Stellar Evolution

Stars form in $\text{nebulae}$, vast clouds of hydrogen and dust. Gravity pulls denser regions together, and as the gas falls inwards its gravitational potential energy becomes kinetic energy, heating it. The collapsing ball is a $\text{protostar}$.

When the core reaches about $10^7$ K, particles move fast enough to overcome electrostatic repulsion and hydrogen nuclei fuse into helium. The energy released creates outward pressure.

Once outward pressure balances gravity, the star is stable and on the $\text{main sequence}$. It stays there for most of its life, fusing hydrogen in its core.

More massive stars have stronger gravity, so their cores are hotter and fuse far faster. A star ten times the Sun’s mass burns through its hydrogen in a few tens of millions of years; the Sun lasts about ten billion.

When core hydrogen runs out, fusion there stops and outward pressure falls. The core contracts and heats, which ignites hydrogen fusion in a shell around it. The extra energy makes the outer layers expand and cool: the star becomes a $\text{red giant}$.

The core eventually becomes hot enough to fuse helium into carbon and oxygen. When that fuel is also exhausted, a Sun-sized star is not massive enough to heat its core further.

The outer layers drift away as a $\text{planetary nebula}$, leaving the hot, dense core as a $\text{white dwarf}$ — about the size of Earth. Fusion has stopped; it is supported against gravity by electron degeneracy pressure and slowly cools.

Stars above about eight solar masses become $\text{red supergiants}$. Their cores are hot enough to fuse successively heavier elements, building up layers like an onion, until the core is iron.

Fusing iron releases no energy, so the core suddenly loses its support and collapses in under a second. The infalling outer layers rebound in a $\text{supernova}$, briefly outshining an entire galaxy and scattering heavy elements into space.

If the remaining core is between about 1.4 and 3 solar masses, electrons and protons are squeezed into neutrons, forming an extremely dense $\text{neutron star}$. Above that, nothing can halt the collapse and a $\text{black hole}$ forms, with gravity so strong that not even light escapes.

The H–R diagram plots luminosity (often relative to the Sun, on a logarithmic scale) against surface temperature. By convention, $\text{temperature increases to the left}$, from about 3000 K to 30 000 K or more.

The $\text{main sequence}$ is a diagonal band from hot, bright stars at top left to cool, dim stars at bottom right. $\text{Red giants}$ and $\text{supergiants}$ lie above it on the right — cool but luminous, because they are enormous. $\text{White dwarfs}$ sit below it on the left — hot but dim, because they are tiny.

As the Sun evolves it will leave the main sequence, move up and right to the red giant region, then drop down and left to become a white dwarf.