Stellar Nurseries
Stars are born in giant molecular clouds — vast regions of cold gas and dust tens of light-years across. When a region of a cloud becomes dense enough, gravity overcomes gas pressure and the cloud collapses. The collapsing core heats up until hydrogen fusion ignites at about 10 million Kelvin, and a star is born. This process takes a few hundred thousand years for massive stars and millions of years for smaller ones.
The Main Sequence
Once hydrogen fusion begins, a star enters the main sequence — the longest and most stable phase of its life. Our Sun has been on the main sequence for 4.6 billion years and will remain there for about 5 billion more. During this phase, the star is in hydrostatic equilibrium: gravity pulling inward is exactly balanced by the pressure from nuclear fusion pushing outward.
Red Giants and Beyond
When a star exhausts the hydrogen in its core, the core contracts and heats up while the outer layers expand enormously, creating a red giant. For Sun-like stars, the core eventually becomes hot enough to fuse helium into carbon. More massive stars continue fusing heavier elements in concentric shells — carbon, neon, oxygen, silicon — until they build an iron core, at which point fusion can no longer sustain them.
Death: Dwarf, Star, or Hole
Low-mass stars gently shed their outer layers as planetary nebulae, leaving behind a white dwarf — an Earth-sized remnant supported by electron degeneracy pressure. Massive stars die violently in core-collapse supernovae, outshining entire galaxies for weeks. If the remnant core is below about 3 solar masses, it becomes a neutron star; above that threshold, nothing can halt the collapse and a black hole forms.