The lives of stars, sorted by mass

Foundations

Most stars do not explode

Ask what happens to a star at the end of its life and many people will say it explodes. A few do, spectacularly, and they matter enormously for what comes later. But the great majority of stars, including the Sun, will never explode. They fade, or they swell and gently shed their outer layers, and what is left cools for the rest of time. Which fate a star meets is decided by one number, fixed at its birth: its mass.

The rule

The previous lesson gave the reason. A more massive star needs a hotter core to hold up its greater weight, and fusion runs faster at higher temperature, far faster. A star of ten solar masses has ten times the Sun’s fuel and burns it several thousand times as fast. So the heaviest stars live a few million years, the Sun about ten billion, and the lightest stars far longer than the universe has existed. Sort stars by mass and you have sorted them by lifetime, by brightness, by colour, by the way they die and by what they leave behind.

The frugal majority

Most stars are smaller than the Sun: red dwarfs of a tenth to half a solar mass, cool and faint, burning hydrogen so slowly that they will last for trillions of years. The universe, at 13.8 billion years, is not nearly old enough for a single one of them to have died. When they do, in a future far beyond the cosmic calendar, they will simply fade, having fused nearly all their hydrogen without ever swelling into giants.

The Sun’s road

A star like the Sun spends ten billion years on the main sequence, fusing hydrogen in its core. When the core runs out of hydrogen, fusion moves to a shell around it, and the star’s outer layers swell enormously and cool: a red giant, a hundred times the Sun’s present size, its surface a dull orange-red. The core, meanwhile, contracts and heats until helium begins to fuse into carbon and oxygen. That buys a few hundred million years. Then the helium runs out too, the star swells again, larger still, and in its final hundred thousand years it puffs off its outer layers in a glowing shell that astronomers, for no good reason, call a planetary nebula.

What remains is the core: about six tenths of a solar mass, made of carbon and oxygen, squeezed into a ball the size of the Earth. It is not fusing anything. It shines only because it is hot, and it will cool, slowly, for tens of billions of years. A white dwarf. This is where the Sun is going, and most stars with it.

The Sun’s own timetable has been worked out. It is brightening by about ten percent every billion years, and in roughly a billion years it will be bright enough to boil the oceans away. In about five billion it becomes a red giant. Around 7.6 billion years from now, at its largest, it most likely swallows the Earth.

The limit, worked out on a ship

A white dwarf holds itself up not by heat but by a quantum effect: electrons refuse to be packed closer than a certain limit. In 1930 a nineteen-year-old from Madras, Subrahmanyan Chandrasekhar, on the ship taking him to Cambridge, worked out that this refusal has a breaking point. Above about 1.4 solar masses, gravity wins, and no white dwarf can exist. Eddington, then the most famous astronomer alive, ridiculed the result in public. Chandrasekhar was right, and the limit that bears his name decides which stars die quietly and which do not.

The stars that explode

A star of more than about eight solar masses does not stop at helium. Its core is heavy enough to keep contracting and heating, and it fuses carbon, then neon, then oxygen, then silicon, each stage faster than the last, building an onion of shells around a growing core of iron. Iron is the end of the line: fusing it consumes energy instead of releasing it. When the iron core passes the Chandrasekhar limit, in a matter of days after silicon burning begins, it collapses in less than a second from the size of the Earth to the size of a city. The collapse rebounds, and the rebound, driven by a flood of neutrinos, blows the rest of the star apart. For a few weeks the supernova outshines its entire galaxy. In 1987 a supernova in a neighbouring galaxy was seen with the naked eye, and detectors on Earth caught two dozen of its neutrinos, which had left the collapsing core hours before the light emerged from the star.

The blast scatters the elements the star made, and makes more in the explosion itself. That is the next chapter.

