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Star

A star is a massive, compact body of plasma Plasma

Plasma may refer to: * Plasma [i], an ionized gas ... 

 in outer space Outer space

Outer space, also simply called space, refers to the relatively empty regions of the universe [i] ou ... 

 that is held together by its own gravity Gravitation

In physics [i], gravitation or gravity is the tendency of objects with mass [i] to accelerate [i] ... 

 and, unlike a planet Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i] ... 

, is sufficiently massive to sustain nuclear fusion Nuclear fusion

In physics [i], nuclear fusion is the process by which multiple nuclei [i] join together ... 

 in a very dense, hot core region. This fusion of atomic nuclei Atomic nucleus

The nucleus of an atom [i] is the very dense region in its center consisting of proton [i]s and neutron [i] ... 

 generates the energy Energy

In general, the concept [i] of energy refers to "the potential for causing changes." The word is used in ... 

 that is continuously radiated from the outer layers of the star during much of its life span. Individual stars differ in their total mass, composition, and age. The total mass of a star is the principal determinant in its evolution and eventual fate. A Hertzsprung-Russell diagram Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram shows the relationship between absolute magnitude [i], luminosity [i], classification [i] ... 

 shows the pattern of the temperature of stars against their absolute magnitude, and can be used to determine the age of a star and the stage in its evolution.

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Timeline

1690   Earliest recorded sighting of the planet Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i] ... 

 Uranus Uranus

Uranus is the seventh planet [i] from the Sun [i]. ... 

, by John Flamsteed John Flamsteed

John Flamsteed, was an English [i] astronomer [i]. ... 

, who mistakenly catalogues it as the star 34 Tauri Taurus (constellation)

Taurus is one of the constellation [i]s of the zodiac [i]. ... 

.



Encyclopedia


A star is a massive, compact body of plasma Plasma

Plasma may refer to:
  • Plasma [i], an ionized gas

... 

 in outer space Outer space

Outer space, also simply called space, refers to the relatively empty regions of the universe [i] ou... 

 that is held together by its own gravity Gravitation

In physics [i], gravitation or gravity is the tendency of objects with mass [i] to accelerate [i] ... 

 and, unlike a planet Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i]... 

, is sufficiently massive to sustain nuclear fusion Nuclear fusion

In physics [i], nuclear fusion is the process by which multiple nuclei [i] join together ... 

 in a very dense, hot core region. This fusion of atomic nuclei Atomic nucleus

The nucleus of an atom [i] is the very dense region in its center consisting of proton [i]s and neutron [i] ... 

 generates the energy Energy

In general, the concept [i] of energy refers to "the potential for causing changes." The word is used in ... 

 that is continuously radiated from the outer layers of the star during much of its life span.

Individual stars differ in their total mass, composition, and age. The total mass of a star is the principal determinant in its evolution and eventual fate. A Hertzsprung-Russell diagram Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram shows the relationship between absolute magnitude [i], luminosity [i], classification [i] ... 

 shows the pattern of the temperature of stars against their absolute magnitude, and can be used to determine the age of a star and the stage in its evolution. Initially, stars are composed primarily of hydrogen Hydrogen

|-
| Triple point [i] || 13.8033 K, 7.042 kPa
... 

, with some helium Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

 and heavier trace elements that determine their metallicity. Over the course of a star's evolution, a portion of the hydrogen is converted into helium and smaller quantities of heavier elements through the process of nuclear fusion. Part of the matter is then recycled into the interstellar environment and used to form a new generation of more metal-rich stars.

Binary Binary star

A binary star is a stellar system [i] consisting of two star [i]s orbit [i]i... 

 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. For example, a nova Nova

A nova is a cataclysmic nuclear explosion caused by the accretion [i] of hydrogen onto the surface of a ... 

 occurs when a white dwarf accretes matter from a companion star.

