Stellar Evolution | Everything Astronomy

Nebula

Vast clouds of gas and dust — the stellar nurseries. Disturbances cause clumps of gas to collapse under gravity and begin a long process of condensation.

Energy Source: gravitational collapse · Timescale: thousands to millions of years

Stage 1 — Birthplace of stars

Protostar

The collapsing core heats up. Pressure climbs and an opaque, glowing protostar forms — nuclear fusion has not yet started at the core.

Energy Source: gravitational contraction · Timescale: ~10^5–10^6 years

Stage 2 — Stellar embryo

Main Sequence

Hydrogen fusion ignites in the core and the star shines steadily. Main sequence lifetime depends strongly on mass — low mass lives long, massive ones short.

Energy Source: hydrogen fusion (pp chain / CNO cycle) · Timescale: millions to billions of years

Stage 3 — Stable fuel burning

Red Giant

After hydrogen is exhausted in the core, fusion shifts outward. The envelope expands vastly and the surface cools—appearance becomes red and luminous.

Energy Source: shell fusion · Timescale: 10^6–10^8 years

Stage 4 — Expansion and cooling

Supernova

For massive stars, core collapse can cause an enormous explosion — producing heavy elements and briefly outshining entire galaxies.

Energy Source: core collapse / explosive nucleosynthesis · Timescale: seconds to months (visible)

Stage 5 — Cataclysmic end

Remnant

Leftovers of the explosion become a neutron star or black hole, while the expelled material seeds future generations of stars and planets.

Outcome: neutron star / black hole · Legacy: heavy elements seeded into space

Stage 6 — Aftermath & recycling