STARDUST
A star is a knot that gravity ties in a cold cloud.
Untying it makes everything you are.
THE COLD
It begins with almost nothing, kept very cold
A molecular cloud is the emptiest thing you will ever call dense. Ten to twenty kelvin. Mostly hydrogen paired into H2. Better vacuum than any laboratory on Earth has ever produced — and yet, compared to the space around it, it is a fog.
- TEMPERATURE
- 10–20K
- DENSITY
- 102–106cm−3
- SPAN
- ~100light-years
Cold is the whole point. Heat is motion, and motion resists gathering. Chill a cloud far enough and its own weight starts to matter more than its pressure. Cross that line — the Jeans criterion — and the cloud can no longer hold itself open. It begins, very slowly, to fall inward.
Something usually pushes it over the edge. A passing spiral arm. A blast of ultraviolet. Most often, the shockwave of a nearby star that has already died.
Every stellar nursery is downwind of a funeral.
COLLAPSE
Falling, and the spin that comes with it
Once it starts, collapse is fast — about a hundred thousand years for the core of a cloud to become a protostar. Astronomically, that is a blink.
The cloud was already turning, imperceptibly. As it shrinks, that rotation has nowhere to go: angular momentum is conserved, so the spin accelerates the way a skater's does when their arms come in. The infall flattens. What was a sphere becomes a disc with a hot lump at its centre.
Material that cannot fall inward is flung out along the one axis left open — the poles — as two opposed jets. And the disc that is feeding the new star is the same disc that will clump into planets. The star and its worlds are built from one act.
The centre is not yet a star. It is a falling weight that has begun to glow.
IGNITION
Ten million kelvin, and the falling stops
Compression heats. When the core passes roughly ten million kelvin, hydrogen nuclei begin to strike each other hard enough to stick. The star turns on.
Four protons become one helium nucleus. The helium weighs about 0.7% less than the four protons did. That missing fraction leaves as energy, on the exchange rate Einstein wrote down. In the Sun, six hundred million tonnes of hydrogen go through this every second, and four million tonnes of it simply stop being matter.
- CORE TEMP
- 15.7million K
- FUSED / SECOND
- 600Mt
- MASS → ENERGY
- 4Mt / s
Now a standoff begins. Gravity pulls in; radiation pushes out; they match, almost exactly, for as long as the fuel lasts. That balance — hydrostatic equilibrium — is not something a star does. It is what a star is.
THE LONG BURN
Mass decides everything
A star spends about ninety per cent of its existence here, on the main sequence, quietly converting hydrogen. How long that lasts is set almost entirely by one number — how much of it there is.
More mass means more weight bearing down on the core, which means a hotter core, which means a violently faster burn. Luminosity climbs roughly as the 3.5th power of mass. So a star ten times the Sun's mass carries ten times the fuel and spends it about three thousand times faster. It is rich and it is doomed.
BUILD A STAR1.00 M☉
Without JavaScript the star builder cannot run. The relationships it demonstrates: luminosity rises as roughly the 3.5th power of mass, so a 10 M☉ star shines about 3,000 times brighter than the Sun and lasts about 30 million years instead of ten billion. Below 0.5 M☉ a star outlives the present age of the universe; above about 8 M☉ it ends as a supernova.
Approximate main-sequence relations (L ∝ M3.5 below 20 M☉, softening to M2 above; R ∝ M0.7). Real stars vary with composition and age. The disc colour is computed from the surface temperature with the same blackbody function the background render uses.
THE SWELL
When the core runs dry, the star gets bigger
This is the counterintuitive part. Running out of fuel does not make a star shrink. It makes it enormous.
With no fusion holding it open, the core contracts — and contracting heats it further, until a shell of hydrogen around the dead core ignites. That shell burns harder than the core ever did. The outer envelope, pushed by the extra light and no longer tightly held, balloons outward and cools as it goes. Hotter core, cooler skin, and a star a hundred times wider than before: a red giant.
Inside, helium finally fuses. Three helium nuclei fuse into one carbon — the triple-alpha process. The carbon in your body is made of ash from this fire.
In about five billion years the Sun will do this. It will swallow Mercury, then Venus. Earth's exact fate is still argued over; none of the arguments end well.
