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A star's life history is determined almost entirely by its initial mass. Low-mass stars (like the Sun) spend billions of years on the main sequence, then expand into red giants as core hydrogen depletes, shed their outer layers as a planetary nebula, and leave behind a white dwarf. High-mass stars burn through their fuel in millions of years, expand into supergiants, and end in core-collapse supernova explosions that disperse heavy elements into the interstellar medium. A star's main-sequence lifetime scales as roughly mass divided by luminosity — since luminosity scales as mass to the ~3.5 power, massive stars live disproportionately shorter lives.
Trace evolutionary tracks on the HR diagram for stars of 0.5, 1, 5, and 10 solar masses. Compare the Sun's expected future (red giant → planetary nebula → white dwarf) with the high-mass pathway (supergiant → supernova → neutron star or black hole).
A star's fate is sealed at birth by a single number: its mass. Everything else — how long it lives, how it dies, what it leaves behind — follows almost inevitably from the initial mass. Understanding stellar evolution means tracing how the balance between gravity and pressure shifts as fusion fuel is consumed, and how each imbalance triggers the next stage.
On the main sequence, a star is in hydrostatic equilibrium: gravity pulling inward is exactly balanced by thermal pressure from fusion pushing outward. This phase lasts as long as core hydrogen supplies hold. For the Sun, that is about 10 billion years; for a 25-solar-mass star, only a few million. The reason is counterintuitive at first — more mass means more fuel, but luminosity scales as roughly mass to the 3.5 power, so massive stars are so much brighter that they consume their hydrogen at a ruinously fast rate.
When core hydrogen runs out, fusion stops in the core but continues in a surrounding shell. The core contracts under gravity, heats up, and the outer layers paradoxically expand and cool — the star becomes a red giant (or supergiant for massive stars). On the HR diagram, this corresponds to the star leaving the main sequence and moving rightward toward lower temperatures and higher luminosities. For the Sun, this red giant phase will occur in about 5 billion years, expanding to perhaps 100 times the Sun's current radius.
For low-mass stars (below ~8 solar masses), the story ends quietly. The helium core ignites briefly, carbon-oxygen accumulates, but core temperatures never get high enough to fuse carbon. The outer layers drift away as a beautiful planetary nebula — the term is an 18th-century misnomer, as it has nothing to do with planets — and the inert carbon-oxygen core remains as a white dwarf, slowly cooling over billions of years.
For massive stars, the story is far more violent. Successive shells of fusion ignite — helium, then carbon, neon, oxygen, silicon — each lasting a shorter time. When silicon fusion produces iron, the game ends: iron fusion consumes energy rather than releasing it. The iron core collapses in less than a second, bounces, and drives a shockwave outward in a core-collapse supernova explosion. The explosion disperses heavy elements synthesized in the star — the carbon, oxygen, and iron in your body were forged in stellar interiors and scattered by such explosions. What remains is a neutron star or, for the most massive progenitors, a black hole.