Are stars just the holes to heaven?

Answer 1

no just touch one and see. you will die.

.....well maybe it is.

what do you have against bk tech anyway.

Answer 2

I can think of a few stars that are definately "holes"; but not in heaven.

Answer 3

No.
A star is a massive, compact body of plasma in outer space that is held together by its own gravity and, unlike a planet, is sufficiently massive to sustain nuclear fusion in a very dense, hot core region. This fusion of atomic nuclei generates the energy that is continuously radiated from the outer layers of the star during much of its life span

Answer 4

Not quite. Stars are planets that burned out millions of years ago and the light that you see from them is their energy source finally arriving through space to be seen. If you go out at night and simply view a number of stars, then go out about five months later, those same exact stars are no longer there. This is because their light has either finally died out or the light has moved forward again.

Answer 5

Stars are the heaven.

Answer 6

Nope.

Answer 7

Hi. Probably feels more like to way to hell.

Answer 8

ONly black holes.

Answer 9

No.
Here you are:
Stars:

This article is about the astronomical object. For other uses, see Star (disambiguation).

The Pleiades, an open cluster of stars in the constellation of Taurus. NASA photo.A star is a massive, compact body of plasma in outer space that is held together by its own gravity and, unlike a planet, is sufficiently massive to sustain nuclear fusion in a very dense, hot core region. This fusion of atomic nuclei generates the energy that is continuously radiated from the outer layers of the star during much of its life span.[1]

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 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, with some helium 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.[2]

Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[3] For example, a nova occurs when a white dwarf accretes matter from a companion star.

Contents [hide]
1 Observation history
1.1 Star designations
2 Units of measurement
3 Formation and evolution
3.1 Protostar formation
3.2 Main sequence
3.3 Post-main sequence
3.3.1 Massive stars
3.4 Collapse
4 Appearance and distribution
5 Age and size
6 Radiation
6.1 Luminosity
6.2 Magnitude
7 Classification
8 Variable stars
9 Structure
10 Nuclear fusion reaction pathways
11 References
12 See also
13 External links



[edit]
Observation history
Stars have been important to every culture. They have been used in religious practices and for celestial navigation and orientation. The Gregorian calendar, used nearly everywhere in the world, is a solar calendar based on the position of the Earth relative to the nearest star, the Sun.

Early astronomers such as Tycho Brahe identified new stars in the heavens, suggesting that the heavens were not immutable. In 1584 Giordano Bruno suggested that the stars were actually other suns, and may have Earth-like planets in orbit around them.[4] 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 who would prompt Isaac Newton to suggest that the stars were equally distributed in every direction, resulting in no net gravitational pull.[5]

The Italian astronomer Geminiano Montanari recorded seeing variability in the star Algol 1667. Edmond Halley 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 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 using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[4]

[edit]
Star designations
Main articles: star designation, astronomical naming conventions, and Star catalogue
The concept of the constellation arose as far back in history as Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology. Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.

As well as certain constellations and the Sun itself, stars as a whole have their own myths.[6] They were thought to be the souls of the dead or gods. An example of this is the star Algol, which was thought to represents the eye of the Gorgon Medusa.

To the Ancient Greeks, some "stars", later identified as planets, represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[6] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)

At about the year 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied greek letters as designations to the stars in each constellation. Later the English astronomer John Flamsteed came up with a system using numbers, which would later be known as the Flamsteed designation. Numerous additional systems have since been created as star catalogues appeared.

The only body which has been recognized by the scientific community as having the authority to name stars or other celestial bodies is the International Astronomical Union (IAU).[7] A number of private companies (for instance, the "International Star Registry") purport to sell names to stars; however, these names are neither recognized by the scientific community nor used by them,[7] and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.[8]

[edit]
Units of measurement
Most stellar parameters are expressed in SI units by convention, but CGS units are also used (e.g., expressing luminosity in ergs per second). Mass, luminosity, and radii are usually given in solar units, based on the characteristics of the Sun:

solar mass: kg[9]
solar luminosity: watts[9]
solar radius: m[10]

Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit (AU) — approximately the mean distance between the Earth and the Sun.

[edit]
Formation and evolution
Main article: stellar evolution
Stars are formed within molecular clouds; large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). These clouds consist of mostly hydrogen with about 23–28% helium and a few percent heavier elements. One example of such a star-forming nebula is the Orion Nebula.[11] As massive stars are formed from these clouds, they powerfully illuminate the clouds from which they formed, creating an H II region.

[edit]
Protostar formation
Main article: star formation
The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shockwaves from supernovae or the collision of two galaxies (as in a starburst galaxy). Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans Instability it begins to collapse under its own gravitational force.

