Unto Orion a star is born: The constellation is a factory, capable of producing millions of sparklers, say Heather Couper and Nigel Henbest

Look up on a clear December night when there is no moon and you'll see a classic tableaux: mighty Orion rising in the east, in hot pursuit of the Seven Sisters (Pleiades) high in the south. In his poem Locksley Hall, Tennyson wrote of the Pleiades as looking like 'fireflies tangled in a silver braid'.

The illusion of mistiness the Pleiades give is the result of so many stars packed in a tiny space. The Seven Sisters (most people can see only six) are genuinely associated with each other, unlike in most constellations. This is because the Pleiades are young and haven't had time to move away from their birthplace.

The link between the Pleiades and Orion goes deeper than mythology. Both groups are very young; in Orion, stars are still being born. Below the three stars of Orion's 'belt', you'll see the faint line of his sword-handle. Look carefully at the middle 'star' in the line, and you'll see that it isn't a star at all, but a misty patch. Binoculars or a small telescope will reveal the patch for what it is: a fan-shaped region of softly glowing gas.

The Orion Nebula is a place where stars are born. In its depths, a clutch of young stars has just come into being, illuminating the surrounding gas with the light of their fires. Although the Nebula is 15 light years across, it is a tiny part of an enormous reservoir of gas (mainly hydrogen) and dust that fills the whole Orion region - material for millions of future stars.

The birth of a star starts off when a reservoir such as this receives a gravitational 'nudge' - perhaps a jolt from a nearby exploding star. It starts to collapse, and grows increasingly dense, particularly in the centre. The middle regions break up and each fragment continues to shrink on its own. Each is then a 'protostar'.

As the collapse continues, the gas in the centres of the protostars grows ever hotter - eventually reaching millions of degrees. At 10 million degrees C, the densely packed hydrogen gas undergoes nuclear fusion. A flood of energy surges through the protostar, halting the collapse. A star is born, along with dozens, even hundreds, of companions.

Fledgling stars are volatile and most are surrounded by dark, swirling discs of dust that intermittently hide them from view. All have powerful stellar winds: violent outflows of gas that help to clear the construction site. Eventually, the surrounding debris is blown to tatters, and a young star cluster emerges to face the Universe.

Many astronomers believe the dusty discs are the raw material of future planets. 'About one in 20 stars will be born with a planetary system,' says David Hughes of Sheffield University. But others, such as Gerry Gilmore of the Institute of Astronomy in Cambridge, are more sceptical. He points out that the environments of young stars are very violent and unstable, and that any surrounding discs would eventually be destroyed.

Furthermore, he argues, young stars spin rapidly, and the easiest way they can lose excess angular momentum is by splitting in two. It turns out that double stars are extremely common. Dr Gilmore thinks they may outnumber single stars by 100 to one. The hitch is that, in terms of gravity, double stars are unstable environments for planetary systems. So although the accepted wisdom is that planets are born alongside stars, they may eventually turn out to be much rarer than previously predicted.

Planets and meteors

Mercury is still visible in dawn skies during the first half of the month, and will be only 10 moonwidths north of the red star Antares on 12 December. Venus is rising less than an hour before the Sun. Jupiter, too, is visible as a brilliant 'star' in the south- east in the early hours.

The only planet on view in evening skies is Saturn, which will be setting about 9pm mid-month. It is not the best time to be observing the Solar System - but there will be compensation by way of the shower of Geminids meteors - tiny grains of rock from an asteroid called Phaethon - late on 13 December. As the Moon is new on 13 December, there couldn't be a more favourable time to observe them. If you are viewing from a dark countryside site, you may see as many as 90 shooting stars an hour.

The stars

It's the best time of year to spot Cetus, the sea-monster or whale, just south of the Pleiades. Cetus terrorised the captive Andromeda before being turned to stone by Perseus. Although a faint constellation, it contains some weird and wonderful stars. One is UV Ceti, a pair of red dwarf stars only 8.9 light years away. One occasionally suffers enormous versions of solar flares, and in just minutes it brightens up more than 100 times (but is still too dim to be seen with the unaided eye).

Another famous star in Cetus is Mira, 'the wonderful'. Mira takes nearly a year to go through its cycle of variation. It is 300 to 400 times bigger than the Sun, and brightens and fades as it swells and shrinks. It was a medium-

bright star in June, but it is now fading. In February, it will be 20 times too dim to be seen with the naked eye.

Diary (all times GMT)

6 3.49pm Moon at Last

Quarter

13 9.27am New Moon; maximum of Geminids meteor shower

20 10.25pm Moon at First Quarter

21 8.26pm Winter Solstice

27 Mars in conjunction (behind Sun)

28 11.05pm Full Moon.

(Map omitted)