From Academic Kids
A star is any massive gaseous body in outer space, just like the Sun. Unlike a planet, a star generates energy through nuclear fusion and therefore emits light. All stars except the Sun appear as shining points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere and their distance from us. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there.
Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos. Astronomers estimate that there are at least 70 sextillion (70×1021) stars in the known universe  (http://news.bbc.co.uk/2/hi/science/nature/3085885.stm). That is 70 000 000 000 000 000 000 000, or 230 billion times as much as the 300 billion in our own Milky Way.
Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the estimated age of the universe. (See Big Bang theory and stellar evolution.) They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the Sun —about 1.6 terametres. However, these have a much lower density than the Sun.
One of the most massive stars known is η Carinae, with 100–150 times as much mass as the Sun. Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The reason for this limit is not precisely known, but the Eddington limit is part of the answer. The very first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive star is long extinct, however, and currently only theoretical.
The smallest known star undergoing fusion in its core is AB Doradus C, a companion to AB Doradus A, which has a mass only 93 times that of Jupiter. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.
The energy produced by stars radiates into space as electromagnetic radiation, as a stream of neutrinos from the star's core, and as a stream of particles from the star's outer layers (its stellar wind). The peak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically far from negligible. The apparent brightness of a star is measured by its apparent magnitude.
Stellar astronomy is the study of stars and the phenomena exhibited by the various forms/developmental stages of stars.
The majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist. Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars.
Star formation and evolution
As learned by star formation astronomers, stars are born in molecular clouds, large regions of slightly higher density of matter (though still less dense than the inside of an earthly vacuum chamber), and form by gravitational instability inside those clouds triggered by shockwaves from supernovae. High mass stars powerfully illuminate the clouds from which they formed. One example of such a nebula is the Orion Nebula.
Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years (far longer than the time elapsed in the universe so far). At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs —although none exist yet.
As most stars exhaust their supply of hydrogen, 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 subsume Mercury and Venus. Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. Larger stars will also fuse heavier elements, all the way to iron, which is the end point of the process.
An average-size star will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. It will fade into a black dwarf over very long stretches of time.
In larger stars, fusion continues until collapse ends up causing the star to explode in a supernova. This is the only cosmic process that happens on human timescales; historically, supernovae have been observed as "new stars" where none existed before. Most of the matter in a star is blown away in the explosion (forming nebulae such as the Crab Nebula) but what remains will collapse into a neutron star (a pulsar or X-ray burster) or, in the case of the largest stars, a black hole.
The blown-off outer layers include heavy elements, which are often converted into new stars and/or planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.
Stellar evolution explains how stars are created and die in greater detail.
There are different classifications of stars ranging from type O which are very large and bright, to M which is often just large enough to start ignition of the hydrogen. Some of the more common classifications are O, B, A, F, G, K, M, and can perhaps be more easily remembered using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Jump Cannon (1863-1941). There are many other mnemonics for star classification; the most frequent addition tacks Right Now Sweetheart for the red dwarf sub-types R, N and S.
Each letter has 9 subclassifications. Our Sun is a G2, which is very near the middle in terms of quantities observed. Most stars fall into the main sequence which is a description of stars based on their absolute magnitude and spectral type. The Sun is taken as the prototypical star (not because it is special in any way, but because it is the closest and most studied star we have), and most characteristics of other stars are usually given in solar units.
For example, the mass of the Sun is
- MSun = 1.9891�1030 kg
The masses of other stars can be given in terms of MSun.
Naming of stars
Most stars are identified only by catalogue numbers; only a few have names as such. The names are either traditional names (mostly from Arabic), Flamsteed designations, or Bayer designations. The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union (IAU). A number of private companies (e.g. the "International Star Registry") purport to sell names to stars; however, these names are not recognized by the scientific community, nor used by them, and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.
Nuclear fusion reaction pathways
A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition (see Stellar nucleosynthesis).
Stars begin as a cloud of mostly hydrogen with about 25% helium and heavier elements in smaller quantities. In the Sun, with a 107 K core, hydrogen fuses to form helium in the proton-proton chain:
- 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 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:
- 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
- Cliff Pickover (2001) "The Stars of Heaven", Oxford University Press
- John Gribbin, Mary Gribbin (2001) "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press.
- List of nearest stars
- List of nearest bright stars
- List of brightest stars (apparent & absolute magnitude)
- List of largest stars (by diameter)
- List of heaviest stars (by solar mass)
- Stellar evolution
- Variable star
- List of stars with confirmed extrasolar planets
- Timeline of stellar astronomy