Look up on a clear night and the stars seem eternal — the same fixed points our great-great-grandparents saw, and their great-great-grandparents before them. It's easy to imagine they have simply always been there and always will be. They haven't, and they won't.
A star is born, it lives for a long, steady middle age, and then it dies — sometimes quietly, sometimes in the most violent explosion in the Universe. Our own Sun is a star, roughly halfway through its life. This page follows a star from a cold cloud of dust all the way to its dramatic end, and shows why the ending depends on just one thing: how much stuff the star is made of — its mass.
Space between the stars is not completely empty. Drifting through it are enormous, cold clouds of dust and gas — mostly hydrogen, the lightest and most common stuff in the Universe. A cloud like this is called a nebula, and a single one can be light-years across.
Left alone, the cloud's own
The protostar keeps collapsing, and its core keeps getting hotter and denser — millions of degrees hot. At a critical point, something switches on that changes everything: nuclear fusion. The hydrogen nuclei in the core are crushed so hard that they begin to join together to make helium:
Each time this happens, a tiny sliver of mass is turned into a huge amount of
energy — this is Einstein's famous
Why does a main sequence star stay the same size for billions of years instead of collapsing or blowing apart? Because of a beautiful tug-of-war between two forces pointing opposite ways:
When these two are exactly balanced, the star holds a steady size — neither shrinking nor swelling. This calm balance is what makes the main sequence such a long, stable chapter: as long as there is hydrogen fuel in the core to keep fusion pushing out, gravity is held off, and the star shines quietly on. The bigger the star, the fiercer it burns its fuel — so, oddly, the most massive stars have the shortest lives.
No fuel lasts forever. After billions of years the core's hydrogen starts to run out, and fusion there falters. Now gravity begins to win, and the core squeezes inward again. But this fresh squeeze heats the star's outer layers, and — surprisingly — the star swells up enormously, ballooning to many times its old size. As it grows its surface spreads out and cools, glowing a deep orange-red.
A swollen, cooling, late-life star like this is a red giant. If the star was very massive to begin with, it puffs up even more, into a red super-giant — so vast that, if it sat where our Sun sits, it could swallow the inner planets whole. When our own Sun reaches this stage, in about five billion years, it will grow large enough to scorch or engulf the Earth.
Here the story splits in two, and which path a star takes depends entirely on its mass.
A Sun-like star (low mass). It hasn't enough gravity for a violent end. Instead it gently puffs its outer layers off into space, forming a glowing shell called a planetary nebula (a misleading name — it has nothing to do with planets). Left behind is the hot, dead core: a tiny, incredibly dense ember called a white dwarf, no bigger than the Earth. With no fusion left, it simply cools and fades over billions of years.
A massive star (high mass). Its enormous gravity crushes the core far harder, until it collapses in a fraction of a second and rebounds in a titanic explosion — a supernova, briefly outshining an entire galaxy. What remains of the crushed core depends on how heavy it was: a city-sized ball of pure crushed matter called a neutron star (a teaspoon of it would weigh billions of tonnes), or — if the core is heavier still — a point of gravity so strong not even light can escape: a black hole.
Choose a Sun-like star or a massive one, then step through the stages and watch the star change. Notice how both paths share the same beginning — nebula, protostar, main sequence — and only split apart at the end. Watch the star swell into a giant, then either puff away into a white dwarf or blow itself apart in a supernova.
When the Universe began, it held almost nothing but the lightest elements — hydrogen and helium. So where did everything else come from: the carbon in your pencil, the oxygen you breathe, the iron in your blood, the calcium in your bones, the gold in a ring?
The answer is that they were forged inside stars. Fusion in a star's core builds heavier elements out of lighter ones, and a dying star — especially in a supernova — scatters these new elements across space. That enriched dust drifts into new nebulae, which collapse into new stars and planets. Our Solar System, the Earth, and every atom in your body are built from the ashes of stars that lived and died long before the Sun was born. As the astronomer Carl Sagan put it, "we are made of star stuff."
These three trip nearly everyone up:
No need to panic. The Sun is a middle-aged main sequence star, about 4.6 billion years old, and it has roughly 5 billion years of steady hydrogen-burning still to go — so it's a little past halfway through its calm life. Only then will it swell into a red giant, puff off a planetary nebula, and settle down as a slowly cooling white dwarf.
Five billion years is an almost unimaginable stretch of time: it is longer than the Earth has already existed. Life on Earth will face other troubles long before then — but the Sun itself will keep shining, steady and bright, for an age far beyond anything human.