The Physics Question Bank

A star begins in a nebula, which is a large cloud of dust and gas.

Gravity pulls the dust and gas together. As the particles move closer, the gas becomes hotter and denser. This forms a protostar.

If the temperature becomes high enough, hydrogen nuclei start to join together. This is called nuclear fusion. Fusion releases energy and the star becomes a main sequence star.

During the main sequence stage, the star is stable. Gravity pulls the star inwards, while the energy from fusion pushes outwards. These two effects are balanced.

What happens next depends on the mass of the star.

A star like the Sun expands into a red giant when it starts to run out of hydrogen. The outer layers are released into space, leaving behind a hot, dense white dwarf. Over a very long time, the white dwarf cools and becomes a black dwarf.

A massive star expands into a red supergiant. It then explodes in a supernova. The core left behind becomes either a neutron star or, if the star is massive enough, a black hole.

Order to remember

All stars:

Nebula → Protostar → Main sequence star

Sun-like star:

Red giant → White dwarf → Black dwarf

Massive star:

Red supergiant → Supernova → Neutron star or Black hole

6-mark answer split into marks

1 mark: A star starts as a nebula, which is a cloud of dust and gas.

2 mark: Gravity pulls the dust and gas together, forming a protostar.

3 mark: The protostar gets hotter until nuclear fusion starts.

4 mark: The star becomes a main sequence star, where gravity pulling inwards is balanced by energy from fusion pushing outwards.

5 mark: A Sun-like star becomes a red giant, then a white dwarf, and finally a black dwarf.

6 mark: A massive star becomes a red supergiant, explodes as a supernova, and leaves behind a neutron star or black hole.