What is left

If the collapsed core is below about two solar masses, it settles as a neutron star: more than the Sun’s mass in a sphere twenty-four kilometres across, so dense that a teaspoon would weigh hundreds of millions of tonnes, spinning up to hundreds of times a second and sweeping a beam of radio waves across the sky. Jocelyn Bell found the first one in 1967 as a ticking signal in her radio data.

If the core is heavier still, nothing can stop the collapse. A black hole forms. The heaviest stars end this way, and in 2015 the LIGO detectors recorded the sound of two of them, of thirty-six and twenty-nine solar masses, spiralling together and merging, a billion light-years away.

Not alone

Most massive stars have companions, close enough to pull gas off each other, and that changes everything: a star can be stripped of its outer layers, or grow beyond what it was born with, or swallow its partner. A white dwarf with a companion can be fed until it crosses the Chandrasekhar limit and detonates completely, in an explosion so uniform from one case to the next that it can be used to measure distances across the universe. Those were the standard candles that revealed dark energy in lesson 1.7.

Deeper

Lifetimes

Main-sequence lifetime scales as the fuel divided by the rate at which it is burned, roughly mass over luminosity. With luminosity going as mass to the power 3.5, lifetime goes as mass to the power minus 2.5: a star of ten solar masses lives about a three-hundredth as long as the Sun, and a star of a tenth of a solar mass about three hundred times longer, with the fully convective mixing of red dwarfs extending this further, to trillions of years.

Post-main-sequence phases for a Sun-like star

When core hydrogen is exhausted, the inert helium core contracts and hydrogen burns in a shell around it. The shell’s energy inflates the envelope; the star ascends the red giant branch. In stars below about two solar masses the helium core becomes electron-degenerate and ignites explosively in a helium flash, unseen from outside, after which the star settles into stable core helium burning on the horizontal branch. When the core helium is spent, the star climbs the asymptotic giant branch with helium and hydrogen burning in shells, undergoes thermal pulses, and loses mass in a strong wind at rates up to a ten-thousandth of a solar mass per year. The exposed hot core ionises the ejected envelope, producing a planetary nebula for a few tens of thousands of years, and then cools as a white dwarf along a track that takes billions of years to reach a few thousand kelvin.

Degeneracy pressure and the limit

At white dwarf densities the electrons are degenerate: the Pauli exclusion principle forbids two from occupying the same quantum state, and squeezing them raises their momentum and hence their pressure, independent of temperature. For non-relativistic electrons this pressure rises steeply enough with density to support any mass, at ever smaller radius. Above about a solar mass the electrons become relativistic, the pressure rises more gently, and there is a maximum supportable mass. Chandrasekhar’s 1931 calculation gives it as about 1.4 solar masses. The same physics, with neutrons in place of electrons plus nuclear forces, gives a maximum mass for neutron stars of around two to two and a half solar masses; the exact value depends on the still-uncertain behaviour of matter at those densities.

Massive-star burning

Carbon burning at about 600 million kelvin lasts centuries; neon burning about a year; oxygen burning months; silicon burning about a day. The final iron core grows to about 1.4 solar masses within a day and collapses. During collapse, electrons are captured by protons, producing neutrons and neutrinos; the inner core reaches nuclear density and bounces; the shock stalls, and is thought to be revived by neutrino heating over a few hundred milliseconds, though the mechanism is still not fully reproduced in simulations. About 99 percent of the energy released, some 10⁴⁶ joules, leaves as neutrinos; the light we see is less than one percent.

Binaries

Roughly seven in ten massive stars will interact with a companion during their lives. Mass transfer can strip a star’s hydrogen envelope, producing helium stars and hydrogen-poor supernovae; it can spin up the accretor; it can lead to mergers. The binary black holes detected by LIGO must have formed through some such channel, either in isolated binaries or by dynamical capture in dense clusters. Stellar evolution is, for massive stars, mostly the evolution of pairs.

