A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion (3×1023)[1] stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.

A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[2] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.[3] In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.[4] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole.

Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[5] Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.

The Sun by the Atmospheric Imaging Assembly of NASA's Solar Dynamics Observatory - 20100819
False-color imagery of the Sun, a G-type main-sequence star, the closest to Earth

Observation history

Dibuix de Leo
People have seen patterns in the stars since ancient times.[6] This 1690 depiction of the constellation of Leo, the lion, is by Johannes Hevelius.[7]
The constellation of Leo as it can be seen by the naked eye. Lines have been added.

Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[6] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[8] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.

The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC.[9] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (c. 1531–1155 BC).[10]

The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[11] The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue.[12] Hipparchus is known for the discovery of the first recorded nova (new star).[13] Many of the constellations and star names in use today derive from Greek astronomy.

In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[14] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[15] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[16] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[17][18][19]

Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[20] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[21] 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 having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[22]

According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[23] Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584, Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[24] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[25] and by medieval Islamic cosmologists[26] such as Fakhr al-Din al-Razi.[27] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[28]

The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.[24]

William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[29] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.

The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into spectral types.[30] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.

Alpha Centauri AB over limb of Saturn PIA10406
Alpha Centauri A and B over limb of Saturn

The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[24] 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. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[31] The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.[32]

Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[33] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[34]

Milky Way IR Spitzer
The infrared image from NASA's Spitzer Space Telescope shows hundreds of thousands of stars in the Milky Way galaxy

With the exception of supernovae, individual stars have primarily been observed in the Local Group,[35] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy).[36] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[37] In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group[38] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars 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 one billion light years[39]—ten times further than the most distant star cluster previously observed.

In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed—about 180 million years after the Big Bang.[40]

In April, 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth.[41][42]

In May 2018, astronomers reported the detection of the most distant oxygen ever detected in the Universe—and the most distant galaxy ever observed by Atacama Large Millimeter Array or the Very Large Telescope—with the team inferring that the signal was emitted 13.3 billion years ago (or 500 million years after the Big Bang. They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only 250 million years after the Universe began, corresponding to a redshift of about 15.[43]


The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[44] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.

As well as certain constellations and the Sun itself, individual stars have their own myths.[45] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[45] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)

Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[46][47]

The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[48] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[49] which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[50][51] The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community.[52] One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[53][54][55][56] and the New York City Department of Consumer Affairs issued a violation against ISR for engaging in a deceptive trade practice.[57][58]

Units of measurement

Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:

nominal solar luminosity: L = 3.828 × 1026 W [59]
nominal solar radius R = 6.957 × 108 m [59]

The solar mass M was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass together (GM) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:

nominal solar mass parameter: GM = 1.3271244 × 1020 m3 s−2 [59]

However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.

Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit—approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m.[59]

Formation and evolution

Star life cycles red dwarf en
Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics

Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions—known as molecular clouds—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[60] Most stars form in groups of dozens to hundreds of thousands of stars.[61] Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.

All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[62]

  • Very low mass stars, with masses below 0.5 M, are fully convective and distribute helium evenly throughout the whole star while on the main sequence. Therefore, they never undergo shell burning, never become red giants, which cease fusing and become helium white dwarfs and slowly cool after exhausting their hydrogen.[63] However, as the lifetime of 0.5 M stars is longer than the age of the universe, no such star has yet 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, do become red giants as their core hydrogen is depleted and they begin to burn helium in core in a helium flash; they develop a degenerate carbon-oxygen core later on the asymptotic giant branch; they finally blow off 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, pass through evolutionary stages similar to low mass stars, but after a relatively short period on the red giant branch they ignite helium without a flash and spend an extended period in the red clump 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 at the core these stars become supergiants and go on to fuse elements heavier than helium. They end their lives when their cores collapse and they explode as supernovae.

Star formation

The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy).[64][65] When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[66]

Witness the Birth of a Star
Artist's conception of the birth of a star within a dense molecular cloud.

As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[67] These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.

W40 star-forming region
A cluster of approximately 500 young stars lies within the nearby W40 stellar nursery.

Early stars of less than 2 M are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[68][69] These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[70]

Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.

Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.[71] A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.

Main sequence

Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[72] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[73]

Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M every year,[74] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M each year, significantly affecting their evolution.[75] Stars that begin with more than 50 M can lose over half their total mass while on the main sequence.[76]

H-R diagram -edited-3
An example of a Hertzsprung–Russell diagram for a set of stars that includes the Sun (center). (See "Classification" below.)

The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.[63] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M[77] are expected to have moved off the main sequence.

Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,[78] which affects the strength of its stellar wind.[79] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.

Post–main sequence

As stars of at least 0.4 M[3] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly 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 roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.[73][80]

As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.[5]

After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.

Massive stars

During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.

When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[81]

The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy. To a very limited degree such a process proceeds, but it consumes energy. Likewise, since they are more tightly bound than all lighter nuclei, such energy cannot be released by fission.[82]


As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 M, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place.[83] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.

Crab Nebula
The Crab Nebula, remnants of a supernova that was first observed around 1050 AD

In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.[84]

A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula.[84] The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M.[85] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood.

The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[84]

Binary stars

The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.


Sirius A and B artwork
A white dwarf star in orbit around Sirius (artist's impression).

Stars are not spread uniformly across 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 (1011) galaxies in the observable universe.[86] In 2010, one estimate of the number of stars in the observable universe was 300 sextillion (3×1023).[1][87] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[88]

A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star 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.[89] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy.

It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[90]

Ngc6397 hst blue straggler
This view contains blue stars known as "Blue stragglers", for their apparent location on the Hertzsprung–Russell diagram

The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive.[91] This is typical of stellar separations in galactic discs.[92] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.

Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[93] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.[94]


Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.


Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old.[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 lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.[97][98]

Lifetimes of stages of stellar evolution in billions of years[99]
Initial Mass (M) Main Sequence Subgiant First Red Giant Core He Burning
1.0 7.41 2.63 1.45 0.95
1.5 1.72 0.41 0.18 0.26
2.0 0.67 0.11 0.04 0.10

Chemical composition

When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[100] as 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, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.[101]

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.[102] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[103] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[104] Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.[105]


Well known stars 2
Some of the well-known stars with their apparent colors and relative sizes.

Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[106]

The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[107]

Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of our sun.[108][109] Betelgeuse, however, has a much lower density than the Sun.[110]


Pleiades large
The Pleiades, an open cluster of stars in the constellation of Taurus. These stars share a common motion through space.[111]

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 motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its 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 milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity 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.[112]

When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[113] A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations.[114]

Magnetic field

Surface magnetic field of SU Aur (a young star of T Tauri type), reconstructed by means of Zeeman–Doppler imaging

The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[115]

Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.[116] During the Maunder Minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.


One of the most massive stars known is Eta Carinae,[117] which, with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M as an upper limit for stars in the current era of the universe.[118] This represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses,[119] but it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M limit on massive star formation.[120]

The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis (center), a variable star with about 3.5 times the mass of the Sun. The black patch of sky is a vast hole of empty space and not a dark nebula as previously thought.

The first stars to form after the Big Bang may have been larger, up to 300 M,[121] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later 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.[122][123]

With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core.[124] For stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ.[125][126] When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[126][127] Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants.

The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[34]


The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to bulge outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart.[128] By contrast, the Sun rotates once every 25–35 days depending on latitude,[129] with an equatorial velocity of 1.93 km/s.[130] A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.[131]

Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[132] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[133] The rotation rate of the pulsar will gradually slow due to the emission of radiation.[134]


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 often estimated from the star's color index.[135] The 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. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core.[136] The temperature in the core region of a star is several million kelvins.[137]

The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[34]

Massive main sequence stars 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 of about 3,600 K; but they also have a high luminosity due to their large exterior surface area.[138]


The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,[139] which streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.

The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.

The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[140] Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.

Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on 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 mass of a single star.[141]) With these parameters, astronomers can also estimate the age of the star.[142]


The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.[143]

Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots. Giant stars have much larger, more obvious starspots,[144] and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[145] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.[146]


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, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).

