Galaxies with cheesy clusters: Heather Couper and Nigel Henbest explain how the holey structure of the Universe is shaped by clumps of stars

MAY, like April, is a month for planets, with brilliant Jupiter riding high in the southern sky and Mars dipping into the west. But far more distant vistas are also on view at this time of year.

Using Jupiter as a guide, look for the Y-shaped constellation of Virgo, now at its highest in the south. If you have good binoculars - or, better still, a small telescope with a wide field of view - slowly sweep the 'bowl' of the Y. Under clear dark skies, you should be able to make out a few faint fuzzy blobs. Each is a galaxy of billions of stars, a few are many times bigger than our own Milky Way, which is thought to contain 200 billion stars.

These galaxies are just the tip of the gigantic Virgo Cluster. About 50 million light years from Earth, this is the closest galaxy-swarm to our Milky Way. More than 1,000 galaxies have been identified, and there must be many thousands too faint to have been detected.

Galaxies like to live in groups, clumped together under the force of gravity. Our Milky Way belongs to a small cluster of about 25 galaxies known as the Local Group, which contains the Andromeda galaxy, the Large and Small Magellanic Clouds and a motley collection of 'dwarf' galaxies.

There are several small galaxy groups near our Local Group. One is the M81 cluster, the largest in Ursa Major, which has just spawned the brightest supernova (exploding star) seen in the northern hemisphere since 1937. 'Bright', alas, is relative: it is just visible through binoculars.

On the cosmic scale, a pattern begins to emerge. The small groups in our vicinity, combined with the Virgo Cluster, make up a cluster of clusters - a 'supercluster'. Our supercluster is flat, with the Local Group out to one edge.

All the galaxies in the Local Supercluster share a motion of 600 kilometres per second (more than 2 million kilometres per hour) towards a large concentration of mass called the Great Attractor. Unfortunately, the latter lies behind one of the dustier parts of our galaxy and no one is ever likely to see it.

The pattern of clusters of galaxies forming superclusters is repeated all over the Universe. On the largest scale, the structure of the Universe vaguely resembles a Swiss cheese, the superclusters forming walls of galaxies around huge bubbles of empty space.

To find out how this structure emerged, we would need to look far enough away in space - and hence back in time - to see galaxies grouping and forming; but we do not yet have a powerful enough telescope to do it.

Results announced a month ago at the National Astronomy Meeting at Leicester University, however, offer some hints. A team from Birmingham University has been using the German Rosat orbiting X-ray telescope to study the clusters. High above the Earth's obscuring atmosphere, Rosat detects X-radiation from the very hot gas driven out by skirmishes among the galaxies. The clumpiness of the gas suggests that the clusters resulted from mergers of many small groups, rather than the fragmentation of larger clusters. The Birmingham results will help researchers to understand the history of groups and clusters, and to predict the course of their evolution.

Whether our small Local Group will one day become a part of a much larger cluster is a moot point, but such an eventuality would lie in the distant

future.

The planets

ALTHOUGH Mercury emerges from behind the Sun in mid- May to become an 'evening star', it will be drowned out by the bright twilight. Venus is rising more than an hour before dawn, and is at its brightest (magnitude minus 4.4), for this apparition, on 7 May. By contrast, Mars is fading as it and the Earth pull apart and its tiny disc shrinks. It has left Gemini (where it spent autumn and winter) for the neighbouring constellation of Cancer. Look for the crescent moon close to Mars on 26 May.

Jupiter, in Virgo, still dominates the night sky when the Moon is not up, while Saturn, which rises about three hours before dawn, looks like a brightish star in the constellation of Aquarius.

Partial eclipse

ON 21 MAY a partial eclipse of the Sun will be visible from most of North America, Iceland, the Arctic, Scotland and northern Europe. The eclipse begins at 12.19 and ends at 16.20 (GMT). Although nearly three-quarters of the Sun will be covered, people in Scotland will notice hardly any difference; keen observers in Shetland might register that one-tenth of the Sun is missing. (Caution: never use an optical aid, such as binoculars, and never look at the Sun directly.)

The stars

BELOW Virgo lies the rectangular constellation of Corvus, the crow. Its five stars rise highest at this time of year and make a surprisingly prominent group so low in the sky. The constellation is an ancient one. The Greeks called it the Raven and linked it with the nearby constellations of the cup (Crater) and the water snake (Hydra).

In legend Apollo sent Corvus to get some water in a cup, but instead it went off in search of figs, returning with a water snake. Corvus, cup and snake were banished, the bird condemned to perpetual thirst.

Diary (all times BST)

6 4.34am full Moon

13 1.20pm Moon at last quarter

16 Mercury at superior conjunction

21 3.07 new Moon; Sun's partial eclipse

28 7.22pm Moon at first quarter