As the cloud collapses, individual Bok globules form with up to 50 solar masses of material. As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached hydrostatic equilibrium, a protostar forms at the core.[12] These pre-main sequence stars are often surrounded by a protoplanetary disk. The protostar then follows a Hayashi track on the Hertzsprung-Russell diagram.[13] 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.[14] The period of gravitational contraction lasts for about 10-15 million years.

Early stars of less than 2 solar masses are called T Tauri 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 objects.[15]

[edit]
Main sequence
Main article: 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. 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.[16] 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.[17]

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,[18] 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.[19] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.[20]

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:[21]


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

Large stars burn their fuel very rapidly and are short-lived. Small stars (called red dwarfs) 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.[22] However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), 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,[23] and modifies the strength of the stellar wind.[24] 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

Betelgeuse is a red giant star approaching the end of its life cycle.As most stars exhaust their supply of hydrogen at their core, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume Mercury and Venus. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit (1 astronomical unit, or AU). However by that time the orbit of the Earth will expand to about 1.7 AUs due to mass loss by the Sun. Our planet will thus escape envelopment.[25]

In a red giant, hydrogen fusion proceeds in a shell-layer surrounding the core.[26] Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. In low mass stars (less than the 1.4 solar mass) 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.[27]

[edit]
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. This process continues, with the successive stages being fueled by oxygen, neon, silicon, and sulfur. 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.[28]

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.[26] 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 Wolf-Rayet stars with a dense stellar wind that sheds the outer atmosphere.

[edit]
Collapse
An average-size star (less than 1.4 solar masses after explosion) will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of electron degenerate matter not massive enough for further compression to take place, supported only by degeneracy pressure, called a white dwarf.[29] 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.


The Crab Nebula, remnants of a supernova that was first observed around 1050 AD.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, 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.[30]

Eventually, most of the matter in a star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula[30]) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars (more than 3 solar masses after explosion), a black hole.[31] 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 planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[30]

[edit]
Appearance and distribution
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 disks of 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. 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.[32]


A white dwarf star in orbit around Sirius (artist's impression). NASA image.It has been a long-held assumption that the majority of stars occur in gravitationally-bound, multiple-star systems, forming binary stars. 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.[33]

Larger groups called star clusters also exist. 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.[34]

Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the known universe.[35] That is 230 billion times as many as the 300 billion in our own Milky Way.

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.[36] Distances like this are typical inside galactic discs, where the solar system is located.[37] Stars can be much closer to each other in the centres of galaxies and in globular clusters, 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.[38] Such collisions can produce what are known as blue stragglers. 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. [39]

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,[40] and also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[41] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.[42]

[edit]
Age and size

The Sun is the nearest star to Earth.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 stars no bigger than a city to supergiants like Betelgeuse in the Orion 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.[43]

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.[44] (See Big Bang theory and stellar evolution.) 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.

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 on the Hooker telescope.[45]

One of the most massive stars known is Eta Carinae,[46] 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.[47] 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,[48] 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.[49] 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.[50][51] 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.[52][51] Smaller bodies are called brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants.

[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[53] (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 deep inside the core of a star, photons of electromagnetic energy are released from the nuclear fusion reaction, which are then converted to visible light in the star’s outer layers.

The peak frequency and color of the visible light depends on the temperature of the star’s outer layers, including its photosphere.[54] 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 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 rotation 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.[55]) With these parameters, astronomers can also estimate the age of the star.[56]

[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 can be approximated by treating the emitted energy as a black body radiation.[57][58] So:


where L is the luminosity, σ 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:


[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.

Number of stars brighter than magnitude Apparent
magnitude Number
of Stars[59]
0 4
1 15
2 48
3 171
4 513
5 1,602
6 4,800
7 14,000
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 (32.6 light-years), 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[58] (2.512 to be precise). 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, which is the faintest star visible to the naked eye.

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 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 = variation in brightness
Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not exactly equivalent for an individual star;[58] for example, the bright star Sirius has an apparent magnitude of -1.44, but it has an absolute magnitude of +1.41.

Our Sun has an apparent magnitude of -26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky, is approximately 23 times more luminous than our 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 our Sun. Despite Canopus being vastly more luminous than Sirius, Sirius appears brighter than Canopus to our eyes, only because it is merely 8.6 light-years away from us, while Canopus is much further away from us at 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 38,000,000 times more luminuous as our own sun.[60] The least luminous stars that are currently known are located in the NGC 6397 cluster. There, the faintest red dwarf star was found, with a magnitude of 26, and a white dwarf of the 28th magnitude. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[61]

[edit]
Classification
Main article: stellar classification
There are different 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 subclassifications numbered (hottest to coldest) 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.[62] (See Wolf-Rayet star.)