Frontier

Who explodes

Simulations of core collapse have struggled for decades to make the shock succeed. Three-dimensional models with detailed neutrino transport now explode in many cases, but not all, and the outcome depends sensitively on the star’s structure at collapse. There is growing evidence that some massive stars collapse directly to black holes with no visible explosion, including the observed disappearance of a red supergiant. Which stars explode, which vanish, and what sets the masses of the resulting neutron stars and black holes is active research.

The heaviest remnants

LIGO has now recorded black holes heavier than the theory of single-star evolution can easily make, in a mass range where pair-instability physics should leave a gap. Either they grew through earlier mergers, or the models of massive-star evolution need revising. Each new detection is a test.

The Sun’s last billion habitable years

The Sun’s brightening is certain; its exact effect on Earth’s climate over the next billion years depends on the carbon cycle, cloud feedbacks and the fate of the oceans, and estimates of when the planet becomes uninhabitable range from several hundred million to about a billion and a half years from now. A question for chapter nine.

Transcript and shot list

Target runtime about 7 minutes. Built as sort by mass: one number fixed at birth decides everything, so the fates run in order, from the stars that have never died to the ones that tear themselves apart. Shot types: ANIM is rendered from code with Manim, AI is a generated shot from Higgsfield with the shared style reference, REAL is agency imagery with credit, CAM is the narrator.

[00:00] S01 · REAL · The Crab Nebula, remnant of the supernova of 1054 (NASA, ESA, Hubble or Webb; confirm credit line).

Ask what happens to a star at the end of its life and most people will say it explodes. A few do. This one did, in the year 1054, and people across Asia wrote it down. But the great majority of stars, including the Sun, will never explode. They fade, or they swell and gently let go of their outer layers. Which fate a star meets is decided by one number, fixed at its birth. Its mass.

[00:36] S02 · ANIM · Cosmic calendar; the Sun’s birth at 1 September; a second calendar page for next year with a marker at 20 July labelled “the Sun dies”.

The Sun was born around the 1st of September. It dies around the 20th of July, next year, on the cosmic calendar. This lesson is about why, and what happens to everything else.

[00:51] S03 · ANIM · Stars on a mass scale from a tenth to a hundred Suns; a lifetime bar beside each: trillions of years, ten billion, twenty million, three million.

The rule comes from the last lesson. A heavier star needs a hotter core to hold up its weight, and fusion runs far faster when it’s hotter. Ten times the fuel, burned thousands of times as fast. So the heaviest stars live a few million years. The Sun, ten billion. And the lightest live far longer than the universe has existed.

[01:19] S04 · ANIM · A field of faint red dwarfs; a clock runs to 13.8 billion years and past it; none of them changes.

Most stars are those lightest ones: red dwarfs, a tenth to half the Sun’s mass, faint and cool and so frugal that they’ll last trillions of years. The universe isn’t remotely old enough for a single one to have died. Not one, ever. When they finally do, far beyond the end of the calendar, they’ll simply fade.

[01:45] S05 · ANIM · A Sun-like star: core hydrogen runs out; the star swells into a red giant a hundred times wider, dull orange; the core contracts and lights helium.

Now the Sun’s road. Ten billion years fusing hydrogen in its core. Then the core runs dry. Fusion moves to a shell around it, and the outer layers swell enormously and cool. A red giant: a hundred times the Sun’s present size, dull orange. Meanwhile the core contracts and heats until helium begins to fuse into carbon and oxygen. That buys a few hundred million years.

[02:16] S06 · ANIM · The giant puffs off its layers in a glowing shell; at the centre a small white point remains; the shell fades; the point dims slowly with a clock in billions of years.

Then the helium runs out too, the star swells again, and in its last hundred thousand years it puffs off its outer layers in a glowing shell. Astronomers call it a planetary nebula, for no good reason. What’s left is the core: six tenths of a solar mass, carbon and oxygen, squeezed into a ball the size of the Earth. Not fusing anything. Shining only because it’s hot, and cooling for tens of billions of years. A white dwarf. That’s where the Sun is going, and most stars with it.

