What will XMM look for? Over its 10-year lifetime, XMM will study the entire sky and will almost certainly discover hundreds of thousands of X-ray sources. Many different types of objects generate X-rays, so XMM will have a long list of targets to investigate.

Artist's impression of the XMM satellite, which forms a central part of the ESA's 'Horizon 2000' programme

The stars Most stars emit X-rays. In the case of the Sun, these usually come from violent storms called solar flares, or from the corona - the Sun's tenuous outer atmosphere - which is visible during a total solar eclipse when the moon blocks out the bright light from the Sun's photosphere - where temperatures exceed 1 million degrees Celsius.

An X-ray image of the Sun's corona, the outer layer of its atmosphere

Some stars are much more violent than our Sun, displaying huge flares which throw giant clouds of hot gas out into space. In the hottest blue stars, the fierce radiation causes a stellar wind - very thin hot gas from the star's surface - which streams into space at thousands of kilometres per second. XMM will allow astronomers to compare many different types of star by measuring changes in their X-ray output and telling us more about the temperature and motions of their X-ray emitting gas.

X-ray bursts Many stars occur in pairs (binaries). There are several types of binary system which emit X-rays. For example, this may occur when one of the stars acts as a cannibal, feeding off its neighbour. If one of the stars is a very small dense star - known as a white dwarf - gas may be drawn from its companion into a spiralling disc before crashing into the white dwarf at high speed. The temperature of the gas in the disc may reach 100 million degrees Celsius.

The bright source on the centre left is the X-ray binary star system LMC X-1 in the Large Magellanic Cloud, a satellite galaxy to our own Milky Way

When this build-up of gas reaches a critical level on the white dwarf's surface, an explosive nuclear reaction takes place. Each time this happens, the dwarf star suddenly brightens, emitting huge amounts of both X-rays and visible light, to become a 'nova'. After a few months the star fades again. Some X-ray binaries are the strongest X-ray sources in our galaxy. They are formed when gas from a nearby star is sucked towards an extremely dense neutron star or black hole. Neutron stars are made almost entirely of neutrons and may contain several times the mass of the Sun within a diameter of about 20 km. Black holes are regions where gravity is so strong that even light cannot escape. In both cases, as the inflowing gas is compressed, it becomes so hot that it radiates X-rays, emitting thousands of times more energy than our Sun. Sometimes, when gas builds up very quickly on the neutron star's surface, a series of rapid nuclear explosions creates brief, intense X-ray bursts every few seconds. These may continue for several days. XMM will tell us much more about such events by studying the gas streaming from one star to another.

Supernovae Extremely hot, massive stars burn their supply of hydrogen and helium very quickly. Once this fuel runs out, they collapse, causing a violent explosion. These 'supernovae' release so much energy that they can outshine entire galaxies. The explosions also eject hot shells of gas at speeds reaching 20,000 km/s. X-rays are emitted by the gaseous clouds while they remain hot, but they rapidly fade as the gas cools. Since the cold gas gives out very little visible light, the shells are difficult to detect with optical telescopes. The only evidence for the existence of an old supernova remnant may be a rapidly rotating neutron star (a pulsar), which may also emit X-rays.

This coloured X-ray image shows the Vela supernova remnant which is about 18,000 years old and lies 1300 light years away from Earth. The colours represent X-ray intensity from blue (lowest) through yellow and red to white (brightest)

A supernova remnant. The material seen here is travelling away from the site of the supernova at 11,000 km/s

One of the most famous examples lies at the heart of the Crab Nebula. Here, electrons emitted by the pulsar energise the surrounding nebula so that it emits X-rays. XMM will find similar supernova remnants in our Milky Way and other nearby galaxies.

The Crab Nebula, a supernova remnant which is formed by gas ejected during the supernova explosion and by interstellar gas which has been swept up and ionised by the shock wave originating from the explosion

Beyond the Milky Way XMM will also probe the most powerful X-ray sources of all - active galaxies and quasars (very luminous, distant objects which look like stars in telescopes) - which lie far beyond the Milky Way. These objects give out huge amounts of radiation. In some cases this is due to tremendous bursts of star formation ('starbursts') which produce numerous supernova explosions and large clouds of hot gas. X-rays may also occur when a supermassive black hole in the centre of a galaxy generates energy by swallowing nearby stars. These black holes may be millions or billions of times more massive than the Sun. Any stars or gas which get too close are trapped and destroyed. Enormous amounts of X-rays are generated as glowing discs of gas swirl around the black hole at speeds which may exceed 2 million km/h.

Evidence for a black hole at the core of galaxy NGC 4261. This false-colour image was made by the Hubble Space Telescope

The X-ray background Whenever astronomers look at the sky, they detect a faint background of X-ray radiation. Some of this is emitted by hot, thin gas between galaxies. Although this gas is extremely sparse, it covers such a vast area that it may account for most of the atoms in the Universe. XMM will allow us to study the gas and discover if this is true.

The intensely hot gases between the galaxies in this cluster radiate X-rays

Much of the remaining X-ray background probably comes from swarms of very young, active galaxies located in the far reaches of the Universe. They are so far away that their X-ray radiation has taken billions of years to reach us. Astronomers believe the X-rays were generated during bursts of star formation as the galaxies came together. XMM will test this theory by studying X-ray sources near the edge of the observable Universe.