In addition, stars may be classified by their "luminosity effects", which correspond to their spatial size. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). Most stars fall into the main sequence which consists of ordinary hydrogen-burning stars. These fall along a narrow band when graphed according to their absolute magnitude and spectral type.[62] 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.[62]

White dwarf stars, which typically fall in the lower left section of the Hertzsprung-Russell diagram, 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.[63]

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


The asymmetrical appearance of Mira, an oscillating variable star. NASA HST image.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, and long-period variables such as Mira.[64]

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[64] 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.[3] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[65] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[64]

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[64] 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, main sequence star is in a state of equilibrium in which the forces in any small volume exactly counterbalance each other. The balancing forces consist of inward directed gravitational force and the opposing pressure from the thermal energy of the plasma gas. For these forces to balance out, the temperature at the core of a typical star to be on the order of 107 °C or higher. 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.[66]

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 fusion occurs in a slowly expanding shell around the degenerate helium core.[67]

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 top half of this illustration shows a cut-away view of the Solar interior. In the bottom half are different views of the Sun's surface as seen in various regions of the electromagnetic spectrum. 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, as near the core, or in areas with high opacity, as in the outer envelope.[66]

The 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.[68] The convective zones will also vary over time as the star ages and the constitution of the interior is modified.[66]

The portion of a main sequence star that is visible to an observer is called the photosphere. This is the layer at which the plasma gas 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 star 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.[69] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[68] 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.

[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 converted into energy, according to the mass-energy relationship E=mc².[1]

In the Sun, with a 107 °K core, hydrogen fuses to form helium in the proton-proton chain reaction:[70]

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)
In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle.[70]

In stars 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:[70]

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.[70]

The example below shows the amount of time required for a star of 20 solar masses to consume 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.[71]