Number of stars brighter than magnitude
of stars[147]
0 4
1 15
2 48
3 171
4 513
5 1,602
6 4,800
7 14,000

Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[148] (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:

Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;[148] for example, the bright star Sirius has an apparent magnitude of −1.44, but it 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 as seen from Earth, is approximately 23 times more luminous 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 more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther 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.[149] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[150]


Surface temperature ranges for
different stellar classes[151]
Class Temperature Sample star
O 33,000 K or more Zeta Ophiuchi
B 10,500–30,000 K Rigel
A 7,500–10,000 K Altair
F 6,000–7,200 K Procyon A
G 5,500–6,000 K Sun
K 4,000–5,250 K Epsilon Indi
M 2,600–3,850 K Proxima Centauri

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.[152] It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000 K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.[153]

Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.[154]

In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[154] The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.

Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[154]

White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the 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.[155]

Variable stars

Mira 1997
The asymmetrical appearance of Mira, an oscillating variable star.

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.

During their stellar evolution, some stars pass through phases where they can become pulsating variables. 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 Cepheid and Cepheid-like stars, and long-period variables such as Mira.[156]

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[156] This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.

Cataclysmic or explosive variable stars are those that undergo 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.[5] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[157] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[156]

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[156] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.[158]


Star types
Internal structures of main sequence stars, convection zones with arrowed cycles and radiative zones with red flashes. To the left a low-mass red dwarf, in the center a mid-sized yellow dwarf, and, at the right, a massive blue-white main sequence star.

The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[159][160]

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M, fusion occurs in a slowly expanding shell around the degenerate helium core.[161]

In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.

The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection 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 radiatative heat transfer inefficient) as in the outer envelope.[160]

The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[162] Red dwarf stars with less than 0.4 M are convective throughout, which prevents the accumulation of a helium core.[3] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[160]

Sun parts big
This diagram shows a cross-section of the Sun.

The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.

Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[163] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[162] Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally 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 extends throughout a bubble-shaped region called the heliosphere.[164]

Nuclear fusion reaction pathways

Overview of the proton-proton chain
CNO Cycle
The carbon-nitrogen-oxygen cycle

A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.[2]

The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[137]

In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction:[165]

41H → 22H + 2e+ + 2νe(2 x 0.4 MeV)
2e+ + 2e → 2γ (2 x 1.0 MeV)
21H + 22H → 23He + 2γ (2 x 5.5 MeV)
23He → 4He + 21H (12.9 MeV)

These reactions result 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 actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.

Minimum stellar mass required for fusion
Element Solar
Hydrogen 0.01
Helium 0.4
Carbon 5[166]
Neon 8

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.[165]

In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[165]

4He + 4He + 92 keV → 8*Be
4He + 8*Be + 67 keV → 12*C
12*C → 12C + γ + 7.4 MeV

For an overall reaction of:

34He → 12C + γ + 7.2 MeV

In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56, an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.[165]

The example below shows the amount of time required for a star of 20 M to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[167]

(million kelvins)
Burn duration
(τ in years)
H 37 0.0045 8.1 million
He 188 0.97 1.2 million
C 870 170 976
Ne 1,570 3,100 0.6
O 1,980 5,550 1.25
S/Si 3,340 33,400 0.0315[168]

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Further reading

External links

Bryce Harper

Bryce Aron Max Harper (born October 16, 1992) is an American professional baseball right fielder for the Philadelphia Phillies of Major League Baseball (MLB). He played in MLB for the Washington Nationals from 2012 through 2018. He has been touted as a "five-tool player".Harper graduated from high school early so that he could attend the College of Southern Nevada, where he won the 2010 Golden Spikes Award. The Nationals selected Harper as the first overall pick in the 2010 MLB Draft. He made his MLB debut with the Nationals on April 28, 2012, at 19 years old. Harper was selected for the 2012 All-Star Game, becoming the youngest position player to perform in an All-Star Game.Harper won the National League (NL) Rookie of the Year Award in 2012 and tied for the NL lead in home runs in 2015. He was named the NL Most Valuable Player for 2015 by unanimous decision of the Baseball Writers' Association of America; at age 23, he became the youngest MLB baseball player to win the award. As a free agent during the 2018–19 off-season, he signed a 13-year, $330 million contract with the Phillies. Harper's deal is the richest contract in the history of North American sports.

