Credit: NASA
OBSERVERS OF THE northern night sky during the early hours of 19 March 2008, would have made little of the flicker of light that appeared briefly in the constellation Bootes. The display lasted a few minutes before fading and then disappearing. As far as astronomical shows go, this was a damp squib.
Yet the display was special. Indeed, it was one of the most remarkable celestial occurrences ever recorded by human beings. That light, which so briefly illuminated the heavens, had taken almost eight billion years to reach our world - but was still visible to the naked eye.
By a very large margin, the spectacle - produced by photons that had travelled half way across the universe - was a record breaker. "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," says Stephen Holland of NASA's Goddard Space Flight Centre in Maryland, USA.
As to the cause, scientists are now certain that the light was associated with a gamma-ray burst, the most energetic event in the universe. Triggered when giant stars destroy themselves, these are the biggest bangs since the Big Bang itself.
"Gamma-ray bursts occur when a giant star uses up all its fuel, collapses, and turns a vast amount of its mass into hard radiation in a few seconds, leaving behind a massive black hole," says Neil Gehrels, another Goddard astrophysicist. The star emits a blast of gamma radiation, followed by a burst of X-rays and finally, a pulse of visible light: "the death throes of a great star and the birth cries of a black hole," as Gehrels describes it.
This is physics at its most extreme. Almost daily, one of these violent stellar paroxysms is detected by astronomers (see "The light fantastic", p73). Yet only a few decades ago, scientists had absolutely no idea of their existence. The story of their discovery is one of the most extraordinary scientific detective tales of modern times and takes us from the intrigues of the Cold War to the setting up, in the 21st century, of the slickest, most complex astronomical operation ever devised: a series of satellites that track these eruptions, alerting astronomers, on 24-hour duty, so they can then turn their huge, mountain-top telescopes to study the glowing leftovers.
Explaining the existence of gamma-ray bursts has also pushed scientists to the limits of their understanding of the laws of physics. And although many mysteries have been solved, some issues still perplex astronomers. A puzzling new class of bursts has been recently discovered, for example, while it has even been suggested that radiation from these objects could have changed the course of evolution here on Earth.
And there is more. Gamma rays are the most energetic and most dangerous type of electromagnetic radiation. Astronomical sources include the Sun, though its weak emissions are blocked by our atmosphere. But the radiation from a nearby gamma-ray burst would be a different matter. "If one went off in our galactic neighbourhood, we would be fried," says astronomer Julian Osborne, from the University of Leicester in England. "The radiation could wipe out life on Earth.
Clearly, gamma-ray bursts are remarkable events. So let us look in detail at their origins, their possible impact on Earth - and the intriguing story of their discovery.
On 2 July 1969, physicists Ray Klebesadel and Roy Olson sat down in a tiny office at the Los Alamos National Laboratory in New Mexico to examine data sent back by the U.S. Vela 4 spy satellites. At the time, the U.S. was afraid the Soviet Union was planning to test atomic bombs in space, far from prying Western eyes, in order to gain a lead in nuclear weapons development. So two Vela satellites were placed in orbits over 100,000 km high and fitted with instruments that could detect gamma rays released by an atomic blast.
Can you point your radiation telescopes to the Pacific Ocean?
Can you point your radiation telescopes to the Pacific Ocean? So we may determine if radiation from Fukushima nuclear plant explosions is reaching the USA!