but consumes energy. Similarly, since nuclei are more closely bound than all lighter nuclei, such energy cannot be released by fission . [ 80 ]
As the core of a star contracts, the intensity of radiation from that surface increases, creating such radiation pressure in the outer layer of gas that it will push those layers together, forming a planetary nebula . If what is left after the outer atmosphere has been released is less than 1.4 M ☉ , it is reduced to a relatively small object. the size of Earth, known as a white dwarf . White dwarfs lack sufficient mass for additional gravitational compression to occur. [ 81 ] The material DegenerateThe electron within a white dwarf is no longer a plasma, even though stars are generally known as plasma spheroids. Eventually white dwarfs fade into black dwarfs over a very long period of time.
In the largest 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 nucleus will suddenly collapse as its electrons are propelled into its protons, forming neutrons, neutrinos and gamma rays in an electron capture and reverse beta decay explosion . The shock wave formed by this sudden collapse causes the rest of the star to explode in a supernova.. These become so bright that they can briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been described by naked-eye observers as "new stars" where apparently none existed before. [ 82 ]
A supernova explosion ejects the outer layers of the star, leaving a remnant such as the Crab Nebula . [ 82 ] The nucleus is compressed into a neutron star that sometimes manifests as a pulsar or X-ray eruption . In the case of the largest stars, the remnant is a black hole larger than 4 M ☉ . [ 83 ] In a neutron star matter is in a state known as neutron degenerate matter, with a more exotic form of degenerate matter, QCD matter., possibly present in the nucleus. Inside a black hole, matter is in a state that cannot be understood today.
Heavy elements are included in the shed outer layers of dying stars that can be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow of supernovae and the stellar wind from large stars play an important role in the formation of the interstellar medium. [ 82 ]
The post-main sequence evolution of binary stars can be significantly different from the evolution of individual stars of the same mass. If the stars in a binary system are close enough, when one of the stars expands to become a red giant it can spill over into its Roche lobe , the region around a star where the material is gravitationally bound to that star, leading to the transfer of material from one to another. A variety of phenomena such as contact binary stars , common envelope binaries , cataclysmic variables , and type Ia supernovae can occur when the Roche lobe is breached .
Stellar grouping and distribution
Stars are not evenly distributed throughout the universe but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (10 11 ) galaxies in the observable universe . [ 84 ] In 2010, an estimate of the number of stars in the observable universe was almost a third of a quadrillion ( 3 × 10 23 ). [ 85 ] Although stars are often believed to only exist within galaxies, intergalactic stars have been discovered. [ 86 ]
A multistellar system consists of two or more gravitationally linked stars that orbit each other. The simplest and most common multistellar system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars. [ 87 ] There are also larger groups, called star clusters, ranging from loose stellar associations with just a few stars to huge globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.
It has long been assumed that most stars are found in gravitationally bound multi-star systems. This is particularly true for very massive class O and B stars, where 80% of the stars are believed to be part of multi-star systems. The proportion of single-star systems increases with decreasing stellar mass, so that only 25% of red dwarfs are known to have stellar companions. Because 85% of all stars are red dwarfs, most of the stars in the Milky Way are possibly unique from birth. [ 88 ]
The closest star to Earth, other than the Sun, is Proxima Centauri , which is 39.9 trillion kilometers, or 4.2 light years away. Traveling at the orbital speed of the space shuttle (8 kilometers per second, almost 30,000 kilometers per hour), it would take about 150,000 years to arrive. [ 89 ] This is typical of stellar separations in galactic discs . [ 90 ] Stars can be much closer to each other at the centers of galaxies and in globular clusters , or much further apart in galactic halos .
Due to the relatively large distances between stars outside the galactic core, collisions between stars are believed to be rare. In denser regions such as the nuclei of globular clusters or the galactic center, collisions may be more common. [ 91 ] Such collisions can produce what are known as blue stragglers . These anomalous stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which they belong. [ 92 ]
Stars can be gravitationally bound to each other to form binary star systems , ternaries, or even larger clusters. A high fraction of the stars in the Milky Way disk belong to binary systems; the percentage is close to 90% for massive stars [ 93 ] and drops to 50% for low-mass stars. [ 94 ] Other times, the stars are grouped in large concentrations ranging from tens to hundreds of thousands or even millions of stars, forming so-called star clusters.. These clusters may be due to variations in the galactic gravitational field or they may be the result of outbreaks of star formation (it is known that most stars form in groups). Traditionally, in the Milky Way , two types were distinguished: (1) globular clusters , which are old, found in the halo and contain hundreds of thousands to millions of stars, and (2) open clusters , which are of recent formation. , are found on the disk and contain fewer stars. Since the end of the 20th century, this classification has been questioned when star clusters were discovered in the disk of the Milky Way.young like Westerlund 1 or NGC 3603 with a star number similar to that of a globular cluster. These young, massive clusters are found in other galaxies as well; some examples are 30 Doradus in the Large Magellanic Cloud and NGC 4214-IA in NGC 4214.
Not all stars maintain stable gravitational bonds; some, like the Sun, travel alone, separating much from the stellar grouping in which they were formed. These isolated stars respond only to the global gravitational field constituted by the superposition of the fields of all objects in the galaxy: black holes , stars, compact objects and interstellar gas .
Stars are not normally evenly distributed in the universe , despite what it may appear to the naked eye, but rather grouped into galaxies . A typical spiral galaxy , like the Milky Way , contains hundreds of billions of stars clustered, most of them in the narrow galactic plane . The terrestrial night sky appears homogeneous to the naked eye because it is only possible to observe a very localized region of the galactic plane. Extrapolating from what is observed in the vicinity of the solar system, it can be said that most of the stars are concentrated in the galactic disk and within this in a central region, the galactic bulge, which is located in the constellation Sagittarius .
Despite the enormous distances that separate the stars, from Earth's perspective their relative positions appear fixed in the sky. Thanks to the precision of their positions, "they are very useful for navigation, for the orientation of astronauts in spacecraft and to identify other stars" ( The American Encyclopedia ). Stars were the only way for sailors to navigate the high seas until the advent of electronic positioning systems in the mid-20th century. See Star (nautical) .
Almost everything about a star is determined by its initial mass, including characteristics such as its luminosity, size, evolution, lifespan, and final destination.
Most stars are between 1 billion and 11 billion years old. Some stars may even be close to 13.8 billion years, the observed age of the universe . The oldest star discovered so far, HD 140283 , nicknamed the Methuselah Star , has an estimated age of 14.46 ± 0.8 billion years. [ 95 ] (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13 799 ± 0.021). [ 95 ] [ 96 ]
The more massive the star, the shorter its life span, mainly because massive stars have greater pressure on their cores, causing them to burn hydrogen more quickly. The most massive stars last an average of a few million years, while the lowest mass stars (red dwarfs) burn their fuel very slowly and can last for tens to hundreds of billions of years. [ 97 ] [ 98 ]
When stars form in today's Milky Way galaxy, they are made up of 71% hydrogen and 27% helium, [ 99 ] measured by mass, with a small fraction of heavier elements. Typically, the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, since iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements can be an indicator of the probability that the star has a planetary system. [ 100 ]
The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1 / 200,000th the iron content of the Sun. [ 101 ] In contrast, the super-metal rich star μ Leonis has almost twice the abundance of iron as the Sun, while the planetary star 14 Herculis has almost three times the iron. [ 102 ]
There are also chemically peculiar stars that show unusual abundances of certain elements in their spectrum, especially chromium and rare earths . [ 103 ] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules. [ 104 ]
Due to their great distance from Earth, all stars except the Sun appear to the naked eye as bright spots in the night sky that twinkle due to the effect of Earth's atmosphere. The Sun is also a star, but it is close enough to Earth to appear as a disk and provide natural light. Apart from the Sun, the star with the largest apparent size is R Doradus , with an angular diameter of only 0.057 arcseconds . [ 105 ]
The disks of most stars are too small in angular size to be observed with current ground-based optical telescopes, so interferometric telescopes are required to image these objects. Another technique for measuring the angular size of stars is through concealment . By accurately measuring the fall in brightness of a star that is obscured by the Moon (or the increase in brightness when it reappears), its angular diameter can be calculated. [ 106 ]
The size of stars ranges from neutron stars, which are 20 to 40 km in diameter, to supergiants like Betelgeuse in the constellation Orion , with a diameter about 1,070 times that of the Sun - about 1,490,171,880. km (925,949,878 mi) - albeit with a density much lower than the sun. [ 107 ]
The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of a star's motion consist of the radial velocity toward or away from the Sun, and the transverse angular motion, which is called proper motion .
