Tuesday, July 6, 2010

Main sequence

Stars spend about 90% of their lifetime fusing hydrogen to produce 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. 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[60]–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.[61]

Every star generates a stellar wind 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,[62] 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.[63] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.[64]

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

The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to fuse and the rate at which it fuses that fuel, i.e. its initial mass and its luminosity. For the Sun, this is estimated to be about 1010 years. Large stars consume their fuel very rapidly and are short-lived. Small stars (called red dwarfs) consume 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.[2] However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no red dwarfs are expected to have yet reached this state.

Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields[65] and modify the strength of the stellar wind.[66] 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 these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.)

[edit] Post-main sequence

Main article: Red giant

As stars of at least 0.4 solar masses[2] exhaust their supply of hydrogen at their core, their outer layers expand greatly and cool to form a red giant. For example, in about 5 billion years, when the Sun is a red giant, it will expand out to a maximum radius of roughly 1 AU (150,000,000 km), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass.[61][67]

In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell-layer surrounding the core.[68] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.[69]

After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.

[edit] Massive stars

Main article: Red supergiant
Betelgeuse is a red supergiant star approaching the end of its life cycle

During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, they can continue to fuse elements heavier than helium.

The core contracts until the temperature and pressure are sufficient 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 can occur along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[70]

The final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound 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.[68] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.

[edit] Collapse

An evolved, average-size star will now shed its outer layers as a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf.[71] 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. White dwarfs will eventually fade into black dwarfs over a very long stretch of time.

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

In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are 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 existed before.[72]

Most of the matter in the star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula[72]) and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole.[73] 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 new star formation. 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.[72]

[edit] Distribution

A white dwarf star in orbit around Sirius (artist's impression). NASA image

In addition to isolated stars, a multi-star system can consist of two or more gravitationally bound stars that orbit around each other. The 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 co-orbiting binary stars.[74] 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.

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 systems are believed to be multiple. However the portion of single star systems increases for smaller stars, 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.[75]

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.[76] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[77] Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the observable universe.[78]

The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (1012) 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 (5 miles per second—almost 30,000 kilometres per hour), it would take about 150,000 years to get there.[79] Distances like this are typical inside galactic discs, including in the vicinity of the solar system.[80] 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.[81] 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 in the cluster .[82]

[edit] Characteristics

The Sun is the nearest star to Earth.

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

[edit] Age

Most 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 oldest star yet discovered, HE 1523-0901, is an estimated 13.2 billion years old.[83][84]

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 (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.[85][86]

[edit] Chemical composition

See also: Metallicity

When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[87] 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. Because the molecular clouds where stars form are steadily enriched by heavier elements from supernovae explosions, a measurement of the chemical composition of a star can be used to infer its age.[88] The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system.[89]

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.[90] 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.[91] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[92]

[edit] Diameter

Stars vary widely in size

Due to their great distance from the Earth, all stars except the Sun appear to the human 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.[93]

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 in order 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.[94]

Stars range in size from neutron stars, which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun—about 0.9 billion kilometres. However, Betelgeuse has a much lower density than the Sun.[95]

[edit] Kinematics

Main article: Stellar kinematics
The Pleiades, an open cluster of stars in the constellation of Taurus. These stars share a common motion through space.[96] NASA photo.

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 is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. By determining the parallax of a star, the proper motion can then be converted into units of velocity. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[97]

Once 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 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.[98] Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds. [99]

[edit] Magnetic field

Main article: Stellar 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, generating magnetic fields that extend throughout 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 fields that reach out into the corona from active regions. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[100]

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, however, functioning as a brake to gradually slow the rate of rotation as the star grows older. 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.[101] During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.

[edit] Mass

One of the most massive stars known is Eta Carinae,[102] 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 suggests that 150 solar masses is the upper limit for stars in the current era of the universe.[103] The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space.

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.NASA image

.

The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,[104] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.[105] 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.[106][107] 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.[107][108] Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants.

The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than 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.[32]

[edit] Rotation

Main article: Stellar rotation

The rotation rate of stars can be approximated through spectroscopic measurement, or more exactly determined by tracking the rotation rate of starspots. Young stars can have a rapid rate of rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial rotation velocity of about 225 km/s or greater, giving it an equatorial diameter that is more than 50% larger than the distance between the poles. This rate of rotation is just below the critical velocity of 300 km/s where the star would break apart.[109] By contrast, the Sun only rotates once every 25 – 35 days, with an equatorial velocity of 1.994 km/s. The star's magnetic field and the stellar wind serve to slow down a main sequence star's rate of rotation by a significant amount as it evolves on the main sequence.[110]

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.[111] 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.[112] The rotation rate of the pulsar will gradually slow due to the emission of radiation.

[edit] Temperature

The surface temperature of a main sequence star is determined by the rate of energy production at the core and the radius of the star and is often estimated from the star's color index.[113] It is normally given as the 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 value, however, as stars actually have a temperature gradient that decreases with increasing distance from the core.[114] The temperature in the core region of a star is several million kelvins.[115]

The stellar temperature will determine the rate of energization or ionization of different 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).[32]

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.[116]

[edit] Radiation

The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind[117] (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star’s outer layers) and as a steady stream of neutrinos 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 of a new heavier element, gamma ray photons are released from the nuclear fusion reaction. This energy is converted to other forms of electromagnetic energy, including visible light, by the time it reaches the star’s outer layers.

The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star’s outer layers, including its photosphere.[118] 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 and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.

