In the 1970's it was discovered that stars in spiral galaxies rotate
around the center in a very unusual way. The stars in the outer
parts of the galaxies seem to be moving too fast for the galaxy
to hold them on circular orbits - they ought to fly off into intergalactic
space. Since galaxies are common and stable enough looking things,
it seems unlikely that we would be lucky enough to be getting a look at
them through our telescopes moments before they fly apart and dissolve
into the depths of space.
This simple observation has been followed up in a great many
ways over the last 30 years. The conclusion most people draw is that
what we see is not all that's out there: galaxies contain much more matter
than we can see directly. This wouldn't be such a big deal if the
amount of this so called dark matter were not so disturbingly large
- many studies indicate that most of the matter in the Universe is so dark
as to be so far invisible to our telescopes.
We think there is a lot of matter out there even if we cannot
see it. As much as 90 percent of the Universe might be dark; indicating
that, in effect, we currently do not know what the Universe
is made of!
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The main evidence
for dark matter comes from rotation curves of disk galaxies. A rotation
curve measures the rate at which stars and/or gas move in their circular
orbits around the center of the galaxy. In general the curves are "flat",
that is, the stars and gas rotate at the same speed from the inner parts
out to the edge. This is not at all what one expects from he visible matter
in the galaxies --- the outer parts of the galaxy should move slower than
the inner parts, like the planets do in the Solar System.
On the left, a spiral
galaxy image, with spiral arms delineated by so-called HII regions. On
the right, the light from a narrow strip running along the major axis
of the galaxy has been spread into a spectrum, between about 6500 and 6800
Angstroms. The rotation of the galaxy is seen in the emission lines from
H alpha at 6563 Angstroms (the brightest line), as well as other fainter
lines in this region due to Nitrogen. The "S" shaped curve allows the rotation
of the galaxy to be measured.
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