Darth Vader

Darth Vader is a fictional character in the Star Wars franchise. He is a primary antagonist in the original trilogy, but, as Anakin Skywalker, is the main protagonist of the prequel trilogy. Star Wars creator George Lucas has collectively referred to the first six episodic films of the franchise as "the tragedy of Darth Vader."Originally a Jedi prophesied to bring balance to the Force, Anakin Skywalker is lured to the dark side of the Force by Palpatine, who is secretly a Sith Lord. After fighting a lightsaber battle with his former mentor Obi-Wan Kenobi in which he is dismembered, Vader is transformed into a cyborg. He then serves the Galactic Empire as Darth Vader until he redeems himself by saving his son, Luke Skywalker, from Palpatine, sacrificing his own life in the process. He is also the father of Princess Leia, the secret husband of Padmé Amidala, and grandfather of Kylo Ren, the main villain of the Star Wars sequel trilogy.

The character has been portrayed by numerous actors. His cinematic appearances span the first six Star Wars films, as well as Rogue One, and he is referenced in both The Force Awakens and The Last Jedi. He also appears in television series (most substantially The Clone Wars) and numerous iterations of the Star Wars Expanded Universe, including video games, novels, and comic books.

Darth Vader has become one of the most iconic villains in popular culture, and has been listed among the greatest villains and fictional characters ever. The American Film Institute listed him as the third greatest movie villain in cinema history on 100 Years... 100 Heroes and Villains, behind Hannibal Lecter and Norman Bates. His role as a tragic hero in the prequel trilogy was met with positive reviews.

Flag of the United States

The flag of the United States of America, often referred to as the American flag, is the national flag of the United States. It consists of thirteen equal horizontal stripes of red (top and bottom) alternating with white, with a blue rectangle in the canton (referred to specifically as the "union") bearing fifty small, white, five-pointed stars arranged in nine offset horizontal rows, where rows of six stars (top and bottom) alternate with rows of five stars. The 50 stars on the flag represent the 50 states of the United States of America, and the 13 stripes represent the thirteen British colonies that declared independence from the Kingdom of Great Britain, and became the first states in the U.S. Nicknames for the flag include the Stars and Stripes, Old Glory, and the Star-Spangled Banner.

Harrison Ford

Harrison Ford (born July 13, 1942) is an American actor. He gained worldwide fame for his starring roles as Han Solo in the Star Wars film series and as the title character of Indiana Jones movie series. Five of his movies are within the 30 top-grossing movies of all time at the US box office (when adjusted for inflation). Ford is also known for playing Rick Deckard in the neo-noir dystopian science fiction film Blade Runner (1982) and its sequel Blade Runner 2049 (2017); John Book in the thriller Witness (1985), for which he was nominated for the Academy Award for Best Actor; and Jack Ryan in the action films Patriot Games (1992) and Clear and Present Danger (1994).

His career spans six decades and includes roles in several Hollywood blockbusters, including the epic war film Apocalypse Now (1979), the legal drama Presumed Innocent (1990), the action film The Fugitive (1993), the political action thriller Air Force One (1997), and the psychological thriller What Lies Beneath (2000). Seven of his films have been inducted into the National Film Registry: American Graffiti (1973), The Conversation (1974), Star Wars (1977), Apocalypse Now (1979), The Empire Strikes Back (1980), Raiders of the Lost Ark (1981) and Blade Runner (1982).

As of 2016, the U.S. domestic box-office grosses of Ford's films total over US$4.7 billion, with worldwide grosses surpassing $6 billion, making Ford the second highest-grossing U.S. domestic box-office star.Ford is married to actress Calista Flockhart.


