[ExI] thick and thicker
thespike at satx.rr.com
Wed Feb 20 20:14:55 UTC 2008
Milky Way is much bigger than we thought
Wednesday, 20 February 2008
by John Pickrell
SYDNEY: By doing simple calculations on data
freely available online, Australian
astrophysicists have discovered that our Milky
Way galaxy may be twice as thick as previously estimated.
Bryan Gaensler of the University of Sydney led
the team who now estimate the disc of our galaxy
to be 12,000 rather than 6,000 light years thick.
Proving not all science requires big, expensive
apparatus, the researchers downloaded data from
the Internet and analysed it in a spreadsheet.
"We were tossing around ideas about the size of
the Galaxy, and thought we had better check the
standard numbers that everyone uses," said
Gaensler. "It took us just a few hours to
calculate this for ourselves. We thought we had
to be wrong, so we checked and rechecked and couldn't find any mistakes."
Their results were presented at a meeting of the
American Astronomical Society in Austin, Texas,
last month, and are now slated for publication in
an academic journal. The new estimate differs
from previous calculations because Gaensler's
team was more selective about the pulsars they used as a data source.
Short for 'pulsating stars', pulsars are rotating
neutron stars - the remnants of massive suns that
have collapsed into extremely dense objects.
Rather like the beam of a lighthouse, they emit
electromagnetic radiation in the form of radio waves as they rotate.
"As light from these pulsars travels to us, it
interacts with electrons scattered between the
stars (the Warm Ionised Medium, or WIM), which
slows the light down," said Gaensler.
In particular, the longer (redder) wavelengths of
these pulses slow down more than the shorter
(bluer) wavelengths, so by seeing how far the red
lags behind the blue we can calculate how much
WIM the pulse has travelled through, he said. The
concept is similar to counting the time between a
crash of thunder and a flash of lightning to measure how far away it is.
By using the light from pulsars to measure where
the WIM ends, we can estimate the edge of the
galaxy, said Gaensler. "If you know the distance
to the pulsar accurately, then you can work out
how dense the WIM is and where it stops in
other words where the galaxy's edge is."
Less, but more
Of the thousands of pulsars known in and around
our galaxy, only about 60 have well known
distances. But to measure the thickness of the
Milky Way, Gaensler's team focussed only on those
sitting directly above or below us. Previous
estimates measured the distance to pulsars
diagonally above and below us in the direction of
the upper and lower edge of the galaxy. But this
yields a less accurate estimate of the thickness
of the galactic disc, said the researchers.
The gradual accumulation of new data has yielded
a larger number of pulsars directly above or
below us, so Gaensler's team was able to focus on
20 to 30 of these. "We used less data, but it's much more reliable."
Coincidentally, a recent study from the Max
Planck Institute for Radio Astronomy in Bonn,
Germany, suggested that there is around five
times as much magnetism in the Milky Way as would
be predicted for a galaxy of our size. Without
any prior knowledge of his work, Gaensler said
that the study proposed that a Milky Way twice as
thick as we thought might be one possible way to explain the discrepancy.
He added that it's not just magnetism, but also
heat, pressure and many other factors about our
understanding of the Milky Way that could be
knocked out of kilter if the new theory proves to
be correct. And by extension, this could effect
our understanding of all galaxies, which itself
is built on knowledge of the Milky Way.
Rate of star formation
"The study is intriguing and it could be an
important result," commented Quentin Parker of
the Anglo-Australian Observatory in Sydney.
He said the findings could mean that either the
galaxy itself is wider than we thought, or the
ionised gas of the WIM simply goes much further
beyond the stars that make up the edge of the
galaxy than we thought. "It's all a matter of
semantics. And depends whether you're talking
about the size of the galaxy in terms of dust, gas or stars."
However, said Parker, even if the results
indicate that the ionised gas goes out further
than we thought, this would mean that the
star-forming region of the galaxy is much bigger
than we thought. "If we've underestimated star
formation in our own galaxy, we've underestimated
it in other galaxies too
and this could affect
our understanding of the rate of star formation
in the history of the universe."
"Some colleagues have come up to me and have said
'That wrecks everything!'" concluded Gaensler.
"And others have said 'Ah! Now everything fits together!'"
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