Stellar evolution and compact objects

For as long as a star is on the main sequence, it is in a stable balance: gravity pulls every layer of the star inward, and the outward pressure from nuclear fusion in the core (hydrogen fusing into helium) pushes back with exactly enough force to hold it up. That balance is what makes stars stable for millions to trillions of years, but it isn't permanent — a star's luminosity depends extremely steeply on its mass (roughly as mass to the third or fourth power), so a star with ten times the Sun's mass shines thousands of times brighter and burns through its proportionally larger fuel supply far faster. Counter to intuition, the most massive stars have the shortest lives, sometimes only a few million years, while a star like the Sun lasts about ten billion.

Once a star's core hydrogen runs out, the pressure that was holding the core up disappears, and gravity briefly wins: the core contracts and heats up until it becomes hot enough to ignite the next available fuel — helium fusing into carbon and oxygen, and for massive enough stars, successively heavier elements after that. Each ignition swells the outer layers of the star into a giant, tracing a path up and to the right on the Hertzsprung-Russell diagram, away from the main sequence. This cycle of exhaustion, collapse, and re-ignition can only continue as long as there is a next fuel to burn and enough gravity to compress the core hot enough to light it.

What is left behind when fusion finally cannot resupply enough pressure depends on the star's initial mass. A Sun-like star ends as a white dwarf: an Earth-sized, extremely dense ember held up not by fusion but by electron degeneracy pressure — a quantum-mechanical effect with nothing to do with temperature. A more massive star's core can collapse further, past the point electron degeneracy pressure can resist, crushing protons and electrons together into neutrons and forming a neutron star just a few kilometres across but as massive as the Sun, often triggering a supernova that blows off the rest of the star. If the collapsing core is still massive enough, not even neutron degeneracy pressure can stop it, and it collapses all the way to a black hole — a region of spacetime whose gravity is unremarkable at a distance, but so intense within a certain radius (the event horizon) that not even light can escape.