Even our unaided eyes tell us that we live in some kind of disk structure' We see the Milky Way in t

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Even our unaided eyes tell us that we live in
some kind of disk structure. We see the Milky
Way in the summer time as we look toward the
center (white arrow). We see a thinner Milky Way
in the winter time as we look opposite the center
(blue arrow) and we see fewer stars in the fall
and spring as we look out of the disk (red
arrows).
What we think we live in
What we really see
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Examples of different spiral galaxies seen from
different orientations.
Satellite Galaxy
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While we can deduce the overall shape of our
galaxy based on visible star counts, it is
difficult to be specific because many of the
stars are obscured by gas and dust. For example,
the actual center of our galaxy is not the
brightest region in the sky because of this fact.
William Herschels 18th century map of the
galaxy
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Because it is difficult to accurately determine
the spectral type of a star and its absolute
luminosity we cant use 1/d2 law very
accurately. A class of stars called RR Lyrae and
Cepheid variables have the unique property of
having periodic pulsations which are related to
their absolute brightness. Thus we can tell
their absolute brightness by simply measuring
their periods. Then we can use 1/d2 law to get
their distance.
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RR Lyrae and Cepheid variables have now increased
our ability to measure distance to galactic sizes.
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With variables stars as a new measuring
technique, astronomers could map out their three
dimensional distribution. Since many variables
stars are found in globular clusters, we map
their distribution and found they map out a
spherical distribution nearly 30 kpc across.
The center was offset from our sun by 8 kpc
which shows the distance the Sun is from the
galactic center.
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What we think our galaxy looks like from
above Our galaxy is 100s of times wider than it
is thick.
How we see our galaxy in the infrared
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Close to the Sun the motions of stars appear
random. On larger scales, the stars in the disk
revolve around the center differentially while
the Halo is still random. It takes the Sun 225
million years to revolve once. Thus, it has been
around the galaxy only 20 times.
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The scenario for the formation of galaxies is
very reminiscent of the formation process for
solar systems. Giant cloud of gas and dust,
shrinks under its own gravity, rotation produces
a flattened disk. The halo objects probably
formed first and retained their spherical
distribution and didnt get pulled into the disk.
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Spectroscopic radio astronomy allows astronomers
to look through our galaxy and see the Doppler
velocities of hydrogen through its 21 cm
spin-flip line. With this information
astronomers can recreate the 3 dimensional
structure of our galaxy including the number of
spiral arms, etc. This is what we think our
galaxy looks like.
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Astronomers still cant explain well why we have
spiral arms in galaxies. The stars, gas, and
dust in the galaxy all obey Keplers laws of
planetary motion. Thus the inner material
rotates faster than the outer material. If
spiral arms were tied to the galaxy then after a
few 100,000,000 years they would all be wound up
and loose their shape.
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The density wave theory says that a compression
front moves through the disk and compresses the
gas and dust to start the star formation process.
In this model new material is being constantly
fed into the density regions as old material
leaves.
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An excellent simulation of the density wave
theory may be found at http//members.lycos.co.u
k/DavideSardella/P16GalaxyFrame.htm
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Since material orbiting the galaxy must obey
Keplers laws, if we could measure the distance
and orbital period of material as a function of
distance from the center , we could determine the
amount of mass inside that distance. When we
reach the edge of the galaxy, we expect the
rotation speed to diminish, but instead it keeps
on increasing as if there is some invisible
(dark) matter.
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One attempt to detect dark matter is to look for
gravitational lensing. Even if the matter is
dark it should disturb the space-time continuum
and produce gravity like any other matter. While
we have seen several such events, they could only
account for about 1/2 the dark matter.
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What is at the center of our Galaxy? Theory
predicts a densely populated galactic bulge, but
we can only see 1/10th of the way to the center
using optical means. Radio astronomy shows
(1) The density at the center of our Galaxy is
about 1 million times greater than the density at
our solar system. (2) There is a bright ring
of molecular gas around the center. There are
filaments near the center, as seen on the sun,
indicating a strong magnetic field. (3) At
the center of our Galaxy there is a bright radio
source Sagittarius A, an object only 10 A.U.
across but containing the mass of 2-3 million
sunsa black hole??