# The formation and life cycle of stars

The life cycle for a particular star depends on its size. The diagram shows the life cycles of stars that are:

• about the same size as the Sun
• far greater than the Sun in size
Life cycles of stars

All stars begin life in the same way. A cloud of dust and gas, also known as a , becomes a protostar, which goes on to become a star. Following this, stars develop in different ways depending on their size.

Stars that are a similar size to the Sun follow the left hand path:

red giant star $$\rightarrow$$ white dwarf $$\rightarrow$$ black dwarf

Stars that are far greater in mass than the Sun follow the right hand path:

red super giant star $$\rightarrow$$ $$\rightarrow$$ neutron star, or a black hole (depending on size)

### A nebula

A star forms from massive clouds of dust and gas in space, also known as a nebula. Nebulae are mostly composed of hydrogen.

Gravity begins to pull the dust and gas together.

### Protostar

As the mass falls together it gets hot. A star is formed when it is hot enough for the hydrogen nuclei to fuse together to make helium. The fusion process releases energy, which keeps the core of the star hot.

### Main sequence star

During this stable phase in the life of a star, the force of gravity holding the star together is balanced by higher pressure due to the high temperatures. The Sun is at this stable phase in its life.

### Red giant star

When all the hydrogen has been used up in the fusion process, larger nuclei begin to form and the star may expand to become a red giant.

### White dwarf

When all the nuclear reactions are over, a small star like the Sun may begin to contract under the pull of gravity. In this instance, the star becomes a white dwarf which fades and changes colour as it cools.

### Supernova

A larger star with more mass will go on making nuclear reactions, getting hotter and expanding until it explodes as a supernova.

An exploding supernova throws hot gas into space.

### Neutron star or black hole

Depending on the mass at the start of its life, a supernova will leave behind either a neutron star or a black hole.