Radial velocity is measured by the Doppler shift of the star's spectral lines and is given in units of km / s . The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milliseconds of arc ( mas ) per year. Knowing the parallax of the star and its distance, the speed of its own movement can be calculated. Together with the radial velocity the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements. [ 109 ]
When both speeds of motion are known, the space velocity of the star can be calculated relative to the Sun or the galaxy. Among nearby stars, the younger stars in population I have generally been found to have slower velocities than the older stars in population II. [ 110 ] Comparing the kinematics of nearby stars allowed astronomers to trace their origin to common points in giant molecular clouds, called stellar associations . [ 111 ]
The magnetic field of a star is generated within the interior regions where convective circulation occurs . This movement of the conducting plasma works like a dynamo, where the movement of electrical charges induces magnetic fields, just like a mechanical dynamo . Those magnetic fields have a great range that extends through and beyond the star. The intensity of the magnetic field varies with the mass and composition of the star, and the amount of surface magnetic activity depends on the speed of the star's rotation. This surface activity produces starspots , which are regions of strong magnetic fields with lower than normal surface temperatures. TheCoronal loops arch lines of magnetic field flux that rise from a star's surface to the star's outer atmosphere, its corona. Coronal loops can be seen due to the plasma they conduct their entire length. The stellar eruptions are bursts of high - energy particles that are emitted due to the same magnetic activity. [ 112 ]
Young, rapidly spinning stars tend to have high levels of surface activity due to their magnetic field. The magnetic field can act on the stellar wind of a star, working as a brake that gradually decreases the speed of rotation over time. Thus, older stars like the Sun have a much slower rotational speed and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical fashion and can be completely interrupted for periods of time. [ 113 ] For example, during the Maunder Minimum , the Sun underwent a period of 70 years with almost no sunspot activity.
One of the most massive known stars is Eta Carinae , [ 114 ] which, with 100-150 times more mass than the Sun, will have a life of only several million years. Studies of the most massive open clusters suggest 150 M ☉ as the upper limit for stars in the current age of the universe. [ 115 ] This represents an empirical value for the theoretical limit on the mass of stars in formation due to the increasing radiation pressure on the accretion gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses, [ 116] But it has been determined that could have been created through the collision and merging of massive stars in close binary systems, avoiding the limit of 150 M☉in the formation of massive stars. [ 117 ]
The first stars to form after the Big Bang may have been larger, up to 300 M ☉ , [ 118 ] due to the complete absence of elements heavier than lithium in their composition. It is likely that this generation of population III supermassive stars existed in the very early universe (i.e., they are observed to have a high red shift) and may have started the production of chemical elements heavier than hydrogen that are necessary. for the subsequent formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60. [ 119 ] [ 120 ]
With a mass only 80 times that of Jupiter ( M J ), 2MASS J0523-1403 is the smallest known star to undergo nuclear fusion at its core. [ 121 ] For stars with similar metallicity of the Sun, theoretical minimum mass the star can have and still suffer fusion in the core, it is estimated to be about 75 M J . [ 122 ] [ 123 ] However, when the metallicity is very low, the minimum size of the star seems to be about 8.3% of the solar mass, or about 87 M J . [123 ] [ 124 ] Smaller bodies called brown dwarfs, occupy a poorly defined gray area between stars and gas giants .
The combination of a star's radius and mass determines its surface gravity. Giant stars have a much lower surface gravity than main sequence stars, whereas the opposite is the case for compact, degenerate stars like white dwarfs. Surface gravity can influence the appearance of a star's spectrum, with greater gravity causing a widening of absorption lines . [ 35 ]
The speed of rotation of stars can be determined through spectroscopic measurement , or more accurately by tracking their starspots . Young stars can rotate more than 100 km / s at the equator. For example, the class B star Achernar has an equatorial velocity of about 225 km / s or more, which makes its equator protrude outward and gives it an equatorial diameter that is more than 50% greater than between the poles. This speed of rotation is just below the critical speed of 300 km / s, the speed at which the star would break. [ 125 ]In contrast, the Sun rotates once every 25-35 days, depending on latitude, with an equatorial speed of 1994 km / s. A main sequence star's magnetic field and stellar wind serve to slow its rotation by a significant amount as it evolves in the main sequence. [ 126 ]
The degenerate stars have contracted into a compact mass, resulting in a rapid rotational speed. However, they have relatively low rates of rotation compared to what would be expected from conservation of angular momentum: the tendency of a rotating body to compensate for a size contraction by increasing its rate of rotation. A large part of the star's angular momentum is dissipated as a result of mass loss by the stellar wind. [ 127 ] Despite this, the rotational speed of a pulsar can be very fast. For example, the pulsar in the heart of the Crab Nebula spins 30 times per second. [ 128 ]The rotation speed of the pulsar will gradually decrease due to the emission of radiation.
The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is usually calculated from the star's color index . [ 129 ] Temperature is normally given in terms of an effective temperature , which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. [ 130 ] The temperature in the central region of a star is several million degrees Kelvin . [ 131 ]
The stellar temperature will determine the ionization rate of various elements, giving rise to characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption characteristics, are used to classify a star (see classification below). [ 35 ]
The largest stars in the main sequence can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures, around 3,600 K; but they also have a high luminosity due to their large outer surface. [ 132 ]
The energy produced by stars, product of nuclear fusion, radiates as much space as electromagnetic radiation and particle radiation . The latter, emitted by a star, manifests itself as the stellar wind, [ 133 ] which flows from the outer layers in the form of electrically charged protons and alpha and beta particles . Although nearly massless, there is also a constant stream of neutrinos emanating from the star's core.
The production of energy in the nucleus is the reason why stars shine so brightly: every time two or more atomic nuclei fuse to form a single atomic nucleus of a new heavier element, photons of gamma rays are released , product of nuclear fusion. This energy is converted into other forms of lower frequency electromagnetic energy , such as visible light when it reaches the outer layers of the star.
The color of a star, determined by the strongest frequency of visible light, depends on the temperature of the star's outer layers, including its photosphere . [ 134 ] In addition to visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye . In fact, the electromagnetic radiation stellar spans the electromagnetic spectrum , from wavelengths longer of radio waves through an infrared, visible light and ultraviolet , until the shorter of the X - rays andgamma rays . From the point of view of the total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the physics of the star.
Using the stellar spectrum , astronomers can also determine the surface temperature, surface gravity , metallicity, and rotation speed of a star. If the distance from the star is found, such as by measuring parallax, then the luminosity of the star can be derived. Mass, radius, surface gravity, and period of rotation can be estimated from stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the individual mass of a star. [ 135 ]) With these parameters, astronomers can also estimate the age of the star. [ 136 ]
The luminosity of a star is the amount of light and other forms of radiant energy radiated per unit of time. It has power units . The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. For example, the fast-rotating star Vega has a higher energy flux (power per unit area) at its poles than along its equator. [ 137 ]
The surface spots of a star with lower temperature and luminosity than average are known as starspots . Small and dwarf stars , like our Sun, generally have essentially featureless spots with only tiny spots. In contrast, giant stars have much larger and more conspicuous starspots, [ 138 ] and also exhibit a strong darkening of the stellar limb. That is, the brightness decreases towards the edge of the stellar disk. [ 139 ] The flashing stars red dwarf such as UV Cetithey may also have prominent characteristic spots. [ 140 ]
The apparent brightness of a star is expressed in terms of its apparent magnitude . It is a function of the star's luminosity, its distance from Earth, and the alteration of the star's light as it passes through Earth's atmosphere. The intrinsic or absolute magnitude is directly related to the luminosity of the star, and is the apparent magnitude of a star if the distance between the Earth and the star were 10 parsecs (32.6 light years).
of stars [ 141 ]
Both the apparent and absolute magnitude scales are logarithmic units : a whole number difference in magnitude is equal to a brightness change of about 2.5 times [ 142 ] (the fifth root of 100 or about 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude star (+2.00), and about 100 times brighter than a sixth magnitude star (+6.00). The fainter stars visible to the naked eye under ideal visual conditions are magnitude +6.