A lesbian is a homosexual woman. The word lesbian is also used for women in relation to their sexual identity or sexual behavior regardless of sexual orientation, or as an adjective to characterize or associate nouns with female homosexuality or same-sex attraction.The concept of "lesbian" to differentiate women with a shared sexual orientation evolved in the 20th century. Throughout history, women have not had the same freedom or independence as men to pursue homosexual relationships, but neither have they met the same harsh punishment as homosexual men in some societies. Instead, lesbian relationships have often been regarded as harmless and incomparable to heterosexual ones unless the participants attempted to assert privileges traditionally enjoyed by men. As a result, little in history was documented to give an accurate description of how female homosexuality was expressed. When early sexologists in the late 19th century began to categorize and describe homosexual behavior, hampered by a lack of knowledge about homosexuality or women's sexuality, they distinguished lesbians as women who did not adhere to female gender roles and incorrectly designated them mentally ill—a designation which has been reversed in the global scientific community.

Women in homosexual relationships responded to this designation either by hiding their personal lives or accepting the label of outcast and creating a subculture and identity that developed in Europe and the United States. Following World War II, during a period of social repression when governments actively persecuted homosexuals, women developed networks to socialize with and educate each other. Greater economic and social freedom allowed them gradually to be able to determine how they could form relationships and families. With second wave feminism and growth of scholarship in women's history and sexuality in the 20th century, the definition of lesbian broadened, sparking a debate about sexual desire as the major component to define what a lesbian is. Some women who engage in same-sex sexual activity may reject not only identifying as lesbians but as bisexual as well, while other women's self-identification as lesbian may not align with their sexual orientation or sexual behavior. Sexual identity is not necessarily the same as one's sexual orientation or sexual behavior, due to various reasons, such as the fear of identifying their sexual orientation in a homophobic setting.

Portrayals of lesbians in the media suggest that society at large has been simultaneously intrigued and threatened by women who challenge feminine gender roles, and fascinated and appalled with women who are romantically involved with other women. Women who adopt a lesbian identity share experiences that form an outlook similar to an ethnic identity: as homosexuals, they are unified by the heterosexist discrimination and potential rejection they face from their families, friends, and others as a result of homophobia. As women, they face concerns separate from men. Lesbians may encounter distinct physical or mental health concerns arising from discrimination, prejudice, and minority stress. Political conditions and social attitudes also affect the formation of lesbian relationships and families in open.

List of Star Wars characters

This list of characters from the Star Wars franchise contains only those which are considered part of the official Star Wars canon. Some of these characters have additional and alternate plotlines in the Star Wars Legends continuity, and characters found in that body of works are compiled in the list of Star Wars Legends characters.

Pornographic film actor

A pornographic actor (or actress for female), adult entertainer, or porn star, is a person who performs sex acts in video that is usually characterized as a pornographic movie. Such videos tend to be made in a number of distinct pornographic subgenres and attempt to present a sexual fantasy and the actors selected for a particular role are primarily selected on their ability to create or fit that fantasy. Pornographic videos are characterized as either "softcore", which does not contain depictions of sexual penetration or "extreme fetishism" and "hardcore", which can contain depictions of penetration or extreme fetishism, or both. The genres and sexual intensity of videos is mainly determined by demand. Depending on the genre of the film, the on-screen appearance, age, and physical features of the main actors and their ability to create the sexual mood of the video is of critical importance. Most actors specialize in certain genres, such as gay sex, lesbian sex, bondage, strap-on sex, anal sex, double penetration, semen swallowing, teenage women, interracial or MILFs. Unless the genre specifies otherwise, most actors are required to appear nude in pornographic videos.

Solar System

The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.

The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, at least four of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.

The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy.


StarPlus is an Indian pay television channel owned by Star India. The network's programming consists of family dramas, comedies, youth-oriented reality shows, shows on crime and television films. It is also distributed internationally by Fox Networks Group, subsidiary of 21st Century Fox.

Star Trek

Star Trek is an American space opera media franchise based on the science fiction television series created by Gene Roddenberry. The first television series, simply called Star Trek and now referred to as "The Original Series", debuted in 1966 and aired for three seasons on NBC. It followed the interstellar adventures of Captain James T. Kirk (William Shatner) and his crew aboard the starship USS Enterprise, a space exploration vessel built by the United Federation of Planets in the 23rd century. The Star Trek canon includes The Original Series, an animated series, five spin-off television series, the film franchise, and further adaptations in several media.