[02:57] S07 · ANIM · Typographic: 1930, a ship, a nineteen-year-old with a notebook; a white dwarf with a mass dial that reads 1.4 and then the star collapses.

A white dwarf holds itself up by a quantum rule: electrons refuse to be packed closer than a certain limit. In 1930 a nineteen-year-old from Madras, Subrahmanyan Chandrasekhar, on the ship taking him to Cambridge, worked out that this refusal has a breaking point. Above 1.4 solar masses, gravity wins. Eddington, the most famous astronomer alive, mocked him in public for it. Chandrasekhar was right, and the limit that carries his name decides which stars die quietly and which don’t.

[03:34] S08 · ANIM · A massive star as an onion of burning shells; the iron core grows to 1.4 solar masses and collapses in a fraction of a second; the star blows apart; a galaxy is briefly outshone.

A star of more than about eight solar masses doesn’t stop at helium. Its core keeps contracting and heating, fusing carbon, then neon, oxygen, silicon, each stage faster than the last, building an onion of shells around a growing core of iron. Iron is the end of the line: fusing it costs energy instead of releasing it. When the iron core passes the Chandrasekhar limit, it collapses in less than a second from the size of the Earth to the size of a city. The collapse rebounds, driven by a flood of neutrinos, and blows the rest of the star apart. For a few weeks, the supernova outshines its whole galaxy.

[04:25] S09 · ANIM · 1987: a supernova in a neighbouring galaxy; a detector underground registers two dozen neutrino hits, hours before the light.

In 1987 one went off in a neighbouring galaxy, visible to the naked eye. Detectors on Earth caught two dozen of its neutrinos, which had left the collapsing core hours before the light fought its way out of the star. The explosion scatters the elements the star made, and makes more in the blast. That’s the next chapter.

[04:52] S10 · ANIM · The remnant: a neutron star twenty-four kilometres across beside a city map; it spins; a lighthouse beam sweeps; then a heavier core collapses to a black hole; two black holes spiral in with a gravitational-wave chirp.

What’s left? If the core is under about two solar masses, it settles as a neutron star: more than the Sun’s mass in a sphere twenty-four kilometres across, a teaspoon of it weighing hundreds of millions of tonnes, spinning up to hundreds of times a second and sweeping a radio beam across the sky. Jocelyn Bell found the first one in 1967 as a tick in her data. If the core is heavier still, nothing can stop it. A black hole. In 2015 the LIGO detectors heard two of them, thirty-six and twenty-nine solar masses, spiral together and merge, a billion light-years away.

[05:40] S11 · ANIM · A pair of stars exchanging gas; a white dwarf fed past 1.4 detonates; a row of identical explosions labelled “standard candles”.

One more thing. Most massive stars have companions close enough to pull gas off each other, and that changes everything. A white dwarf with a partner can be fed until it crosses the Chandrasekhar limit and detonates completely, in an explosion so uniform from one case to the next that it can measure distances across the universe. Those were the candles that revealed dark energy.

[06:10] S12 · ANIM · Labels: ESTABLISHED for the mass rule and the fates; ACTIVE RESEARCH for which stars explode and the heaviest black holes.

The scorecard. That mass decides a star’s life and death, and what each kind of star leaves behind: established, and confirmed by remnants we can see and neutrinos we’ve caught. Exactly which massive stars explode and which collapse silently, and how the heaviest black holes are made: active research.

[06:32] S13 · AI into ANIM · A sky of stars, each briefly labelled with its future: fade, white dwarf, supernova; the calendar returns.

Look up tonight and every star you see has a fate written in its mass. Most will fade. The Sun will swell and let go. A few, the brightest, will blow themselves apart, and everything heavier than helium in you came out of one that already did. Next lesson: where those stars live. Galaxies.

Shot list

One AI shot, eleven code-rendered shots, one agency image. Estimated Higgsfield use: 2 to 4 generations including retries.