Stars & Planets

July: what caused one of the biggest explosions since the big bang?

On bright summer evenings, the constellation Leo is sinking into the west before the sky becomes really dark. The easiest way to find it is by the bright planet Jupiter, which lies near Leo's brightest star, Regulus.

On bright summer evenings, the constellation Leo is sinking into the west before the sky becomes really dark. The easiest way to find it is by the bright planet Jupiter, which lies near Leo's brightest star, Regulus.

Earlier this year - on 29 March - Leo was the site for a scene of astonishing cosmic violence. It wasn't easily visible to ordinary telescopes, but a massive explosion sent a violent burst of gamma rays - the most energetic kind of radiation - flooding out through space. It was one of the biggest explosions in the universe since the Big Bang itself.

The burst was detected by a Nasa satellite which orbits the Earth looking for eruptions like this. This gamma-ray burst was among the most powerful detected by any satellite in the 30 years since these eruptions were picked up by a Pentagon satellite that was monitoring nuclear tests.

Ever since then, gamma-ray bursts have been one of the biggest mysteries in astronomy. What kind of explosion are they? Some astronomers have said that the gamma-ray burst comes from a minor eruption within our Milky Way galaxy; others that it is a vast cataclysm further off in space.

In the years leading up to March this year, evidence has been building that gamma-ray bursts have their origin far beyond our Milky Way. This makes them far more energetic than any other kind of explosion, including supernova eruptions where a star blows itself apart at the end of its life. And the burst of gamma rays on 29 March has at last revealed a critical link between these two mighty kinds of astronomical explosions.

If your eyes were sensitive to gamma rays instead of light, and you'd been looking towards Leo on that date, the gamma-ray burst would have blazed in the sky more brilliantly than the sun. For just half a minute - then it faded from sight.

But a team of researchers was on its track. Astronomers in New South Wales, Australia, turned an optical telescope to this part of the sky. They found a fading star in the region of the burst. Researchers at the European Southern Observatory in Chile then swung one of the world's biggest telescopes towards this star. From the spectrum of its light, they worked out it was 2,650 million light-years away from us.

This distance may be huge by ordinary standards, but it's very close for a gamma-ray burst. By comparing this spectrum with the light from supernovae in the past, the astronomers could piece together what was happening.

And the observations dovetailed with a theory proposed by the US astronomer Stan Woosley 10 years ago. It says that gamma-ray bursts originate in star explosions that are even more powerful than ordinary supernovae: they are hypernovae.

According to this theory, we are seeing the death of one of the heaviest stars in the universe. It began more than 25 times more massive than our Sun. Over its lifetime, the brilliantly shining star lost most of its outer layers. Just before the explosion, all that was left was the star's superhot core. Even this remnant was 10 times more massive than our Sun.

Then the star suddenly collapsed under its gravity. The central core shrank in an instant to become a black hole - a region just a few kilometres across where gravity was so intense that nothing could escape. Around this black hole, the interior layers of the star swirled round in a disc.

Magnetic fields were twisted around in the swirling disc, building up energy like twisted elastic bands. The energy was released in two powerful jets that punched through the star's outer layers. As they emerged into space, the jets released their energy as a massive burst of gamma rays.

When the explosion has subsided, just the black hole will be left. From the most brilliant explosion in the universe has come the darkest object it's possible to imagine.

What's Up

Jupiter, the "star" of the evening skies since last winter, is now slipping down into twilight in the north-west. It won't reappear on our starcharts until the end of the year. Mars rises just after 11pm towards the end of the month. Now's a great time to spot summer constellations. If you've got a clear southern horizon, you'll easily see blood-red Antares, "the rival of Mars".