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 known, 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 measured directly for stars in binary systems. The technique of gravitational microlensing will also yield the mass of a star.[119]) With these parameters, astronomers can also estimate the age of the star.[120]

[edit] Luminosity

In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. The luminosity of a star is determined by the radius and the surface temperature. However, many stars do not radiate a uniform flux—the amount of energy radiated per unit area—across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux at its poles than along its equator.[121]

Surface patches with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. Larger, giant stars have much bigger, much more obvious starspots,[122] and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[123] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.[124]

[edit] Magnitude

Main articles: Apparent magnitude and Absolute 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. 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
Apparent
magnitude		 Number
of Stars[125]
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[126] (the 5th root of 100 or approximately 2.512). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star. 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 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:

Δm = mf − mb
2.512Δm = ΔL

Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not equivalent for an individual star;[126] 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.[127] 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.[128]

[edit] Classification

Surface Temperature Ranges for
Different Stellar Classes[129]
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
Main article: Stellar classification

The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[130] It was not known at the time that the major influence on the line strength was temperature; the hydrogen line strength reaches a peak at over 9000 K, and is weaker at both hotter and cooler temperatures. When the classifications were reordered by temperature, it more closely resembled the modern scheme.[131]

There are different single-letter classifications of stars 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 have 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: class O0 and O1 stars may not exist.[132]

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 the surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[132] Our Sun is a main sequence G2V yellow dwarf, being of intermediate temperature and ordinary size.

Additional 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.[132]

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 index.[133]

[edit] Variable stars

Main article: Variable star
The asymmetrical appearance of Mira, an oscillating variable star. NASA HST image

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.[134]

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[134] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.

Cataclysmic or explosive variables 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.[4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[135] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[134]

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[134] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days.

[edit] Structure

Main article: Stellar structure

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.[136][137]

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 solar masses, fusion occurs in a slowly expanding shell around the degenerate helium core.[138]

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.

This diagram shows a cross-section of a solar-type star. NASA image

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, such as near the core or in areas with high opacity as in the outer envelope.[137]

The occurrence of convection in the outer envelope of a main sequence star depends on the 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.[139] Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core.[2] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[137]

The portion of a star that is visible to an observer is called the photosphere. 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 out into space. It is within the photosphere that sun spots, or 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 is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km. Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[140] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[139] Despite its high temperature, the corona emits very little light. 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, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.[141]

[edit] Nuclear fusion reaction pathways

Main article: Stellar nucleosynthesis
Overview of the proton-proton chain
The carbon-nitrogen-oxygen cycle

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. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is released as electromagnetic energy, according to the mass-energy equivalence relationship E = mc².[1]

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 K for a small M-class star to 40 million K for a massive O-class star.[115]

In the Sun, with a 10 million K core, hydrogen fuses to form helium in the proton-proton chain reaction:[142]

41H → 22H + 2e+ + 2νe (4.0 MeV + 1.0 MeV)
21H + 22H → 23He + 2γ (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.

Minimum stellar mass required for fusion
Element Solar
masses
Hydrogen 0.01
Helium 0.4
Carbon 5[143]
Neon 8

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon—the carbon-nitrogen-oxygen cycle.[142]

In evolved stars with cores at 100 million K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[142]

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. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.[142]

The example below shows the amount of time required for a star of 20 solar masses 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.[144]


Fuel
material		 Temperature
(million kelvins)		 Density
(kg/cm³)		 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[145]

