A star is a self-gravitating sphere of plasma, in hydrostatic equilibrium, that generates energy in its interior, via nuclear fusion. Energy, from this process, radiates into space as electromagnetic radiation and neutrinos. With the exception of the Sun, the nearest star to Earth is Proxima Centauri, at a distance of about 4 x 1016 metres (4.2 light years).
As learned by star formation astronomers, stars are born in molecular clouds, regions of higher density of matter, and form by gravitational instability inside those clouds. High mass stars illuminate powerfully the clouds from which they formed. One example of such reflection nebulae is the Orion Nebula. Nebulae such as the Crab Nebula are the remnants of a high mass star which has exploded. When a star dies it returns material to interstellar space. This includes heavy elements which are often converted into new stars and/or planets.
Many stars are gravitationally bound to other stars, forming binary stars. Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars.
The sun is taken as the prototypical star (not because it is special in any way, but because 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
The masses of all other stars are given in terms of Msun.
Stellar evolution explains how stars are created and then die.
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", invented by Annie Jump Cannon[?] (1863-1941). There are many other mnemonics for star classification. 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.
Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high pressure reactions near the core. Such stars are said to be on the main sequence. As these stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. 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.
In smaller stars, the outer layers are eventually shed leaving the core, which is not massive enough for further fusion to take place and so is supported only by degeneracy pressure. This is called a white dwarf. The surrounding material stays visible for a while as a planetary nebula.
The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.
Most stars are identified only by catalog 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. 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. (Many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.) See star designations for more information on how stars are named.
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:
These reactions result in the overall reaction:
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:
For an overall reaction of:
See also: Blue straggler