Fuel
material Temperature
(million Kelvin) Density
(kg/cm3) 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
[edit]
References
^ a b Bahcall, John N. (2000-06-29). How the Sun Shines (in English). Nobel Foundation. Retrieved on 2006-08-30.
^ Stellar Evolution & Death (in English). NASA Observatorium. Retrieved on 2006-06-08.
^ a b Iben, Icko, Jr. (1991). "Single and binary star evolution". Astrophysical Journal Supplement Series 76: 55-114.
^ a b Drake, Stephen A. (2006-08-17). A Brief History of High-Energy (X-ray & Gamma-Ray) Astronomy (in English). NASA HEASARC. Retrieved on 2006-08-24.
^ Hoskin, Michael (1998). The Value of Archives in Writing the History of Astronomy (in English). Space Telescope Science Institute. Retrieved on 2006-08-24.
^ a b Coleman, Leslie S.. Myths, Legends and Lore (in English). Frosty Drew Observatory. Retrieved on 2006-08-13.
^ a b The Naming of Stars (in English). National Maritime Museum. Retrieved on 2006-08-13.
^ Adams, Cecil (1998-04-01). Can you pay $35 to get a star named after you? (in English). The Straight Dope. Retrieved on 2006-08-13.
^ a b I.-J. Sackmann, A. I. Boothroyd (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.
^ S. C. Tripathy, H. M. Antia (1999). "Influence of surface layers on the seismic estimate of the solar radius". Solar Physics 186 (1/2): 1-11.
^ P. R. Woodward (1978). "Theoretical models of star formation". Annual review of astronomy and astrophysics 16: 555-584.
^ Seligman, Courtney. Slow Contraction of Protostellar Cloud (in English). Retrieved on 2006-09-05.
^ C. Hayashi (1961). "Stellar evolution in early phases of gravitational contraction.". Publications of the Astronomical Society of Japan 13: 450-452.
^ L. G. Henyey, R. Lelevier, R. D. Levée (1955). "The Early Phases of Stellar Evolution". Publications of the Astronomical Society of the Pacific 67 (396): 154.
^ J. Bally, J. Morse, B. Reipurth (1996). Piero Benvenuti, F.D. Macchetto, and Ethan J. Schreier "The Birth of Stars: Herbig-Haro Jets, Accretion and Proto-Planetary Disks". Science with the Hubble Space Telescope - II. Proceedings of a workshop held in Paris, France, December 4-8, 1995, 491, Space Telescope Science Institute. Retrieved on 2006-07-14.
^ J. G. Mengel, P. Demarque, A. V.Sweigart, P. G. Gross (1979). "Stellar evolution from the zero-age main sequence". Astrophysical Journal Supplement Series 40: 733-791.
^ Sackmann, I.-Juliana, Arnold I. Boothroyd, Kathleen E. Kraemer (11 1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457.
^ B. E. Wood, H.-R. Müller, G. P. Zank, J. L. Linsky (2002). "Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity". The Astrophysical Journal 574: 412-425.
^ C. de Loore, J. P. de Greve, H. J. G. L. M. Lamers (1977). "Evolution of massive stars with mass loss by stellar wind". Astronomy and Astrophysics 61 (2): 251-259.
^ The evolution of stars between 50 and 100 times the mass of the Sun (in English). Royal Greenwich Observatory. Retrieved on 2006-09-07.
^ Richmond, Michael. Stellar evolution on the main sequence (in English). Retrieved on 2006-08-24.
^ Richmond, Michael. Late stages of evolution for low-mass stars (in English). Rochester Institute of Technology. Retrieved on 2006-08-04.
^ N. Pizzolato, P. Ventura, F. D'Antona, A. Maggio, G. Micela, S. Sciortino (2001). "Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests". Astronomy & Astrophysics 373: 597-607.
^ Mass loss and Evolution (in English). UCL Astrophysics Group (2004-06-18). Retrieved on 2006-08-26.
^ I.J. Sackmann, A.I. Boothroyd, K.E. Kraemer, "Our Sun. III. Present and Future.", Astrophysical Journal, vol. 418, pp. 457.
^ a b Hinshaw, Gary (2006-08-23). The Life and Death of Stars (in English). NASA WMAP Mission. Retrieved on 2006-09-01.
^ Alan C. Edwards (1969). "The hydrodynamics of the helium flash". Monthly Notices of the Royal Astronomical Society 146: 445.
^ What is a star? (in English). Royal Greenwich Observatory. Retrieved on 2006-09-07.
^ J. Liebert (1980). "White dwarf stars". Annual review of astronomy and astrophysics 18 (2): 363-398.
^ a b c Introduction to Supernova Remnants (in English). Goddadr Space Flight Center (2006-04-06). Retrieved on 2006-07-16.
^ C. L. Fryer (2003). "Black-hole formation from stellar collapse". Classical and Quantum Gravity 20: S73-S80.
^ "The Biggest Star in the Sky", ESO, 1997-03-11. Retrieved on 2006-07-10.
^ Harvard-Smithsonian Center for Astrophysics (2006-01-30). Most Milky Way Stars Are Single. Press release. Retrieved on 2006-07-16.
^ What is a galaxy? How many stars in a galaxy / the Universe?. Royal Greenwich Observatory. Retrieved on 2006-07-18.
^ "Astronomers count the stars", BBC News, 2003-07-22. Retrieved on 2006-07-18.
^ 3.99 × 1013 km / (3 × 104 km/h × 24 × 365.25) = 1.5 × 105 years.
^ J. Holmberg, C. Flynn (2000). "The local density of matter mapped by Hipparcos". Monthly Notices of the Royal Astronomical Society 313 (2): 209-216. Retrieved on 2006-07-18.
