A giant cloud in space is emitting regular flashes of laser light, astronomers have shown. The laser is powered by the spinning corpse of a dead star.
The discovery is the first direct proof that laser mechanisms operate in interstellar clouds, says Joel Weisberg of Carleton College in Northfield, Minnesota, US. In this case, the laser emits radiation at radio frequencies and is known as a `maser`.
When a massive star explodes at the end of its life and leaves behind a stellar core slightly more massive than the Sun, the core can collapse into a neutron star - a super-dense spinning ball of neutrons about the size of a city. Neutron stars have strong magnetic fields and emit intense beams of radio waves from their poles. If the neutron star is orientated in such a way that these beams sweep across the Earth like lighthouse beams as the star rotates, we can detect regular radio `blips`. In that case, the neutron star is called a pulsar.
In September 2004, Weisberg`s team observed a pulsar called B1641-45 using the 64-metre Parkes radio telescope in New South Wales, Australia. The pulsar lies about 15,000 light years away and rotates twice every second.
Between the pulsar and the Earth lies a cloud of interstellar gas. Weisberg`s team found that the cloud absorbs some of the pulsar beam`s radio waves as they pass through it. But at the specific frequency of 1720 megahertz, the cloud acted like a maser. Hydroxyl (OH) molecules in the cloud amplified the pulsar beam, emitting additional, identical radiation.
"Humans invented masers in the 1950s, but nature invented them first," Weisberg notes. Astronomers have been pretty sure that masers operate in interstellar gas since the 1960s. They noticed that the clouds often emit radiation at pin-sharp frequencies that only masers could create. But this is the first maser seen flashing to the tune of a pulsar`s beat.
"We see the maser flashing on and off exactly when the pulsar pulse does, hence there can be no question about its being a maser," says Weisberg.
He adds that the discovery backs an earlier suggestion that interstellar clouds could be useful for galactic communications. If alien civilisations wanted to contact us by sending radio signals, they could use the clouds to make their message `louder` without distorting it.
"Our particular maser only amplifies the signals by about 5%, but other clouds could cause a much stronger magnification of the signal," says Weisberg.
"Since everything in space is so far apart, a `free` amplification scheme for a signal would be very helpful in making it detectable across large distances."