Observation history

Stars have been important to every culture. They have been used in religious practices and for celestial navigation Celestial navigation

Celestial navigation, also known as astronavigation, is a position fixing [i] technique that was d ... 

 and orientation. The Gregorian calendar Gregorian calendar

The Gregorian calendar is the calendar [i] that is used nearly everywhere in the world.... 

, used nearly everywhere in the world, is a solar calendar based on the position of the Earth Earth

Earth is the third planet [i] in the solar system [i] in terms of distance from the Sun [i], and the fi ... 

 relative to the nearest star, the Sun Sun

|+ The Sun   |+
|-
... 

.

Early astronomers such as Tycho Brahe Tycho Brahe

Tycho Brahe , born Tyge Ottesen Brahe , was a Danish [i] nobleman [i] best know ... 

 identified new stars in the heavens, suggesting that the heavens were not immutable. In 1584 Giordano Bruno Giordano Bruno

Giordano Bruno was an Italian [i] philosopher [i], priest [i], astronomer [i]/astrologer [i], and ... 

 suggested that the stars were actually other suns, and may have Earth-like planets in orbit around them. By the following century the idea of the stars as distant suns was reaching a consensus among astronomers, and it would be the theologian Richard Bentley Richard Bentley

Richard Bentley was an English [i] theologian [i], classical scholar [i] and critic [i] ... 

 who would prompt Isaac Newton Isaac Newton

[i] [[[Old Style and New Style dates|OS]] [i]: [[25 December]] [i] [[1642]] [i]... 

 to suggest that the stars were equally distributed in every direction, resulting in no net gravitational pull.

The Italian astronomer Geminiano Montanari Geminiano Montanari

Geminiano Montanari was an Italian [i] astronomer [i], lens [i]-maker, and proponen ... 

 recorded seeing variability in the star Algol Algol

Algol is a bright star [i] in the constellation [i] Perseus [i]. ... 

 1667. Edmond Halley Edmond Halley

Edmond Halley was an English [i] astronomer [i], geophysicist [i], mathematician [i], meteorologist [i] ... 

 would then publish the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed position from the time of the ancient Greek astronomers Ptolemy Ptolemy

Claudius Ptolemaeus , known in English as Ptolemy, was a Greek-speaking geographer [i], astronomer [i]... 

 and Hipparchus. But it would not be until 1838 that the first direct measurement of the distance to the star 61 Cygni was made by Friedrich Bessel Friedrich Bessel

Friedrich Wilhelm Bessel was a German [i] mathematician [i], astronomer [i], and systematizer o ... 

 using the parallax Parallax

Parallax, or more accurately motion parallax is the change of angular position [i]... 

 technique. Parallax measurements demonstrated the vast separation of the stars in the heavens. These pre-main sequence stars are often surrounded by a protoplanetary disk Protoplanetary disk

A protoplanetary disk is a rotating disk of dense gas surrounding a young newly formed star.... 

. The protostar then follows a
Hayashi track on the Hertzsprung-Russell diagram Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram shows the relationship between absolute magnitude [i], luminosity [i], classification [i] ... 

. The contraction will proceed until the Hayashi boundary is reached, and thereafter contraction will continue on a Kelvin-Helmholtz timescale with the temperature remaining stable. Stars with less than 0.5 solar masses thereafter join the main sequence. For more massive protostars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the Henyey track. The period of gravitational contraction lasts for about 10-15 million years.

Early stars of less than 2 solar masses are called T Tauri T Tauri star

[i]
[i]
... 

 stars, while those with greater mass are Herbig Ae/Be stars. These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro object Herbig-Haro object

Herbig-Haro objects are small patches of nebulosity [i] associated with newly-born star [i]s, and ... 

s.

Main sequence

Stars spend about 90% of their lifetime fusing Nuclear fusion

In physics [i], nuclear fusion is the process by which multiple nuclei [i] join together ... 

 hydrogen Hydrogen

|-
| Triple point [i] || 13.8033 K, 7.042 kPa
... 

 to produce helium Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

 in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence Main sequence

The main sequence of the Hertzsprung-Russell diagram [i] is the curve [i] along which the majority of star [i] ... 