[edit] See also

He1523a.jpg Star portal

[edit] References

  1. ^ a b Bahcall, John N. (2000-06-29). "How the Sun Shines". Nobel Foundation. http://nobelprize.org/nobel_prizes/physics/articles/fusion/index.html. Retrieved 2006-08-30.
  2. ^ a b c d Richmond, Michael. "Late stages of evolution for low-mass stars". Rochester Institute of Technology. http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html. Retrieved 2006-08-04.
  3. ^ "Stellar Evolution & Death". NASA Observatorium. http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_intro.html. Retrieved 2006-06-08.
  4. ^ a b Iben, Icko, Jr. (1991). "Single and binary star evolution". Astrophysical Journal Supplement Series 76: 55–114. doi:10.1086/191565. http://adsabs.harvard.edu/abs/1991ApJS...76...55I.
  5. ^ a b Forbes, George (1909) (Free e-book from Project Gutenberg). History of Astronomy. London: Watts & Co.. http://www.gutenberg.org/etext/8172.
  6. ^ Hevelius, Johannis (1690). Firmamentum Sobiescianum, sive Uranographia. Gdansk.
  7. ^ Tøndering, Claus. "Other ancient calendars". WebExhibits. http://webexhibits.org/calendars/calendar-ancient.html. Retrieved 2006-12-10.
  8. ^ von Spaeth, Ove (2000). "Dating the Oldest Egyptian Star Map". Centaurus International Magazine of the History of Mathematics, Science and Technology 42 (3): 159–179. http://www.moses-egypt.net/star-map/senmut1-mapdate_en.asp. Retrieved 2007-10-21.
  9. ^ North, John (1995). The Norton History of Astronomy and Cosmology. New York and London: W.W. Norton & Company. pp. 30–31. ISBN 0393036561.
  10. ^ Murdin, P. (November 2000). "Aristillus (c. 200 BC)". Encyclopedia of Astronomy and Astrophysics. doi:10.1888/0333750888/3440. http://adsabs.harvard.edu/abs/2000eaa..bookE3440. Retrieved 2009-06-02.
  11. ^ Grasshoff, Gerd (1990). The history of Ptolemy's star catalogue. Springer. pp. 1–5. ISBN 0387971815.
  12. ^ Pinotsis, Antonios D.. "Astronomy in Ancient Rhodes". Section of Astrophysics, Astronomy and Mechanics, Department of Physics, University of Athens. http://conferences.phys.uoa.gr/jets2008/historical.html. Retrieved 2009-06-02.
  13. ^ Clark, D. H.; Stephenson, F. R. (June 29, 1981). "The Historical Supernovae". Supernovae: A survey of current research; Proceedings of the Advanced Study Institute. Cambridge, England: Dordrecht, D. Reidel Publishing Co. pp. 355–370. http://adsabs.harvard.edu/abs/1982sscr.conf..355C. Retrieved 2006-09-24.
  14. ^ Zhao, Fu-Yuan; Strom, R. G.; Jiang, Shi-Yang (2006). "The Guest Star of AD185 Must Have Been a Supernova". Chinese Journal of Astronomy and Astrophysics 6 (5): 635–640. doi:10.1088/1009-9271/6/5/17.
  15. ^ "Astronomers Peg Brightness of History’s Brightest Star". NAOA News. March 5, 2003. http://www.noao.edu/outreach/press/pr03/pr0304.html. Retrieved 2006-06-08.
  16. ^ Frommert, Hartmut; Kronberg, Christine (August 30, 2006). "Supernova 1054 - Creation of the Crab Nebula". SEDS. University of Arizona. http://www.seds.org/messier/more/m001_sn.html.
  17. ^ Duyvendak, J. J. L. (April 1942). "Further Data Bearing on the Identification of the Crab Nebula with the Supernova of 1054 A.D. Part I. The Ancient Oriental Chronicles". Publications of the Astronomical Society of the Pacific 54 (318): 91–94. doi:10.1086/125409. Bibcode: 1942PASP...54...91D.
    Mayall, N. U.; Oort, Jan Hendrik (April 1942). "Further Data Bearing on the Identification of the Crab Nebula with the Supernova of 1054 A.D. Part II. The Astronomical Aspects". Publications of the Astronomical Society of the Pacific 54 (318): 95–104. doi:10.1086/125410. Bibcode: 1942PASP...54...95M.
  18. ^ Brecher, K.; et al. (1983). "Ancient records and the Crab Nebula supernova". The Observatory 103: 106–113. Bibcode: 1983Obs...103..106B.
  19. ^ Kennedy, Edward S. (1962). "Review: The Observatory in Islam and Its Place in the General History of the Observatory by Aydin Sayili". Isis 53 (2): 237–239. doi:10.1086/349558.
  20. ^ Jones, Kenneth Glyn (1991). Messier's nebulae and star clusters. Cambridge University Press. p. 1. ISBN 0521370795.
  21. ^ Zahoor, A. (1997). "Al-Biruni". Hasanuddin University. Archived from the original on 2008-06-26. http://web.archive.org/web/20080626074150/http://www.unhas.ac.id/~rhiza/saintis/biruni.html. Retrieved 2007-10-21.
  22. ^ Montada, Josep Puig (September 28, 2007). "Ibn Bajja". Stanford Encyclopedia of Philosophy. http://plato.stanford.edu/entries/ibn-bajja. Retrieved 2008-07-11.
  23. ^ a b Drake, Stephen A. (August 17, 2006). "A Brief History of High-Energy (X-ray & Gamma-Ray) Astronomy". NASA HEASARC. http://heasarc.gsfc.nasa.gov/docs/heasarc/headates/heahistory.html. Retrieved 2006-08-24.
  24. ^ "Exoplanets". ESO. July 24, 2006. http://www.eso.org/outreach/eduoff/edu-prog/catchastar/CAS2004/casreports-2004/rep-226/. Retrieved 2006-10-11.
