around the Solar Neighborhood & our Local Chimney
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.
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.
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.
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."
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.
is presenting the findings on Thursday, May 29, at the
American Astronomical Society meeting in Nashville,
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.
have known since the 1970s that the Solar Neighborhood
lies in the middle of an enormous "Local Bubble"
of million-degree, ionized hydrogen gas, surrounded
by a wall of colder, denser neutral gas. Within this
hot bubble, gas density is much sparser, with some 100
to 1,000 times fewer hydrogen atoms, than the average
density of the rest of the Milky Way's spiral disk.
The Local Bubble was thought, at first, to be an asymmetric
cavity of 330 to 490 light-years (ly) -- 100 to 150
parsecs (pc) -- in diameter.
A Tubular Cavity
Penetrating the Local Disk
May 29, 2003, a team of astronomers (including Barry
Welsh, Rosine Lallement, Francoise Crifo, Daphne Sfeir,
and and Jean-Luc Vergely) presented the first detailed
map of interstellar space within 1,000 ly (300 pc) of
the Solar System. They also announced confirmation of
the hypothesis that the so-called Local Bubble of the
spiral disk's gas clouds actually pierces the disk of
the galaxy, perhaps as the result of exploding stars
around one or two million years ago (CNRS press release).
The tubular cavity of hot, low-density gas was found
to be irregular with tunnels branching off through the
surrounding dense gas wall, which is suspected of having
interconnecting cavities and tunnels like an interstellar
sponge, as first suggested by astronmers nearly 30 years
ago (Cox and Smith, 1974). The astronomers hypothesize
that energetic supernova explosions created fast-moving
expanding bubbles of hot gas that collided with the
surrounding cold gas of interstellar space, which in
turn became compressed into thin shells. Eventually,
these shells of cold gas met other expanding hot cavities
and broke up to form small tunnels or pathways between
the expanding voids (U.C. Berkeley press release; and
Welsh et al, 1999). Such "hot chimneys" have
been detected in other galaxies.
One portion of the Local Bubble's wall appears to have
collided and merged with the shell of another enormous
bubble of hot, ionized gas that's called (Radio) Loop
I. Located far above the galactic plane, within 490
ly of the Local Bubble, Loop I's brightest feature is
the North Polar Spur, which is thought to be created
by supernovae and stellar winds from the 13-million-year-old,
Scorpius-Centaurus Association of young and massive,
OB-type stars (more discussion and illustrations). In
addition to Loop I, astronomers have also detected also
two other expanding bubbles nearby, called LOOP II and
LOOP III (more discussion).
the last five to 10 million years, the Solar System
has been moving through the lower density region of
interstellar gas of the Local Bubble. As a result, Earth
and its lifeforms have avoided dangerous flows of cosmic
radiation and gas. Astronomers, however, have discovered
a denser cloud of interstellar gas about 25 ly (7.7
pc) in diameter called the "Local Fluff" (or
"Local Interstellar Cloud") that is moving
towards the Solar System. Stretched out towards Constellation
Cygnus, the stellar winds of young stars in a star-forming
region of the Scorpius-Centaurus Association near the
Aquila Rift (a high-density molecular cloud) have been
blowing the Local Fluff so that its denser parts may
reach Sol's heliosphere in around 50,000 years (Straizys
et al, 2003).