NASHVILLE -- Objects in space seldom conform to the neat shapes mapped out in theoretical models. Upon close examination of the real thing -- whatever it is -- bumps, wiggles, bulges and even stranger structures are often found.
Sometimes theory is way off, as appears to be the case with the Local Bubble, said to surround the Sun and nearby stars.
A new study shows that the hole of sorts -- in which there are fewer hydrogen atoms than outside the hole -- is far from spherical in dimension. It's more like the internal structure of a sponge, researchers announced here today at a meeting of the American Astronomical Society.
In the first detailed map of gas within 1,000 light-years of Earth, scientists found we do in fact inhabit a sort of void in the galaxy. But the thing is an irregular cavity, surrounded by similar, smaller cavities and interconnecting tunnels, said Barry Welsh of the University of California, Berkeley.
The unexpected discovery could be the result of several exploded stars, Welsh said, and the complex structure perhaps reflect the merger of gas those stars long ago flung into space. Or the intricate cavities could have been carved by strong winds from several hot stars. Or maybe a passing star disrupted things.
Most of the gas in space is hydrogen. A new NASA satellite, called
, launched in December and is designed to map hydrogen in the local universe. That effort is expected to yield more detail on the Local Bubble, which apparently now needs a new name.Welsh and his colleagues, meanwhile, spent five years examining this nearby region of the galaxy with five ground-based telescopes. The observatories were not sensitive enough to spot hydrogen. So the researchers mapped sodium, which is known to exist along with hydrogen in relative quantities.
The nearest wall is less than 200 light-years away.
"This thin shell of dense gas surrounding the local void is broken in many places," said Ranoise Crifo, an astronomer at the University of Paris who worked on the discovery. "In several directions in the galaxy, our local cavity seems to be linked with other similar empty regions by pathways or tunnels in the interstellar medium."
There are at least two people who won't be surprised. Don Cox and Barry Smith at the University of Wisconsin theorized about this sponge-like distribution of gas three decades ago.
Their theory called for exploding stars, or supernovae, to create rapidly expanding bubbles of hot gas that would collide with colder gas in the surrounding medium. Cold shells would form. These shells would later interact with other expanding hot shells, then break apart to form the tunnels.
The new research shows the overall network of cavities and tubes extends above and below the main plane of the Milky Way Galaxy. It might serve as a chimney to vent hot gas out of the disk and into a vast, sparse and presumed spherical structure called the galactic halo, which surrounds the entire galaxy.
The findings will be detailed in the journal Astronomy and Astrophysics.
One bit of work remains: The astronomers aim to find out if the nearest wall is moving toward us or away.
"If the wall is approaching us, it means that a distant explosive force is pushing it towards us," Welsh said. "If it is expanding away from the Sun, then it seems possible that a supernova explosion took place about a million years ago that was located relatively close to our Sun."
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