  25. ^ Ahmad, I. A. (1995). "The impact of the Qur'anic conception of astronomical phenomena on Islamic civilization". Vistas in Astronomy (ScienceDirect) 39 (4): 395–403 [402]. doi:10.1016/0083-6656(95)00033-X (inactive 2010-03-19).
  26. ^ Setia, Adi (2004). "Fakhr Al-Din Al-Razi on Physics and the Nature of the Physical World: A Preliminary Survey". Islam & Science 2. http://findarticles.com/p/articles/mi_m0QYQ/is_2_2/ai_n9532826/. Retrieved 2010-03-02.
  27. ^ Hoskin, Michael (1998). "The Value of Archives in Writing the History of Astronomy". Space Telescope Science Institute. http://www.stsci.edu/stsci/meetings/lisa3/hoskinm.html. Retrieved 2006-08-24.
  28. ^ Proctor, Richard A. (1870). "Are any of the nebulæ star-systems?". Nature 1: 331–333. doi:10.1038/001331a0. http://digicoll.library.wisc.edu/cgi-bin/HistSciTech/HistSciTech-idx?type=div&did=HISTSCITECH.0012.0052.0005&isize=M.
  29. ^ MacDonnell, Joseph. "Angelo Secchi, S.J. (1818–1878) the Father of Astrophysics". Fairfield University. http://www.faculty.fairfield.edu/jmac/sj/scientists/secchi.htm. Retrieved 2006-10-02.
  30. ^ Aitken, Robert G. (1964). The Binary Stars. New York: Dover Publications Inc..
  31. ^ Michelson, A. A.; Pease, F. G. (1921). "Measurement of the diameter of Alpha Orionis with the interferometer". Astrophysical Journal 53: 249–259. doi:10.1086/142603. http://adsabs.harvard.edu/abs/1921ApJ....53..249M.
  32. ^ a b c Unsöld, Albrecht (1969). The New Cosmos. New York: Springer-Verlag.
  33. ^ e. g. Battinelli, Paolo; Demers, Serge; Letarte, Bruno (2003). "Carbon Star Survey in the Local Group. V. The Outer Disk of M31". The Astronomical Journal 125 (3): 1298–1308. doi:10.1086/346274. http://adsabs.harvard.edu/abs/2003AJ....125.1298B. Retrieved 2007-02-04.
  34. ^ "Millennium Star Atlas marks the completion of ESA's Hipparcos Mission". ESA. 1997-12-08. http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=esa_msa. Retrieved 2007-08-05.
  35. ^ Villard, Ray; Freedman, Wendy L. (1994-10-26). "Hubble Space Telescope Measures Precise Distance to the Most Remote Galaxy Yet". Hubble Site. http://hubblesite.org/newscenter/archive/releases/1994/1994/49/text/. Retrieved 2007-08-05.
  36. ^ "Hubble Completes Eight-Year Effort to Measure Expanding Universe". Hubble Site. 1999-05-25. http://hubblesite.org/newscenter/archive/releases/1999/19/text/. Retrieved 2007-08-02.
  37. ^ "UBC Prof., alumnus discover most distant star clusters: a billion light-years away.". UBC Public Affairs. 2007-01-08. http://www.publicaffairs.ubc.ca/media/releases/2007/mr-07-001.html. Retrieved 2007-08-02.
  38. ^ Koch-Westenholz, Ulla; Koch, Ulla Susanne (1995). Mesopotamian astrology: an introduction to Babylonian and Assyrian celestial divination. Carsten Niebuhr Institute Publications. 19. Museum Tusculanum Press. p. 163. ISBN 8772892870.
  39. ^ a b Coleman, Leslie S. "Myths, Legends and Lore". Frosty Drew Observatory. http://www.frostydrew.org/observatory/courses/myths/booklet.htm. Retrieved 2006-08-13.
  40. ^ "Naming Astronomical Objects". International Astronomical Union (IAU). http://www.iau.org/public_press/themes/naming/. Retrieved 2009-01-30.
  41. ^ "Naming Stars". Students for the Exploration and Development of Space (SEDS). http://seds.org/~spider/spider/Misc/naming.html. Retrieved 2009-01-30.
  42. ^ Lyall, Francis; Larsen, Paul B. (2009). Space Law: A Treatise. Ashgate Publishing, Ltd. p. 176. ISBN 0754643905.
  43. ^ "Star naming". Scientia Astrophysical Organization.. 2005. http://www.astrometry.org/starnaming.php. Retrieved 2010-06-29.
  44. ^ "Disclaimer: Name a star, name a rose and other, similar enterprises". British Library. The British Library Board. http://www.bl.uk/names.html. Retrieved 2010-06-29.
  45. ^ Andersen, Johannes. "Buying Stars and Star Names". International Astronomical Union. http://www.iau.org/public/buying_star_names/. Retrieved 2010-06-24.
  46. ^ Pliat, Phil (September/October 2006). "Name Dropping: Want to Be a Star?". Skeptical Inquirer 30.5. http://www.csicop.org/si/show/name_dropping_want_to_be_a_star/. Retrieved 2010-06-29.
  47. ^ Adams, Cecil (April 1, 1998). "Can you pay $35 to get a star named after you?". The Straight Dope. http://www.straightdope.com/classics/a3_385.html. Retrieved 2006-08-13.
  48. ^ Golden, Frederick; Faflick, Philip (January 11, 1982). "Science: Stellar Idea or Cosmic Scam?". Times Magazine (Time Inc.). http://www.time.com/time/magazine/article/0,9171,925195,00.html. Retrieved 2010-06-24.
  49. ^ Di Justo, Patrick (December 26, 2001). "Buy a Star, But It's Not Yours". Wired. Condé Nast Digital. http://www.wired.com/techbiz/media/news/2001/12/49345. Retrieved 2010-06-29.
