The Life Cycle of Stars

A star may seem the very image of permanence. The same points of light have guided travellers and inspired stories for as long as human beings have looked up at the night sky, and within a single lifetime they appear not to change at all. Yet stars are not eternal. Each one is born, shines for a long but limited time, and eventually dies, passing through a series of stages that astronomers have pieced together by studying the vast numbers of stars now visible at every point in their lives. A star's whole existence is a slow struggle between two opposing forces, and its history is written in how that struggle unfolds.

Stars are born in enormous clouds of gas and dust that drift between the existing stars of a galaxy. Where such a cloud is dense enough, its own gravity begins to pull it inward, and it slowly collapses. As the material falls together, the centre grows hotter and more tightly packed. If enough matter gathers, the core becomes so hot and so compressed that a new process begins: the nuclei of hydrogen atoms start to join together to form helium, releasing a tremendous amount of energy. This process, called nuclear fusion, is the moment a true star ignites, and it is the source of the light and heat the star will pour out for the rest of its life.

Once fusion has begun, a stable balance is established. The energy produced in the core pushes outward and tends to make the star expand, while gravity continues to pull all the material inward. For most of a star's life these two forces are almost exactly matched, and the star holds a steady size and brightness. Our own Sun is in this long, stable phase now, quietly converting hydrogen into helium in its core. A star spends the greatest part of its existence in this condition, which is why most of the stars we see in the sky are found in it.

The length of this stable period depends above all on the star's mass. It might be expected that the heaviest stars, having the most fuel, would last the longest, but in fact the opposite is true. A very massive star burns through its hydrogen at such a furious rate that it exhausts its supply in a comparatively short time, while a small, dim star uses its fuel so sparingly that it can shine steadily for an immense span. Mass, in other words, sets the pace of a star's life as well as its power.

When the hydrogen in the core at last runs low, the balance is broken and the star begins to change dramatically. The core contracts while the outer layers swell enormously, and the star becomes a red giant, far larger and cooler at its surface than before. What happens next depends once again on mass. A star about the size of the Sun will, in this late stage, gently shed its outer layers into space, leaving behind a small, dense, slowly cooling core known as a white dwarf. This ember gives out no new energy and fades over an extremely long period.

The most massive stars meet a far more violent end. After swelling into giants, their cores collapse suddenly and the star is torn apart in a colossal explosion called a supernova, which for a short time can outshine an entire galaxy. Such explosions are not merely spectacular; they are among the most important events in the universe, because they scatter into space the heavier chemical elements that were forged inside the star. The atoms of carbon, oxygen and iron found on Earth and in our own bodies were created in earlier generations of stars and spread abroad by these explosions.

What is left behind after a supernova is stranger still. The crushed core may become a neutron star, an object so dense that a spoonful of its material would weigh more than a mountain. If the original star was heavier yet, the collapse can go further, producing a region whose gravity is so overwhelming that not even light can escape from it. The great range of endings, from the quiet fading of a white dwarf to the formation of these extraordinary objects, all flows from the single factor of how much matter the star contained at birth.

Seen across their whole span, then, stars are not fixed lamps but slowly evolving bodies, each shaped by the contest between gravity pulling in and fusion pushing out. Their births seed galaxies with light, their deaths seed space with the raw material of planets and living things, and the recycled clouds go on to form new stars in turn. The night sky, far from being a changeless backdrop, is a record of this endless cycle caught at every one of its stages.