THE FORGE
Everything on this chart was made somewhere
A massive star burns through a ladder of fuels, and each rung is shorter than the one below it. For a star of twenty-five solar masses, hydrogen lasts seven million years. Silicon lasts one day.
- HYDROGEN
- 7Myr
- HELIUM
- 700kyr
- CARBON
- 600yr
- OXYGEN
- 6months
- SILICON
- 1day
The ladder stops at iron. Iron-56 sits at the bottom of the nuclear energy well — the most tightly bound nucleus there is. Fusing anything into iron releases energy; fusing iron into anything costs it. A star that reaches iron has built the one thing it cannot burn.
So the elements had to come from different places. Hover or tap any square to see where it was made.
ORIGIN OF THE ELEMENTSZ = 1–92
Without JavaScript the periodic table cannot be drawn. In summary: hydrogen and helium came from the Big Bang; lithium, beryllium and boron from cosmic rays splitting heavier nuclei; carbon and nitrogen mostly from dying low-mass stars; oxygen through zinc from exploding massive stars; much of the iron from exploding white dwarfs; and gold, platinum and the other heaviest stable elements from merging neutron stars.
Each square shows the dominant source; most elements have more than one. Categories follow the standard nucleosynthesis picture popularised by Jennifer Johnson's origin-of-the-elements chart.
THE FLASH
One second, and the star is gone
The iron core grows until it passes about 1.4 solar masses — the Chandrasekhar limit — and the quantum pressure holding it up gives way.
What follows takes less than a second. The core falls inward at roughly a quarter of the speed of light, until it reaches the density of an atomic nucleus and can compress no further. Then it rebounds. The infalling star meets the rebound and detonates.
Almost all of the energy — around 99% — escapes as neutrinos, particles so unwilling to interact that they pass straight through the dying star. The explosion bright enough to briefly rival an entire galaxy is the leftover one per cent.
In 1987, a supernova went off in the Large Magellanic Cloud. Detectors on Earth caught its neutrinos about three hours before anyone saw the light.
In that second, the shockwave slams neutrons into nuclei faster than they can decay, and the periodic table is finished in the wreckage. Then everything the star ever made is thrown outward at ten thousand kilometres per second, into the cold, where it will eventually get cold enough to fall together again.
WHAT REMAINS
Two ways to leave a corpse
The core does not get thrown clear. What it becomes depends, again, on mass.
A neutron star, if the remnant is light enough. Around 1.4 solar masses crushed into a sphere about twenty kilometres across — protons and electrons pressed together into neutrons. A sugar-cube of it would weigh about a billion tonnes. Some spin hundreds of times a second; the fastest known turns 716 times every second. Jocelyn Bell Burnell found the first one in 1967, in the form of a pulse nobody could explain.
A black hole, if it is heavier. Past roughly two to three solar masses, nothing known can hold the core open, and it collapses without limit. Its horizon is not a surface — it is the distance at which leaving would require exceeding light. For the Sun's mass that distance would be about three kilometres. For Earth's, about nine millimetres.
- M87* IMAGED
- 2019
- SGR A* IMAGED
- 2022
- FIRST MERGER HEARD
- 2015
The bright circle behind this text is not decoration. Light that passes close enough to a black hole is bent into an orbit and then released — so you see the far side of the disc lifted over the top of something that emits nothing at all. The dark disc is about 2.6× wider than the horizon itself. On 14 September 2015 two of these merged, and the resulting ripple in spacetime stretched a four-kilometre detector on Earth by less than the width of a proton.
YOU
This already happened, and you are the receipt
The hydrogen in you is original — made in the first minutes after the Big Bang and never since. Almost everything else in your body was assembled inside a star and released when that star came apart.
By mass, a human body is about 65% oxygen, 18.5% carbon, 9.5% hydrogen, 3.2% nitrogen, 1.5% calcium, 1.0% phosphorus and 0.006% iron. Only the hydrogen predates the stars.
The oxygen in the breath you just took was forged in the core of a massive star. The calcium holding your skeleton rigid was made in the same place. The iron in your blood — the atom that binds the oxygen and carries it — came from a stellar explosion, or from two dead stars colliding.
Nothing here was made on Earth. Earth is only where it settled.
A cold cloud fell together, burned for ten billion years, swelled, and tore itself apart — and some of the debris is currently reading a sentence about itself.