Science news

21 March 2008

Biggest star explosion ever seen

Early on Wednesday morning NASA’s Swift satellite detected an explosion from deep space that was so powerful its afterglow could be seen by the naked eye. But the explosion happened halfway across the visible universe.

Never before has anything so far away come close to being visible by the naked eye. The explosion was so distant that it took its light 7,500,000,000 (7.5 billion) years to reach Earth. It happened so long ago in fact that Earth had not yet been born.

"No other known object or type of explosion could be seen by the naked eye at such an immense distance," says Swift science team member Stephen Holland of NASA’s Goddard Space Flight Center.

Caption: Afterglow of GRB 080319B imaged by Swift's X-ray Telescope (left) and Optical/Ultraviolet Telescope (right).

Credit:NASA/Swift/Stefan Immler et al.

"We don’t know yet if anyone was looking at the afterglow at the time it brightened to peak visibility. But if someone just happened to be looking at the right place at the right time, they saw the most distant object ever seen by human eyes without optical aid.

On a dark, moonless night with clear skies the farthest object that can be see by the naked eye is a spiral galaxy called M33. This is far distant by Earth standards, but it is nearby on a cosmic distance scale. The light from M33 takes 2,900,000 (2.9 million) years to reach Earth, making it thousands of times closer than the March 19 explosion.

The explosion itself is of a kind known as gamma ray bursts, since much of the energy is in the form of gamma rays. These are a type of electromagnetic radiation with energy far higher than visible light. Most gamma ray bursts are caused by huge stars dying violently. Instead of blowing material in all directions, they channel an immense amount of energy into two high-speed jets, like turbocharged cosmic blowtorches

When these jets plough into surrounding clouds of gas, they light it up. This generates an afterglow that can be seen by telescopes in space and on Earth. The afterglows are often fairly bright, but never before has an afterglow been visible to the naked eye.

The March 19 gamma ray burst’s afterglow was bright enough for about 30 seconds to be seen without a telescope or binoculars.

"Even by the standards of gamma ray bursts, this burst was a whopper," says Swift lead scientist Neil Gehrels of NASA Goddard. "It blows away every gamma ray burst we’ve seen so far."

Astronomers have just begun their scientific analysis of the burst, so they do not know yet why it was so powerful, or why the afterglow was so much brighter than other afterglows. Perhaps the burst itself was more energetic than other bursts, or perhaps its jets were very narrow and aimed directly at Earth.

Swift normally detects about two gamma ray bursts a week. But March 19 was special. The satellite detected four bursts on that day. This is a Swift record. “Coincidentally, the passing of Arthur C. Clarke the day before seems to have set the universe ablaze with gamma ray bursts,” says Swift science team member Judith Racusin of Penn State University.

More help with words

What's it all about?

1. What detected the explosion?
2. The explosion happened very far away but it could be seen without a telescope. Why?
3. When did the explosion actually happen?
4. Light travels very fast but light from this explosion took billions of years to get here. Why?
5. Before this new explosion what was the farthest object that could be seen in the night sky without using a telescope?
6. This new explosion is of a kind called gamma ray bursts, because they produce a big burst of gamma rays. What are they?
7. What causes a gamma ray burst?
8. The light from gamma ray bursts doesn't come mainly from the explosion itself. Where does it come from?
9. Why was the afterglow only able to be seen for a short time?
10. The astronomers don't know yet why this burst was so much more powerful than other gamma ray bursts, but they have a couple of suggestions. State one of these.
11. Do you think it is possible, from so far away, to decide which if any of these suggestions is true?
12. Why is coming up with ideas to explain things you don't understand an important part of doing science?
13. Once scientists have one or two possible explanations for something they don't understand, what do they do next?
14. If you were these scientists think of one thing you would like to do next.
15. What question would that be trying to answer?