LOOK at the sun. On second thoughts, don't. The light will burn your eyes. But if your eyes registered gamma rays rather than light, you could gaze at it safely—for there is practically nothing to see. Except during violent solar flares, the sun gives out almost no gamma rays. In fact, in this part of the spectrum, it is dimmer even than the moon, which spits a few of them off when its rocks are hit by cosmic rays.

Through a gamma-ray telescope, therefore, the sky looks nothing like the familiar sea of points that human eyes perceive at night. Gamma rays are at the high-energy end of the electromagnetic spectrum (visible light is in the middle of it). Creating them in quantities that can be seen from far away requires extreme conditions. Few things in the known universe are fierce enough. Some gamma rays shine from exotic objects, such as pulsars (which pour all kinds of radiation into space and can therefore be seen with other sorts of telescopes). But some appear to come from nowhere.

Since 1973 astronomers have seen thousands of gamma-ray bursts (GRBs)—events that last from milliseconds to minutes or even hours—without a coincidental flash of light, or any other signal, to match them to. But on February 28th an Italian-Dutch satellite called Beppo-SAX saw a faint trickle of X-rays coming from a part of the sky where it had spotted an 80-second GRB eight hours earlier. A team led by Jan van Paradijs of the University of Amsterdam took a visible-light picture of the same part of the sky shortly afterwards. In the next picture, taken eight days later, the team found that one of the stars had faded.

Baffled by the burst

Over the past seven weeks, therefore, several telescopes have been turned on the point occupied by the fading “star” in the hope that it is, indeed, the remains of whatever caused the GRB. Preliminary reports of what happened are now coming in.

There are competing theories on what causes GRBs, but most agree that some kind of explosion is involved. It is thought that debris from the explosion slams into the gas and dust surrounding whatever it was that blew up, and that this collision generates gamma rays. But the impact should also give off some less-energetic radiation—X-rays, ultraviolet light and visible light—particularly once it has weakened enough to stop emitting gamma rays.

The failure until now to detect these other kinds of radiation has been frustrating. Knowing what might cause a GRB depends on knowing how far away it is. And that depends on being able to see the lower-energy radiation, in order to pinpoint what it was that exploded (gamma rays are impossible to focus, so the location of a burst is known only approximately).

If bursts occur within the Milky Way, the galaxy that contains the earth's solar system, they could be relatively small affairs—quakes on the surfaces of neutron stars is one suggestion. But if they originate in other galaxies, thousands of times farther away, they would have to be the brightest objects in the universe. To be seen from the earth they would need to put out a billion times more power even than a supernova (a stellar explosion which, in turn, can outshine an entire galaxy of 100 billion stars). In this case the source would have to be something far more exotic, such as a neutron star being swallowed by a black hole or colliding with another neutron star.

Until the recent results, the only clue to the origins of the bursts was that they seemed to be spread evenly over the heavens. This suggests either that they are happening outside the Milky Way or that they occur in the spherical halo that surrounds its main disc. If they took place within the disc, they would appear from earth to be concentrated in the band of stars that gives the Milky Way its name.

The dying glow of light from the February burst provides further clues. Now that Dr van Paradijs's team, and another led by Kailash Sahu at the Space Telescope Science Institute in Baltimore, have had time to look properly at the fading point of light, they have noticed a faint fuzz around it that looks as if it could be a galaxy playing host to a dramatic explosion.

On the other hand, there is evidence that the object which burst forth so spectacularly is actually quite close. Patrizia Caraveo and her colleagues, at the Istituto di Fisica Cosmica in Milan have recently reported that in two pictures taken 12 days apart by the Hubble space telescope the point of light had shifted perceptibly. If it was in a distant galaxy, it would have to be travelling at several thousand times the speed of light to move as much as it has. That, she says, suggests the burst came from a single neutron star just a few hundred light years away.

Dr Sahu, however, cannot see any movement between the two pictures. And, unfortunately, there will be no new evidence to settle the question for a while. Viewed from earth, the disputed source is now too near the sun for big telescopes to observe. Several months must pass before it can be gazed upon again. If the source is then still bright enough to be visible, it will be easy to see if there has been any movement. It will also be possible to examine the fuzzy patch in more detail to find out whether it really is a galaxy. And if a few more such events can be found, that should settle once and for all whether gamma-ray bursts are merely little local upheavals or the biggest noises in creation.