Death of a star: Astronomers get grandstand view of supernova

May 22, 2008 12:00 am

, PARIS, May 22 – Astronomers on Wednesday said they had witnessed for the first time how a dying star erupted into a supernova, one of the mightiest yet most elusive sights in the Universe.

Supernovae are massive stars that run out of fuel, collapse under the weight of their own gravity and become an ultra-dense object known as a neutron star.

They then send out a shockwave that rips the star apart and leaves a sizzling cauldron of radiation.

These colossal events are among the most thrilling phenomena for astronomers but are also agonisingly hard to detect.

They are usually only spotted days or weeks after they occur, becoming visible when material from the blast collides with gas and dust that the star had shed earlier. This high-energy smash creates a dramatic flare which eventually fades.

In January, though, a "once-in-a-lifetime" stroke of luck enabled astronomers to spot a supernova in its infancy, the skygazers report in the British journal Nature.

Princeton University’s Alicia Soderberg had asked NASA mission controllers to orientate their orbital X-ray telescope, Swift, to observe the glowing remnants of a two-week-old supernova in a spiral galaxy called NGC 2770, 90 million light years from Earth.

But, by sheer good fortune, another supernova was brewing nearby, sending out a burst of X-rays that Swift instantly picked up and started to record.

"It’s a really lucky chain of events. It was all over in a matter of minutes," said Soderberg, leading a team of 39 astronomers who are poring over Swift’s observation.

The satellite’s precious 530 seconds of data confirm a theory postulated nearly four decades ago that a supernova would be heralded by an X-ray burst.

Under this hypothesis, the newborn neutron star first compresses itself and then rebounds, sending out a shockwave that rips through the star’s gassy outer layers, heating them to X-ray emitting temperatures. Then the shock blows the star to smithereens.

"The results are stunning, and illuminate one of the last frontiers in stellar death," said co-author Joshua Bloom of the University of California at Berkeley.

"What happens early on speaks directly to the nature of the star and the ways in which massive stars die."

Six other high-powered terrestrial telescopes and the orbiting US Chandra X-ray and Hubble observatories swiftly trained their eyes on the event, and followed it up for 109 days.

The astronomers estimate that the supernova, which has been dubbed SN 2008D, was caused by a star that was probably 30 times the mass of the Sun but of the same radius — a so-called Wolf-Rayet star, meaning that it has lost its outer layer of hydrogen.

2008D falls into the category of a "Type 1b" supernova, in which the collapse of the stellar core is followed by the telltale rebound.

Galaxies usually play host to a supernova only once or twice every century, which means that, across the Universe, the event occurs several hundred times a year.

The hope is that the signature X-ray burst that occurs at the moment of "shock breakout" can be used to calibrate future orbital telescopes so that they can spot other Type 1b supernovae at the earliest stages, following them from cradle to grave.

Supernovae have long been viewed with awe, with soothsayers seeing in them portents of good or ill omen.

Records of them date back to AD 185, when a Chinese journal, the Astrological Annals of the Houhanshu, recorded "scintillating, variegated colours" in a "guest star [that] emerged within the Southern Gate."

A supernova that occurred in 1572 observed by the Danish astronomer Tycho Brahe was traumatic for many thinkers at the time, for it destroyed the notion, set down by Aristotle, that the Universe was immutable.

In spite of the lethal energy released in their death, supernovae are also creators, for they forge heavy elements such as oxygen and calcium which are essential for life as we know it.


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