The birth of a supernova has been captured for the first time by a satellite that happened to be pointing in the right direction when the explosion took place.

The Nasa Swift satellite was investigating a two-week-old supernova when another massive star within its field of view exploded as the satellite was recording.

It detected a five-minute burst of intense X-rays that alerted astronomers to the probability that a dying star had just exploded, to be reborn as a supernova.

“We were in the right place, at the right time, with the right telescope on January 9 and witnessed history,” said Alicia Sonderberg, of the Carnegie Institution and Princeton University in the United States.

Researchers said it was serendipity that the supernova was detected at its birth. Galaxies typically produce only one or two every 100 years so the chances of observing the start of one were regarded as vanishingly small.

An X-ray burst was predicted as the first sign of a supernova almost 40 years ago but the observations on January 9 this year were the first time that the event was witnessed and the prediction proved.

The dying star that turned into supernova SN 2008D was about the same size as the Sun but 30 times its mass. It exploded some 90 million light years away in a spiral galaxy called NGC 2770.

Supernovae are created by dying stars which, when they run out of nuclear fuel, collapse in on themselves to become a neutron star or a black hole. The collapse causes a shock wave so powerful that it can blow the star up and can travel at up to 70 per cent of the speed of light. The X-rays detected by Swift were emitted as the shock wave broke through the star’s surface.

Astronomers monitoring the X-ray burst alerted eight other orbiting and ground-based telescopes, which were realigned to watch and record the supernova’s birth in detail. The findings are reported in the journal Nature.

Neil Gehrels, the lead investigator for the Swift satellite, said the observations made of the explosion so early on in the process should enable scientists to learn more about the physics of massive stars.

He said: “This first instance of catching the X-ray signature of stellar death is going to help us fill in a lot of gaps about the properties of massive stars, the birth of neutron stars and black holes, and the impact of supernovae on their environments.

“We also now know what X-ray pattern to look for. Hopefully we will be able to find many more supernovae at this critical moment.”

Joshua Bloom, of the University of California, Berkeley, said: “The results are stunning and illuminate one of the last frontiers in stellar death. What happens early on speaks directly to the nature of the star and the ways in which massive stars die.”

Professor David Burrows, of Penn State University, Pennsylvania, said: “All the data we are collecting will help us more fully understand why some supernovae make gamma-rays and others do not.”

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