Stars are born, grow up, and eventually die.  The exact way that stars change as they age (or how they evolve) depends on how massive they are at birth.

Stars are born when large gas clouds collapse under gravity. They form hot cores that gather more and more gas and dust until a protostar is formed.  If only a small amount of gas is around, then only a small star will form;  if a large amount of gas is present, then a massive star will form.  Usually, the gas cloud is large enough for the cloud to break up into fragments so that multiple stars form of various sizes. The Sun formed from a very average sized gas cloud, and the bits left over stuck together under gravity to form the planets.

When stars initially form, they are still surrounded by gas and dust and so are hidden from the view of optical telescopes.  However, infrared light can pass straight through these clouds, enabling astronomers to use telescopes such as the Herschel Space Telescope or UKIRT (the United Kingdom Infra Red Telescope) to peer deep inside these star forming regions. 

Star forming regions (credit: ESA/Herschel)

Our star, the Sun, is an average star.  It's been around for about 5 billion years, and will be around for 5 billion more.  When it eventually dies, it will expand to a form known as a 'red giant' and then all the outer layers of the Sun will gradually blow out into space leaving only a small White Dwarf star behind about the size of the Earth.

Massive stars (more than eight times the mass of the Sun) live fast and die young, with a life span as little as 3 million years. They die with a tremendous supernova explosion which occurs when the star runs out of nuclear fuel.  The core that remains can collapse into either a neutron star, or (if the initial star was very large indeed, greater than 15-20 times the mass of the Sun) a black hole. The following cartoon explains how stars die in supernovae and what happens as a result.

ACTIVITY: Use the interactive activity to explore stellar evolution further. (Flash player required)