. Starting at zero age main sequence, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity. The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.

Every star generates a stellar wind Solar wind

[i]s, [[carrot]... 

 of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible. The sun loses 10-14 solar masses every year, or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10-7 to 10-5 solar masses each year, significantly affecting their evolution. Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.

The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to burn and the rate at which it burns that fuel. In other words, its initial mass and its luminosity. For the Sun, this is estimated to be about 1010 years. However, the luminosity of a star is also determined by its mass. Consequently the total main sequence lifetime of a star can be estimated from its mass relative to the Sun's as follows:

where is the mass of the star and is the star's estimated main sequence lifetime in years.

Large stars burn their fuel very rapidly and are short-lived. Small stars burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs. However, since the lifespan of such stars is greater than the current age of the universe , no black dwarfs exist yet.

Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. This metallicity can influence the duration that a star will burn its fuel; control the formation of magnetic fields, and modifies the strength of the stellar wind. Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed.

Post-main sequence


As most stars exhaust their supply of hydrogen at their core, their outer layers expand and cool to form a red giant Red giant

According to the Hertzsprung-Russell diagram [i], a red giant is a large non-main sequence [i] star [i] ... 

. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume Mercury and Venus Venus

Venus is the second-closest planet [i] to the Sun [i], orbiting it every 224.7 Earth days. ... 

. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit . By that time, however, the orbit of the Earth will expand to about 1.7 AUs due to mass loss by the Sun and thus the Earth will escape envelopment.

In a red giant, hydrogen fusion proceeds in a shell-layer surrounding the core. Eventually the core is compressed enough to start helium Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

 fusion, and the star heats up and contracts. In low mass stars the helium fusion process begins with an explosive burst of energy generation known as a helium flash. The energy resulting from this event is equivalent to the luminosity of 108 Suns, but it lasts only a few minutes. However, this energy goes into the elimination of the electron degeneracy at the core, and is not visible from the exterior.
Massive stars
Very high mass stars with more than nine solar masses can continue to fuse elements heavier than helium. The core contracts until the temperature and pressure are sufficient to fuse carbon Carbon

Carbon is a chemical element [i] in the periodic table [i] that has the symbol [i] C' ... 

. This process continues, with the successive stages being fueled by oxygen Oxygen

Oxygen is a chemical element [i] with the chemical symbol O and atomic number [i] 8.... 

, neon Neon

[i] in the periodic table that has the symbol Ne and [[atomic number]... 

, silicon Silicon

Silicon is the chemical element [i] in the periodic table [i] that has the symbol Si and atomic number [i] ... 

, and sulfur Sulfur

Sulfur or sulphur is the chemical element [i] in the periodic table [i] that has the symbol S' ... 

. Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell burns a different element, with the outermost shell burning hydrogen; the next shell burning helium, and so forth.

The final stage is reached when the star begins producing iron Iron

Iron is a chemical element [i] with the symbol Fe and atomic number [i] 26. ... 

. Since iron nuclei are more tightly bound Binding energy

Binding energy is the energy [i] required to disassemble a whole into separate parts. ... 

 than any heavier nuclei, if they are fused they do not release energy — the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission Nuclear fission

For the generation of electrical power by fission, see Nuclear power plant [i]
... 

. These too will fade into brown, and then black dwarfs over a very long stretch of time. Electron degenerate matter is not plasma, even though stars are generally referred to as being spheres of plasma.


In larger stars, defined as having more than 1.4 solar masses after explosion, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay Beta decay

In nuclear physics [i], beta decay is a type of radioactive [i] decay in which a beta particle [i] is e ... 

, or electron capture. The shockwave Shock wave

A shock wave is a type of propagating disturbance.... 

 formed by this sudden collapse causes the rest of the star to explode in a supernova Supernova

A supernova is a stellar [i] explosion [i] which produces an extremely bright [i] ... 

. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way Milky Way

The Milky Way , is a barred spiral galaxy [i] which forms part of the Local Group [i]. ... 

, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.

Eventually, most of the matter in a star is blown away by the supernovae explosion and what remains will be a neutron star Neutron star

A neutron star is one of the few possible endpoints [i] of stellar evolution [i]. ... 

  or, in the case of the largest stars , a black hole Black hole

A black hole is an object predicted by general relativity [i] with a gravitational field so strong that ... 

. In neutron stars and black holes, the star is not in a plasma state of matter, but either neutron degenerate matter or a state of matter not currently understood within the black hole.

The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planet Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i]... 

s. The outflow from supernovae and the stellar wind Solar wind

[i]s, [[carrot]... 

 of large stars play an important part in shaping the interstellar medium.


It has been a long-held assumption that the majority of stars occur in gravitationally-bound, multiple-star systems, forming binary star Binary star

A binary star is a stellar system [i] consisting of two star [i]s orbit [i]i... 

s. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the portion of single star systems increases for smaller stars, so that only 25% of red dwarf Red Dwarf

Red Dwarf is a British [i] science fiction [i] sitcom [i] that ran for eight series, ... 

s 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.

Larger groups called star cluster Star cluster

Star clusters are groups of star [i]s which are gravitationally [i] bound.... 

s also exist. Stars are not spread uniformly across the universe Universe

The term universe has a variety of meanings, based on the context in which it is used.... 

, but are normally grouped into galaxies Galaxy

A galaxy is a huge gravitationally bound [i] system of star [i]s, interstellar gas and dust [i] ... 

 along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion galaxies in the observable universe.

Astronomers estimate that there are at least 70 sextillion  stars in the known universe. That is 230 billion times as many as the 300 billion in our own Milky Way Milky Way

The Milky Way , is a barred spiral galaxy [i] which forms part of the Local Group [i]. ... 

.

The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years away. Light from Proxima Centauri takes 4.2 years to reach Earth. Travelling at the orbital speed of the Space Shuttle Space Shuttle

NASA [i]'s Space Shuttle, officially called Space Transportation System , is the United States [i] ... 

 , it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the solar system is located. Stars can be much closer to each other in the centres of galaxies and in globular cluster Globular cluster

A globular cluster is a spherical [i] collection of star [i]s that orbits a galactic core [i] ... 

s, or much farther apart in galactic halos.

Because of their low density, collisions of stars in the galaxy are thought to be rare. However in dense regions such as the core of stellar clusters or the galactic center, collisions can be more common. Such collisions can produce what are known as blue straggler Blue straggler

Blue stragglers are star [i]s in open [i] or globular clusters [i] that ar ... 

s. These abnormal stars appear on a different part of the evolutionary track of the HR-diagram, effectively forming a merged star that has a higher surface temperature than the other main sequence stars in the cluster with the same luminosity.

Small, dwarf stars such as the Sun Sun

|+ The Sun   |+
|-
... 

 generally have essentially featureless disks with only small starspots Sunspot

A sunspot is a region on the Sun [i]'s surface that is marked by a lower temperature than its surroundi ... 

. Larger, giant stars have much bigger, much more obvious starspots, and also exhibit strong stellar limb darkening Limb darkening

Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the cente... 

. That is, the brightness decreases towards the edge of the stellar disk. Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.

Age and size


Almost everything about a star is determined by its initial mass, including its destiny and fate, as well as its essential characteristics, such as lifespan, luminosity, and size. Stars range in size from neutron star Neutron star

A neutron star is one of the few possible endpoints [i] of stellar evolution [i]. ... 

s no bigger than a city to supergiants like Betelgeuse Betelgeuse

Betelgeuse is a semiregular variable star [i] located 427 light-year [i]s away . ... 

 in the Orion constellation Orion (constellation)

Orion, a constellation [i] often referred to as The Hunt [i]er, is a prominent constellation,... 

