The lives of the stars

Star formation

Stars are born in dark clouds of dust and gas. Unlike people, stars are usually not born singly but in clutches, with the dark clouds containing enough material to form hundreds or even thousands of stars.

Star formation begins when a giant cloud of gas collapses under its own gravitational attraction. As the cloud collapses it may fragment into hundreds of smaller clouds, each of which continues to collapse. As the fragments collapse, they heat up. When they reach a temperature of about 30 K they are warm enough to be studied with infrared telescopes.

Over a period of perhaps a million years the fragment continues to collapse, getting hotter and denser as it does so. Eventually the centre of the fragment reaches a temperature and density that are high enough for nuclear reactions to start. At this point, the hot core, which will eventually form the visible star, begins to blow away its dusty cocoon, allowing its ultraviolet radiation to reach into the surrounding gas. This radiation first breaks the molecules in the nearby gas into their constituent atoms, and then strips away the outer electrons from the atoms, in a process called ionisation. The interactions of the free electrons and the ionised atoms produce radio waves, which can be studied with radio telescopes.

At this point, the star and its surrounding ionised gas (known to astronomers as HII regions) may still be invisible to optical telescopes, being buried deep inside a dusty cloud. Eventually, even this is blown away from the new stars, and they emerge from their cocoons.

There are still many questions to be answered about this process: for instance, what triggers star formation, and what happens in some of its early phases.

Life and death

The nuclear fusion of hydrogen to helium stars when the star's internal temperature reaches about ten million degrees. Other fusion processes follow. This nuclear furnace produces an enormous amount of energy that, moving out through the star, exerts a pressure that just balances the star's tendency to collapse under gravity. The star will remain in this state for many millions of years.

Red giants and white dwarfs

When all the star's hydrogen has been converted to helium, the outward radiation pressure is no longer great enough to stop the star collapsing. After it collapses, new nuclear reactions take place that can turn the star into a hugely swollen red giant.. This will be our Sun's fate, and it will expand out to about the size of the Earth's orbit, possible engulfing the Earth.

During the red giant phase, a star will shed large amounts of debris ­ dust and gas ­ into the space around it. Molecules in these shells of dust and gas can be detected by the radio waves they emit. Studies of these shells by radio telescopes have shown that red giants go through this shedding phase in a quite short space of time ­ only a few tens of thousands of years.

Eventually the star's entire atmosphere is sloughed off, exposing its hot core. This core appears as a very compact, hot blue star. It cools relatively quickly ­ over about 100,000 years ­ to become a white dwarf star. The expelled envelope forms a glowing shell a little less than a light-year in diameter around the hot blue star. These shells are known as planetary nebulae because they resembled a planet when seen through early telescopes, but they have nothing to do with planets.

Finally, the circumstellar nebula is dispersed into the surrounding space. The white dwarf cools to the point where it emits no further light and exists, perhaps forever, as an unseen black dwarf.

Supernovae, neutron stars and black holes

If the original star is very massive, more than a few times the mass of the Sun, the collapse will be extremely rapid and the release of energy so great the star explodes. Its outer parts, now rich in heavy elements, will be blown far out into interstellar space, where they will be visible as a supernova remnant. After 100 000 years or so the debris will have mingled with the interstellar gas, enriching it with heavy elements. From this gas a new generation of stars will be born.

The core of the star going supernova is squeezed in on itself, with its matter reaching incredible densities. Depending on the initial mass of the star, a neutron star or a black hole is formed. A neutron star emitting a beam of electromagnetic radiation (radio waves, light or X-rays) as it spins is called a pulsar.