Star Life Cycle Model

Sizes are not to scale. A real red giant is about 100 times wider than the stable star, and a white dwarf is about 100 times narrower.

The two paths side by side

	Low mass (like the Sun)	High mass
1	Interstellar cloud of gas and dust
2	Protostar
3	Stable star
4	Red giant	Red supergiant
5	Planetary nebula	Supernova
6	White dwarf	Neutron star or black hole

How to write it in the exam

A star forms from an interstellar cloud of gas and dust that contains hydrogen.
A protostar is the cloud collapsing and increasing in temperature because of its own gravitational attraction.
It becomes a stable star when the inward force of gravitational attraction is balanced by the outward force due to the high temperature in the centre.
All stars eventually run out of hydrogen as fuel for the nuclear reaction.
When most of the hydrogen in the centre has been converted to helium, most stars expand to form red giants. More massive stars form red supergiants.
A red giant from a less massive star forms a planetary nebula with a white dwarf at its centre.
A red supergiant explodes as a supernova. This forms a nebula containing hydrogen and new heavier elements, and leaves a neutron star or a black hole at its centre.
The nebula from a supernova may form new stars with orbiting planets.

The life cycle of a star