The short answer
Stars form when gravity compresses a cloud of gas and dust until the core is hot enough to fuse hydrogen. They spend most of their lives on the main sequence, then expand, shed mass, and leave behind a compact remnant. What that remnant is depends almost entirely on how massive the star was when it formed.
Stage 1 — Stellar nursery
Stars are born inside molecular clouds — vast, cold regions of gas (mostly hydrogen and helium) and dust. A gravitational disturbance — perhaps the shockwave from a nearby supernova — causes part of the cloud to collapse. As the gas falls inward, it heats up and spins faster. The result is a protostar surrounded by a rotating disc of material. The ESA Herschel space observatory mapped stellar nurseries across the Milky Way.
Stage 2 — Main sequence (the long middle)
Once the core temperature reaches about 10 million K, nuclear fusion ignites and the star joins the main sequence. Fusion converts hydrogen to helium, releasing energy that provides the outward pressure to balance gravity. This equilibrium — called hydrostatic equilibrium — is remarkably stable. The Sun has maintained it for 4.6 billion years and has roughly another 5 billion left. Massive stars burn much hotter and exhaust their fuel in millions, not billions, of years.
Stage 3 — Red giant or red supergiant
When the hydrogen in the core is exhausted, fusion stops and gravity wins temporarily. The core contracts and heats; a shell of hydrogen outside the core ignites. This extra energy causes the outer layers to expand enormously. For a Sun-like star, the result is a red giant up to 200 times the Sun's current diameter. For very massive stars, the result is a red supergiant — Betelgeuse in Orion is one, large enough to engulf Jupiter's orbit.
Stage 4 — What happens next (mass decides everything)
Low-mass stars (like the Sun)
The red giant sheds its outer layers as a glowing planetary nebula. The remaining core — a white dwarf the size of Earth but with roughly half the Sun's mass — slowly cools over billions of years. It cannot sustain fusion.
High-mass stars (>8 solar masses)
The core fuses heavier and heavier elements (carbon, oxygen, neon, silicon) until it reaches iron — which cannot release energy by fusion. The core collapses in milliseconds, triggering a supernova explosion. The remnant is either a neutron star or, if massive enough, a black hole. NASA Chandra studies these remnants in X-ray light.
The importance of stellar recycling
When stars die they return enriched material — carbon, oxygen, iron, and heavier elements forged in supernova explosions — to the interstellar medium. This material eventually forms new molecular clouds, new stars, and new planets. Every atom of carbon in your body was forged inside a star that died before the Sun was born. The ESO's research on stellar nucleosynthesis traces exactly which stars produce which elements.