The life cycle of a star
From nebula to main sequence to red giant β then white dwarf, neutron star or black hole.
A star's life starts as a nebula β a vast cloud of hydrogen gas and dust. Gravity pulls the material together; the centre heats up; once temperature and pressure get high enough, hydrogen nuclei fuse into helium and a new star ignites.
For most of its life (millions to billions of years) a star sits on the main sequence, balancing inward gravity against outward radiation pressure from fusion. Our Sun is currently a main-sequence star and has been for ~4.6 billion years.
When hydrogen runs out:
Low/medium-mass stars (like the Sun):
- Hydrogen ends β core contracts β outer layers expand β red giant.
- Outer layers drift off as a glowing planetary nebula.
- Core left behind is a dense, hot white dwarf β slowly cooling over billions of years.
High-mass stars (>~8 solar masses):
- Hydrogen ends β bigger and brighter β red supergiant.
- Fusion of heavier elements continues for a short time.
- Iron is the end of the line β fusion stops; core collapses; outer layers blast off in a supernova.
- What's left: a tiny neutron star (~20 km wide, made entirely of neutrons), or β for the most massive β a black hole.
- Stars form from nebulae collapsing under gravity.
- Main sequence: hydrogen fusion β helium.
- Low-mass end: red giant β planetary nebula β white dwarf.
- High-mass end: red supergiant β supernova β neutron star OR black hole.