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University of California-Berkeley

Media Contacts:

Robert Sanders
(510) 643-6998, (510) 642-3734
rls{at}pa.urel.berkeley.edu

Additional Resources:

Barry Welsh
(510) 642-0305, bwelsh{at}ssl.berkeley.edu

FOR IMMEDIATE RELEASE: Thursday, May 29, 2003

3-D map of local interstellar space shows sun lies in middle of hole
piercing galactic plane

Part 1 of 2

Nashville, Tenn. -- The first detailed map of space within about
1,000 light years of Earth places the solar system in the middle of a
large hole that pierces the plane of the galaxy, perhaps left by an
exploding star one or two million years ago.

The new map, produced by University of California, Berkeley, and
French astronomers, alters the reigning view of the solar
neighborhood. In that picture, the sun lies in the middle of a hot
bubble -- a region of million-degree hydrogen gas with 100-1,000
times fewer hydrogen atoms than the average gas density in the Milky
Way -- and is surrounded by a solid wall of colder, denser gas.

Instead, said astronomer Barry Welsh of UC Berkeley's Space Sciences
Laboratory, the region around the sun is an irregular cavity of
low-density gas that has tunnels branching off through the
surrounding dense gas wall. Welsh and his French colleagues suspect
that the interconnecting cavities and tunnels, analogous to the holes
in a sponge, were created by supernovas or very strong stellar winds
that swept out large regions and, when they encountered one another,
merged into passageways. 

"When we started mapping gas in the galaxy, we found a deficit of
neutral gas within about 500 light years, suggesting that we are in a
bubble-shaped cavity perhaps filled with hot, ionized gas," Welsh
said. "But the Local Bubble is shaped more like a tube and should be
called the Local Chimney." 

If this system of interlocking, gaseous cavities is characteristic of
the entire galaxy, it presents a dramatic confirmation of a
30-year-old theory of the Milky Way, Welsh said.

Welsh is presenting the findings on Thursday, May 29, at the American 
Astronomical Society meeting in Nashville, Tenn.

At the moment, the origin of the cavities is anybody's guess, Welsh
said. The local cavity has been around for a few million years and
could easily have been caused by a supernova punching through the top
and bottom of the galactic disk, the intense stellar winds from 10 or
so hot stars, a powerful gamma-ray burst, or even a large star moving
through the area. Each of these could theoretically sweep dense gas
out of the region, leaving only tenuous, ionized hydrogen. 

Three recently developed satellites could shed light on the mystery.
The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) satellite,
built at UC Berkeley's Space Sciences Laboratory, was launched last
December to look for hot, 500,000-degree Celsius gas near our solar
system. The UC Berkeley-built SPEAR (Spectroscopy of Plasma Evolution
from Astrophysical Radiation) instrument, to be launched later this
year as the primary payload of the Korean KAISTSAT-4 satellite
mission, will detect the presence of warm gas -- about 250,000
degrees Celsius -- in the solar neighborhood. NASA's Far Ultraviolet
Spectroscopic Explorer (FUSE) satellite is also currently searching
for this hot gas. 

With only ground-based telescopes at their disposal, Welsh and his
colleagues could not look directly for cold neutral hydrogen (H),
since the density is about 10 times too low for radio telescopes to
detect. Instead, they looked for a surrogate -- cold neutral sodium,
which is found wherever cold, dense hydrogen is found. Using five
separate telescopes, they searched for the cavity walls where the
density of cold neutral sodium becomes high enough to detect. 

"We used several ground-based telescopes, including the Observatoire
de Haute Provence in France, the European Southern Observatory in
Chile and the Lick Observatory in California, to detect atoms of gas
in interstellar space towards over a 1,000 nearby stars," said Dr.
Rosine Lallement, the project leader at the Centre National de la
Recherche Scientifique (CNRS) in Paris. "In collaboration with Dr.
Barry Welsh at UC Berkeley, this project has taken over five years to
accumulate and analyze all the data."

A key factor in mapping the local interstellar space was recent data
from the European Hipparcos satellite, which has provided highly
accurate distances to nearby stars, improving significantly over
distances obtained through ground-based measurement of parallax.

By locating stars that showed no absorption by sodium and those that
did, they were able to construct a three-dimensional picture of the
edge of the low-density region surrounding our solar system. The
1,005 stars they looked at were all hot, blue Type A and B stars,
because it's easier to pick out sodium absorption lines from their
spectra. 

"Eventually, our measurements towards more distant stars started to
pick up large numbers of sodium atoms, indicating that we had
stumbled across a dense neutral-gas boundary, or 'wall,' to our local
cavity," Welsh said. The nearest wall is 175-190 light years from
Earth, in the direction of the center of our galaxy.

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