On both the apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; conversely, the higher the magnitude number, the fainter the star. The brightest stars, on any scale, have numbers of negative magnitudes. The variation in brightness (Δ L ) between two stars is calculated by subtracting the magnitude number of the brightest star ( m b ) from the magnitude number of the fainter star ( m f ), using the difference as an exponent for the number of base 2,512; that is to say:
In relation to luminosity and distance from Earth, the absolute magnitude of a star ( M ) and the apparent magnitude ( m ) are not equivalent; [ 142 ] For example, the bright star Sirius has an apparent magnitude of –1.44, but has an absolute magnitude of +1.41.
The Sun has an apparent magnitude of -26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky seen from Earth, is approximately 23 times brighter than the Sun, while Canopus , the second brightest star in the night sky with an absolute magnitude of –5.53, is approximately 14,000 times brighter than the Sun. However, although Canopus is much brighter than Sirius, it appears brighter than Canopus. This is because Sirius is only 8.6 light years from Earth, while Canopus is much further away, at a distance of 310 light years.
As of 2006 the star with the highest known absolute magnitude is LBV 1806-20 , with a magnitude of –14.2. This star is at least 5,000,000 times more luminous than the Sun. [ 143 ] The least luminous stars known as of 2017 are in the cluster NGC 6397 . The faintest red dwarfs in the cluster were of magnitude 26, while a white dwarf of magnitude 28 was also discovered. These faint stars are so dark that their light would be as dim as a birthday candle on the Moon seen from Earth. . [ 144 ]
|Classification||Color||Temperature (° C)||Example|
|B||Bluish white||25 000||Spica|
|R||Reddish orange||2600||CW Leonis|
The first stellar classification was made by Hipparchus of Nicea and preserved in Western culture through Ptolemy , in a work called Almagest . This system classified stars by the intensity of their apparent brightness as seen from Earth . Hipparchus defined a decreasing scale of magnitudes, where the brightest stars are of the first magnitude and the less bright, almost invisible to the naked eye, are of the sixth magnitude. Although it is no longer used, it formed the basis for the current classification.
The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line . [ 145 ] The resistance of the hydrogen line was thought to be a simple linear function of temperature. Although more complicated, it strengthened with increasing temperature, reaching its maximum near 9000 K, and then decreasing at higher temperatures. When the classifications were rearranged based on temperature, it was more like the modern scheme. [ 146 ]
In addition, stars can be classified by the effects of luminosity found in their spectral lines, which correspond to their spatial size and are determined by their surface gravity. These range from 0 ( hypergiants ) to III ( giants ), to V (main sequence dwarfs); also some authors add VII (white dwarfs). Most stars belong to the main sequence , which is made up of ordinary hydrogen-burning stars .
These are divided along a narrow, diagonal band when plotted based on their magnitude and absolute spectral type. [ 147 ]
The Sun is a G2V-type main sequence yellow dwarf of intermediate temperature and of ordinary size.
There is an additional nomenclature, in the form of lowercase letters added to the end of the spectral type, for the purpose of indicating peculiar characteristics of the spectrum. For example, an " e " can indicate the presence of broadcast lines; " m " represents unusually high levels of metals, and " var " can mean variations in spectral type. [ 147 ]
The white dwarf stars have their own class that begins with the letter D . This is subdivided into classes DA , DB , DC , DO , DZ, and DQ , depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature. [ 148 ]
The Harvard classification of spectral types does not uniquely determine the characteristics of a star. Stars with the same temperature can have very different sizes, which implies very different luminosities . To distinguish them, the brightness classes were defined in Yerkes. In this classification system, the stellar spectrum is again examined and spectral lines sensitive to the star's gravity are searched for . In this way it is possible to estimate its size.
Both classification systems are complementary.
About 10% of all stars are white dwarfs, 70% are M-type stars, 10% are K-type stars, and 4% are G-type stars like the Sun. Only 1% of the stars are of greater mass and types A and F. Wolf-Rayet stars are extremely rare. The brown dwarfs , stars that projects were half because of their small mass, could be very abundant but prevents its weak luminosity make a proper census.
Gravitational classification of stars
Stars can be classified according to four gravitational criteria established by the International Astronomical Union in 2006. This IAU stellar classification is the most accepted and commonly used.
Classification by stellar gravitational center
The first criterion is the presence or absence of a stellar gravitational center, that is, if they are part of a stellar system . The stars that are part of a stellar system (presence of stellar gravitational center) are called systemic stars . Stars that are not part of a stellar system (absence of stellar gravitational center) are called lonely stars .
Systemic star ranking by position
Systemic stars (which are part of a star system) can in turn be of two types. The central stars are those systemic stars that act as the gravitational center of other stars. This means that other stars orbit them. Systemic stars that orbit a central star are called satellite stars .
Star classification by gravitational grouping
This classification of stars is based on distinguishing two types of stars depending on whether they are grouped with other stars by gravitational attractive forces. This classification refers to two types of stars (cumular and independent) according to whether or not they are united to other stars and, furthermore, this union is not due to the presence of a stellar gravitational center; that is, no star revolves around another and yet they are gravitationally bound.
The cumulares stars are those that form star clusters . If the cluster is globular , the stars are attracted by gravity (the stars attract each other). If the cluster is open , the stars are attracted by gravitation where the gravitational center is the center of mass of the cluster (the stars orbit a common gravitational center that holds them together). The independent stars are those that do not form star clusters with any other star. However, there are independent stars that are part of a star system because they orbit stars or are the center of others. This would be the case for systemic-independent stars.
Star classification by planetary system
The stars that are part of a planetary system are called planetary stars , the planetary system being understood as the set of the star or central stellar system and the different celestial bodies (planets, asteroids, comets) that orbit around it. On the contrary, unique stars are called those that do not have other bodies that orbit them.
Variable stars have periodic or random changes in luminosity due to intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three main groups.
During their stellar evolution, some stars go through phases where they can become pulsating variables. The pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes stars such as Cepheid and Cepheid -like variables, and long-period variables, such as Mira . [ 149 ]
The eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events. [ 149 ] This group includes protostars , Wolf-Rayet stars , and flare stars , as well as giant and supergiant stars .
The cataclysmic variable stars or explosive are those who experience a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a type 1a supernova. [ 6 ] The explosion is created when the white dwarf accumulates hydrogen from the companion star, gaining mass until the hydrogen undergoes fusion. [ 150 ] Some novae are also recurrent, presenting periodic outbreaks of moderate amplitude. [ 149 ]
Stars can also vary in luminosity due to extrinsic factors, such as eclipsing binaries , as well as spinning stars that produce extreme spots. [ 149 ] A notable example of an eclipsing binary is Algol , which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days.
The interior of a stable star is in a state of hydrostatic equilibrium : the forces on any small volume are almost exactly balanced against each other. The balanced forces are the gravitational force inward and an outward force due to the pressure gradient within the star. The pressure gradient is established by the plasma temperature gradient; the outer part of the star is cooler than the core. The temperature in the core of a main sequence star or giant star is at least of the order of 10 7 K. The resulting temperature and pressure in the hydrogen burning core of a main sequence star are sufficient for the occurrence ofnuclear fusion and for enough energy to be produced to prevent further collapse of the star. [ 151 ] [ 152 ]
As atomic nuclei fuse into the nucleus, they emit energy in the form of gamma rays . These photons interact with the surrounding plasma, adding to the thermal energy in the core. Main sequence stars convert hydrogen to helium, creating a slow but steady proportion of helium in the nucleus. Eventually the helium content becomes predominant, and energy production in the nucleus ceases. In contrast, for stars larger than 0.4 M ☉ , fusion occurs in a slowly expanding shell around the degenerate helium core . [ 153 ]
In addition to hydrostatic equilibrium, the interior of a stable star will also maintain a thermal equilibrium energy balance . There is a radial temperature gradient through the interior that results in a flow of energy flowing outward. The outgoing flow of energy left by any layer inside the star will exactly match the incoming flow from below.
