SCIENCE : Going out with an extremely big bang

Heather Couper and Nigel Henbest chart the cataclysmic suicide and gradual rebirth of a star

Almost exactly eight years ago, a new star appeared in the southern hemisphere, where it brightened to become one of the most prominent objects. It reached maximum brightness in May 1987, then faded back into obscurity. But radio astronomers in Australia have found it is making a comeback.

"SN 1987A" was actually a mature star on its way out. And it went by the most spectacular means of suicide in the universe - going "supernova", literally blowing itself apart. 1987A was the first supernova visible to the unaided eye for nearly 400 years. The last was observed by the mathematician Johannes Kepler in 1604 - four years before Galileo turned the first telescope skywards. But 1987A's timing was immaculate: it appeared just as astronomy was undergoing a hi-tech revolution. Every telescope, satellite and detector that could be pressed into service was trained on the supernova.

Even before the blast became visible, two underground tanks of water, in Ohio and Japan, started to scintillate with flashes early on 23 February. The flashes were caused by a flood of penetrating particles, neutrinos, which hardly ever interact with matter. But the Japanese detector stopped 12 neutrinos; the Ohio tank, eight. To catch this number, something like 300 million million neutrinos must have passed through each tank.

The flood of neutrinos confirmed ideas about what makes certain stars explode. All stars are giant nuclear reactors. In its central core, a star converts hydrogen to helium through nuclear fusion, and the energy created by the reaction emerges as heat and light. But when a star like the Sun runs out of hydrogen fuel, that is more or less it. Using its powerful gravity, it may be able to "squeeze" its helium core and generate a little more energy for a short time, but its gravity is insufficient to take things further.

In the case of massive stars - more than 10 times heavier than the Sun - not only do these giants rip through their fuel in record time; they can also generate more "squeezing power". When their helium is used up, they can fuse the resulting carbon "ash" into neon and oxygen.

Squeeze tighter, and you get a core of silicon. Eventually, the star ends up with a core of iron. When the star tries to squeeze its iron core to generate energy, all hell breaks loose. To fuse iron, you must take in energy, which the star tries to obtain by squeezing its core even tighter. In seconds, the core catastrophically collapses, and in the searing heat, iron nuclei are broken down into subatomic particles. Among these are billions of neutrinos, which flood out of the dying star at the speed of light, and lift off its outer layers.

This is the fate that overtook SN 1987A. Since then, debris from the supernova has been hurtling outwards into space. Astronomers are picking up an ever-increasing signal of radio waves. These waves are generated as gases from the supernova crash into the surrounding interstellar gas. As time passes, this "remnant" will grow in radio brightness until it looks like a glowing ring.

Thousands of years on, the remnant will have swept up so much gas that starbirth becomes possible. Among the ashes of the supernova, young stars will come into being. But they are subtly different from the generation that preceded them. Their gases are enriched with the elements such as silicon and carbon that the old star forged to go on living. They also contain rare elements - such as gold - that can be created only in the fury of the supernova explosion.

It was a supernova that made our own existence possible. The carbon in our bones, the minerals that make up Earth's rocks, the precious metals we adorn ourselves with - all are the products of a suicidal star.

The Moon

The moon moves in front of the bright star Spica on 18 March and hides it from view for more than half an hour. This is known as an "occultation" of Spica.

From London, Spica disappears at 11.06pm and reappears at 11.42; from Edinburgh it vanishes at 11.01, reappearing at 11.46. From the South-west, the occultation will be much shorter; from Cornwall, the Moon only just hides Spica; people in the Scilly Isles will see nothing at all.

The planets

Mars dominates the evening sky, shining as a deep orange "star", high in the south. Earth is pulling away from its slower-moving neighbour, and as a result Mars is rapidly fading: its brightness drops by half during March.

If you have a moderate telescope, this will be your last chance to see any details on the planet's tiny disc.

Jupiter rises in the South-east around 2am and is brilliant in the pre- dawn skies. Early in the month, you may spot the even brighter Venus to the lower left of Jupiter as the sky brightens.

Mercury is technically a "morning star" this month as well, but from Britain it will be lost in the morning twilight. It will be a splendid object as seen from the southern hemisphere. Saturn passes behind the Sun on 6 March and is not visible.

The stars

To the north, the seven-star Plough is hanging almost upside down. In North America it is known as the Big Dipper, and to many British schoolchildren - who have never seen a plough - it is the Saucepan!

In classical Greece and Rome this shape was part of a larger constellation, the Great Bear (Ursa Major).

Diary

1 11.48am new moon; Mercury at greatest western elongation

6 Saturn at conjunction

9 10.13am moon at first quarter

17 1.26am full moon

18 11.06-11.42pm occultation of Spica

21 2.14am spring equinox

23 8.10pm moon at last quarter

31 2.08am new moon

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