Star

:This article is about celestial bodies. {{otheruses}}

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.

Related Topics:
Star formation - Molecular cloud - Vacuum chamber - Supernova - Nebula - Orion Nebula

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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.

Related Topics:
Fusing - Hydrogen - Helium - Main sequence

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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.

Related Topics:
Red dwarf - Black dwarf

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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.

Related Topics:
Mercury - Venus - Helium - Iron

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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.

Related Topics:
Planetary nebula - Degenerate matter - Degeneracy pressure - White dwarf

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It will fade into a black dwarf over very long stretches of time.

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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.

Related Topics:
Collapse - Supernova - Crab Nebula - Neutron star - Pulsar - X-ray burster - Black hole

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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.

Related Topics:
Element - Planet - Stellar wind

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Stellar evolution explains how stars are created and die in greater detail.

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