The radiation zone is the region of the stellar interior where the outward energy flow depends on radiant heat transfer, since connective heat transfer is inefficient in that area. In this region the plasma will not be disturbed, and any mass movement will be extinguished. However, if this is not the case, then the plasma becomes unstable and convection occurs, forming a convective zone . This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiative heat transfer inefficient) such as in the outer envelope. [ 152 ]
The occurrence of convection in the outer envelope of a main sequence star depends on the mass of the star. Stars with several times the mass of the Sun have a deep convection zone in the interior and a radiative zone in the outer layers. [ 154 ] Red dwarf stars with less than 0.4 M ☉ are convective throughout, preventing the accumulation of a helium nucleus. [ 4 ] For most stars, the convective zones also vary with time, as the age and constitution of the stars change. [ 152 ]
The photosphere is the portion of a star that is visible to an observer. This is the layer in which the star's plasma becomes transparent to photons of light. From here, the energy generated in the nucleus is released, to propagate into space. It is within the photosphere where sunspots appear , regions of lower than average temperature.
Above the level of the photosphere is the stellar atmosphere . In a main sequence star like the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin region of the chromosphere , where spicules appear and also where stellar flares begin .
Above it is the transition region, where the temperature rises rapidly over a distance of just 100 kilometers (62 mi). Beyond is the corona , a volume of superheated plasma that can extend outward for up to several million kilometers. [ 155 ] Despite its high temperature, the corona emits very little light, due to its low gas density. Normally, the corona region of the Sun is only visible during a solar eclipse .
From the corona, a stellar wind of plasma particles expands outward from the star until it interacts with the interstellar medium . For the Sun, the influence of its solar wind stretches across a bubble-shaped region called the heliosphere . [ 156 ]
Nuclear fusion reaction pathways
A variety of nuclear fusion reactions take place in the cores of stars, depending on their mass and composition. When the cores are fused, the mass of the fused product is less than the mass of the original parts. This lost mass is converted into electromagnetic energy, according to the equivalence relation between mass and energy E = mc 2 . [ 3 ]
The hydrogen fusion process is temperature sensitive, so a moderate increase in core temperature will lead to a significant increase in the fusion rate. As a result, the internal temperature of the main sequence stars only varies from 4 million kelvin for a small class M star 40 million degrees Kelvin for a massive star class O. [ 131 ]
- 4¹H → 2²H + 2e+ + 2νe (4.0 MeV + 1,0 MeV)
- 2e+ + 2e− → 2γ (2 x 1,0 MeV)
- 2¹H + 2²H → 2³He + 2γ (5,5 MeV)
- 2³He → 4He + 2¹H (12,9 MeV)
These reactions are reduced in the overall reaction:
- 4¹H → 4He + 2e+ + 2γ + 2νe (26,7 MeV)
Where e + is a positron , γ is a gamma ray photon, νe is a neutrino , and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts , which is really only a small amount of energy. However, an enormous number of these reactions are constantly occurring, producing all the energy necessary to sustain the radiation output from the star. By comparison, burning two hydrogen gas molecules with one oxygen gas molecule only releases 5.7 eV.
|Element|| Masas |
In stars whose nuclei are at 100 million degrees K and whose masses range from 0.5 to 10 M ☉ , the helium resulting from the first reactions can be transformed into carbon through the triple-alpha process :
- 4He + 4He + 92 keV → 8*Be
- 4He + 8*Be + 67 keV → 12*C
- 12*C → 12C + γ + 7,4 MeV
The overall reaction is:
- 34He → 12C + γ + 7,2 MeV
In massive stars, heavier elements can also be produced in a core combustion shrinkage by the combustion processes neon and oxygen combustion . The final phase of the stellar nucleosynthesis process is the silicon combustion process that results in the production of stable isotopic iron-56, an energy-consuming endothermic process, whereby additional energy can only be produced through gravitational collapse. [ 157 ]
The following example shows the amount of time required for a 20 M ☉ star to consume all of its nuclear fuel. As a star of main sequence or class, it would be 8 times the solar radius and 62,000 times the luminosity of the sun. [ 159 ]
(million degrees kelvin)
(kg / cm³)
Duration of combustion |
(τ in years)
|S / Si||3.340||33.400||0,0315|
- Portal: Astronomy . Content related to Astronomy .
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- "The Sun" . www.astromia.com . Retrieved July 7, 2017 .
- "The stars" . www.agenciaelvigia.com.ar . Retrieved July 7, 2017 .
- Bahcall, John N. (June 29, 2000). "How the Sun Shines" (in English) . Nobel Foundation . Retrieved August 30, 2006 .
- Richmond, Michael. «Late stages of evolution for low-mass stars» (en inglés). Rochester Institute of Technology. Consultado el 4 de agosto de 2006.
- "Stellar Evolution & Death" (in English) . NASA Observatorium. Archived from the original on February 10, 2008 . Retrieved August 8, 2006 .
- Iben, Icko, Jr. (1991). «Single and binary star evolution». Astrophysical Journal Supplement Series (en inglés) 76: 55-114. Bibcode:1991ApJS...76...55I. doi:10.1086/191565.
- Forbes, George (1909). History of Astronomy (en inglés). Londres: Watts & Co. ISBN 1-153-62774-4.
- Hevelius, Johannes (1690). Sobiescianum support or Uranographia (en inglés) . Gdansk.
- Tøndering, Claus. "Other ancient calendars' (in English) . WebExhibits . Retrieved December 10, 2006 .
- von Spaeth, Ove (2000). «Dating the Oldest Egyptian Star Map». Centaurus International Magazine of the History of Mathematics, Science and Technology (en inglés) 42 (3): 159-179. Bibcode:2000Cent...42..159V. doi:10.1034/j.1600-0498.2000.420301.x. Consultado el 21 de octubre de 2007.
- North, John (1995). The Norton History of Astronomy and Cosmology (en inglés). Nueva York y Londres: W.W. Norton & Company. pp. 30-31. ISBN 0-393-03656-1.
- Murdin, P. (Noviembre de 2000). «Aristillus (c. 200 BC)». Encyclopedia of Astronomy and Astrophysics (en inglés). Bibcode:2000eaa..bookE3440.. ISBN 0-333-75088-8. doi:10.1888/0333750888/3440.
- Grasshoff, Gerd (1990). The history of Ptolemy's star catalogue (en inglés). Springer. pp. 1-5. ISBN 0-387-97181-5.
- Pinotsis, Antonios D. «Astronomy in Ancient Rhodes» (en inglés). Section of Astrophysics, Astronomy and Mechanics, Department of Physics, University of Athens. Consultado el 2 de junio de 2009.
- Clark, D. H.; Stephenson, F. R. (29 de junio de 1981). «The Historical Supernovae». Supernovae: A survey of current research; Proceedings of the Advanced Study Institute. Cambridge, Inglaterra: Dordrecht, D. Reidel Publishing Co. pp. 355-370. Bibcode:1982sscr.conf..355C.
- Zhao, Fu-Yuan; Strom, R. G.; Jiang, Shi-Yang (2006). «The Guest Star of AD185 Must Have Been a Supernova». Chinese Journal of Astronomy and Astrophysics (en inglés) 6 (5): 635-640. Bibcode:2006ChJAA...6..635Z. doi:10.1088/1009-9271/6/5/17.
- "Astronomers Peg Brightness of History's Brightest Star" (in English) . NAOA News. March 5, 2003 . Retrieved June 8, 2006 .
- Frommert, Hartmut; Kronberg, Christine (30 August 2006). «Supernova 1054 - Creation of the Crab Nebula» . SEDS (in English) . University of Arizona.
- Duyvendak, J. J. L. (abril de 1942). «Further Data Bearing on the Identification of the Crab Nebula with the Supernova of 1054 A.D. Part I. The Ancient Oriental Chronicles». Publications of the Astronomical Society of the Pacific (en inglés) 54 (318): 91-94. Bibcode:1942PASP...54...91D. doi:10.1086/125409.