, which has a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometres. However, Betelgeuse has a much lower density than the Sun Sun

|+ The Sun   |+
|-
... 

.

Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old — the observed age of the universe. 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 about one million years, while stars of minimum mass burn their fuel very slowly and last tens to hundreds of billions of years.

Most of our understanding of stars comes from theoretical models and simulations based on spectral observations and measurements of the diameters of stars. The first measurement of the diameter of a star other than the Sun was made in 1921 by Albert Abraham Michelson Albert Abraham Michelson

Albert Abraham Michelson was a Polish [i]-born German [i]-American [i] physicist [i] known fo ... 

 on the Hooker telescope Mount Wilson Observatory

The Mount Wilson Observatory is an astronomical observatory [i] in Los Angeles County, California [i]. ... 

.

One of the most massive stars known is Eta Carinae Eta Carinae

Eta Carinae is a very large, very luminous [i] star [i]. ... 

, with 100–150 times as much mass as the Sun; its lifespan is very short — only several million years at most. A recent study of the Arches cluster Arches Cluster

... 

 suggests that 150 solar masses is the upper limit for stars in the current era of the universe. The reason for this limit is not precisely known, but is partially due to Eddington luminosity.

The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium Lithium

|-
| colspan="6" align="center" | 6Li content may be as low as 3.75% innatural samples.... 

 in their composition. This generation of supermassive, population III stars Population III star

Population III stars are a hypothetical population of extremely massive star [i]s that are believed to h ... 

 is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter Jupiter

Jupiter is the fifth planet [i] from the Sun [i] and the largest [i] within the solar system [i] ... 

, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter Jupiter

Jupiter is the fifth planet [i] from the Sun [i] and the largest [i] within the solar system [i] ... 

. When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter. and as a steady stream of neutrino Neutrino

The neutrino is an elementary particle [i]. ... 

s emanating from the star’s core.

The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus Atomic nucleus

The nucleus of an atom [i] is the very dense region in its center consisting of proton [i]s and neutron [i] ... 

 of a new heavier element deep inside the core of a star, photon Photon

In modern physics [i], the photon is the elementary particle [i] responsible for electromagnetic phenomena [i] ... 

s of electromagnetic energy are released from the nuclear fusion reaction, which are then converted to visible light Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum [i] that is visible [i] ... 

 in the star’s outer layers.

The peak frequency Frequency

[i] of the number of times that a repeated event occurs per unit of [[time]... 

 and color Color

Color or colour is the visual perceptual property corresponding in humans to the categories... 

 of the visible light depends on the temperature of the star’s outer layers, including its photosphere Photosphere

The photosphere of an astronomical [i] object [i] is the region at which t ... 

. Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans across the entire electromagnetic spectrum, from the longest wavelength Wavelength

The wavelength is the distance between repeating units of a wave [i] pattern. ... 

s of radio wave Radio frequency

Radio [i] frequency [i], or RF, refers to that portion of the electromagnetic spectrum [i] in whic ... 

s and infrared Infrared

Infrared radiation is electromagnetic radiation [i] of a wavelength [i] longer than that of visible light [i] ... 

 to the shortest wavelengths of ultraviolet Ultraviolet

Ultraviolet light is electromagnetic radiation [i] with a wavelength [i] shorter than that of visible l ... 

, X-ray X-ray

X-rays are a form of electromagnetic radiation [i] with a wavelength [i] in the range of 10 to 0.01 nanometre [i] ... 

s, and gamma ray Gamma ray

Gamma rays are an energetic form of electromagnetic radiation [i] produced by radioactive decay [i] or ... 

s. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.

Using the stellar spectrum Astronomical spectroscopy

Astronomical spectroscopy is the technique of spectroscopy [i] used in astronomy [i]. ... 

