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
Flow chart showing the life cycle of a star, starting with a protostar and ending with either a white dwarf, neutron star or black hole.Life cycles of stars

All stars begin life in the same way. A cloud of dust and gas, also known as a nebula, becomes a protostar, which goes on to become a main sequence 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 supernova \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.

Image of nebula, (cloud and dust in space that pull together to form a protostar)

Gravity begins to pull the dust and gas together.

Image of nebula, (cloud and dust in space that pull together to form a protostar) but some of the cloud and dust start to pull together, and a spherical outline can be seen.

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.

Image of a protostar, rocks and dust have been pulled together to form a strong outline of a sphere, though not a full sphere yet.

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.

Image of a main sequence star, a yellow sphere, wiith the occasional flare coming out. The core is a darker shade than the outer core, and the flares coming out a light still.

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.

Image of a red giant star. A red sphere in space, it is all one shade of red.

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.

Image of a white dwarf. A white sphere in space, and purposefully small in comparison to the overall image.

Supernova

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

Image of a supernova. A red sphere in space, and purposefully larger in comparison to the overall image.

An exploding supernova throws hot gas into space.

Image of a supernova. A red sphere in space, and purposefully larger in comparison to the overall image. Also there are smaller faded red spheres around the main sphere across.

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.

Image of a neutron star. A full black sphere, on a grey background representing space.