Mayall, N. U.; Oort, Jan Hendrik (abril de 1942). «Further Data Bearing on the Identification of the Crab Nebula with the Supernova of 1054 A.D. Part II. The Astronomical Aspects». Publications of the Astronomical Society of the Pacific (en inglés) 54 (318): 95-104. Bibcode:1942PASP ... 54 ... 95M . doi : 10.1086 / 125410 .
- Brecher, K. et al. (1983). «Ancient records and the Crab Nebula supernova». The Observatory (en inglés) 103: 106-113. Bibcode:1983Obs...103..106B.
- Kennedy, Edward S. (1962). Review: The Observatory in Islam and Its Place in the General History of the Observatory by Aydin Sayili (en inglés) 53 (2). Isis. pp. 237-239. doi:10.1086/349558.
- Jones, Kenneth Glyn (1991). Messier's nebulae and star clusters (en inglés). Cambridge University Press. p. 1. ISBN 0-521-37079-5.
- Zahoor, A. (1997). "Al-Biruni" (in English) . Hasanuddin University. Archived from the original on June 26, 2008 . Retrieved October 21, 2007 .
- Montada, Josep Puig (September 28, 2007). "Ibn Bajja" (in English) . Stanford Encyclopedia of Philosophy . Retrieved July 11, 2008 .
- Drake, Stephen A. (August 17, 2006). "A Brief History of High-Energy (X-ray & Gamma-Ray) Astronomy ' (in English) . NASA HEASARC . Retrieved August 24, 2006 .
- Greskovic, Peter; Rudy, Peter (July 24, 2006). "Exoplanets" (in English) . THAT . Retrieved June 15, 2012 .
- Ahmad, I. A. (1995). «The impact of the Qur'anic conception of astronomical phenomena on Islamic civilization». Vistas in Astronomy (en inglés) 39 (4): 395-403 . Bibcode:1995VA.....39..395A. doi:10.1016/0083-6656(95)00033-X.
- Setia, Adi (2004). «Fakhr Al-Din Al-Razi on Physics and the Nature of the Physical World: A Preliminary Survey». Islam & Science (en inglés) 2 (2) – via Questia.
- Hoskin, Michael (1998). «The Value of Archives in Writing the History of Astronomy» (en inglés). Space Telescope Science Institute. Consultado el 24 de agosto de 2006.
- Proctor, Richard A. (1870). «Are any of the nebulæ star-systems?». Nature (en inglés) 1 (13): 331-333. Bibcode:1870Natur...1..331P. doi:10.1038/001331a0.
- MacDonnell, Joseph. "Angelo Secchi, SJ (1818-1878) the Father of Astrophysics' (in English) . Fairfield University . Archived from the original on July 21, 2011 . Retrieved October 2, 2006 .
- Aitken, Robert G. (1964). The Binary Stars (en inglés). Nueva York: Dover Publications Inc. p. 66. ISBN 0-486-61102-7.
- Michelson, A. A.; Pease, F. G. (1921). «Measurement of the diameter of Alpha Orionis with the interferometer». Astrophysical Journal (en inglés) 53: 249-259. Bibcode:1921ApJ....53..249M. doi:10.1086/142603.
- « " Payne-Gaposchkin, Cecilia Helena. " CWP » (in English) . University of California . Archived from the original on October 12, 2012 . Retrieved February 21, 2013 .
- Unsöld, Albrecht (2001). The New Cosmos (in English) (5th edition). New York: Springer. pp. 180-185, 215-216. ISBN 3-540-67877-8 .
- p. ej. Battinelli, Paolo; Demers, Serge; Letarte, Bruno (2003). «Carbon Star Survey in the Local Group. V. The Outer Disk of M31». The Astronomical Journal (en inglés) 125 (3): 1298-1308. Bibcode:2003AJ....125.1298B. doi:10.1086/346274.
- "Millennium Star Atlas marks the completion of ESA's Hipparcos Mission" (in English) . THAT. December 8, 1997 . Retrieved June 13, 2007 .
- Villard, Ray; Freedman, Wendy L. (October 26, 1994). "Hubble Space Telescope Precise Distance Measures to the Most Remote Galaxy Yet" (in English) . Hubble Site . Retrieved August 5, 2007 .
- "Hubble Completes Eight-Year Effort to Measure Expanding Universe" (in English) . Hubble Site. May 25, 1999 . Retrieved August 2, 2007 .
- "UBC Prof., alumnus discover most distant star clusters: a billion light-years away." (in English) . UBC Public Affairs. January 8, 2007 . Retrieved June 28, 2015 .
- Koch-Westenholz, Ulla; Koch, Ulla Susanne (1995). Mesopotamian astrology: an introduction to Babylonian and Assyrian celestial divination. Carsten Niebuhr Institute Publications (en inglés) 19. Museum Tusculanum Press. p. 163. ISBN 87-7289-287-0.
- Coleman, Leslie S. "Myths, Legends and Lore" (in English) . Frosty Drew Observatory . Retrieved June 15, 2012 .
- "Naming Astronomical Objects" (in English) . International Astronomical Union (IAU) . Retrieved January 30, 2009 .
- «Naming Stars» (en inglés). Students for the Exploration and Development of Space (SEDS). Consultado el 30 de enero de 2009.
- Lyall, Francis; Larsen, Paul B. (2009). «Chapter 7: The Moon and Other Celestial Bodies». Space Law: A Treatise (en inglés). Ashgate Publishing, Ltd. p. 176. ISBN 0-7546-4390-5.
- "IAU Working Group on Star Names (WGSN)" (in English) . Retrieved May 22, 2016 .
- "Star naming" (in English) . Scientia Astrophysical Organization. 2005. Archived from the original on June 17, 2010 . Retrieved June 29, 2010 .
- "Disclaimer: Name a star, name a rose and other, similar enterprises" . British Library (in English) . The British Library Board. Archived from the original on January 19, 2010 . Retrieved June 29, 2010 .
- Andersen, Johannes. "Buying Stars and Star Names" (in English) . International Astronomical Union . Retrieved June 24, 2010 .
- Pliat, Phil (September-October 2006). Name Dropping: Want to Be a Star? (in English) . 30.5. Skeptical Inquirer. Archived from the original on July 20, 2010 . Retrieved June 29, 2010 .
- Adams, Cecil (April 1, 1998). "Can you pay $ 35 to get a star named after you?" (in English) . The Straight Dope . Retrieved August 13, 2006 .
- Golden, Frederick; Faflick, Philip (January 11, 1982). "Science: Stellar Idea or Cosmic Scam?" . Time ( Time Inc.) . Retrieved June 24, 2010 .
- Di Justo, Patrick (December 26, 2001). "Buy a Star, But It's Not Yours . " Wired ( Condé Nast Digital) . Retrieved June 29, 2010 .
- Plait, Philip C. (2002). Bad astronomy: misconceptions and misuses revealed, from astrology to the moon landing "hoax" (en inglés). John Wiley and Sons. pp. 237-240. ISBN 0-471-40976-6.
- Sclafani, Tom (May 8, 1998). "Consumer Affairs Commissioner Polonetsky Warns Consumers:" Buying A Star Won't Make You One " " (in English) . National Astronomy and Ionosphere Center, Aricebo Observatory. Archived from the original on January 11, 2006 . Retrieved June 24, 2010 .
- Prsa, A.; Harmanec, P.; Torres, G.; Mamajek, E.; et al. (2016). «Nominal values for selected solar and planetary quantities: IAU 2015 Resolution B3». Astronomical Journal (en inglés) 152 (2): 41. Bibcode:2016AJ....152...41P. arXiv:1605.09788. doi:10.3847/0004-6256/152/2/41.
- Woodward, P. R. (1978). «Theoretical models of star formation». Annual Review of Astronomy and Astrophysics (en inglés) 16 (1): 555-584. Bibcode:1978ARA&A..16..555W. doi:10.1146/annurev.aa.16.090178.003011.
- Lada, C. J.; Lada, E. A. (2003). «Embedded Clusters in Molecular Clouds». Annual Review of Astronomy and Astrophysics (en inglés) 41 (1): 57-115. Bibcode:2003ARA&A..41...57L. arXiv:astro-ph/0301540. doi:10.1146/annurev.astro.41.011802.094844.