, astronomers can also determine the surface temperature, surface gravity, metallicity and rotation Rotation

Rotation is the movement of an object in a circular motion.... 

 velocity of a star. If the distance of the star is known, such as by measuring the parallax Parallax

Parallax, or more accurately motion parallax is the change of angular position [i]... 

, 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. With these parameters, astronomers can also estimate the age of the star.

Luminosity

In astronomy, luminosity is the amount of light Light

Light is electromagnetic radiation [i] with a wavelength [i] that is visible to the eye [i] or, in a technical [i] ... 

, and other forms of radiant energy, a star radiates per unit of time Time

Two distinct views exist on the meaning of time.... 

. The luminosity of a star can be approximated by treating the emitted energy as a black body Black body

In physics [i], a black body is an object that absorbs all electromagnetic radiation [i] that falls onto ... 

 radiation. So:




where L is the luminosity, s is the Stefan-Boltzmann constant, R is the stellar radius and T is the effective temperature. This same formula can be used to compute the approximate radius of a main sequence star relative to the sun:



Magnitude

The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star’s luminosity, distance from Earth, and the altering of the star’s light as it passes through Earth’s atmosphere.
Number of stars brighter than magnitude
Apparent
magnitude
Number 
of Stars
04
115
248
3171
4513
51,602
64,800
714,000


Intrinsic or absolute magnitude is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs , and it is directly related to a star’s luminosity, measured from the standard distance of 10 parsecs.

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

Classification

There are different classifications of stars according to their spectra Astronomical spectroscopy

Astronomical spectroscopy is the technique of spectroscopy [i] used in astronomy [i]. ... 

 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 have special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarf Brown dwarf

Brown dwarfs are sub-stellar [i] objects with a mass below that necessary to maintain hydrogen [i]-... 

s. Each letter has 10 subclassifications numbered from 0 to 9. This system matches closely with temperature, but breaks down at the extreme hottest end; class O0 and O1 stars may not exist.

In addition, stars may be classified by their "luminosity effects", which correspond to their spatial size. These range from 0 through III to V and VII . Most stars fall into the main sequence Main sequence

The main sequence of the Hertzsprung-Russell diagram [i] is the curve [i] along which the majority of star [i] ... 

 which consists of ordinary hydrogen-burning stars. These fall along a narrow band when graphed according to their absolute magnitude and spectral type Stellar classification

In astronomy, stellar classification is a classification of star [i]s based initially on photospheric temperature [i] ... 

.tional nomenclature, in the form of lower-case letters, can follow 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.

Variable stars


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.


Pulsating variables are stars that vary in radius over time, expanding and contracting as a result of the stellar aging process. This category includes Cepheid and cepheid-like stars Cepheid variable

A Cepheid variable or Cepheid is a member of a particular class of variable star [i]s, notable fo ... 

, and long-period variables such as Mira.

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.clysmic or explosive variables undergo a dramatic change in their properties. This group includes novae Nova

A nova is a cataclysmic nuclear explosion caused by the accretion [i] of hydrogen onto the surface of a ... 

 and supernovae Supernova

A supernova is a stellar [i] explosion [i] which produces an extremely bright [i] ... 

. 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. Some novae are also recurrent, having periodic outbursts of moderate amplitude.s can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.

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 a shell within the star will exactly match the incoming flux.


The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. 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, as near the core, or in areas with high opacity, as in the outer envelope.occurrence of convection in the outer envelope of a main sequence star depends on the spectral type. Massive stars 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. The convective zones will also vary over time as the star ages and the constitution of the interior is modified. The existence of a corona appears to be dependent on a convective zone in the outer layers of the star. the corona, a stellar wind Solar wind

[i]s, [[carrot]... 

 of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium.

Nuclear fusion reaction pathways


A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of stellar nucleosynthesis Stellar nucleosynthesis

Stellar nucleosynthesis is the collective term for the nuclear [i] reactions taking place ... 

. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is converted into energy, according to the mass-energy relationship E=mc² E=mc²

In physics [i] E = mc2 is an important and well-known equation [i], which states an equivalence betw ... 

