Carbon made in stars

Section 1 of 4

Every carbon atom in your body was made inside a star. Stars build carbon out of helium, three nuclei at a time.

The Big Bang, 13.8 billion years ago, filled the universe with hydrogen and helium, the two lightest elements, and a trace of lithium. Every heavier atom, carbon included, has been made since then, in stars.

A star shines by joining small nuclei into bigger ones: nuclear fusion. For most of its life a star fuses hydrogen into helium in its core, as the Sun's core, at about 15 million °C, is doing now. When the hydrogen in the core runs low, the core shrinks and grows hotter, the outer layers swell, and the star becomes a red giant. At about 100 million °C, the helium starts to fuse.

Three helium nuclei make one carbon

A helium-4 nucleus holds 2 protons and 2 neutrons. Two of them can join into beryllium-8, with 4 of each, but beryllium-8 falls apart again in about 10⁻¹⁶ seconds. In the crowded core of a red giant, a third helium nucleus sometimes hits it within that moment, and the three make carbon-12: 6 protons and 6 neutrons, the nucleus of the carbon atom you met at the very start of the course. Physicists call a helium-4 nucleus an alpha particle, so this is the triple-alpha process.

Three helium nuclei join to make a carbon nucleus Two rows. Row 1: two helium-4 nuclei, each 2 protons and 2 neutrons, join into beryllium-8, drawn as the two helium nuclei side by side. Beryllium-8 lasts only about 10 to the minus 16 seconds before it falls apart. Row 2: a beryllium-8 nucleus and a third helium-4 nucleus join into carbon-12, a nucleus of 6 protons and 6 neutrons. 1. Two helium nuclei join + + + + + + + + + helium-4 helium-4 beryllium-8 lasts 10⁻¹⁶ s 2. A third joins in time + + + + + + + + + + + + + beryllium-8 helium-4 carbon-12
The triple-alpha process. Two helium-4 nuclei join into beryllium-8, which falls apart almost at once unless a third helium-4 nucleus hits it first. The three together make carbon-12. Protons carry a + sign.

Three helium-4 nuclei weigh a little more than one carbon-12 nucleus. The missing mass, less than a tenth of 1%, comes out as energy, which keeps the star shining. Some of the new carbon catches a fourth helium nucleus and becomes oxygen-16, with 8 protons and 8 neutrons.

Spreading the carbon

Stars up to about eight times the Sun's mass end as red giants and blow their outer layers away into space. In many of them, carbon stirred up from deep inside goes too. The gas round the dying star glows for a few tens of thousands of years as a planetary nebula, and the core cools into a small, dense white dwarf. Bigger stars end in huge explosions, supernovae. The gas they all give back drifts between the stars, and new stars and planets form from it. The Sun and Earth formed in this way about 4.6 billion years ago.

Where the main atoms in your body were made
ElementShare of your massMade in
oxygenabout 65%big stars, spread when they explode
carbonabout 18%red giants and exploding big stars
hydrogenabout 10%the Big Bang
nitrogenabout 3%red giants

Quick check

How many helium-4 nuclei join to make one carbon-12 nucleus?