The Lives of Stars

The long, stable middle age of a star, during which it fuses hydrogen into helium in its core. This is where stars spend most of their lives — the Sun for roughly ten billion years — and a star's mass fixes where it sits along the main sequence, how brightly it burns, and how long it lasts.

A star born with more than roughly eight solar masses fuses ever-heavier elements until it builds an inert iron core. Iron cannot release energy by fusing, so once the core exceeds its stability limit it collapses in under a second, rebounds, and blows the star apart as a core-collapse supernova, leaving a neutron star or a black hole.

At the end of the asymptotic giant branch a dying star sheds its outer layers into space, exposing the hot stellar core beneath. The core's ultraviolet light lights up the expanding shell as a glowing planetary nebula — a name from the eighteenth century that has nothing to do with planets — before it fades to a white dwarf.

The stage between a protostar and a fully-fledged star. Still contracting and drawing energy from gravity rather than fusion, the young star — a T Tauri star at low mass, a Herbig Ae/Be star at higher mass — is wrapped in an accretion disc and drives energetic jets, until its core grows hot enough to ignite hydrogen.

Stars are born when the densest cores of a giant molecular cloud become unable to support themselves against their own gravity and collapse. As a core falls inward it heats, spins up into a disc, and grows a central protostar — a process that can be triggered by the shock of a nearby supernova or the squeeze of a spiral arm.

A star's rotation and convection together drive a magnetic dynamo, giving rise to starspots, flares, and hot outer atmospheres, often waxing and waning in activity cycles. The Sun is the star whose magnetic activity we can watch in closest detail, but the same physics operates across the cool stars.

Throughout their lives, and especially near the end, stars shed mass in winds — driven by radiation pressure on spectral lines in hot, luminous stars, and on dust grains in cool giants and supergiants. Mass loss reshapes a star's fate, stripping envelopes and returning enriched gas to the interstellar medium.

Stars spin, and how fast they spin shapes their lives. Rotation mixes fresh fuel into the core, flattens the star, powers magnetic dynamos, and drives mass loss; it can be measured from the broadening of spectral lines and, for the internal rotation hidden beneath the surface, from asteroseismology.

Late in a low- or intermediate-mass star's life, both a hydrogen shell and a helium shell burn around a carbon–oxygen core. Recurring thermal pulses dredge freshly-made elements to the surface and drive heavy mass loss, as the star climbs the asymptotic giant branch toward the ejection of its envelope.

In stars below about two solar masses, the helium core becomes so dense that it is held up by electron degeneracy pressure. When helium finally ignites, the degenerate gas cannot expand to regulate itself, so fusion runs away in a brief, violent flash — over in minutes, and hidden deep inside the star.

After helium ignites, a low-mass star settles into a stable phase of quiet core helium burning, sitting on the horizontal branch of the colour–magnitude diagram at roughly constant luminosity. Stars crossing the instability strip here pulsate as RR Lyrae variables.

When a star exhausts the hydrogen in its core, fusion moves to a shell around an inert helium core. The envelope swells enormously and cools to a red glow, and the star climbs the red-giant branch — brightening as it ascends until helium ignites at its tip.