In creating Star Trek, Roddenberry was inspired by the Horatio Hornblower novels, the satirical book Gulliver's Travels, and Westerns such as the television series Wagon Train. These adventures continued in the 22-episode Star Trek: The Animated Series and six feature films. Five other television series were eventually produced: Star Trek: The Next Generation follows the crew of a new starship Enterprise, set a century after the original series; Star Trek: Deep Space Nine and Star Trek: Voyager are set contemporaneously with The Next Generation, and Enterprise is set before the original series in the early days of human interstellar travel. The most recent Star Trek TV series, entitled Star Trek: Discovery, aired exclusively on the digital platform CBS All Access. The adventures of The Next Generation crew continued in four additional feature films. In 2009, the film franchise underwent a "reboot" set in an alternate timeline, or "Kelvin Timeline," entitled simply Star Trek. This film featured a new cast portraying younger versions of the crew from the original show; their adventures were continued in Star Trek Into Darkness (2013). Its sequel, Star Trek Beyond (2016), was released to coincide with the franchise's 50th anniversary.

Star Trek has been a cult phenomenon for decades. Fans of the franchise are called Trekkies or Trekkers. The franchise spans a wide range of spin-offs including games, figurines, novels, toys, and comics. Star Trek had a themed attraction in Las Vegas that opened in 1998 and closed in September 2008. At least two museum exhibits of props travel the world. The series has its own full-fledged constructed language, Klingon. Several parodies have been made of Star Trek. In addition, viewers have produced several fan productions. As of July 2016, the franchise had generated $10 billion in revenue, making Star Trek one of the highest-grossing media franchises of all time.

Star Trek is noted for its cultural influence beyond works of science fiction. The franchise is also noted for its progressive civil rights stances. The Original Series included one of television's first multiracial casts. Star Trek references may be found throughout popular culture from movies such as the submarine thriller Crimson Tide to the animated series South Park.

Star Wars

Star Wars is an American epic space opera franchise, created by George Lucas and centered around a film series that began with the eponymous 1977 movie. The saga quickly became a worldwide pop culture phenomenon.

The first film, later subtitled Episode IV: A New Hope, was followed by two successful sequels, Episode V: The Empire Strikes Back (1980) and Episode VI: Return of the Jedi (1983); forming the original Star Wars trilogy. A subsequent prequel trilogy, consisting of Episode I: The Phantom Menace (1999), Episode II: Attack of the Clones (2002) and Episode III: Revenge of the Sith (2005), was met with mixed reactions from critics and fans. Finally, a sequel trilogy began with Episode VII: The Force Awakens (2015), continued with Episode VIII: The Last Jedi (2017), and will end with Episode IX in 2019. The first eight films were nominated for Academy Awards (with wins going to the first two released) and were commercially successful, with a combined box office revenue of over US$8.5 billion. Together with the theatrical spin-off films The Clone Wars (2008), Rogue One (2016), and Solo (2018), Star Wars is the second-highest-grossing film series of all time.The film series has spawned into other media, including books, television shows, computer and video games, theme park attractions and lands, and comic books, resulting in significant development of the series' fictional universe. Star Wars holds a Guinness World Records title for the "Most successful film merchandising franchise". In 2018, the total value of the Star Wars franchise was estimated at US$65 billion, and it is currently the fifth-highest-grossing media franchise.

Star Wars (film)

Star Wars (later retitled Star Wars: Episode IV – A New Hope) is a 1977 American epic space opera film written and directed by George Lucas. It is the first film in the original Star Wars trilogy and the beginning of the Star Wars franchise. Starring Mark Hamill, Harrison Ford, Carrie Fisher, Peter Cushing, Alec Guinness, David Prowse, James Earl Jones, Anthony Daniels, Kenny Baker, and Peter Mayhew, the film focuses on the Rebel Alliance, led by Princess Leia (Fisher), and its attempt to destroy the Galactic Empire's space station, the Death Star.

