The life cycle of a star
Physics · Key Stage 3 · The School
Born from a cloud of gas and dust
A star starts inside a NEBULA — a huge cloud of gas and dust. Gravity slowly pulls the thickest parts of the cloud inward. As it squeezes together, it heats up. Once the centre is hot and dense enough, NUCLEAR FUSION starts. Hydrogen joins together to form helium. This releases huge energy. That is the moment a star is born.
A balance that lasts billions of years
For most of its life, a star sits in a stable balance. The push outward from fusion balances the pull inward from its own gravity. This stage is called the MAIN SEQUENCE. The Sun has been in it for about 4.6 billion years already. In time the hydrogen fuel at the core runs low, and the balance breaks.
A Sun-like star's gentle end
When a Sun-like star runs low on fuel, its core shrinks. Its outer layers puff out and cool, turning it into a RED GIANT. In time those outer layers drift away for good. What is left behind is a small, very dense core called a WHITE DWARF. It has no fuel left. It just cools down, slowly, for a very long time.
A star far more massive than the Sun burns through its fuel much faster. So it lives a much shorter life. When its fuel runs out, its core does not shrink gently. It collapses fast. The outer layers blast outward in a SUPERNOVA, briefly outshining a whole galaxy. What is left depends on the star's original mass. It could be a NEUTRON STAR, packed incredibly dense. For the biggest stars of all, it could be a BLACK HOLE, where gravity is so strong that not even light can escape. Mass decided the whole story, right from birth to this very different end.