- Kwok, Sun (2000). The origin and evolution of planetary nebulae. Cambridge astrophysics series (en inglés) 33. Cambridge University Press. pp. 103-104. ISBN 0-521-62313-8.
- Adams, Fred C.; Laughlin, Gregory; Graves, Genevieve J. M. «Red Dwarfs and the End of the Main Sequence». Gravitational Collapse: From Massive Stars to Planets (en inglés). Revista Mexicana de Astronomía y Astrofísica. p. 46.49. Bibcode:2004RMxAC..22...46A. Consultado el 24 de junio de 2008.
- Elmegreen, B. G.; Lada, C. J. (1977). «Sequential formation of subgroups in OB associations». Astrophysical Journal, Part 1 (en inglés) 214: 725-741. Bibcode:1977ApJ...214..725E. doi:10.1086/155302.
- Getman, K. V. et al. (2012). «The Elephant Trunk Nebula and the Trumpler 37 cluster: contribution of triggered star formation to the total population of an H II region». Monthly Notices of the Royal Astronomical Society (en inglés) 426 (4): 2917-2943. Bibcode:2012MNRAS.426.2917G. arXiv:1208-1471. doi:10.1111/j.1365-2966.2012.21879.x.
- Smith, Michael David (2004). The Origin of Stars (en inglés). Imperial College Press. pp. 57-68. ISBN 1-86094-501-5.
- Seligman, Courtney. "Slow Contraction of Protostellar Cloud" . Self-published (in English) . Archived from the original on June 23, 2008 . Retrieved September 5, 2006 .
- Bally, J.; Morse, J.; Reipurth, B. (1996). «The Birth of Stars: Herbig-Haro Jets, Accretion and Proto-Planetary Disks». En Benvenuti, Piero; Macchetto, F. D.; Schreier, Ethan J., eds. Science with the Hubble Space Telescope – II. Proceedings of a workshop held in Paris, France, December 4–8, 1995 (en inglés). Space Telescope Science Institute. p. 491. Bibcode:1996swhs.conf..491B.
- Smith, Michael David (2004). The origin of stars (en inglés). Imperial College Press. p. 176. ISBN 1-86094-501-5.
- Tom, Megeath (May 11, 2010). "Herschel finds a hole in space" (in English) . ESA . Retrieved May 17, 2010 .
- Duquennoy, A.; Mayor, M. (1991). «Multiplicity among solar-type stars in the solar neighbourhood. II - Distribution of the orbital elements in an unbiased sample». Astronomy & Astrophysics (en inglés) 248 (2): 485-524. Bibcode:1991A&A...248..485D.
- Mengel, J. G. et al. (1979). «Stellar evolution from the zero-age main sequence». Astrophysical Journal Supplement Series (en inglés) 40: 733-791. Bibcode:1979ApJS...40..733M. doi:10.1086/190603.
- Sackmann, I. J.; Boothroyd, A. I.; Kraemer, K. E. (1993). «Our Sun. III. Present and Future». Astrophysical Journal (en inglés) 418: 457. Bibcode:1993ApJ...418..457S. doi:10.1086/173407.
- Wood, B. E. et al. (2002). «Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity». The Astrophysical Journal (en inglés) 574 (1): 412-425. Bibcode:2002ApJ...574..412W. arXiv:astro-ph/0203437. doi:10.1086/340797.
- de Loore, C.; de Greve, J. P.; Lamers, H. J. G. L. M. (1977). «Evolution of massive stars with mass loss by stellar wind». Astronomy and Astrophysics (en inglés) 61 (2): 251-259. Bibcode:1977A&A....61..251D.
- "The evolution of stars between 50 and 100 times the mass of the Sun" (in English) . Royal Greenwich Observatory. Archived from the original on November 18, 2015 . Retrieved November 17, 2015 .
- «Main Sequence Lifetime». Swinburne Astronomy Online Encyclopedia of Astronomy (en inglés). Swinburne University of Technology.
- Pizzolato, N. et al. (2001). «Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests». Astronomy & Astrophysics 373 (2): 597-607. Bibcode:2001A&A...373..597P. doi:10.1051/0004-6361:20010626.
- "Mass loss and Evolution" (in English) . UCL Astrophysics Group. June 18, 2004. Archived from the original on November 22, 2004 . Retrieved August 26, 2006 .
- Sackmann, I. J.; Boothroyd, A. I.; Kraemer, K. E. (1993). «Our Sun. III. Present and Future». Astrophysical Journal (en inglés) 418: 457. Bibcode:1993ApJ...418..457S. doi:10.1086/173407.
- Schröder, K.-P .; Smith, Robert Connon (2008). "Distant future of the Sun and Earth revisited". Monthly Notices of the Royal Astronomical Society (English) 386 (1): 155-163. Bibcode : 2008MNRAS.386..155S . arXiv : 0801.4031 . doi : 10.1111 / j.1365-2966.2008.13022.x . See also Palmer, Jason (February 22, 2008). "Hope dims that Earth will survive Sun's death . " NewScientist.com news service (in English) . Retrieved March 24, 2008 .
- «The Evolution of Massive Stars and Type II Supernovae» (en inglés). Penn Stats College of Science. Consultado el 5 de enero de 2016.
- Christopher, Sneden (8 de February 2001). Astronomy: The age of the Universe . Nature (en inglés) 409 (6821): 673-675. ISSN 0028-0836 . doi : 10.1038 / 35055646 .
- Liebert, J. (1980). «White dwarf stars». Annual Review of Astronomy and Astrophysics (en inglés) 18 (2): 363-398. Bibcode:1980ARA&A..18..363L. doi:10.1146/annurev.aa.18.090180.002051.
- "Introduction to Supernova Remnants" (in English) . Goddard Space Flight Center. April 6, 2006 . Retrieved July 16, 2006 .
- Fryer, C. L. (2003). «Black-hole formation from stellar collapse». Classical and Quantum Gravity (en inglés) 20 (10): S73-S80. Bibcode:2003CQGra..20S..73F. doi:10.1088/0264-9381/20/10/309.
- «What is a galaxy? How many stars in a galaxy / the Universe? » (in English) . Royal Greenwich Observatory . Retrieved July 18, 2006 .
- Borenstein, Seth (December 1, 2010). "Universe's Star Count Could Triple" . CBS News (in English) . Retrieved July 14, 2011 .
- "Hubble Finds Intergalactic Stars" (in English) . Hubble News Desk. January 14, 1997 . Retrieved November 6, 2006 .
- Szebehely, Victor G.; Richard B., Curran (1985). Stability of the Solar System and Its Minor Natural and Artificial Bodies (en inglés). Springer. ISBN 90-277-2046-0.
- "Most Milky Way Stars Are Single" (in English) . Harvard-Smithsonian Center for Astrophysics. January 30, 2006 . Retrieved July 16, 2006 .
- 3.99 × 10 13 km / (3 × 10 4 km / h × 24 × 365.25) = 1.5 × 10 5 years.
- Holmberg, J.; Flynn, C. (2000). «The local density of matter mapped by Hipparcos». Monthly Notices of the Royal Astronomical Society (en inglés) 313 (2): 209-216. Bibcode:2000MNRAS.313..209H. arXiv:astro-ph/9812404. doi:10.1046/j.1365-8711.2000.02905.x.
- "Astronomers: Star Collisions are rampant, catastrophic" (in English) . CNN News. June 2, 2000. Archived from the original on January 7, 2007 . Retrieved January 21, 2014 .
- Lombardi, Jr., J. C. et al. (2002). «Stellar Collisions and the Interior Structure of Blue Stragglers». The Astrophysical Journal (en inglés) 568 (2): 939-953. Bibcode:2002ApJ...568..939L. arXiv:astro-ph/0107388. doi:10.1086/339060.
- Mason, BD et al. 1998, AJ 115, 821
- Kraus, A. L.; White, R. J. y Hillenbrand, L. A. 2005, ApJ 633, 452
- H. E. Bond; E. P. Nelan; D. A. VandenBerg; G. H. Schaefer; D. Harmer (2013). «HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang». The Astrophysical Journal Letters (en inglés) 765 (1): L12. Bibcode:2013ApJ...765L..12B. arXiv:1302.3180. doi:10.1088/2041-8205/765/1/L12.
