How stars make elements — Astronomy, 14–17 years
Stars change lighter atomic nuclei into heavier ones. This explains where many of the elements in planets, rocks and living things came from.
The idea
Inside a star, enormous pressure and heat push atomic nuclei together. Hydrogen nuclei can fuse into helium, and later reactions can make heavier elements. A star is not creating matter from nothing: it is rearranging nuclei and releasing some mass as energy.
Why it matters
People once had a puzzle: why are some elements common while others are rare, and where did elements beyond hydrogen and helium come from? Nuclear reactions in stars provide the answer for many of them. Exploding and dying stars then spread these new atoms into space.
A worked example
A helium-4 nucleus contains two protons and two neutrons. Four hydrogen nuclei start with a slightly greater total mass than one helium-4 nucleus. The missing mass becomes energy, according to E = mc²; in the Sun, this happens continually and supplies its light and heat.
The common trap
It is easy to imagine that every element is made in the same way inside every star. That seems reasonable because stars all begin mostly with hydrogen and helium, but their masses and temperatures differ. Fusion in ordinary stars stops at different points, while some rare elements need explosions or neutron-rich events.
Where it is used
Astronomers compare the amounts of elements in stars to reconstruct the history of a galaxy. The carbon in a pencil, the calcium in bones and the iron in blood were made by earlier generations of stars or by events linked to them. Stellar chemistry is therefore a clue to cosmic history.
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