Two decades ago astronomers discovered that our solar system and a few thousand neighboring stars lie just inside a vast bubble, within which the thin gas of interstellar space is much thinner still--and also hotter. But in the years since that discovery, astronomers haven’t been able to explain the origin of what they’ve come to call the Local Bubble. Now they may finally have solved the mystery. The Local Bubble may be the remnant of a supernova that exploded roughly 300,000 years ago and less than 200 light-years away. Seen from Earth--and our hominid ancestors would very likely have seen it--that stellar catastrophe would have been as bright as the full moon.
The proposed solution to the Local Bubble mystery depended on, and came on the heels of, the resolution of another long-standing astronomical puzzle. Late in 1972, astronomers detected a powerful source of gamma rays, the most energetic form of electromagnetic radiation, in the constellation Gemini. Researchers tried to pin the gamma rays on some object they already knew--something that emitted X-rays, radio waves, or visible light--but for a long time they failed. Astronomers vented some of their frustration when they named the mysterious source, calling it Geminga, from an expression in Italian dialect meaning it’s not there.
Last spring, astronomers finally figured out what sort of beast Geminga is. Jules Halpern of Columbia University and Stephen Holt of NASA, using the orbiting X-ray telescope ROSAT, found a pulsing X-ray source in the same part of the sky as Geminga. That led David Bertsch of NASA to reexamine records of Geminga’s gamma rays for evidence that they, too, were coming in pulses. Sure enough, he found that Geminga was pulsing--and at the same rate, every two-tenths of a second, as the X-ray source.
That meant the X-ray source and Geminga were one and the same pulsar: the dense, rapidly spinning core of a star that exploded as a supernova. As a pulsar spins, it emits a narrow stream of radiation, like the beam from a lighthouse; astronomers detect it only if the beam happens to sweep past Earth.
So what does all this have to do with the Local Bubble? In February a team of Italian astronomers announced that Geminga was moving so rapidly across our sky that it must be fairly close to Earth--and to the Local Bubble. The finding struck Neil Gehrels, an astronomer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, as more than a coincidence.
Gehrels and his colleague Wan Chen have proposed that the supernova that gave birth to Geminga also created the Local Bubble. When Geminga’s parent star blew up, says Gehrels, the explosion cleared our neck of the galaxy of most interstellar gas and dust, creating a wind sock-shaped bubble some 600 light-years across. A supernova shock wave is like a snowplow pushing through the interstellar medium, pushing the more dense gas out, says Gehrels.
The Geminga supernova, he says, exploded at the right time and place to carve out the Local Bubble. Judging from how much Geminga’s rotation has slowed down since it was born--astronomers think a newborn pulsar rotates a thousand times a second or so--it is no more than 340,000 years old. That is in the same ballpark, at least, as the age of the Local Bubble. A supernova-spawned bubble, says Gehrels, probably lasts no longer than a million years: The pressure on the outside of the bubble is more than the pressure on the inside. So once the supernova shock wave has used all its energy to push the gas away, the gas outside starts pushing back in, and the bubble collapses.
And although at present Geminga lies outside the Local Bubble, that’s only because the pulsar has moved since the explosion, perhaps because the explosion sent it rocketing through space. Using the Italian observations of Geminga’s motion, Gehrels and Chen traced the pulsar back to its place of birth. We played the movie backward and saw where this star would have been 300,000 years ago, says Gehrels. Ground zero for Geminga’s fiery explosion turned out to be less than 200 light-years away in the constellation Orion--right at the mouth of the wind sock, and in just the right place to create the bubble that now envelops the sun, the planets, and us.