Night Sky: Death of a red giant: Some collapse and fade, others explode. Heather Couper and Nigel Henbest on what happens when stars run out of fuel

The winter sky is alive with stories. There are the legends of the constellations: Orion, the mighty hunter and his two dogs, and Gemini, the twins who would not be parted even in death. But this month's sky tells the tale of how stars die.

Of Orion's brilliant stars, one stands out particularly. Most are blue-white, but Betelgeuse, which marks the hunter's left shoulder, is distinctly red. Betelgeuse is a star close to the end of the road. It is a red giant - a star fast running out of fuel. So, too, is Aldebaran, the red eye of Taurus the Bull, which you can find by extending the line of Orion's belt up and to the right.

Stars like the Sun generate energy through nuclear fusion reactions. The heart of a star is so dense and hot - at over 15 million C - that hydrogen turns to helium, releasing energy that makes the star shine.

But a star's supply of fuel is finite. An average star lasts for billions of years, but eventually all the hydrogen in the core gets used up and the star ends up with a heart of helium.

Most stars can go no further than this. Fusion stops: energy no longer flows out of the core. Now gravity, the force that created the star in the first place, squeezes the core tight, making it heat up. The star's atmosphere expands, blowing out and cooling. The star swells to 100 times its former size, becoming a red giant.

But it is a red giant for only an instant of cosmic time - a few million years at most. The weakly attached atmosphere boils away into space, eventually leaving the collapsed core exposed. This body - as heavy as a star, but shrunk to the size of a planet - is so dense that a teaspoonful would weigh a ton.

White dwarf stars like this start off white-hot. The brightest star, Sirius, the Dog Star, has a white dwarf companion ('the Pup') which can just be seen through a telescope as a tiny point of light next to its brilliant partner. But a white dwarf has no nuclear energy reserves. As the years go by, the Pup cools steadily to become a cold, black globe - a fate that will also befall our Sun.

But not all stars die with a whimper. The heaviest and brightest, such as Rigel in Orion, will go out with a bang. Stars heavier than about 10 times the Sun's weight have huge gravitational 'squeezing power'. After exhausting their hydrogen, they can go on to fuse helium. The helium turns to carbon, giving the star a new lease of life. The star can continue, fusing carbon to oxygen - and then all the way up to iron.

But iron fusion, instead of giving out energy, takes it in. The only way the supermassive star can get this energy is by collapsing - a course of action leading to a colossal explosion that destroys the star. One such 'supernova' exploded in Taurus in 1054. A small telescope reveals the twisted wreckage today - the Crab Nebula - just above the Bull's lower horn.

Even an explosion this huge might not destroy a star altogether. The core could just survive as an ultra-compressed neutron star, or perhaps a black hole. And a supernova explosion is not just a destructive event. The elements created in the star before the explosion, plus new elements forged in the heat of the supernova itself, are flung into space. The carbon in your bones, the gold in your ring, the oxygen we breathe - all owe their origins to the death throes of supermassive stars.

The planets

Mercury makes one of its infrequent appearances in the evening sky towards the end of the month. In the last few days of January, it sets one and a half hours after the Sun. Saturn is finishing a long period of visibility in the night sky, setting just over an hour after the Sun by the end of the month. But the planet scheduled to dominate the skies this spring is the giant Jupiter, currently rising at about 2am. As the year goes on it will rise earlier.

To make up for a rather planet-poor month, a shower of meteors may well put up a good show. On 3 January, the Quadrantids - which made an impressive display in 1992 - may yield up to 100 meteors an hour. The meteors, which are debris shed by a comet whose identity is not certain, peak sharply in their numbers - blink and you may miss them]

The stars

Overhead on winter evenings is the constellation of Auriga, the Charioteer. It is dominated by the sixth brightest star in the sky, Capella, which has the same surface temperature as the Sun but is 100 times brighter. Capella - 'little she-goat' - was named after the mythological nanny goat Amalthea, who nursed the baby Jupiter.

Next to Capella is a triangle of faint stars - the Haedi, or 'kids'. At the apex is Epsilon Aurigae, whose light stays constant for 27 years and then dims for two years. Epsilon is an 'eclipsing binary', whose dimmings are caused not by the star itself, but by a mysterious companion that passes in front. It may be a huge disc of gas surrounding a much smaller star.

Diary (all times GMT)

2 Earth at perihelion (closest to Sun)

3 Maximum of Quadrantids meteor shower

5 12.01am Moon at last quarter

11 11.10pm New Moon

17 Venus at superior conjunction (behind the Sun)

19 8.26pm Moon at first quarter

27 1.23pm Full Moon

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