Our solar system sits inside a bubble of space that is mysteriously empty. Now, an astronomer suggests that the area was blasted clean by several massive explosions from stars in a cluster that has since migrated away from us. The theory, reported in the 10 October issue of Astrophysical Journal, could settle a long-standing debate about the origin of the so-called Local Bubble and offer clues about how many such bubbles exist inside our galaxy.
In the mid-1970s, astronomers began to recognize that the space around the solar system has a density less than 1% of normal. Many believed that only powerful explosions from dying stars, or supernovae, could explain this interstellar void. But there are so few stars nearby that it's highly unlikely that three or more would explode within the required few million years, and those low odds have mystified scientists.
Now, astronomer Jesús Maíz-Apellániz of the Space Telescope Science Institute in Baltimore, Maryland, may have found the answer. Maíz-Apellániz analyzed the movements, velocities, and present positions of stars in the Scorpius-Centaurus cluster, just outside the bubble. Extrapolating back, he found that 2 million to 8 million years ago, dozens of the stars were inside what's now the Local Bubble. That number was high enough for Maíz-Apellániz to estimate that about six exploded, demolishing almost everything nearby. In the same time period, the sun and other stars moved in, although the bubble remains relatively empty.
"It's the most likely theory I've heard in the last 5 or 10 years," says Steven Snowden, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Snowden believes that Maíz-Apellániz's theory could help astronomers determine the frequency of such bubbles-giving us a better understanding of what our galaxy looks like.