- Planck Collaboration (2015). «Planck 2015 results. XIII. Cosmological parameters (Ver Tabla 4 en página 31 del pfd).». Astronomy & Astrophysics (en inglés) 594: A13. Bibcode:2016A&A...594A..13P. arXiv:1502.01589. doi:10.1051/0004-6361/201525830.
- Naftilan, S. A.; Stetson, P. B. (13 de julio de 2006). «How do scientists determine the ages of stars? Is the technique really accurate enough to use it to verify the age of the universe?» (en inglés). Scientific American. Consultado el 11 de mayo de 2007.
- Laughlin, G.; Bodenheimer, P.; Adams, F. C. (1997). «The End of the Main Sequence». The Astrophysical Journal (en inglés) 482 (1): 420-432. Bibcode:1997ApJ...482..420L. doi:10.1086/304125.
- Irwin, Judith A. (2007). Astrophysics: Decoding the Cosmos (en inglés). John Wiley and Sons. p. 78. ISBN 0-470-01306-0.
- Fischer, D. A.; Valenti, J. (2005). «The Planet-Metallicity Correlation». The Astrophysical Journal (en inglés) 622 (2): 1102-1117. Bibcode:2005ApJ...622.1102F. doi:10.1086/428383.
- "Signatures Of The First Stars" (in English) . ScienceDaily. April 17, 2005 . Retrieved October 10, 2006 .
- Feltzing, S.; Gonzalez, G. (2000). «The nature of super-metal-rich stars: Detailed abundance analysis of 8 super-metal-rich star candidates». Astronomy & Astrophysics (en inglés) 367 (1): 253-265. Bibcode:2001A&A...367..253F. doi:10.1051/0004-6361:20000477.
- Gray, David F. (1992). The Observation and Analysis of Stellar Photospheres (en inglés). Cambridge University Press. pp. 413-414. ISBN 0-521-40868-7.
- Jørgensen, Uffe G. (1997), «Cool Star Models», en van Dishoeck, Ewine F., ed., Molecules in Astrophysics: Probes and Processes, International Astronomical Union Symposia. Molecules in Astrophysics: Probes and Processes (en inglés) 178, Springer Science & Business Media, p. 446, ISBN 079234538X
- "The Biggest Star in the Sky" (in English) . THAT. March 11, 1997 . Retrieved July 10, 2006 .
- Ragland, S.; Chandrasekhar, T.; Ashok, N. M. (1995). «Angular Diameter of Carbon Star Tx-Piscium from Lunar Occultation Observations in the Near Infrared». Journal of Astrophysics and Astronomy (en inglés) 16: 332. Bibcode:1995JApAS..16..332R.
- Davis, Kate (December 1, 2000). "Variable Star of the Month-December, 2000: Alpha Orionis" (in English) . AAVSO. Archived from the original on July 12, 2006 . Retrieved August 13, 2006 .
- Loktin, A. V. (Setiembre de 2006). «Kinematics of stars in the Pleiades open cluster». Astronomy Reports (en inglés) 50 (9): 714-721. Bibcode:2006ARep...50..714L. doi:10.1134/S1063772906090058.
- "Hipparcos: High Proper Motion Stars" (in English) . THAT. September 10, 1999. Archived from the original on June 9, 2011 . Retrieved October 10, 2006 .
- Johnson, Hugh M. (1957). «The Kinematics and Evolution of Population I Stars». Publications of the Astronomical Society of the Pacific (en inglés) 69 (406): 54. Bibcode:1957PASP...69...54J. doi:10.1086/127012.
- Elmegreen, B .; Efremov, YN (1999). "The Formation of Star Clusters" . American Scientist (in English) 86 (3): 264. BIBCODE : 1998AmSci..86..264E . doi : 10.1511 / 1998.3.264 . Archived from the original on March 23, 2005 . Retrieved August 23, 2006 .
- Brainerd, Jerome James (July 6, 2005). "X-rays from Stellar Coronas' (in English) . The Astrophysics Spectator . Retrieved June 21, 2007 .
- Berdyugina, Svetlana V. (2005). "Starspots: A Key to the Stellar Dynamo" (in English) . Living Reviews . Retrieved June 21, 2007 .
- Smith, Nathan (1998). "The Behemoth Eta Carinae: A Repeat Offender" . Mercury Magazine (English) (Astronomical Society of the Pacific) 27 : 20 . Retrieved August 13, 2006 .
- Weidner, C.; Kroupa, P. (11 de febrero de 2004). «Evidence for a fundamental stellar upper mass limit from clustered star formation» (PDF). Monthly Notices of the Royal Astronomical Society (en inglés) 348 (1): 187-191. Bibcode:2004MNRAS.348..187W. ISSN 0035-8711. arXiv:astro-ph/0310860. doi:10.1111/j.1365-2966.2004.07340.x.
- Hainich, R.; Rühling, U.; Todt, H.; Oskinova, L. M.; Liermann, A.; Gräfener, G.; Foellmi, C.; Schnurr, O. et al. (2014). «The Wolf-Rayet stars in the Large Magellanic Cloud». Astronomy & Astrophysics (en inglés) 565: A27. Bibcode:2014A&A...565A..27H. arXiv:1401.5474. doi:10.1051/0004-6361/201322696.
- Banerjee, Sambaran; Kroupa, Pavel; Oh, Seungkyung (21 de octubre de 2012). «The emergence of super-canonical stars in R136-type starburst clusters». Monthly Notices of the Royal Astronomical Society (en inglés) 426 (2): 1416-1426. Bibcode:2012MNRAS.426.1416B. ISSN 0035-8711. arXiv:1208.0826. doi:10.1111/j.1365-2966.2012.21672.x.
- "Ferreting Out The First Stars" (in English) . Harvard-Smithsonian Center for Astrophysics. September 12, 2005 . Retrieved September 5, 2006 .
- Sobral, David; Matthee, Jorryt; Darvish, Behnam; Schaerer, Daniel; Mobasher, Bahram; Röttgering, Huub J. A.; Santos, Sérgio; Hemmati, Shoubaneh (4 de junio de 2015). «Evidence For POPIII-Like Stellar Populations In The Most Luminous LYMAN-α Emitters At The Epoch Of Re-Ionisation: Spectroscopic Confirmation». The Astrophysical Journal (en inglés) 808: 139. Bibcode:2015ApJ...808..139S. arXiv:1504.01734. doi:10.1088/0004-637x/808/2/139.
- Overbye, Dennis (June 17, 2015). "Astronomers Report Finding Earliest Stars That Enriched Cosmos" . The New York Times (English) . Retrieved June 17, 2015 .
- "2MASS J05233822-1403022" (in English) . SIMBAD - Strasbourg Astronomical Data Center . Consultado el 14 de diciembre de 2013 .
- Boss, Alan (April 3, 2001). "Are They Planets or What?" . Carnegie Institution of Washington. Archived from the original on September 28, 2006 . Retrieved June 8, 2006 .
- Shiga, David (August 17, 2006). "Mass cut-off between stars and brown dwarfs revealed" . New Scientist (in English) . Archived from the original on November 14, 2006 . Accessed September 23, 2006 .
- Leadbeater, Elli (August 18, 2006). "Hubble glimpses faintest stars" (in English) . BBC . Retrieved August 22, 2006 .
- "Flattest Star Ever Seen" (in English) . THAT. June 11, 2003 . Retrieved October 3, 2006 .
- Fitzpatrick, Richard (February 13, 2006). "Introduction to Plasma Physics: A graduate course" (in English) . The University of Texas at Austin. Archived from the original on January 4, 2010 . Retrieved October 4, 2006 .
- Villata, Massimo (1992). «Angular momentum loss by a stellar wind and rotational velocities of white dwarfs». Monthly Notices of the Royal Astronomical Society (en inglés) 257 (3): 450-454. Bibcode:1992MNRAS.257..450V. doi:10.1093/mnras/257.3.450.
