A stellar flare that originated before the dawn of recorded human history delivered a mighty burst of energy to the Earth last December, astronomers announced Friday.
The burst, in the form of high-energy gamma rays, carried tremendous force, said Bryan M. Gaensler, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
\It gave off more energy in 0.2 seconds than the sun does in 100,000 to 200,000 years,"" Gaensler told The New York Times.
The flare was emitted by the star SGR 1806-20 in the constellation Sagittarius, about 50,000 light-years from Earth. That means the burst occurred 50,000 years ago but only reached us now, even though it was traveling at the speed of light.
The flare was the brightest ever recorded, and though it was not in the visible spectrum of light, it was detected by NASA's Swift telescope, only one month after the instrument was launched. Matt Bershady, a UW-Madison associate professor of astronomy, said the flare would only be detectable from space.
""Earth's atmosphere is opaque to gamma rays, so we couldn't have detected it directly on Earth. Maybe we could have measured it indirectly if the gamma rays interacted with Earth's atmosphere,"" he said.
A gamma ray is a high-energy particle, with more energy than ultraviolet rays and X-rays. Gamma rays are typically produced by neutron stars that explode and form black holes, but another type of star, called a magnetar, is also known to release periodic bursts of gamma rays. Magnetars, such as SGR 1806-20, are so named because they are stars with tremendously powerful magnetic fields, on the order of one million billion times stronger than Earth's magnetic field.
""A magnet this strong, located at about half the distance to the moon, would easily erase your credit cards and suck pens out of your pocket,"" according to NASA's website.
This discovery is legendary for several reasons. First, astronomers believe such powerful gamma ray bursts are rare, occuring perhaps once a millennium, though Bershady says that belief might reflect the fact that we never had technology to detect the bursts more frequently.
""Maybe we got incredibly lucky, or maybe our whole model of how common these bursts are is wrong,"" he mused.
Second, the burst was tremendously powerful.
""If it happened next door to us, meaning a light-year or two away, it could damage, if not destroy, life on Earth,"" Bershady said. ""But even if it's really close, it has to be pointed right at you for this to happen."" A gamma burst doesn't radiate out equally in all directions like the sun's light does. Instead, it flares out in more of a cone shape, expanding out from a point in an ever-widening circle. So even if a burst were released near Earth, if it didn't happen to be pointed in our exact direction, we'd be safe.
Third, scientists know they were fortunate to have detected the burst. Astronomers have difficulty focusing gamma rays to figure out their origin; while we can detect them, we don't always know from where they came. In this case, the point of origin was clear.
SGR 1806-20 is about one and a half times more massive than our sun, though that mass is compressed to the size of a city, meaning the star is extremely dense. It spins around its axis once every 7.5 seconds and scientists have detected it giving off smaller gamma ray bursts since 1979.
""This [observation] might be a once-in-a-lifetime event for astronomers, as well as for the neutron star,"" said David Palmer of Los Alamos National Laboratory, lead author on a paper describing the Swift observation, on the NASA website. ""We know of only two other giant flares in the past 35 years, and this December event was 100 times more powerful.""
Dinesh Ramde is a graduate student in journalism. E-mail him your brilliant science questions at dramde@wisc.edu.