  50. ^ Plait, Philip C. (2002). Bad astronomy: misconceptions and misuses revealed, from astrology to the moon landing 'hoax'. John Wiley and Sons. pp. 237–240. ISBN 0471409766.
  51. ^ Sclafani, Tom (May 8, 1998). "Consumer Affairs Commissioner Polonetsky Warns Consumers: "Buying A Star Won't Make You One"". National Astronomy and Ionosphere Center, Aricebo Observatory. http://www.naic.edu/~gibson/starnames/isr_news.html. Retrieved 2010-06-24.
  52. ^ a b Sackmann, I.-J.; Boothroyd, A. I. (2003). "Our Sun. V. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars". The Astrophysical Journal 583 (2): 1024–1039. doi:10.1086/345408. http://adsabs.harvard.edu/abs/2003ApJ...583.1024S.
  53. ^ Tripathy, S. C.; Antia, H. M. (1999). "Influence of surface layers on the seismic estimate of the solar radius". Solar Physics 186 (1/2): 1–11. doi:10.1023/A:1005116830445. http://adsabs.harvard.edu/abs/1999SoPh..186....1T.
  54. ^ Woodward, P. R. (1978). "Theoretical models of star formation". Annual review of astronomy and astrophysics 16: 555–584. doi:10.1146/annurev.aa.16.090178.003011. http://adsabs.harvard.edu/abs/1978ARA&A..16..555W.
  55. ^ Smith, Michael David (2004). The Origin of Stars. Imperial College Press. pp. 57–68. ISBN 1860945015.
  56. ^ Seligman, Courtney. "Slow Contraction of Protostellar Cloud". Self-published. Archived from the original on 2008-06-23. http://web.archive.org/web/20080623190408/http://courtneyseligman.com/text/stars/starevol2.htm. Retrieved 2006-09-05.
  57. ^ Bally, J.; Morse, J.; Reipurth, B. (1996). "The Birth of Stars: Herbig-Haro Jets, Accretion and Proto-Planetary Disks". in Piero Benvenuti, F.D. Macchetto, and Ethan J. Schreier. Science with the Hubble Space Telescope - II. Proceedings of a workshop held in Paris, France, December 4–8, 1995. Space Telescope Science Institute. p. 491. http://adsabs.harvard.edu/abs/1996swhs.conf..491B. Retrieved 2006-07-14.
  58. ^ Smith, Michael David (2004). The origin of stars. p. 176. ISBN 1860945015.
  59. ^ Megeath, Tom (May 11, 2010). "Herschel finds a hole in space". ESA. http://www.esa.int/esaCP/SEMFEAKPO8G_index_0.html. Retrieved 2010-05-17.
  60. ^ Mengel, J. G.; Demarque, P.; Sweigart, A. V.; Gross, P. G. (1979). "Stellar evolution from the zero-age main sequence". Astrophysical Journal Supplement Series 40: 733–791. doi:10.1086/190603. http://adsabs.harvard.edu/abs/1979ApJS...40..733M.
  61. ^ a b Sackmann, I. J.; Boothroyd, A. I.; Kraemer, K. E. (1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457. doi:10.1086/173407. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...418..457S.
  62. ^ Wood, B. E.; Müller, H.-R.; Zank, G. P.; Linsky, J. L. (2002). "Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity". The Astrophysical Journal 574 (1): 412–425. doi:10.1086/340797. http://www.journals.uchicago.edu/doi/full/10.1086/340797.
  63. ^ de Loore, C.; de Greve, J. P.; Lamers, H. J. G. L. M. (1977). "Evolution of massive stars with mass loss by stellar wind". Astronomy and Astrophysics 61 (2): 251–259. http://adsabs.harvard.edu/abs/1977A&A....61..251D.
  64. ^ "The evolution of stars between 50 and 100 times the mass of the Sun". Royal Greenwich Observatory. http://www.nmm.ac.uk/server/show/conWebDoc.727. Retrieved 2006-09-07.
  65. ^ Pizzolato, N.; Ventura, P.; D'Antona, F.; Maggio, A.; Micela, G.; Sciortino, S. (2001). "Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests". Astronomy & Astrophysics 373: 597–607. doi:10.1051/0004-6361:20010626. http://www.edpsciences.org/articles/aa/abs/2001/26/aah2701/aah2701.html.
  66. ^ "Mass loss and Evolution". UCL Astrophysics Group. 2004-06-18. Archived from the original on 2004-11-22. http://web.archive.org/web/20041122143115/http://www.star.ucl.ac.uk/groups/hotstar/research_massloss.html. Retrieved 2006-08-26.
  67. ^ Schröder, K.-P.; Smith, Robert Connon (2008). "Distant future of the Sun and Earth revisited". Monthly Notices of the Royal Astronomical Society 386: 155. doi:10.1111/j.1365-2966.2008.13022.x. See also Palmer, Jason (2008-02-22). "Hope dims that Earth will survive Sun's death". NewScientist.com news service. http://space.newscientist.com/article/dn13369-hope-dims-that-earth-will-survive-suns-death.html?feedId=online-news_rss20. Retrieved 2008-03-24.
  68. ^ a b Hinshaw, Gary (2006-08-23). "The Life and Death of Stars". NASA WMAP Mission. http://map.gsfc.nasa.gov/m_uni/uni_101stars.html. Retrieved 2006-09-01.
  69. ^ Iben, Icko, Jr. (1991). "Single and binary star evolution". Astrophysical Journal Supplement Series 76: 55–114. doi:10.1086/191565. http://adsabs.harvard.edu/abs/1998RPPh...61...77K. Retrieved 2007-03-03.
  70. ^ "What is a star?". Royal Greenwich Observatory. http://www.nmm.ac.uk/server/show/conWebDoc.299/. Retrieved 2006-09-07.
