Most of the light and radiation we can observe in the Universe originates in stars - individual stars, clusters of stars, nebulae lit by stars and galaxies composed of billions of stars. Stars are spheres of glowing hydrogen and other chemical elements which produce their prodigious energy output by converting lighter elements to heavier ones through nuclear processes similar to those in hydrogen bombs. Like human beings they are born, mature and eventually die, but their lifetimes are vastly longer than our own.

Hubble has gone beyond what can be achieved by other observatories by linking together studies of the births, lives and deaths of individual stars with theories of stellar evolution. In particular Hubble's ability to probe stars in other galaxies enables scientists to investigate the influence of different environments on the lives of stars. This is crucial in order to be able to complement our understanding of the Milky Way galaxy with that of other galaxies.

Hubble was the first telescope to directly observe white dwarfs in globular star clusters. White dwarfs are stellar remnants and provide a 'fossil' record of their progenitor stars which shone so brightly that they long ago exhausted their nuclear fuel. Through these measurements it is possible to determine the ages of these ancient clusters which is an important cosmological tool.

Another area where Hubble's work has been widely acknowledged is the linking of star formation (also see pages 28-29) with stellar evolution. Hubble's infrared instrument, NICMOS, is capable of looking through the dust surrounding newly born stars. Some of the most surprising discoveries so far have come about by peering through the clouds of dust surrounding the centre of our Milky Way. Astronomers found that this centre, which was thought to be a calm and almost 'dead' region, is in fact populated with massive infant stars gathered into clusters.

Gerard Gilmore
Astronomer, University of Cambridge

"Hubble has in my view revolutionised the study of globular clusters - especially those in other galaxies. These objects are so dense and the stars so tightly packed together that it is almost impossible to separate the stars from each other with ground-based telescopes. We have been able to measure what kind of stars they are composed of, how they evolve and how gravity works in these complex systems."

Formation of Stars