From: Spudboy100@aol.com
Date: Fri May 30 2003 - 08:49:02 MDT
Thoughts on the following article"
1) I wonder if, the description of our local portion of the galaxy means
that such technologies as mag sails, and the like; will become the
transportation of choice as sails react with ionized interstellar gas?
2) Does having a "swiss-cheese" galaxy promote or hinder the
development of other intelligent species. or neither?
3) Would large regions of ionized interstellar gas, affect the propagation or
detection of radio signals? If so, this would explain Fermi's paradox?
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2003-05-30
http://www.sciencedaily.com/releases/2003/05/030530081808.htm
3-D Map Of Local Space Shows Sun Lies In Middle Of Hole Piercing Galactic
Plane
Berkeley -- 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.
"This thin shell of dense gas surrounding the local void is broken in many
places," said Dr. Francoise Crifo, an astronomer at the Paris Observatory. "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."
The existence of a network of tunnels of hot gas that thread interstellar
space was first suggested nearly 30 years ago by Don Cox and Barry Smith of the
University of Wisconsin. In their model, energetic galactic supernova
explosions create fast-moving expanding bubbles of hot gas that collide with the
surrounding cold gas of interstellar space, which in turn becomes compressed into
thin shells. Eventually, these shells of cold gas meet other expanding hot
cavities and break up to form small tunnels or pathways between the expanding
voids, Welsh said.
"If you look at the new map of cold, local interstellar gas in the galactic
plane, you see an inner empty region with finger-like extensions or tunnels
that poke through the surrounding wall and reach into nearby regions of our
galaxy, such as the Lupus-Norma, Auriga-Perseus and Scorpius-Centaurus associations
of young stars," he said. "These regions are known to contain large amounts
of hot and ionized low-density gas that can be detected at radio and
ultraviolet wavelengths."
The new results also show that the local void of gas extends out of the
galactic disk and stretches into the overlying galactic halo region. In this view
of the galaxy, the cavity appears as a tube-like "chimney" that links the gas
in the galactic disk with that of the halo. Galactic chimneys have been widely
observed in other galaxies and are thought to be responsible for venting the
hot and ionized gas expelled in supernova explosions into galaxy halos. Active
vents seen in other galaxies have been labeled "galactic fountains" and the
gas falling back into the disk, "galactic rain."
"We have been searching for signs of hot gas in this local chimney using the
NASA FUSE satellite, and so far, we have found tentative signs of hot gas that
seems to be coming towards us, which could be part of a fountain that is
falling back onto the galactic plane," Welsh said.
The results of the research are soon to be published in the journal Astronomy
and Astrophysics, along with a 3-D map of the distribution of cold sodium gas
out to 1,000 light years. Lallement noted, however, that the project is not
yet complete.
"We also have information on the motion of the atoms of sodium gas we have
detected along the 1,005 sight-lines," she said. "Soon we will be able to say
whether the gas wall that surrounds our local void is coming towards the sun and
squeezing our local interstellar space, or whether it is moving away from us
such that the local void is getting larger."
"Either of these scenarios is fascinating," Welsh said. "If the wall is
approaching us, it means that a distant explosive force is pushing it towards us.
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."
Other colleagues are Daphne Sfeir, who published a preliminary map in 1999
using 350 stars, and J. L. Vergely of ACRI, a high-tech company based in
Sofia-Antipolis, France. The group used data from two other telescopes -- the Kitt
Peak telescope in Arizona and the recently destroyed Mt. Stromlo telescope in
Australia.
The research was supported by a National Aeronautics and Space Administration
contract with the Johns Hopkins University
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