  71. ^ Liebert, J. (1980). "White dwarf stars". Annual review of astronomy and astrophysics 18 (2): 363–398. doi:10.1146/annurev.aa.18.090180.002051. http://adsabs.harvard.edu/abs/1980ARA&A..18..363L.
  72. ^ a b c "Introduction to Supernova Remnants". Goddard Space Flight Center. 2006-04-06. http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html. Retrieved 2006-07-16.
  73. ^ Fryer, C. L. (2003). "Black-hole formation from stellar collapse". Classical and Quantum Gravity 20: S73–S80. doi:10.1088/0264-9381/20/10/309. http://www.iop.org/EJ/abstract/0264-9381/20/10/309.
  74. ^ Szebehely, Victor G.; Curran, Richard B. (1985). Stability of the Solar System and Its Minor Natural and Artificial Bodies. Springer. ISBN 9027720460.
  75. ^ Harvard-Smithsonian Center for Astrophysics (2006-01-30). "Most Milky Way Stars Are Single". Press release. http://www.cfa.harvard.edu/press/pr0611.html. Retrieved 2006-07-16.
  76. ^ "What is a galaxy? How many stars in a galaxy / the Universe?". Royal Greenwich Observatory. http://www.nmm.ac.uk/server/show/ConWebDoc.20495. Retrieved 2006-07-18.
  77. ^ "Hubble Finds Intergalactic Stars". Hubble News Desk. 1997-01-14. http://hubblesite.org/newscenter/archive/releases/1997/02/text/. Retrieved 2006-11-06.
  78. ^ "Astronomers count the stars". BBC News. 2003-07-22. http://news.bbc.co.uk/2/hi/science/nature/3085885.stm. Retrieved 2006-07-18.
  79. ^ 3.99 × 1013 km / (3 × 104 km/h × 24 × 365.25) = 1.5 × 105 years.
  80. ^ Holmberg, J.; Flynn, C. (2000). "The local density of matter mapped by Hipparcos". Monthly Notices of the Royal Astronomical Society 313 (2): 209–216. doi:10.1046/j.1365-8711.2000.02905.x. http://adsabs.harvard.edu/abs/2000MNRAS.313..209H. Retrieved 2006-07-18.
  81. ^ "Astronomers: Star collisions are rampant, catastrophic". CNN News. 2000-06-02. http://archives.cnn.com/2000/TECH/space/06/02/stellar.collisions/. Retrieved 2006-07-21.
  82. ^ Lombardi, Jr., J. C.; Warren, J. S.; Rasio, F. A.; Sills, A.; Warren, A. R. (2002). "Stellar Collisions and the Interior Structure of Blue Stragglers". The Astrophysical Journal 568: 939–953. doi:10.1086/339060. http://adsabs.harvard.edu/abs/2002ApJ...568..939L.
  83. ^ Frebel, A.; Norris, J. E.; Christlieb, N.; Thom, C.; Beers, T. C.; Rhee, J (2007-05-11). "Nearby Star Is A Galactic Fossil". Science Daily. http://www.sciencedaily.com/releases/2007/05/070510151902.htm. Retrieved 2007-05-10.
  84. ^ Frebel, Anna, et al (May, 2007). "Discovery of HE 1523-0901, a Strongly r-Process-enhanced Metal-poor Star with Detected Uranium". Astrophysical Journal Letters 660 (2): L117–L120. doi:10.1086/518122. Bibcode: 2007ApJ...660L.117F.
  85. ^ Naftilan, S. A.; Stetson, P. B. (2006-07-13). "How do scientists determine the ages of stars? Is the technique really accurate enough to use it to verify the age of the universe?". Scientific American. http://www.scientificamerican.com/article.cfm?id=how-do-scientists-determi. Retrieved 2007-05-11.
  86. ^ Laughlin, G.; Bodenheimer, P.; Adams, F. C. (1997). "The End of the Main Sequence". The Astrophysical Journal 482: 420–432. doi:10.1086/304125. http://adsabs.harvard.edu/abs/1997ApJ...482..420L. Retrieved 2007-05-11.
  87. ^ Irwin, Judith A. (2007). Astrophysics: Decoding the Cosmos. John Wiley and Sons. p. 78. ISBN 0470013060.
  88. ^ "A "Genetic Study" of the Galaxy". ESO. 2006-09-12. http://www.eso.org/outreach/press-rel/pr-2006/pr-34-06.html. Retrieved 2006-10-10.
  89. ^ Fischer, D. A.; Valenti, J. (2005). "The Planet-Metallicity Correlation". The Astrophysical Journal 622 (2): 1102–1117. doi:10.1086/428383. http://adsabs.harvard.edu/abs/2005ApJ...622.1102F.
  90. ^ "Signatures Of The First Stars". ScienceDaily. 2005-04-17. http://www.sciencedaily.com/releases/2005/04/050417162354.htm. Retrieved 2006-10-10.
  91. ^ Feltzing, S.; Gonzalez, G. (2000). "The nature of super-metal-rich stars: Detailed abundance analysis of 8 super-metal-rich star candidates". Astronomy & Astrophysics 367: 253–265. doi:10.1051/0004-6361:20000477. http://adsabs.harvard.edu/abs/2001A&A...367..253F. Retrieved 2007-11-27.
  92. ^ Gray, David F. (1992). The Observation and Analysis of Stellar Photospheres. Cambridge University Press. ISBN 0521408687.
  93. ^ "The Biggest Star in the Sky". ESO. 1997-03-11. http://www.eso.org/outreach/press-rel/pr-1997/pr-05-97.html. Retrieved 2006-07-10.
  94. ^ Ragland, S.; Chandrasekhar, T.; Ashok, N. M. (1995). "Angular Diameter of Carbon Star Tx-Piscium from Lunar Occultation Observations in the Near Infrared". Journal of Astrophysics and Astronomy 16: 332. http://adsabs.harvard.edu/abs/1995JApAS..16..332R. Retrieved 2007-07-05.
  95. ^ Davis, Kate (2000-12-01). "Variable Star of the Month—December, 2000: Alpha Orionis". AAVSO. http://www.aavso.org/vstar/vsots/1200.shtml. Retrieved 2006-08-13.
