The life cycle of a star

In a nutshell

Stars are formed from clouds of dust and gas called nebulae. The gravitational forces pull the cloud together to form a protostar. This protostar undergoes nuclear fusion and eventually becomes a main sequence star. A main sequence star will either become a dwarf star or a supernovae depending on its mass. A supernovae forms a neutron star or a black hole.

Forming a star

A nebula is a cloud of dust and gas (mostly hydrogen). The gravitational forces pulls the particles in the nebula closer together. The cloud becomes denser and the particles collide with each other more frequently. This causes the temperature of the cloud to increase. This hot ball of gas is called a protostar.

When the temperature is high enough, nuclear fusion of hydrogen atoms to form helium nuclei takes place. Nuclear fusion releases large amounts of energy as radiation which keeps the core of the protostar at a high temperature.

This forms a main sequence star.

Main sequence star

After its formation, the star enters a stable period. This typically lasts for several billion years. During this time the star is known as a main sequence star.

In the stable period, the forces of gravity pulling the particles inwards is balanced out by the pressure from the hot gases. This pressure is caused by the continuing fusion reactions taking place.

Example

Currently the Sun is in the middle of its long stable period.

When the star starts to run out of hydrogen, it reaches the end of its stable period.

End of a star

Smaller stars

The long, stable period of stars of a similar size to the Sun is around $10$ billion years.

When the hydrogen runs out, the outward pressures are not strong enough to counteract the force of gravity. The star collapses in on itself but the outer layers expand to form a red giant star.

A red giant star is much larger than the original main sequence star. It is called red because the outer layers cool and emit red wavelengths of light. Inside a red giant other fusion reactions occur. This includes the helium nuclei combining to form other heavier elements.

Eventually the star will become unstable and ejects the outer shell of dust and gas. The force of gravity pulls together the rest of the star and it collapses to form a white dwarf star.

No fusion reactions take place in a white dwarf star. It cools over a period of around a billion years and turns into a black dwarf star. A black dwarf star only emits a relatively small amount of energy.

Larger stars

Stars that are much bigger than the Sun follow a different cycle. These stars are hotter and brighter than the Sun.

After the hydrogen runs out, the star collapses in on itself and the outer layers expand to form a red super giant. The red super giant undergoes more fusion and expands and contracts multiple times. This fusion produces elements up to iron.

At the end of this period, the star quickly collapses in on itself and explodes in a supernova. This forms elements that are heavier than iron. These elements are ejected into the universe and they go on to form more planets and stars.

As the supernova explodes it releases the outer shells of dust and gas. The extremely dense core remains behind. For a smaller red super giant (still massive in comparison to the Sun), this is a neutron star.

If the red super giant star was massive enough (had enough mass) the dense core turns into a black hole. A black hole is a very dense point in space which has a strong gravitational attraction towards it. Not even light can escape a black hole.

1.	Stellar nebula
2.	Protostar
3.	Main sequence star
4.	Red giant star
4a.	White dwarf star
4b.	Black dwarf star
5.	Red super giant
5a.	Supernova
5b.	Black hole
5c.	Neutron star