.he Sun, with a 107 °K core, hydrogen fuses to form helium in the proton-proton chain reaction Proton-proton chain reaction

The proton-proton chain reaction is one of two fusion [i] reactions by which star [i]s co ... 

:
41H Hydrogen atom

A hydrogen atom is an atom [i] of the chemical element hydrogen [i]. ... 

 ? 22H Deuterium

Deuterium, also called heavy hydrogen, is a stable isotope [i] of hydrogen [i] with a natural abundance [i] ... 

 + 2e+ Positron

The positron is the antiparticle [i] or the antimatter [i] counterpart of the electron [i]. ... 

 + 2?e Neutrino

The neutrino is an elementary particle [i]. ... 

21H + 22H ? 23He + 2? Gamma ray

Gamma rays are an energetic form of electromagnetic radiation [i] produced by radioactive decay [i] or ... 

23He ? 4He + 21H


These reactions result in the overall reaction:

41H ? 4He + 2e+ + 2? + 2?e


In more massive stars, helium is produced in a cycle of reactions catalyzed Catalyst

In chemistry, a catalyst is a substance [i] that decreases the activation energy [i] ... 

 by carbon Carbon

Carbon is a chemical element [i] in the periodic table [i] that has the symbol [i] C' ... 

, the carbon-nitrogen-oxygen cycle CNO cycle

The CNO cycle
is one of two fusion [i] reactions [i] by which star [i]s ... 

.tars with cores at 108 K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process Triple-alpha process

The triple alpha process is the process by which three helium [i] nuclei are transformed into carbon [i] ... 

:p>4He + 4He + 92 keV ? 8*Be
4He + 8*Be + 67 keV ? 12*C
12*C ? 12C + ? + 7.4 MeV


For an overall reaction of:

34He ? 12C + ? + 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 Silicon burning process

In astrophysics [i], silicon burning is a one-day-long sequence of nuclear fusion [i] reactions that occ ... 

 that results in the production of the stable isotope iron-56 Iron

Iron is a chemical element [i] with the symbol Fe and atomic number [i] 26. ... 

. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.

Fuel
material
Temperature
Density
Burn duration
τ
H 37 0.0045 8.1 million years
He 188 0.97 1.2 million years
C 870 170 976 years
Ne 1,570 3,100 0.6 years
O 1,980 5,550 1.25 years
S/Si 3,340 33,400 11.5 days

References


See also

General topics:
  • List of mnemonics for star classification
  • Lists of stars
  • Overview of star constellations Constellation

    A constellation is any one of the 88 areas into which the sky - or the celestial sphere [i] - is divided ... 

  • Star count
  • Stellar astronomy
  • Timeline of stellar astronomy


Unusual stars:
  • Blue straggler Blue straggler

    Blue stragglers are star [i]s in open [i] or globular clusters [i] that ar ... 

  • Carbon star
  • Hypernova Hypernova

    Hypernova refers to an exceptionally large star that collapses at the end of its lifespan —for exa... 

  • Hypervelocity star
  • Magnetar Magnetar

    A magnetar is a neutron star [i] with an extremely powerful magnetic field [i], the decay of which power ... 

  • Runaway star


Time and navigation:
  • Sidereal clock Sidereal time

    Sidereal time is time measured by the apparent diurnal motion [i] of the vernal equinox [i], which is ve ... 

  • Star clocks
  • Stellar navigation Celestial navigation

    Celestial navigation, also known as astronavigation, is a position fixing [i] technique that was d ... 




Other:
  • Nursery rhyme Nursery rhyme

    A nursery rhyme is a traditional song [i] or poem [i] taught to young children, originally in the nursery [i] ... 

     Twinkle twinkle little star Twinkle Twinkle Little Star

    "Twinkle, Twinkle, Little Star" is one of the most popular English [i] nursery rhyme [i]s. ... 



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