The Life of a Star

Every star is a battle between two forces. Gravity pulls inward, trying to crush the star into the smallest possible ball. The heat of nuclear fusion pushes outward. For most of a star's life the two are balanced, and the star shines steadily. When the fuel runs low, gravity wins, and what happens next depends entirely on how much the star weighs.

Birth in a cold cloud

Stars form in enormous clouds of gas and dust, mostly hydrogen, drifting between the stars of a galaxy. These clouds are bitterly cold, only ten or twenty degrees above absolute zero, and that is exactly why they can collapse: cold gas has little pressure to resist gravity. A passing shock wave, perhaps from a nearby exploding star, nudges a clump into collapsing. As it shrinks it heats up, and after a few hundred thousand years a hot dense core, a protostar, sits at the centre.

When the core reaches about 10 million degrees, hydrogen nuclei begin to fuse into helium, releasing energy. The newborn star lights up and blows away the remaining gas around it. A single cloud usually makes hundreds of stars at once, which is why young stars are found in clusters.

The long middle age

A star spends about 90 percent of its life fusing hydrogen in its core. Astronomers call this the main sequence. How long it lasts depends on mass, and in a way that surprises most people: heavier stars die younger. A star of twenty solar masses has twenty times more fuel, but it burns that fuel thousands of times faster, because its core is squeezed to far higher temperatures. It lasts perhaps ten million years. The Sun, a middling star, will manage about ten billion. A red dwarf one tenth the Sun's mass sips its hydrogen so slowly that it will shine for trillions of years, longer than the universe has so far existed.

The Sun turns about 600 million tonnes of hydrogen into helium every second. The mass that goes missing in that process, around four million tonnes a second, becomes the sunlight that warms the Earth.

Swelling into a giant

When the hydrogen in the core runs out, the core contracts and heats, and a shell of hydrogen around it begins to burn furiously. The outer layers balloon outward and cool, turning the star into a red giant. When the Sun does this, in about five billion years, it will swell past the orbit of Mercury and Venus and roast the Earth.

Inside the giant, the core becomes hot enough to fuse helium into carbon and oxygen. For a star like the Sun that is the end of the line. It cannot squeeze its core hard enough to go further. Eventually the outer layers drift away into space as a glowing shell called a planetary nebula, and the exposed core, a white dwarf the size of the Earth but half the mass of the Sun, slowly cools for eternity.

The death of giants

Stars more than about eight times the Sun's mass have a far more dramatic ending. Their cores are hot enough to keep fusing: carbon into neon, neon into oxygen, oxygen into silicon, silicon into iron. Each stage burns faster than the last. The silicon stage lasts about a day.

Iron is the end. Fusing iron absorbs energy rather than releasing it, so when the core turns to iron the outward push vanishes. In less than a second the core collapses from the size of the Earth to a ball twenty kilometres across. The collapse rebounds and tears the rest of the star apart in a supernova, one of the most violent events in nature, briefly outshining a hundred billion stars. The explosion forges gold, silver, uranium and every element heavier than iron, and flings them into space.

What remains is either a neutron star, a city-sized sphere so dense that a teaspoon of it would weigh a billion tonnes, or, if the original star was heavy enough, a black hole.

Why it matters

The early universe made only hydrogen, helium and a little lithium. Everything else, the carbon in your cells, the oxygen you breathe, the calcium in your bones and the iron in your blood, was built inside stars and scattered by their deaths. The cloud that collapsed to form the Sun and Earth 4.6 billion years ago was already enriched by generations of stars that had lived and died before. You are, quite literally, made of star dust.