Science: Stars and Planets: May

ON MAY evenings, the large, ancient constellation of Virgo takes centre stage in the southern sky. Shaped like a "Y", it bears little resemblance to a maiden bearing an ear of corn - but within Virgo's mundane outline is a treasure trove of delights. With binoculars, sweep the "bowl" of the Y, and if your skies are clear and dark you should pick out a number of fuzzy patches. These are just a handful of the thousands of galaxies, many much bigger than our own Milky Way, making up the giant Virgo Cluster.

We have known for some time that the basic building-blocks of the Universe are clusters and superclusters of galaxies. These reflect the distribution of matter that came out of the Big Bang. More important, they also reveal the location of the mysterious "dark matter" that may make up more than 90 per cent of the mass of the Universe. The visible galaxies ride the dark matter like surfers on a wave.

To track down the dark matter, and to get an accurate picture of how the galaxies are distributed in 3D, astronomers have latterly been conducting their own censuses of the Universe. One of the first was the British/ Australian "2dF" (2-degree field) survey, conducted with the Anglo-Australian Telescope in New South Wales. This looks at an area of the sky some two degrees across, or four moon-widths. Small though this may seem, it is enormous compared with the field of view of the Hubble Telescope (one- 30th the Moon's diameter), and allows the distances to 400 galaxies to be measured simultaneously. Astronomers aim to end up with a 3-D map of 250,000 galaxies.

On the other side of the world at Apache Point Observatory in New Mexico, the Sloan Digital Sky Survey shares the same technology as the 2dF survey, even down to the size of the area surveyed. At the heart of the instrument - placed at the focus of a 2.5-metre telescope - is an array of optical fibres. By studying existing sky-images, astronomers can position the fibres exactly where the galaxies lie. Plug in the fibres, point the telescope, and light from each galaxy tumbles down its own fibre to a sensitive spectrograph, which measures the distance.

The project leader, Jim Gunn of Princeton University, points out that surveys such as this wouldn't have been possible even 10 years ago. "We've got state-of-the-art detectors - silicon CCDs - which detect 70 per cent of the light that falls on them, instead of the 1 per cent registered by photographic plates. Plus, the database will produce 10 terabytes, something that daunted us when we started planning - but not now."

Over a five-year period, Sloan will perform a 3D survey of a million galaxies in our "near" neighbourhood - out to about 3 billion light years. Its data will be complementary to that obtained on much more distant galaxies by the orbiting Hubble Telescope, so astronomers can pinpoint evolution in both galaxies and structures as the Universe grows older. "This will allow us to understand what the Universe is like now, how it has been, and what it will be like in the far future," says Gunn.

As well as mapping the distribution of galaxies and the underlying dark matter - both fossil relics of conditions following the Big Bang some 13 billion years ago - surveys such as the 2dF and Sloan are going to put a considerable amount of flesh on the largely theoretical skeleton that describes our Universe. "Cosmology has been a subject in which there have been lots and lots of ideas and very, very little data," Gunn explains.

"Finally, there is going to be lots and lots of data. We will perhaps be able to understand the Universe - and among some of those theories there may even be one that's right."

What's up this month

Venus is looking like a brilliant lantern hanging in the western twilight, and it is growing still brighter as it draws closer to the Earth in its orbit. Mid-month, it sets three-and-a-half hours after the Sun. On 18 May it will appear close to the crescent moon in the sky.

Mars, too, is putting on a good performance this month, and is on view nearly all night long in the south. Although only one-tenth as bright as Venus, it still outshines most of the stars, and its lurid red colour makes it unmistakable.

Mercury is too close to the Sun to be visible now; Jupiter and Saturn are also hard to see, emerging above the horizon within an hour of sunrise.