^ "Astronomers: Star collisions are rampant, catastrophic", CNN News, 2000-06-02. Retrieved on 2006-07-21.
^ J. C. Lombardi, Jr., J. S. Warren, F. A. Rasio, A. Sills, A. R. Warren (2002). "Stellar Collisions and the Interior Structure of Blue Stragglers". The Astrophysical Journal 568: 939-953.
^ A. A. Michelson, F. G. Pease (2005). "Starspots: A Key to the Stellar Dynamo". Living Reviews in Solar Physics.
^ A. Manduca, R. A. Bell, B. Gustafsson (1977). "Limb darkening coefficients for late-type giant model atmospheres". Astronomy and Astrophysics 61 (6): 809-813.
^ P. F. Chugainov (1971). "On the Cause of Periodic Light Variations of Some Red Dwarf Stars". Information Bulletin on Variable Stars 520: 1-3.
^ Davis, Kate (2000-12-01). Variable Star of the Month—December, 2000: Alpha Orionis (in English). AAVSO. Retrieved on 2006-08-13.
^ Whitehouse, Dr. David, "'Oldest' star found in galaxy", BBC News, 2002-10-31. Retrieved on 2006-08-13.
^ A. A. Michelson, F. G. Pease (1921). "Measurement of the diameter of Alpha Orionis with the interferometer". Astrophysical Journal 53: 249-259.
^ Nathan, Smith (1998). The Behemoth Eta Carinae: A Repeat Offender (in English). Astronomical Society of the Pacific. Retrieved on 2006-08-13.
^ "NASA's Hubble Weighs in on the Heaviest Stars in the Galaxy", NASA News, 2005-03-09. Retrieved on 2006-08-04.
^ "Ferreting Out The First Stars", Harvard-Smithsonian Center for Astrophysics, 2005-09-22. Retrieved on 2006-09-05.
^ "Weighing the Smallest Stars", ESO, 2005-01-19. Retrieved on 2006-08-13.
^ Boss, Alan (2001-04-03). Are They Planets or What? (in English). Carnegie Institution of Washington. Retrieved on 2006-06-08.
^ a b Shiga, David (2006-08-17). Mass cut-off between stars and brown dwarfs revealed (in English). New Scientist. Retrieved on 2006-08-23.
^ "Hubble glimpses faintest stars", BBC, 2006-08-17. Retrieved on 2006-08-22.
^ Roach, John, "Astrophysicist Recognized for Discovery of Solar Wind", National Geographic News, 2003-08-27. Retrieved on 2006-06-13.
^ The Colour of Stars (in English). Australian Telescope Outreach and Education. Retrieved on 2006-08-13.
^ "Astronomers Measure Mass of a Single Star — First Since the Sun", Hubble News Desk, 2004-07-15. Retrieved on 2006-05-24.
^ D. R. Garnett, H. A. Kobulnicky (2000). "Distance Dependence in the Solar Neighborhood Age-Metallicity Relation". The Astrophysical Journal 532: 1192-1196.
^ Stefan-Boltzmann Law (in English). University of Central Lancashire. Retrieved on 2006-08-13.
^ a b c Luminosity of Stars (in English). Australian Telescope Outreach and Education. Retrieved on 2006-08-13.
^ Magnitude (in English). National Solar Observatory—Sacramento Peak. Retrieved on 2006-08-23.
^ Aaron Hoover (2004-01-05). Star may be biggest, brightest yet observed (in English). HubbleSite. Retrieved on 2006-06-08.
^ Faintest Stars in Globular Cluster NGC 6397 (in English). HubbleSite (2006-08-17). Retrieved on 2006-06-08.
^ a b c MacRobert, Alan M.. The Spectral Types of Stars (in English). Sky and Telescope. Retrieved on 2006-07-19.
^ White Dwarf (wd) Stars (in English). White Dwarf Research Corporation. Retrieved on 2006-07-19.
^ a b c d Types of Variable Stars (in English). AAVSO. Retrieved on 2006-07-20.
^ Cataclysmic Variables (in English). NASA Goddard Space Flight Center (2004-11-01). Retrieved on 2006-06-08.
^ a b c Schwarzschild, Martin (1958). Structure and Evolution of the Stars. Princeton University Press. ISBN 0-691-08044-5.
^ Formation of the High Mass Elements (in English). Smoot Group. Retrieved on 2006-07-11.
^ a b What is a Star? (in English). NASA (2006-09-01). Retrieved on 2006-07-11.
^ ESO (2001-08-01). The Glory of a Nearby Star: Optical Light from a Hot Stellar Corona Detected with the VLT. Press release. Retrieved on 2006-07-10.
^ a b c d G. Wallerstein, I. Iben Jr., P. Parker, A.M. Boesgaard, G.M. Hale, A. E. Champagne, C.A. Barnes, F. KM-dppeler, V.V. Smith, R.D. Hoffman, F.X. Timmes, C. Sneden, R.N. Boyd, B.S. Meyer, D.L. Lambert (1999). "Synthesis of the elements in stars: forty years of progress" (pdf). Reviews of Modern Physics 69 (4): 995-1084. Retrieved on 2006-08-04.
^ S. E. Woosley, A. Heger, T. A. Weaver (2002). "The evolution and explosion of massive stars". Reviews of Modern Physics 74 (4): 1015-1071.
Cliff Pickover (2001) "The Stars of Heaven", Oxford University Press ISBN 0-19-514874-6
John Gribbin, Mary Gribbin (2001) "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press. ISBN 0-300-09097-8
[edit]
See also
General topics:

List of mnemonics for star classification
Lists of stars
Overview of star constellations
Star count
Stellar astronomy
Timeline of stellar astronomy
Unusual stars:

Blue straggler
Carbon star
Hypernova
Hypervelocity star
Magnetar
Runaway star
Time and navigation:

Sidereal clock
Star clocks
Stellar navigation
Other:

Nursery rhyme Twinkle twinkle little star