  96. ^ Loktin, A. V. (September 2006). "Kinematics of stars in the Pleiades open cluster". Astronomy Reports 50 (9): 714–721. doi:10.1134/S1063772906090058. Bibcode: 2006ARep...50..714L.
  97. ^ "Hipparcos: High Proper Motion Stars". ESA. 1999-09-10. http://www.rssd.esa.int/hipparcos/properm.html. Retrieved 2006-10-10.
  98. ^ Johnson, Hugh M. (1957). "The Kinematics and Evolution of Population I Stars". Publications of the Astronomical Society of the Pacific 69 (406): 54. doi:10.1086/127012. http://adsabs.harvard.edu/abs/1957PASP...69...54J.
  99. ^ Elmegreen, B.; Efremov, Y. N. (1999). "The Formation of Star Clusters". American Scientist 86 (3): 264. doi:10.1511/1998.3.264. http://www.americanscientist.org/template/AssetDetail/assetid/15714/page/1. Retrieved 2006-08-23.
  100. ^ Brainerd, Jerome James (2005-07-06). "X-rays from Stellar Coronas". The Astrophysics Spectator. http://www.astrophysicsspectator.com/topics/observation/XRayCorona.html. Retrieved 2007-06-21.
  101. ^ Berdyugina, Svetlana V. (2005). "Starspots: A Key to the Stellar Dynamo". Living Reviews. http://solarphysics.livingreviews.org/Articles/lrsp-2005-8/. Retrieved 2007-06-21.
  102. ^ Smith, Nathan (1998). "The Behemoth Eta Carinae: A Repeat Offender". Mercury Magazine (Astronomical Society of the Pacific) 27: 20. http://www.astrosociety.org/pubs/mercury/9804/eta.html. Retrieved 2006-08-13.
  103. ^ "NASA's Hubble Weighs in on the Heaviest Stars in the Galaxy". NASA News. 2005-03-03. http://www.nasa.gov/home/hqnews/2005/mar/HQ_05071_HST_galaxy.html. Retrieved 2006-08-04.
  104. ^ "Ferreting Out The First Stars". Harvard-Smithsonian Center for Astrophysics. 2005-09-22. http://cfa-www.harvard.edu/press/pr0531.html. Retrieved 2006-09-05.
  105. ^ "Weighing the Smallest Stars". ESO. 2005-01-01. http://www.eso.org/outreach/press-rel/pr-2005/pr-02-05.html. Retrieved 2006-08-13.
  106. ^ Boss, Alan (2001-04-03). "Are They Planets or What?". Carnegie Institution of Washington. http://www.carnegieinstitution.org/News4-3,2001.html. Retrieved 2006-06-08.
  107. ^ a b Shiga, David (2006-08-17). "Mass cut-off between stars and brown dwarfs revealed". New Scientist. Archived from the original on 2006-11-14. http://web.archive.org/web/20061114221813/space.newscientist.com/article/dn9771-mass-cutoff-between-stars-and-brown-dwarfs-revealed.html. Retrieved 2006-08-23.
  108. ^ "Hubble glimpses faintest stars". BBC. 2006-08-18. http://news.bbc.co.uk/1/hi/sci/tech/5260008.stm. Retrieved 2006-08-22.
  109. ^ "Flattest Star Ever Seen". ESO. 2003-06-11. http://www.eso.org/outreach/press-rel/pr-2003/pr-14-03.html. Retrieved 2006-10-03.
  110. ^ Fitzpatrick, Richard (2006-02-13). "Introduction to Plasma Physics: A graduate course". The University of Texas at Austin. http://farside.ph.utexas.edu/teaching/plasma/lectures/lectures.html. Retrieved 2006-10-04.
  111. ^ Villata, Massimo (1992). "Angular momentum loss by a stellar wind and rotational velocities of white dwarfs". Monthly Notices of the Royal Astronomical Society 257 (3): 450–454. http://adsabs.harvard.edu/abs/1992MNRAS.257..450V.
  112. ^ "A History of the Crab Nebula". ESO. 1996-05-30. http://hubblesite.org/newscenter/newsdesk/archive/releases/1996/22/astrofile/. Retrieved 2006-10-03.
  113. ^ Strobel, Nick (2007-08-20). "Properties of Stars: Color and Temperature". Astronomy Notes. Primis/McGraw-Hill, Inc.. Archived from the original on 2007-06-26. http://web.archive.org/web/20070626090138/http://www.astronomynotes.com/starprop/s5.htm. Retrieved 2007-10-09.
  114. ^ Seligman, Courtney. "Review of Heat Flow Inside Stars". Self-published. http://cseligman.com/text/stars/heatflowreview.htm. Retrieved 2007-07-05.
  115. ^ a b "Main Sequence Stars". The Astrophysics Spectator. 2005-02-16. http://www.astrophysicsspectator.com/topics/stars/MainSequence.html. Retrieved 2006-10-10.
  116. ^ Zeilik, Michael A.; Gregory, Stephan A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. p. 321. ISBN 0030062284.
  117. ^ Roach, John (2003-08-27). "Astrophysicist Recognized for Discovery of Solar Wind". National Geographic News. http://news.nationalgeographic.com/news/2003/08/0827_030827_kyotoprizeparker.html. Retrieved 2006-06-13.
  118. ^ "The Colour of Stars". Australian Telescope Outreach and Education. http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html. Retrieved 2006-08-13.
  119. ^ "Astronomers Measure Mass of a Single Star—First Since the Sun". Hubble News Desk. 2004-07-15. http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/24/text/. Retrieved 2006-05-24.
  120. ^ Garnett, D. R.; Kobulnicky, H. A. (2000). "Distance Dependence in the Solar Neighborhood Age-Metallicity Relation". The Astrophysical Journal 532: 1192–1196. doi:10.1086/308617.
