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starquake that left earth 49,990 light years from AN EXPLOSION that lit up the sky just after Christmas was the biggest bang in the Galaxy for 400 years. Astronomers around the world have described it as unprecedented.

A tiny neutron star suddenly erupted, releasing a huge amount of energy in a single burst, most of it in X-rays and gamma rays. Had the event occurred within ten light years of Earth, the radiation could have destroyed the atmosphere and eliminated life. Fortunately it was 50,000 light years away.

Rob Fender, an astronomer from Southampton University, said: “We have observed an object only 20 kilometres across, on the other side of our Galaxy, releasing more energy in a tenth of a second than the Sun does in 100,000 years. It was the biggest explosion in our Galaxy since Kepler observed his supernova in 1604.”

The object that caused the flare was a magnetar, a tiny star made up of matter collapsed to a huge density, and possessing a powerful magnetic field. It is called SGR 1806-20, the SGR standing for soft gamma repeater — a class of objects that periodically flare up and release bursts of gamma rays.

Bryan Gaensler, of the United States Harvard-Smithsonian Centre for Astrophysics, described the burst as “maybe a once per century or millennium event in our galaxy”.

“Astronomically speaking, this explosion happened in our back yard. If it were in our living room, we’d be in big trouble. Had this happened within ten light years of us, it would have severely damaged our atmosphere and triggered a mass extinction.” The nearest known magnetar to Earth, 1E 2259+586, is about 13,000 light years away.

Magnetars, like other neutron stars, are the relics of larger stars that have consumed all their fuel and collapsed under the force of gravity, creating a supernova explosion that blows off the outer layers and compresses the core. So squeezed together is the matter that remains that a pinhead weighs as much as a battleship. If a neutron star created in this way is spinning, it also generates a huge magnetic field.

A magnetar has a surface crust of iron nuclei one kilometre thick, stressed by unbearable forces from its magnetic field. From time to time the crust deforms violently under these forces. The magnetic field generated by these objects, only a dozen or so of which are known, is about 1,000 trillion gauss, powerful enough to strip information from a credit card halfway to the Moon. The Earth’s magnetic field, for comparison, is a puny 0.5 gauss.

The burst detected on December 27 was first seen by the orbiting X-ray observatory Swift. Observatories around the world were alerted, and followed what happened. The UK’s linked network of radio telescopes, Merlin, is continuing to observe the object.

The event raises a number of interesting questions. For the past 30 years astronomers have been detecting bursts of gamma rays, apparently from the outer reaches of the universe. But SGR 1806-20 suggests that the source of some of these gamma ray bursts may be much closer to home — in our own Galaxy, the Milky Way.

The second puzzle is exactly how SGR 1806-20 generated the huge energy output. One possibility is that the outburst is caused by magnetic fields. Another is that they are caused by the surface of the star suddenly distorting in a “starquake”.

# Large as the explosion of SGR 1806-20 was, the 1604 supernova, witnessed by the German astronomer Johannes Kepler, was bigger still. One of only six supernova explosions in our Galaxy in the past 1,000 years, it appeared as a new star in the sky on October 17. Within days it was the brightest object in the night sky, except for the Moon. It remained visible to the naked eye for more than a year.