A star is a bomb that never goes off, held together by its own weight. This is the story of the balance, and of how it ends.
Work that out over the Sun's lifetime and you get about 6 × 1026 kg, which is roughly the mass of 100 Earths. The Sun has turned a hundred Earths' worth of matter into sunlight, and that is only about 0.03% of its total mass. It has barely started.
You already know a star is a ball of gas. This lesson is about the part most people never get told: why it doesn't fall in on itself, why it eventually has to, and how the way it dies depends almost entirely on one number.
In the core, four hydrogen nuclei become one helium nucleus. The helium weighs 0.7% less than the four hydrogens did. That missing 0.7% is the energy. This is why mass is fuel: for a star, a kilogram of hydrogen is worth about 6 × 1014 joules.
The light made in the core does not fly straight out. It is absorbed and re-emitted billions of times, wandering in random directions like someone lost in a crowd, through gas so dense that light can barely get anywhere. Estimates of how long one packet of energy takes to escape range from about 10,000 to 170,000 years. Then it crosses space to us in 8 minutes 20 seconds. The sunlight on your face left the core before humans had cities.
| Birth mass | Lifespan | Ending | What is left |
|---|---|---|---|
| Under about 0.5 M☉ red dwarfs | Trillions of years | Fade slowly | A helium white dwarf. The universe is only 13.8 billion years old, so no red dwarf has ever died. |
| 0.5 to about 8 M☉ includes the Sun | Billions of years | Swell into a red giant, blow off the outer layers as a glowing shell of gas (a planetary nebula) | A white dwarf: roughly the Sun's mass in an Earth-sized ball. |
| About 8 to 20 M☉ | Millions of years | Core collapse supernova | A neutron star. |
| Above about 20 M☉ | A few million years | Supernova, or sometimes a quiet direct collapse | A black hole (next lesson). |
These boundaries are blurry. Rotation, chemical makeup and whether the star has a companion all move the lines, and astronomers are still arguing about exactly where they fall. Treat the numbers as a map, not a law.
The Sun will not explode. In about one billion years it will be bright enough to boil Earth's oceans. In about 5 billion years it will swell into a red giant that swallows Mercury and Venus, and Earth's fate is uncertain. Then it will shed its outer layers and settle as a white dwarf.
A heavy star does not stop at helium. When one fuel runs out, the core contracts, heats up, and starts fusing the ash into something heavier. It builds up like an onion: hydrogen on the outside, then helium, carbon, neon, oxygen, silicon, and an iron core in the centre.
Each step is faster than the last. Iron is where it stops. Fusing iron does not release energy, it costs energy. The core has no more heat to make, so nothing holds up the weight above it. An iron core with about 1.4 times the Sun's mass (the Chandrasekhar limit) falls from roughly Earth-sized to about 20 km across in less than a second. The collapse stops when the core becomes a ball of neutrons, the infalling outer layers bounce off it, and the star explodes.
About 99% of the energy of that explosion leaves as neutrinos, ghostly particles that pass through almost everything. On 23 February 1987, detectors on Earth caught about two dozen of them from Supernova 1987A, a star 168,000 light-years away in the Large Magellanic Cloud. The neutrinos arrived about three hours before the light, because the neutrinos escape the core instantly but the blast wave needs hours to reach the star's surface.
| Remnant | Size | One teaspoon weighs |
|---|---|---|
| White dwarf | About Earth-sized | About 5 tonnes |
| Neutron star | About 20 km across, 1.4–2 M☉ | Billions of tonnes |
A neutron star is a single giant atomic nucleus the size of a city. Many spin, and their beams sweep across space like lighthouses. In 1967 a graduate student, Jocelyn Bell Burnell, found a signal pulsing every 1.337 seconds so regularly that the team half-jokingly labelled it LGM-1, for Little Green Men. It was the first pulsar. The fastest known spins about 716 times a second, with a surface moving at a large fraction of the speed of light.
The Big Bang made almost only hydrogen and helium. Everything else was cooked, in stars, and thrown into space when they died. Tap an element to see where it likely came from.
Drag the mass. Everything else (brightness, colour, lifetime, ending) follows from it. The curve is the main sequence: the long, steady stage of a star's life when it is fusing hydrogen, where stars spend most of their lives. Formulas are textbook approximations, good to a factor of two or so.
A star about 25 times the Sun's mass. Press Next fuel to step through its life. Bar lengths are on a log scale because the times are so different.
Five questions. Wrong answers explain themselves, so read them.
Explain this in your own words, as if to a friend who knows nothing: why doesn't the Sun collapse, and why doesn't it explode? Use the words gravity, pressure, fusion and balance. Type it, or just say it aloud, then compare with the Hook page.