Stellar evolution

Stellar evolution is the sequence of physical and chemical changes a star undergoes from formation to its final remnant or destruction. Its path depends chiefly on the star’s initial mass and composition.

Hydrostatic equilibrium: A state in which inward gravity is balanced by outward pressure within a fluid body. This balance supports a star against collapse during most of its life.
Stellar mass: The amount of matter contained in a star, usually expressed in units of the Sun’s mass. Initial mass is the strongest predictor of a star’s lifetime and final state.
Red dwarf: A small, cool, low-mass star that burns hydrogen slowly and can remain on the main sequence for trillions of years. Its tiny fuel-consumption rate gives it a far longer main-sequence life than the Sun.
Molecular cloud: A cold, dense region of interstellar gas and dust where molecules form and stars can originate. Gravitational collapse within these clouds supplies the material for new stars.
Chemical enrichment: The increase of heavy elements in a cosmic environment through stellar nucleosynthesis and material ejected by stars. Stellar ejecta seed later generations of stars and planets with elements made in earlier stars.
Stellar nucleosynthesis: The production of chemical elements by nuclear reactions inside stars and during stellar explosions. Fusion changes a star’s fuel supply and builds heavier elements as it evolves.
Metallicity: The proportion of a star’s material made of elements heavier than helium. Composition affects opacity, stellar winds, and the rate at which stars consume fuel.
Sun: The star at the center of the Solar System, a roughly 4.6-billion-year-old G-type main-sequence star. The Sun illustrates the expected evolution of a star with about one solar mass.
Jeans instability: The condition under which a gas cloud’s self-gravity overcomes pressure support and causes collapse. It describes when a cloud fragment can begin collapsing toward protostars.
Planetary nebula: An expanding shell of ionized gas expelled by a low- or intermediate-mass star late in its life. It reveals the outer layers being shed before the star becomes a white dwarf.

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