Cosmic bagels are a gas for undiscovered stars : SCIENCE

Heather Couper and Nigel Henbest follow an astronomical discovery that could shed light on how planets are formed

Due south on these winter evenings strides the figure of Orion, one of the most distinctive constellations. The great hunter of Greek mythology is marked by a rectangle of stars for his shoulders and knees, with a tilted line of three stars for hi s belt.

A small group of stars below his belt forms Orion's sword. On a clear night the sword appears fuzzy round the edges. Binoculars will reveal this to be a nebula, a glowing cloud of gas.

The Orion Nebula is beautiful when seen through a telescope, with wreaths of glowing gas surrounding a tight cluster of blue-white stars. For decades, astronomers have known that these are very young stars, lighting up the tatters of the gas cloud from which they were born. But pictures from the Hubble Space Telescope - refurbished a year ago - have provided startling new views.

First, the Orion Nebula is not illuminated by all the stars that lie within it. The gases are fluorescing in ultraviolet light - like a white shirt does in the UV lamps at a disco. And only one star in the nebula is capable of pumping out so much UV.

This powerhouse of a star, Theta-IC Orionis, is not so brilliant at producing ordinary light, so hasn't drawn attention to itself in the past. It is 30 times heavier than the Sun, and shines 5,000 times more brilliantly - largely at ultraviolet wavelengths. The international X-ray satellite Rosat has found powerful X-rays from this star, too. Without the radiation from this one star, it would not be possible to see the Orion Nebul with the naked eye or through binoculars.

And astronomers have now discovered that Theta-IC Orionis is lighting up some objects within the Orion Nebula that we might otherwise never have known about. Robert O'Dell, of Rice University, has used the Hubble Space Telescope to investigate 110 small objects in the nebula. In fact, 56 are not stars at all. They are rings of gas and dust - looking rather like bagels - lit up by radiation from Theta-IC Orionis.

Astronomers from the Max Planck Institute in Heidelberg then found heat radiation coming from a hidden star in the centre of each cosmic bagel. The discovery fits exactly astronomers' theory of how planetary systems - like our solar system - were formed.

A young star should be surrounded by a ring of gas and dark dust, which often obscures the central star itself. If it is lit up by a neighbouring star, the gas in the outer part of the ring would look like a bagel. The material in the ring would eventua lly condense into a set of planets circling the star.

Mr O'Dell is so convinced of his discovery that he has named it "proplyds", an abbreviation of protoplanetary disks. And the statistics - 56 out of 110 - suggest that as many as half of all stars are born with planetary systems.

That's good news for researchers looking for life elsewhere in the universe, including an American team seeking radio waves from "out there". Since its budget was axed by Congress the team has raised several million dollars from private sponsors and is starting its search, fittingly named Project Phoenix, in Australia this month.

Planets In mid-January, look to the south-west after sunset for a glimpse of Mercury. It's low down in the twilight glow, so you'll need a clear horizon to spot this elusive planet.

Higher up in south-west is Saturn. It's been prominent since summer, but this will be your last chance to see the ringed planet for several months, as Saturn slips down into the Sun's glow.

Taking over as "planet of the evening" is Mars. It's now rising in the east at 8pm, and you can't miss its bright orange colour.Seen through binoculars, its colour contrasts strongly with the neighbouring blue-white star Regulus, which marks the heart ofLeo. Mars grows brighter this month as the Earth approaches its closest point to the planet next month. In the early hours, look to the east for the two most brilliant planets, Venus and Jupiter. Venus is moving down into the morning twilight, and passes Jupiter on 14 January.

Meteors The night of 3-4 January may bring the year's most spectacular shower of shooting stars. They'll spread out from a point near the tail of the Great Bear (Ursa Major), where a faint star pattern called Quadrons (the quadrant) once existed. Although this constellation is no longer in use, the meteors are still called the Quadrantids.

The Moon sets early that night, so the sky will be dark around midnight when most meteors are expected. The British Astronomical Association predicts the Quadrantids will be "very favourable", with perhaps two meteors every minute.

Stars This month, the evening sky sports six of the seven most brilliant stars we can ever see from Britain. Sirius, towards the south, is the brightest star of all. On frosty nights, it seems to flash all colours of the rainbow as its light is twisted by winds in the Earth's atmosphere, but, in reality, it is pure white.

Vega is the second brightest on show: but it's a star of summer nights and is currently near the northern horizon. During winter months, its place overhead is taken by third-ranked Capella, the main star in Auriga. Blue-white Rigel in Orion is next, followed by Procyon.

Two red giant stars finish off this ranking of the January stars, Betelgeuse in Orion's shoulder, and Aldebaran, marking the angry eye of Taurus (the bull).

Diary (all times GMT)

1 January 10.56am New Moon 3 10pm Quarantid meteors 4 Earth at perihelion (closest to Sun)

8 3.46pm Moon at first quarter 16 8.26pm Full Moon 19 Mercury at greatest eastern elongation 24 4.58am Moon at last quarter 30 10.48pm New Moon