Page last updated at 13:15 GMT, Tuesday, 28 April 2009 14:15 UK

Cosmic blast sets distance mark

By Jonathan Amos
Science reporter, BBC News

GRB 090423 (Gemini Observatory/NSF/AURA/D.Fox/A.Cucchiara)
The colossal star probably gave way to a black hole, scientists say

The cataclysmic explosion of a giant star early in the history of the Universe is the most distant single object ever detected by telescopes.

The colossal blast was picked up first by Nasa's Swift space observatory which is tuned to see the high-energy gamma-rays emitted from extreme events.

Other telescopes then followed up the signal, confirming the source to be more than 13 billion light-years away.

Scientists say the star's destruction probably resulted in a black hole.

"This gets us into a realm where we've never been before," said Professor Nial Tanvir, of the University of Leicester, UK.

"This is the most remote gamma-ray burst (GRB) ever detected, and also the most distant object ever discovered."

Fast mover

The Swift satellite was launched in 2004 to investigate the energetic flashes that characterise some of the Universe's most violent happenings.

Artist's impression of GRB production (ESO)
Models assume GRBs arise when giant stars burn out and collapse
During collapse, super-fast jets of matter burst out from the stars
Collisions occur with gas already shed by the dying behemoths
The interaction generates the energetic signals detected by Swift
Remnants of the huge stars end their days as black holes

Led by the US space agency, the mission has significant UK and Italian contributions.

It is a three-in-one observatory. Its Burst Alert Telescope is set up to catch the initial flood of gamma rays. The spacecraft then swings itself to look directly into the flash with X-ray and ultraviolet/visible telescopes.

This longer wavelength afterglow lasts on the order of hours to days and Swift also calls up ground-based observatories to join the spectacle. Indeed, it is the ground campaign that establishes the distance.

This burst, dubbed GRB 090423, was detected by Swift on 23 April.

Follow-up observations were led by the United Kingdom Infrared Telescope and the Gemini North Telescope, both on Mauna Kea, Hawaii.

Cosmic expansion

Analysis of the light spectrum confirmed the blast had a redshift of 8.2. Redshift is a measure of the degree to which light has been "stretched" by the expansion of the Universe. The greater the redshift, the more distant the object and the earlier it is being seen in cosmic history.

Ground facilities like the UK Infrared Telescope were also involved

The figure 8.2 equates to a distance of 13.035 billion light-years. Put another way, the explosion is being viewed when the Universe was only 630 million years old, a mere one-20th of its current age (estimated to be 13.7 billion years old).

The previous record holder was a GRB witnessed, also by Swift, in September 2008. It had a redshift of 6.7, making it 190 million light-years closer than GRB 090423.

Scientists have seen what they believe may be faint galaxies at redshifts 8-10, but their true nature is still being investigated.

Researchers are very keen to probe these great distances because they will learn how the early Universe evolved, and that will help them explain why the cosmos looks like it does now.

'Fried' gas

Scientists believe the super-hot conditions that existed after the Big Bang eventually cooled sufficiently to allow protons, neutrons and electrons to form neutral atoms of hydrogen and helium.

When the first stars formed, from infalling clouds of hydrogen and helium, they lit up the cosmos.

Swift (Nasa)
Swift swings itself around to look at the direction of a blast

What is more, these hot, young stars produced so much intense ultraviolet radiation, they "fried" the gas around them - tearing electrons off neutral atoms to leave the diffuse plasma we detect between the stars even today.

The star which ended its days as GRB 090423 was very probably among that early population of giants responsible for this re-ionisation.

Professor Tanvir said: "The re-ionisation era was a change that occurred in the Universe when the first stars switched on; but the actual timescale for that and the processes at work are poorly understood because we have so few observations. We're now pushing into that epoch."

Professor Gerry Gilmore, from Cambridge University, UK, commented: "[This] was probably one of the first stars that ever formed in the Universe; and it will have been one of the first things that ever created stuff like carbon, nitrogen and oxygen that led then to normal stars like our Sun and the planets forming much, much later on.

"This is really a crucial stage of the Universe. The fact that we can see that long ago is just astonishing," he told BBC News.

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