Stars are not permanent. Every one of them switched on at some point, and every one of them will stop. What decides how a star ends is settled before it even starts shining — by how much gas it managed to gather. Nothing else about a star matters half as much.
A star is a ball of gas in an argument with itself, and the argument is what keeps it alive.
Gravity pulls every bit of the gas towards the middle. If gravity were the only thing acting, the whole ball would fall inwards and be over within hours. But squeezing gas makes it hot, and in the very middle it gets hot enough — around fifteen million degrees — that hydrogen atoms slam together hard enough to stick. That is fusion, and it releases enough energy to hold everything up.
So the star settles into a standoff: falling in, pushing out, neither one winning. A star is not a thing that burns. It is a collapse that has been stopped, and is being held open from the inside.
The whole life story below is one question asked over and over: when the fuel in the middle runs low and the pushing weakens, what does gravity do next? The answer depends entirely on how much there is to pull.
Stars begin somewhere unexpected: in the coldest, darkest places there are. A giant cloud of gas and dust sits in space at around −260 °C, doing nothing much for millions of years. Then something nudges it — a passing shockwave, or a nearby star exploding — and a patch of it becomes slightly denser than its surroundings.
That patch pulls in a little more gas, which makes it denser still, which pulls in more. Once that starts it does not stop. The cloud breaks into clumps, each clump collapses, each collapse heats up in the middle, and if a clump gathered enough material, its centre eventually reaches the temperature where fusion starts. The moment it does, the star switches on and blows away the leftovers.
You can look straight into this happening. The Orion Nebula, the Eagle Nebula and the Lagoon Nebula are clouds currently in the act of making stars, glowing because the first stars to switch on are lighting them from inside.
Because a whole cloud fragments at once, stars are born in batches, not one at a time. The Pleiades and the Beehive Cluster are litters of young stars still drifting apart. The Sun had brothers and sisters too. They scattered long ago and nobody knows which stars they are now.
Not all of the leftover gas gets blown off. Some settles into a flat disc going round the new star, and that disc is where planets come from. Around Beta Pictoris the disc is still there and still visibly clumping. That is how the Earth started.
Here is the spine of the whole page. Find the row your star belongs in, and read across.
The last row is not a birth mass at all — it is what happens when two stars that have already ended run into each other, much later. The numbers on the left are rough, and the boundaries between the rows are exactly the part that is still being argued about.
Below about eight per cent of the Sun's mass, a collapsing clump never squeezes its middle hard enough. It gets warm, it glows a dull red, and that is as far as it goes. This is a brown dwarf — not a star, not a planet, and it has nothing to look forward to except slowly cooling down for the rest of time. Luhman 16 is a pair of them, and they are among the closest things to the Sun of any kind.
Eight per cent of the Sun is still about eighty times the mass of Jupiter, which gives you a feel for how far Jupiter is from being a star. It is not a near miss.
Most stars in the galaxy are red dwarfs: small, dim, cool, and by far the most common thing there is. Proxima Centauri, the nearest star to the Sun, is one, and you cannot see it without a telescope. TRAPPIST-1 is another — barely wider than Jupiter, yet it holds seven Earth-sized planets.
A red dwarf is stingy with its fuel and stirs itself all the way through, so it can use practically all of it. That makes it last for trillions of years. Here is the striking part: the universe is about 13.8 billion years old, which is not even one per cent of a red dwarf's lifetime. So although we are fairly confident they will end as white dwarfs, that is a prediction, not an observation. Not one red dwarf anywhere has ever ended. They have all been alive since they were born, and every one of them still is.
The Sun is about halfway through its hydrogen. In roughly five billion years the middle will run out, and the standoff will tip. The core shrinks and gets hotter; the outside responds by swelling enormously. The Sun becomes a red giant, wide enough to swallow Mercury and Venus and to leave the Earth scorched and airless.
You can see one now: Arcturus, one of the brightest stars in the northern sky, has about the Sun's mass but is roughly twenty-five times as wide. It is the Sun's future, already happening, and it is bright orange because a puffed-up star has a cooler surface.
A red giant holds on loosely. Its outer layers drift off over a few tens of thousands of years, and the hot core left in the middle lights them up from inside. The result is one of the prettiest things in the sky and one of the worst-named: a planetary nebula, which has nothing whatever to do with planets. Somebody in the 1780s thought they looked like discs of planets through a small telescope, and the name stuck. The Ring Nebula, the Dumbbell and the Helix are three of them, and in a small telescope all three really are within reach.
The shell thins out and disappears within about twenty thousand years — a blink — and what remains is the bare core: a white dwarf. It has no fuel and makes no new energy. It is simply a very hot object with nothing left to do, cooling off. Sirius B is one, going round the brightest star in the night sky.
The word "small" does not carry this. Each panel below is drawn at a different zoom, and the zoom is written between them.
Everything in the last panel weighs more than everything in the first. A teaspoon of the material on the right would have the mass of a mountain.
A white dwarf on its own just cools quietly. But most stars have a companion, and a white dwarf with a neighbour can pull gas off it. You can watch this happening at SS Cygni, where the stolen gas piles up and flares every few weeks.
