Stars

A series of MERLIN images of the expanding gas surrounding Nova Cygni in 1992

When we look at the sky at night it appears to be full of stars. Astronomers with optical telescopes have studied the stars for hundreds of years. Radio astronomers study stars too, but in a very different way.

Cosmic lighthouses

Thanks to the power of the Lovell Telescope, Jodrell Bank is one of the world's leading centres for the study of pulsars. Discovered at Cambridge in 1967, pulsars are rapidly rotating neutron stars; the collapsed cores of supergiant stars that have exploded as supernovae, blowing most of their material out into space.

Neutron stars are exceedingly dense - a teaspoon of neutron star would weigh hundreds of millions of tons - and strongly magnetised. As the star spins, radio waves emerge in a beam from above each magnetic pole. As the beams sweep across the Earth we see pulses much like the flashes from a lighthouse.

More than 650 pulsars have been catalogued, all but one within our own Galaxy. with pulse periods ranging from a few seconds down to little more than a millisecond. Because pulsars are so fast, only the largest radio telescopes are sensitive enough to follow the rapid fluctuations.

The Lovell Telescope has been at the forefront of pulsar research for almost 30 years. Almost two thirds of known pulsars have been discovered by Jodrell Bank astronomers, often in collaboration with overseas observatories, and almost 100 with the Lovell Telescope itself. Routine monitoring of pulsars is a most important activity and a rich source of research material. With 300 pulsars under regular observation, no other observatory can match Jodrell Bank in its contribution to probing these enigmatic objects.

Among the highlights of this work is the role of the Lovell Telescope in the discovery of ultrafast 'millisecond' pulsars, spinning hundreds of times a second. Millisecond pulsars are believed to be long-dead pulsars which have been brought back to life in binary systems. Gas from a companion star spills on to the neutron star, spinning it up like a top to very high speeds.

One of the most surprising discoveries of the Lovell Telescope was the finding of pulsars in globular star clusters. Globular clusters are spherical swarms of hundreds of thousands of old stars. Because the clusters are among the oldest objects in the Galaxy, astronomers had assumed that any pulsars would have faded away long ago. So in 1986 it came as a great surprise when the Lovell Telescope discovered a millisecond pulsar in a cluster known as Messier 28. Many other globular clusters are now known to harbour pulsars (as many as eleven in one cluster) and it is believed they have been recycled in binary star systems formed by chance encounters between the close-packed stars in the cores of the clusters.

This globular cluster, 47 Tucanae, contains no less than 11 pulsars

Interferometry also has its part to play in pulsar work. With the Lovell Telescope linked into MERLIN, Jodrell Bank astronomers can measure precise positions of pulsars. Over a period of years the positions of the nearer pulsars are seen to be changing as they move across the sky. Astronomers were very surprised to find that about half of all new-born pulsars are moving so fast that they will escape the gravitational pull of our Galaxy and be lost in the darkness of intergalactic space.

A not so new star

On 19 February 1992 a bright, new star appeared in the constellation of Cygnus. Although called a nova, it was not really a new star at all, but a very faint star which had suddenly flared up, increasing its brightness by many thousands of times.

Astronomers now believe that a nova consists of two stars in tight orbits around each other. The normal star, the one we can see, is accompanied by a tiny, invisible white dwarf (see box below). The stars are so close together that gas from the normal star spills over and falls towards the dwarf, forming a swirling disk of material around it. If this disk becomes unstable, large amounts of gas can suddenly drop on to the white dwarf causing a thermonuclear explosion that we see as a nova.

Novae are completely unpredictable and astronomers have to seize the opportunity to observe them when they can. About 80 days after the explosion of the nova in Cygnus the expanding cloud of hot gas had grown big enough for MERLIN to resolve it, the first radio telescope to do so. High-resolution observations showed that the gas cloud was not round but elongated. This may show the orientation of the binary system. The early images show a temperature of 7000-10 000 C, but in later observations the temperature had risen towards 100 000 C. This is a startling result and indicates that the hot gas emitting the radio waves must have received energy directly from the thermonuclear explosions.

Just over three seconds of pulses from the pulsar PSR 0329+54 observed with the Lovell Telescope

Though nova explosions are vastly more powerful than anything seen on Earth, they are dwarfed by much rarer supernova explosions, in which an entire star is disrupted and its matter is blown into space.

Lives of the Stars

The birth, life and death of stars are complex processes, but we now have a good idea of what happens throughout the life of a star. Stars are born out of vast clouds of cold dust and gas. A large enough cloud will contract under its own weight, shrinking and heating until it starts to shine as a protostar. As the core reaches 10 million degrees C, nuclear reactions begin and the outpouring of energy halts the contraction.

Hydrogen is the nuclear fuel that powers most stars, and for most of their lives stars steadily convert hydrogen to helium just as the Sun is doing today. When the hydrogen runs out, the star swells to become a gaint, and more complex energy-producing reactions continue in the core.

What happens the depends on the mass of the star. A heavier star eventually explodes as a supernova, producing a glowing mass of debris (a supernova remnant) with a rapidly spinning pulsar at its centre. A less massive star, like the Sun, puffs off its outer layers and ends its life as a compact, burned-out hot cinder known as a white dwarf.

The Pleiades, a cluster of young stars still partly shrouded in the dust and gas out of which they were born.

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