In the nineteenth century, the physicist Lord Kelvin calculated how long the Sun could shine if its energy came from slowly shrinking under its own gravity. His answer, a few tens of millions of years, alarmed geologists and followers of Darwin, who needed hundreds of millions of years for rocks to form and life to evolve. Kelvin's physics was sound but incomplete. The Sun runs on a source of energy no one in his time knew about, and finding it unlocked the life story of every star.

What powers a star

Deep in the Sun's core, at about 15 million degrees Celsius, hydrogen nuclei crash together hard enough to fuse into helium. In 1920 the astronomer Arthur Eddington suggested that this process could power the stars, and in 1939 the physicist Hans Bethe worked out the detailed nuclear reactions. Each helium nucleus weighs slightly less than the hydrogen that made it, and the missing mass becomes energy according to Einstein's famous E = mc². The Sun converts about four million tonnes of matter into energy every second, and it has enough hydrogen to keep doing so for billions of years. The geologists had been right: the Sun is old enough for evolution after all.

Sorting the stars

At the Harvard College Observatory around 1900, a team of women, hired as "computers" and paid far less than men, examined hundreds of thousands of stellar spectra on glass photographic plates. Annie Jump Cannon classified around 350,000 stars and created the system still used today, which sorts stars by temperature into the classes O, B, A, F, G, K and M, from hottest to coolest. Our Sun is a middling G star.

In 1925 Cecilia Payne used the new physics of atoms to show in her doctoral thesis that stars are made overwhelmingly of hydrogen and helium, not of the same mix as Earth, as most astronomers assumed. The leading astronomer Henry Norris Russell persuaded her to describe the result as almost certainly not real. Four years later he reached the same conclusion himself, and Payne's finding became a foundation of astrophysics.

Between 1911 and 1913 Ejnar Hertzsprung and Henry Norris Russell independently plotted stars by brightness and temperature. Instead of a random scatter, most stars fell along a diagonal band called the main sequence, with smaller groups of giant stars and faint white dwarfs elsewhere. This Hertzsprung-Russell diagram became the astronomer's map of stellar life, because a star's place on it reveals its stage of life.

How stars live and die

The most important fact about a star is its mass. Massive stars burn hot, bright and blue and use up their fuel in a few million years. Small red stars sip their fuel so slowly that they can shine for trillions of years.

A star like the Sun spends about 10 billion years on the main sequence. When the hydrogen in its core runs low, it swells into a red giant, puffs off its outer layers as a glowing shell of gas, and leaves behind a white dwarf, a hot core about the size of Earth that slowly cools.

Stars more than about eight times the Sun's mass end violently. When their core can fuse no further, it collapses in a fraction of a second, and the star explodes as a supernova, briefly outshining an entire galaxy. Chinese astronomers recorded a "guest star" in 1054 that was visible in daylight for weeks. Today we see its remains as the Crab Nebula. The collapsed core becomes a neutron star, about the width of a city yet heavier than the Sun, so dense that a teaspoon of it would weigh billions of tonnes. In 1967 the graduate student Jocelyn Bell Burnell noticed a strange, perfectly regular radio pulse, jokingly labeled LGM-1 for "little green men." It turned out to be a spinning neutron star, a pulsar. Her supervisor shared the 1974 Nobel Prize for the discovery; she did not. If the collapsing core is heavy enough, nothing can stop it, and it becomes a black hole, a region where gravity is so strong that not even light escapes.

We are made of star stuff

The Big Bang made almost nothing but hydrogen and helium. Nearly every other element was made later, inside stars. Fusion in stellar cores builds carbon, oxygen and elements up to iron. Supernovae and the collisions of neutron stars forge heavier ones, such as gold and uranium, and scatter them into space, where they join new clouds that form new stars and planets. The carbon in your cells, the oxygen you breathe and the iron in your blood were made in stars that died before the Sun was born. Carl Sagan liked to sum this up by saying that our bodies are built from material forged in stars.

Worked example: the future of the Sun

Use what you have learned to tell the Sun's life story.

• Now: the Sun is about 4.6 billion years old, roughly halfway through its 10 billion years on the main sequence.
• Too small to explode: it has nowhere near eight times its own mass, so it will never become a supernova or a black hole.
• A slow brightening: as it ages, the Sun grows gradually brighter. In about a billion years it will likely be hot enough to boil away Earth's oceans.
• Red giant: in about five billion years it will swell enormously, swallowing Mercury and Venus and perhaps Earth.
• White dwarf: finally it will shed its outer layers and fade as an Earth-sized ember.

Astronomers see stars at every stage, much as you could learn the human life cycle from one afternoon in a crowded park, and the Sun fits the pattern.