Topics for discussion
Biggest explosion ever seen
Its afterglow could be seen by the naked eye even though the explosion happened halfway across the visible universe.
They saw the most distant object ever seen by human eyes without optical aid.
The farthest object that can be see by the naked eye is a spiral galaxy called M33
The explosion itself is of a kind known as gamma ray bursts
Never before has an afterglow been visible to the naked eye.
Perhaps the burst itself was more energetic than other bursts, or perhaps its jets were very narrow and aimed directly at Earth.
The words "visible universe" suggest there are other parts of the universe that are invisible. Take a look at what astronomers mean by "horizon" and explain it to your classmates. (The first few paragraphs here is a good place to start, and this is also useful.)
What is the most distant object ever seen with optical aid (which means telescope)? Think up a good way to give someone a feeling for how far away it is. Here is a good place to start.
What is M33, where is it and how is it moving? How do we know each of these things? Find out one other interesting fact about M33. Give a short presentation.
Find out enough about gamma rays to explain what they are to someone who knows no physics. Why were gamma ray bursts a big mystery for years? Has that mystery been completely solved?
Suggest why not. Explain why your suggestion is a good one.
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 - even though 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
| coincidence | energy | experiment | gravity | orbit | particles |
| prediction | planet | radiation | tentative | vibration | waves |