Galactic Street Sweeper

18 April 2007 (All day)

Martin Barstow

Clean sweep.
In this map of interstellar dust surrounding the sun, the white areas are nearly free of gas, while the dark areas contain gas of increasing density, suggesting something cleared out the region sometime in the past.

A team of astronomers looking for hot gas between the sun and its galactic neighbors has come up empty. The gas's absence perpetuates an x-ray mystery and could be linked to the reason why Earth remains congenial for life.

Although we think of interstellar space as a void, it’s often a messy place. The immense distances between stars can teem with dust, gas, and radiation, all leftovers from star and planet formation, supernovas, and other cosmic events. Some of this debris was thought to lurk within a few light-years of the sun and generate x-rays that have been detected by space probes and ground-based observations. Researchers suspected that these emissions originated in thin, sizzling clouds of interstellar gas.

Astrophysicist Martin Barstow of the University of Leicester in the U.K. and colleagues used the Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft, which studies cosmic rays and similar phenomena, to gauge the density of clouds located out to about 300 light-years from the sun. FUSE can detect intervening gas by analyzing the incoming light from nearby stars. But at the Royal Astronomical Society's National Astronomy Meeting in Preston, U.K., yesterday, Barstow said the FUSE data indicate the gas just isn't there. This region of interstellar space, he said, is empty. According to a summary of Barstow’s talk released by the Royal Astronomical Society, the most probable explanation for the missing gas is that the area was swept clear of material by a nearby star that went supernova within the last few million years.

The gas's absence requires a new explanation for the x-rays, said Barstow during his talk. One possibility, he noted, is the radiation is caused by interactions between charged particles at the boundary between the sun's magnetic field and interstellar space. (Barstow could not be reached for comment for this story.)

Astrophysicist Jeffrey Linsky of the University of Colorado in Boulder agrees that other recent observations of the x-ray emissions support this hypothesis. Linsky says this new picture of the galactic neighborhood containing extremely little hot gas, and correspondingly little background radiation, "will change our understanding of the environment of the sun," because there must be another explanation for the x-rays.

Evidence for a blast hasn't been noticed before because "most vestiges of such an explosion would have vanished by now," says astrophysicist Robert Petre of NASA's Goddard Space Flight Center in Greenbelt, Maryland. It's possible that the reduced amount of background radiation in the solar neighborhood could have been a factor in the emergence of humans, he adds.

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