In the universe, there are billions (perhaps trillions!) of stars similar to our Sun. They are of different sizes; the Sun is a medium-sized star. All stars, including the Sun, will die one day. Their life cycle is at least a million years. The smallest stars are neutron stars, they are only around 20 km in diameter.
How are stars formed?
- A star starts its life as a cloud of gas and dust; they are called nebula clouds.
- Gravity pulls nebula clouds together, as a spinning ball of gas.
- The gas ball is so compacted that it gets hot and starts to glow, forming a protostar. At the beginning, protostars glow but cannot be seen, because they are surrounded by dust.
- After millions of years, the temperature in their core is so high that nuclear fusion (joining of hydrogen nuclei to form helium) takes place. This releases a massive amount of energy.
- The star is now visible and remains so for millions of years if it is small, and even longer (billions of years) if it is a big star. A star like our Sun may live for around 10 billion years.
- What happens at the end of a star's life depends on the size of the star.
How do stars die?
- The core of a medium-sized star (like our Sun) will eventually contract, while the outer part will expand.
- It changes colour from yellow to red and becomes a red giant. The Sun, for example, will become so big that it will cover Mercury and Venus and even reach Earth. Shells of gas thrown out from a red giant are called planetary nebula.
- Nebula from the Sun will stretch to the edge of the Solar System.
- The core of the original star shrinks to about the size of the Earth; it is very hot and shines brightly as a white dwarf.
- It does not make any energy, so it cools down and becomes yellow and then red and then forms a black dwarf.
- The core of massive stars contracts and the outer part expands forming a red supergiant, before it collapses, causing the whole star to explode and be thrown outwards.
- This explosion is called supernova.
- The remaining core is a neutron star.
- The material thrown out collides with gas and dust in space and it is called supernova remnant.
- More and more gas and dust merges with the remnant and eventually a new star is formed.
- The core continues to collapse, becomes so dense and gravitational forces are so big that not even light can escape. It becomes a black hole.