- "A History of the Crab Nebula" (in English) . THAT. May 30, 1996 . Retrieved October 3, 2006 .
- Strobel, Nick (August 20, 2007). "Properties of Stars: Color and Temperature" . Astronomy Notes (in English) . Primis / McGraw-Hill, Inc. Archived from the original on June 26, 2007 . Retrieved October 9, 2007 .
- Seligman, Courtney. "Review of Heat Flow Inside Stars" . Self-published (in English) . Retrieved July 5, 2007 .
- "Main Sequence Stars" (in English) . The Astrophysics Spectator. February 16, 2005 . Retrieved October 10, 2006 .
- Zeilik, Michael A.; Gregory, Stephan A. (1998). Introductory Astronomy & Astrophysics (en inglés) (4ta edición). Saunders College Publishing. p. 321. ISBN 0-03-006228-4.
- Koppes, Steve (20 de junio de 2003). «University of Chicago physicist receives Kyoto Prize for lifetime achievements in science» (en inglés). The University of Chicago News Office. Consultado el 15 de junio de 2012.
- "The Color of Stars" (in English) . Australian Telescope Outreach and Education. Archived from the original on March 10, 2012 . Retrieved August 13, 2006 .
- "Astronomers Measure Mass of a Single Star-First Since the Sun" (in English) . Hubble News Desk. July 15, 2004 . Retrieved May 24, 2006 .
- Garnett, D. R.; Kobulnicky, H. A. (2000). «Distance Dependence in the Solar Neighborhood Age-Metallicity Relation». The Astrophysical Journal (en inglés) 532 (2): 1192-1196. Bibcode:2000ApJ...532.1192G. arXiv:astro-ph/9912031. doi:10.1086/308617.
- Staff (January 10, 2006). "Rapidly Spinning Star Vega has Cool Dark Equator" (in English) . National Optical Astronomy Observatory . Retrieved November 18, 2007 .
- Michelson, A. A.; Pease, F. G. (2005). «Starspots: A Key to the Stellar Dynamo». Living Reviews in Solar Physics (en inglés) (Max Planck Society).
- Manduca, A.; Bell, R. A.; Gustafsson, B. (1977). «Limb darkening coefficients for late-type giant model atmospheres». Astronomy and Astrophysics (en inglés) 61 (6): 809-813. Bibcode:1977A&A....61..809M.
- Chugainov, P. F. (1971). «On the Cause of Periodic Light Variations of Some Red Dwarf Stars». Information Bulletin on Variable Stars (en inglés) 520: 1-3. Bibcode:1971IBVS..520....1C.
- "Magnitude" (in English) . National Solar Observatory — Sacramento Peak. Archived from the original on February 6, 2008 . Retrieved August 23, 2006 .
- "Starlight" (in English) . Australian Telescope Outreach and Education. Archived from the original on August 9, 2014 . Retrieved August 13, 2006 .
- Hoover, Aaron (January 15, 2004). 'Star May be biggest, brightest yet Observed " (in English) . HubbleSite. Archived from the original on August 7, 2007 . Retrieved June 8, 2006 .
- «Globular Cluster faintest Stars in NGC 6397" (in English) . HubbleSite. August 17, 2006 . Retrieved June 8, 2006 .
- Fowler, A. (Febrero de 1891). «The Draper Catalogue of Stellar Spectra». Nature (en inglés) 45: 427-8. Bibcode:1892Natur..45..427F1. doi:10.1038/045427a0.
- Jaschek, Carlos; Jaschek, Mercedes (1990). The Classification of Stars (en inglés). Cambridge University Press. pp. 31-48. ISBN 0-521-38996-8.
- MacRobert, Alan M. "The Spectral Types of Stars" (in English) . Sky and Telescope. Archived from the original on July 28, 2011 . Retrieved July 19, 2006 .
- "White Dwarf (wd) Stars" (in English) . White Dwarf Research Corporation. Archived from the original on October 8, 2009 . Retrieved July 19, 2006 .
- "Types of Variable" (in English) . AAVSO. May 11, 2010 . Retrieved August 20, 2010 .
- "Cataclysmic Variables" (in English) . NASA Goddard Space Flight Center. November 1, 2004 . Retrieved June 8, 2006 .
- Hansen, Carl J.; Kawaler, Steven D.; Trimble, Virginia (2004). Stellar Interiors (en inglés). Springer. pp. 32-33. ISBN 0-387-20089-4.
- Schwarzschild, Martin (1958). Structure and Evolution of the Stars (en inglés). Princeton University Press. ISBN 0-691-08044-5.
- "Formation of the High Mass Elements" (in English) . Smoot Group . Retrieved July 11, 2006 .
- "What is a Star?" (in English) . POT. September 1, 2006-09-01 . Retrieved July 11, 2006 .
- "The Glory of a Nearby Star: Optical Light from Stellar Crown Hot Detected With the VLT ' (in English) . THAT. August 1, 2001 . Retrieved July 10, 2006 .
- Burlaga, L. F. et al. (2005). «Crossing the Termination Shock into the Heliosheath: Magnetic Fields». Science (en inglés) 309 (5743): 2027-2029. Bibcode:2005Sci...309.2027B. PMID 16179471. doi:10.1126/science.1117542.
- Wallerstein, G. et al. (1999). «Synthesis of the elements in stars: forty years of progress» (PDF). Reviews of Modern Physics (en inglés) 69 (4): 995-1084. Bibcode:1997RvMP...69..995W. doi:10.1103/RevModPhys.69.995. Consultado el 4 de agosto de 2006.
- Girardi, L.; Bressan, A.; Bertelli, G.; Chiosi, C. (2000). «Evolutionary tracks and isochrones for low- and intermediate-mass stars: Form 0.15 to 7 Msun, and from Z=0.0004 to 0.03». Astronomy and Astrophysics Supplement (en inglés) 141 (3): 371-383. Bibcode:2000A&AS..141..371G. arXiv:astro-ph/9910164. doi:10.1051/aas:2000126.
- Woosley, S. E.; Heger, A.; Weaver, T. A. (2002). «The evolution and explosion of massive stars». Reviews of Modern Physics (en inglés) 74 (4): 1015-1071. Bibcode:2002RvMP...74.1015W. doi:10.1103/RevModPhys.74.1015.
- 11 days and 12 hours days equals 0.0315 years.
- Davies, Paul: The Unbridled Universe . Salvat Editores, 1993. ISBN 84-345-8895-1 .
- Ekrutt, Joachim: Stars and Planets . Everest Pub, 1996 ISBN 84-241-2746-3 .
- Murdin, Pavy and Lesley: Supernovae . Promotora General de Estudios, 1989 ISBN 84-86505-22-4 .
- WIDMANN, Walter and SCHÜTTE, Karl. Guide to the stars . Barcelona: Omega Editions, 05/1989. ISBN 84-282-0843-3 and ISBN 978-84-282-0843-7 .
- HERRMANN, Joachim. Stars . Second edition. Collection "Blume Nature Guides". Barcelona: Naturart, 04/1990. ISBN 84-87535-13-5 and ISBN 978-84-87535-13-0 .
- NARLIKAR, Jayant . The structure of the universe . Madrid, Alianza Universidad, 1987. ISBN 84-206-2485-3 .
- Cliff, Pickover (2001). The Stars of Heaven. Oxford University Press. ISBN 0-19-514874-6.
- Gribbin, John; Gribbin, Mary (2001). Stardust: Supernovae and Life—The Cosmic Connection (en inglés). Yale University Press. ISBN 0-300-09097-8.
- Hawking, Stephen (1988). A Brief History of Time (en inglés). Bantam Books. ISBN 0-553-17521-1.
- Langer, N .: The life and death of the stars . Múnich, 1995 ISBN 3-406-39720-4 .
- Scheffler, H. y Elsässer, Hans: Physics of the stars and the sun ISBN 3-411-14172-7 .
- Voigt, HH: Abriß der Astronomie ISBN 3-411-03148-4 .
- Wikimedia Commons hosts a multimedia category on stars .
- Wikiquote hosts famous quotes from or about stars .
- Wiktionary has definitions and other information about star .
- The Dictionary of the Royal Spanish Academy has a definition for star .