Many stars are millions of kilometres across, although they are so far away that they only appear as pinpoints of light to us. Stars may be immensely old, but they are always changing. By looking at many stars, astronomers have been able to piece together the life story of a star.

The space between stars contains gas and dust. This can gather into clouds known as nebulae. When enough gas and dust collects in a nebula it quickly collapses into one or more stars. The gas and dust are drawn ever more closely into a tight ball by the effects of gravity. This releases enormous amounts of heat. Eventually it becomes hot enough to shine.

During much of its life a star burns hydrogen gas and shines brightly – little change appears to happen for billions of years.

Eventually the hydrogen fuel is used up and only helium is left to burn. As helium burns at much higher temperatures than hydrogen, the star gets brighter. At the same time, the outer part of the star begins to expand again, forming a red giant star.

A supernova is a red giant that explodes. It suddenly increases in brightness by a factor of many billions, but even within a few weeks it begins to fade. The Crab Nebula (shown in this picture and some 8,000 light years away) consists of material ejected by the supernova of 1054. A supernova may radiate more energy in a few days than the Sun does in 100 million years. The stellar remnant left behind after the explosion is a star only a few kilometres in diameter but with an enormously high density.

Eventually the star literally blows apart and produces the spectacular 'fireworks' in space called a supernova. The remnant star then contracts as a neutron star or white dwarf, spinning quickly and sending out pulses of radio waves like a galactic beacon. This is why a neutron star is also sometimes called a pulsar. Eventually the neutron star dies, its heat is lost and it ceases to shine. Although it still exists, it is dark, cold and can no longer be seen, and it is now called a black dwarf.