The life cycle of stars — Astronomy, 11–13 years
Stars change over time rather than shining unchanged forever. Their mass decides whether they end quietly or in a powerful explosion.
Stars have stages
A star begins when gravity gathers gas into a dense, hot cloud. For most of its life, it makes energy by joining hydrogen atoms in its centre. Later, the star changes as its fuel is used up. Small stars can fade into white dwarfs; very massive stars may explode and leave a neutron star or black hole.
Why do stars change?
A star must balance two effects: gravity pulls its material inward, while energy from its hot interior pushes outward. This balance can last for a very long time, but the supply of hydrogen is not endless. When the balance shifts, the star swells, shrinks or explodes. This explains why stars have different stages and colours.
The future of a Sun-like star
Take a star with about the Sun’s mass. First, it spends billions of years turning hydrogen into helium. Later, it becomes a red giant and sheds its outer gas. The hot core that remains is a white dwarf, which slowly cools. It does not explode as a supernova because it is not massive enough.
Bigger does not mean longer-lived
A reasonable guess is that a massive star should last longer because it contains more fuel. It actually uses that fuel much faster: its hotter centre drives a stronger energy-producing reaction. So very massive stars shine intensely but may live only millions of years, while smaller stars can last far longer.
The elements in us
Stars make many elements heavier than hydrogen, such as carbon, oxygen and iron. When some stars shed their outer layers or explode, these elements spread through space and can become part of new stars, planets and living things. The calcium in bones and iron in blood were made in earlier cosmic stars.
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