There is a hard limit to how much a white dwarf can hold up — about one and four-tenths times the Sun's mass. Feed it past that and it does not collapse gently. The whole object detonates at once, in a Type Ia supernova, and this is the one kind of stellar death that leaves nothing behind. No neutron star, no black hole. Just wreckage flying outwards and an empty space where a star used to be.
Both of the last two supernovae anybody saw with the naked eye in our own galaxy were this kind: Tycho's in 1572 and Kepler's in 1604. Tycho's was the one that broke the ancient belief that the heavens never change — a new star appeared, stayed put for months, and then faded. It has been over four hundred years since the last one, and we are overdue.
Because the limit is always the same number, every Type Ia explodes with almost exactly the same true brightness. So when astronomers spot one in a distant galaxy, how faint it looks tells them how far away that galaxy is. This is rung 6 of the distance ladder, and it is how the expansion of the universe was measured. A dying star that is always the same brightness turns out to be a ruler.
A star of eight Suns or more lives fast: bright, blue-white, and finished inside a few tens of millions of years while a red dwarf has hardly got going. When its hydrogen runs out it does not quit. It fuses the helium, then the carbon, then the oxygen, each stage hotter and quicker than the last — the final ones lasting days rather than years.
Then it makes iron, and stops dead. Fusing iron takes energy in instead of giving it out, so the moment the core is iron, the pushing-out stops completely. There is no warning and no slowdown. The core collapses in about a quarter of a second, the rest of the star falls in behind it, hits the now-solid centre and rebounds. The star blows itself apart, and for a few weeks outshines every other star in its galaxy put together.
SN 1987A is the closest one seen in modern times, and it did something remarkable: its neutrinos arrived at detectors on Earth three hours before its light did. They come straight out of the collapsing core, while the light has to fight its way out through the rest of the star.
What is left in the middle is a neutron star: the core, squeezed until the atoms themselves give way. RX J1856.5-3754 is the nearest known one. Many of them spin fast and sweep a beam of radio waves round like a lighthouse; catch the beam and you have a pulsar. The Crab Pulsar turns thirty times a second, and it sits in the middle of the Crab Nebula — the still expanding wreck of a star that Chinese astronomers watched explode in 1054 and recorded as a "guest star" bright enough to see in daylight.
Heavier still, and the core is too heavy even to hold itself up as a neutron star. Nothing known can stop it, and it keeps going: a black hole. The nearest one found so far is Gaia BH1, which gives itself away only because a perfectly ordinary star nearby is visibly orbiting something that is not there.
Some heavy stars may skip the fireworks entirely: the core collapses, and instead of bouncing, the whole star falls in after it. The star does not explode. It goes out.
N6946-BH1 is the best candidate anyone has. Astronomers had been photographing a nearby galaxy for years, keeping an eye on its biggest stars. One of them — about twenty-five times the Sun's mass — brightened a little in 2009, then faded, and by 2015 it was simply missing from the pictures. There was never an explosion.
At the very top of the scale, above roughly a hundred and thirty Suns, the core gets so hot that its light starts turning into pairs of particles. That sounds harmless and is not: the light was part of what held the star up, and turning it into matter takes the support away. The star contracts, ignites all at once, and is destroyed outright — no core, no remnant, nothing. These are rare now, but the very first stars in the universe were far heavier than anything forming today, and some of them are likely to have ended this way.
Neutron stars sometimes come in pairs, orbiting each other. Such a pair very slowly loses energy — it leaks away as ripples in space itself — so the orbit tightens over millions of years, then over centuries, then over minutes. At the end they are going round each other hundreds of times a second, and they merge.
This is a kilonova, and it is where a great deal of the gold in the world came from. Smashing neutron stars together makes elements that no ordinary star can: gold, platinum, uranium. It gets flung out into space and ends up, eventually, in planets and in rings and in wires.
One has been watched: GW170817, in 2017. It was felt before it was seen — detectors measured the ground stretching by less than the width of an atom's nucleus as the ripples passed through the Earth — and only then did telescopes turn and find the new point of light. It faded away in a fortnight.
Almost none of the atoms you are made of existed at the start of the universe. Hydrogen did, and helium. Everything else was made inside stars and then let go when they died.
All of that was floating about as gas and dust in a cloud, and then, four and a half billion years ago, part of that cloud collapsed and became the Sun and the Earth and eventually you. Being made of dead stars is not a poetic way of putting it. It is just where the atoms came from.
On a clear winter evening find Orion — three bright stars in a short straight row, with four more marking the corners around them. Let your eyes adjust for a few minutes, then look carefully at the colour of two of them:
Now find Aldebaran, the orange star up and to the right of Orion, marking the eye of Taurus the bull. Compare it with Betelgeuse. To your eye they are nearly the same colour.
But here is the catch, and it is the point of this whole page. Betelgeuse and Aldebaran look the same and are not the same. Aldebaran has about the Sun's mass and will end quietly as a white dwarf. Betelgeuse has something like sixteen times the Sun's mass and will end as a supernova — bright enough to cast shadows here. Colour tells you the surface. Only mass tells you the ending, and mass is not something you can see.