  121. ^ Staff (2006-01-10). "Rapidly Spinning Star Vega has Cool Dark Equator". National Optical Astronomy Observatory. http://www.noao.edu/outreach/press/pr06/pr0603.html. Retrieved 2007-11-18.
  122. ^ Michelson, A. A.; Pease, F. G. (2005). "Starspots: A Key to the Stellar Dynamo". Living Reviews in Solar Physics (Max Planck Society). http://www.livingreviews.org/lrsp-2005-8.
  123. ^ Manduca, A.; Bell, R. A.; Gustafsson, B. (1977). "Limb darkening coefficients for late-type giant model atmospheres". Astronomy and Astrophysics 61 (6): 809–813. http://adsabs.harvard.edu/abs/1977A&A....61..809M.
  124. ^ Chugainov, P. F. (1971). "On the Cause of Periodic Light Variations of Some Red Dwarf Stars". Information Bulletin on Variable Stars 520: 1–3. http://adsabs.harvard.edu/abs/1977A&A....61..809M.
  125. ^ "Magnitude". National Solar Observatory—Sacramento Peak. Archived from the original on 2008-02-06. http://web.archive.org/web/20080206074842/http://www.nso.edu/PR/answerbook/magnitude.html. Retrieved 2006-08-23.
  126. ^ a b "Luminosity of Stars". Australian Telescope Outreach and Education. http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_luminosity.html. Retrieved 2006-08-13.
  127. ^ Hoover, Aaron (2004-01-05). "Star may be biggest, brightest yet observed". HubbleSite. Archived from the original on 2007-08-07. http://web.archive.org/web/20070807035239/http://www.napa.ufl.edu/2004news/bigbrightstar.htm. Retrieved 2006-06-08.
  128. ^ "Faintest Stars in Globular Cluster NGC 6397". HubbleSite. 2006-08-17. http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/37/image/a. Retrieved 2006-06-08.
  129. ^ Smith, Gene (1999-04-16). "Stellar Spectra". University of California, San Diego. http://casswww.ucsd.edu/public/tutorial/Stars.html. Retrieved 2006-10-12.
  130. ^ Fowler, A. (1891–2). "The Draper Catalogue of Stellar Spectra". Nature, a Weekly Illustrated Journal of Science 45: 427–8.
  131. ^ Jaschek, Carlos; Jaschek, Mercedes (1990). The Classification of Stars. Cambridge University Press. ISBN 0521389968.
  132. ^ a b c MacRobert, Alan M. "The Spectral Types of Stars". Sky and Telescope. http://www.skyandtelescope.com/howto/basics/3305876.html. Retrieved 2006-07-19.
  133. ^ "White Dwarf (wd) Stars". White Dwarf Research Corporation. http://www.physics.uq.edu.au/people/ross/ph3080/whitey.htm. Retrieved 2006-07-19.
  134. ^ a b c d "Types of Variable Stars". AAVSO. http://www.aavso.org/vstar/types.shtml. Retrieved 2006-07-20.
  135. ^ "Cataclysmic Variables". NASA Goddard Space Flight Center. 2004-11-01. http://imagine.gsfc.nasa.gov/docs/science/know_l2/cataclysmic_variables.html. Retrieved 2006-06-08.
  136. ^ Hansen, Carl J.; Kawaler, Steven D.; Trimble, Virginia (2004). Stellar Interiors. Springer. ISBN 0387200894.
  137. ^ a b c Schwarzschild, Martin (1958). Structure and Evolution of the Stars. Princeton University Press. ISBN 0-691-08044-5.
  138. ^ "Formation of the High Mass Elements". Smoot Group. http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html. Retrieved 2006-07-11.
  139. ^ a b "What is a Star?". NASA. 2006-09-01. http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html. Retrieved 2006-07-11.
  140. ^ ESO (2001-08-01). "The Glory of a Nearby Star: Optical Light from a Hot Stellar Corona Detected with the VLT". Press release. http://www.eso.org/outreach/press-rel/pr-2001/pr-17-01.html. Retrieved 2006-07-10.
  141. ^ Burlaga, L. F.; Ness, N. F.; Acuña, M. H.; Lepping, R. P.; Connerney, J. E. P.; Stone, E. C.; McDonald, F. B. (2005). "Crossing the Termination Shock into the Heliosheath: Magnetic Fields". Science 309 (5743): 2027–2029. doi:10.1126/science.1117542. PMID 16179471.
  142. ^ a b c d Wallerstein, G.; Iben Jr., I.; Parker, P.; Boesgaard, A. M.; Hale, G. M.; Champagne, A. E.; Barnes, C. A.; KM-dppeler, F.; Smith, V. V.; Hoffman, R. D.; Timmes, F. X.; Sneden, C.; Boyd, R. N.; Meyer, B. S.; Lambert, D. L. (1999). "Synthesis of the elements in stars: forty years of progress" (PDF). Reviews of Modern Physics 69 (4): 995–1084. doi:10.1103/RevModPhys.69.995. http://www.cococubed.com/papers/wallerstein97.pdf. Retrieved 2006-08-04.
  143. ^ Girardi, L.; Bressan, A.; Bertelli, G.; Chiosi, C. (2000). "Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 Msun, and from Z=0.0004 to 0.03". Astronomy and Astrophysics Supplement 141: 371–383. doi:10.1051/aas:2000126.
  144. ^ Woosley, S. E.; Heger, A.; Weaver, T. A. (2002). "The evolution and explosion of massive stars". Reviews of Modern Physics 74 (4): 1015–1071. doi:10.1103/RevModPhys.74.1015. http://adsabs.harvard.edu/abs/2002RvMP...74.1015W.
  145. ^ 11.5 days is 0.0315 years.
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