Star Wars was released in theatres in the United States on May 25, 1977. It earned $461 million in the U.S. and $314 million overseas, totaling $775 million. It surpassed Jaws (1975) to become the highest-grossing film of all time until the release of E.T. the Extra-Terrestrial (1982). When adjusted for inflation, Star Wars is the second-highest-grossing film in North America, and the third-highest-grossing film in the world. It received ten Academy Award nominations (including Best Picture), winning seven. It was among the first films to be selected as part of the U.S. Library of Congress's National Film Registry as being "culturally, historically, or aesthetically significant". At the time, it was the most recent film on the registry and the only one chosen from the 1970s. In 2004, its soundtrack was added to the U.S. National Recording Registry. Today, it is regarded as one of the most important films in the history of motion pictures.

The film has been reissued multiple times at Lucas's behest, incorporating many changes including modified computer-generated effects, altered dialogue, re-edited shots, remixed soundtracks and added scenes. It launched an industry of tie-in products, including spin-off TV series, novels, comic books, video games, amusement park attractions, and merchandise including toys, games, and clothing. The film's success led to two critically and commercially successful sequels, The Empire Strikes Back in 1980 and Return of the Jedi in 1983, and later to a prequel trilogy, a sequel trilogy, and two anthology films.


The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers (864,000 miles), or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System.

Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.

The Sun is roughly middle-aged; it has not changed dramatically for more than four billion years, and will remain fairly stable for more than another five billion years. It currently fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to eventually become a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.

The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today.

The Star-Spangled Banner

"The Star-Spangled Banner" is the national anthem of the United States. The lyrics come from the Defence of Fort M'Henry, a poem written on September 14, 1814, by the then 35-year-old lawyer and amateur poet Francis Scott Key after witnessing the bombardment of Fort McHenry by British ships of the Royal Navy in Baltimore Harbor during the Battle of Baltimore in the War of 1812. Key was inspired by the large U.S. flag, with 15 stars and 15 stripes, known as the Star-Spangled Banner, flying triumphantly above the fort during the U.S. victory.

The poem was set to the tune of a popular British song written by John Stafford Smith for the Anacreontic Society, a men's social club in London. "To Anacreon in Heaven" (or "The Anacreontic Song"), with various lyrics, was already popular in the United States. Set to Key's poem and renamed "The Star-Spangled Banner", it soon became a well-known U.S. patriotic song. With a range of 19 semitones, it is known for being very difficult to sing. Although the poem has four stanzas, only the first is commonly sung today.

"The Star-Spangled Banner" was recognized for official use by the United States Navy in 1889, and by U.S. President Woodrow Wilson in 1916, and was made the national anthem by a congressional resolution on March 3, 1931 (46 Stat. 1508, codified at 36 U.S.C. § 301), which was signed by President Herbert Hoover.

Before 1931, other songs served as the hymns of U.S. officialdom. "Hail, Columbia" served this purpose at official functions for most of the 19th century. "My Country, 'Tis of Thee", whose melody is identical to "God Save the Queen", the United Kingdom's national anthem, also served as a de facto national anthem. Following the War of 1812 and subsequent U.S. wars, other songs emerged to compete for popularity at public events, among them "America the Beautiful", which itself was being considered before 1931, as a candidate to become the national anthem of the United States.

William Shatner

William Shatner, (born March 22, 1931) is a Canadian actor, author, producer, director and singer. In his seven decades of television, Shatner became a cultural icon for his portrayal of James T. Kirk, captain of the USS Enterprise, in the Star Trek franchise. He has written a series of books chronicling his experiences playing Captain Kirk and being a part of Star Trek, and has co-written several novels set in the Star Trek universe. He has also written a series of science fiction novels called TekWar that were adapted for television.

Shatner also played the eponymous veteran police sergeant in T.J. Hooker (1982–1986) and hosted the reality-based television series Rescue 911 (1989–1996), which won a People's Choice Award for the Favorite New TV Dramatic Series. Shatner also appeared in seasons 4 and 5 of the NBC series 3rd Rock from the Sun as the "Big Giant Head" that the alien characters reported to. From 2004 until 2008, he starred as attorney Denny Crane both in the final season of the legal drama The Practice and in its spinoff series Boston Legal, a role that earned him two Emmy Awards. He appeared in both seasons of the comical NBC real-life travelogue with other male companions "of a certain age" in Better Late Than Never, from 2016 to 2017.

Luminosity class
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