Life Cycle of a Star

A star spends most of its life on the main sequence, steadily fusing hydrogen into helium in its core. How long it stays there depends on its mass. Bigger stars burn hotter and faster, so they use up their hydrogen sooner than smaller stars do.

When the hydrogen in the core finally runs out, the balance changes. The core shrinks and heats up, while the outer layers of the star swell and cool. The star grows into a red giant, much larger and redder than before. Inside the hotter core, helium can now fuse into even heavier elements like carbon. Our Sun will do this one day, billions of years from now.

Scientists track these changes with a tool called the H-R diagram, which plots stars by brightness and temperature. As a star ages, its point on the diagram moves, showing its journey from the main sequence to the red giant stage.

What happens after a star like the Sun becomes a red giant? Eventually the core runs low on helium fuel too. The star becomes unstable and gently pushes its outer layers out into space. Those glowing shells of gas are called a planetary nebula. The name is confusing, because it has nothing to do with planets. Early astronomers just thought the round puffs looked like planets through small telescopes.

What is left behind is the star's hot, dense core, called a white dwarf. It no longer makes energy by fusion. It simply glows with stored heat and slowly cools, like embers after a fire.

These late stages do something amazing. They create new chemical elements inside the star and then spread them into space. The carbon and oxygen made in old stars become part of new stars, new planets, and even living things.

Stars much heavier than the Sun live fast and die young. Their strong gravity squeezes their cores harder, so they burn through their fuel far more quickly. A massive star might live only a few million years, while the Sun gets about ten billion.

Because their cores get so hot, massive stars can fuse heavier and heavier elements. Hydrogen becomes helium, helium becomes carbon, and the chain continues through oxygen, neon, silicon, and finally iron. The star ends up layered like an onion, with a different element being fused in each shell.

Iron is the end of the line. Fusing iron does not release energy, so the star can no longer support itself. This is where the elements heavier than hydrogen and helium come from. Almost everything around you, from the calcium in your bones to the iron in your blood, was forged inside a star.

When a star with a mass less than about 1.4 times the Sun's reaches the end, it leaves behind a white dwarf. A white dwarf packs about the mass of the Sun into a ball roughly the size of Earth. A single teaspoon of its matter would weigh tons.

What keeps it from collapsing further? The electrons inside are packed so tightly that they push back with a special pressure, a bit like an invisible spring. This holds the star up even though no fusion is happening anymore.

Over time the white dwarf cools and fades. It grows dimmer and dimmer, like a dying campfire, until it becomes a cold dark cinder drifting through space. Every low-mass star, including our Sun far in the future, will end its story this way.

A massive star ends with a bang. Once its core fills with iron, fusion stops giving off energy, and nothing is left to hold the star up against its own gravity. The core collapses in less than a second, then rebounds in a gigantic explosion called a supernova. For a few weeks, one exploding star can outshine an entire galaxy.

The explosion blasts the star's outer layers, full of elements made during its life, out into space. The collapsing core left behind becomes one of two strange objects. If the core is small enough, it becomes a neutron star, a city-sized ball so dense that a spoonful would weigh billions of tons. If the core is larger, gravity wins completely and it becomes a black hole, where not even light can escape.

A supernova nearby would be dangerous, but luckily the stars close to us are not the kind that explode.