How stars shine and change — Astronomy, 14–17 years

A star shines because its hot core releases energy through nuclear fusion. Its mass controls how long it lives and which stages it passes through before its final remnant.

Stars are changing suns

A star begins as a huge cloud of gas pulled together by gravity. Pressure and heat rise in its centre until hydrogen nuclei fuse. The released energy pushes outward against gravity and makes the star shine.

The problem of starlight

Stars look like small points, so early observers could not tell whether they were tiny lights or distant suns. Explaining their steady energy also required a source stronger than ordinary burning. Fusion solved both puzzles.

Mass and lifetime

Imagine two stars forming from the same kind of gas. A star with 10 times the Sun’s mass has far more fuel, but it burns through it much faster. It may live millions of years, while the Sun lives about 10 billion.

The biggest is not the oldest

It is natural to think that a larger star has a longer life because it contains more fuel. Its stronger gravity raises the core temperature and makes fusion much faster, so massive stars usually use their fuel sooner.

The atoms in us

The early universe made mostly hydrogen and helium. Stars created many heavier elements, and explosions spread some of them into space. Carbon, oxygen and iron in Earth and our bodies came through this cosmic history.

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