Lifecycle of a star

Question 1

How does a star become a star?

Answer 1

1. Nebula -cloud of dust and gas (high proportion of hydrogen)
2. Gravity pulls the dust and gas together, which spiral to form a protostar
3. Gravitational energy converts to heat which fuses hydrogen nuclei to form helium, giving heat and light
4. Main sequence star stays stable because gravity and fusion pressure are equal

Question 2

What happens to a star larger than the sun?

Answer 2

5. Swells to red supergiant, heavier elements up to iron are fused
6. Explodes into a supernova forming elements like gold and platinum and ejecting them
7. Supernova throws outer layers into space, leaving a very dense core called a neutron star. Very big stars become black holes

Question 3

What happens to a star like our sun or smaller?

Answer 3

5. When hydrogen runs out, helium is fused, and the star swells to a red giant
6. The star becomes unstable and collapses, ejecting its outer layer as a planetary nebula
7. A dense, solid and very hot core called a white dwarf is left behind, which cools to form a black dwarf, and eventually disappears