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The Question
(Submitted July 14, 1998)
What proof do astronomers have when saying our galaxy looks a certain way?
All evidence would be indirect I would think. What is this evidence?
The Answer
You have a very good point here: How can we have any idea
what our galaxy looks like from the standpoint of an outside
observer, when we ourselves are embedded in the middle of it?
As you surmised, some detective work is required. If we were
to rely only on optical light in trying to formulate an
educated guess as to the answer, it would be next to impossible
because of all the obscuration due to dust in the plane of our
galaxy. When one goes to other wavelengths, however, the
project becomes do-able.
The first work which mapped out the
spiral structure of our galaxy was done at radio wavelengths
by studying the "21 cm" line which is due to the "spin-flip"
transition of the hydrogen atom. Basically, this means that
the hydrogen atom can have slightly different energy states
depending on whether the spin of the nucleus is parallel
or anti-parallel with the spin of the whole atom. This change
from one state to the other produces this emission, which
passes through the dust. By investigating the strength of this
emission as a function of point on the sky, early workers
(i) determined that we live in a spiral galaxy, and
(ii) mapped out the spiral arms. In addition to measuring
the strength of the signal at different points of the sky,
one also makes use of the Doppler shift of the signal to
infer the velocity structure.
More recently it has become feasible to perform this
exercise at other wavelengths. For instance, the COMPTEL
instrument on the Compton Gamma Ray Observatory has mapped
out the spatial distribution of radioactive aluminum 26
which produces an emission line at 1.8 million electron volts.
To see what kind of galactic structure they have inferred,
take a look at the figure in the following paper. (The
paper itself is rather technical, but the figure just shows
a picture of the spiral structure of the galaxy in our
vicinity.) The paper is just 2 pages, with one figure.
(i) Go to this site
http://adsabs.harvard.edu/abstract_service.html
(ii) In the author field enter: Chen, Gehrels, Diehl, Hartmann,
then select the Boolean "and" search option, and click
"send query". This will get you the following paper:
"On the spiral arm interpretation of COMPTEL 26 Al map features"
Astronomy & Astrophysics Supplement Series vol. 120, 315. (1996)
It has just been pointed out to me that it may be difficult
for you to access this reference in the manner I suggest if you
are on other than a UNIX machine. In that event, it may be
necessary to find a university library to get this paper.
Of possible interest, here is the Multiwavelength Milky Way
poster site
http://adc.gsfc.nasa.gov/mw/milkyway.html
J.K. Cannizzo,
K. Smale,
D. Palmer
for Ask an Astrophysicist
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