Ch. 20 - Astro1010

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UVU Survey of Astronomy
Our Milky Way
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Chapter 21
Chapter 20
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The picture below is a 360o panoramic view
of our home Galaxy the Milky Way. In as
much as we are on the inside of the Galaxy ,
our view is incomplete.
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From Earth, see relatively few stars when
looking out of galaxy (red arrows), many when
looking into the disk (blue arrows) of the
galaxy. When looking toward the bulge at the
center, most our view is blocked by dust and
dense gas.
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One of the first attempts to measure the size
and shape of the Milky Way was done by
Herschel using visible stars. Unfortunately, he
was not aware that most of the Galaxy,
particularly the center, is blocked from view by
vast clouds of gas and dust.
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Giant stars, after the
Helium Flash, pass into
the Instability Strip.
The variability of these
stars comes from a
dynamic imbalance
between gravity and
pressure – they have
oscillations around
stability.
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The upper plot is an
RR Lyrae star. All such
stars have about the
same luminosity curve,
with periods from 0.5 to
1 day.
The lower plot is a
Cepheid variable;
Cepheid periods range
from about 1 to 100
days.
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The usefulness of these stars comes
from their period–luminosity
relation
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The Period-Luminosity Relation allows us to measure
the distances to these bright giant stars.
• RR Lyrae stars all have about the same luminosity;
knowing their apparent magnitude and using the
inverse square law allows us to calculate the distance.
• Cepheids have a luminosity that is strongly
correlated with the period of their oscillations; once
the period is measured, the luminosity is known and
we can proceed as above.
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We have now
expanded our
cosmic distance
ladder one more
step .. this time a
giant step
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Many RR Lyrae stars are
found in globular clusters.
These clusters are not all in
the plane of the Galaxy, so
they are not obscured by
dust and can be measured.
This yields a much more
accurate picture of the
extent of our Galaxy and our
place within it.
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Once we could measure distances in the Milky Way, we
could determine our position in it. This artist’s conception
shows the various
parts of our Galaxy,
and location of our
Sun. Harlow Shapely
and his students
were instrumental in
determining the size
and shape of the
Galaxy.
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The Galactic Halo is made of globular clusters
and formed very early. The halo is essentially
spherical. All the stars in the halo are very old,
and there is no gas and dust in the clusters.
The Galactic disk is where the youngest stars
are, as well as star forming regions (emission
nebulae), and large clouds of gas and dust.
Surrounding the Galactic center is the
Galactic bulge, which contains a mix of older
and younger stars.
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Stellar orbits in the
disk are in a plane
and in the same
direction; orbits in
the halo and bulge
are much more
random.
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This infrared view of our Galaxy shows much more
detail of the Galactic center than the visible-light view
does, as infrared seems to penetrate the gas and
dust.
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Any theory of galaxy formation must be able to
account for all the properties listed below
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Measurement of
the position and
motion of gas
clouds shows that
the Milky Way has
a spiral form
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One method that astronomers have used to calculate
the Mass of the Milky Way galaxy is to use Kepler's
3rd law. We take the radius of our orbit and the
period. Plugging those numbers into Kepler's 3rd law
we can estimate the mass inside our orbit. This gives
just shy of 100 billion solar masses. Other mass
estimates of the Milky Way are up to 1 trillion solar
masses.
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Once all the Galaxy is within an orbit, the Orbital
Velocity should diminish with distance, as the dashed
curve shows. It doesn’t; more than twice the mass of
the Galaxy would have to be outside the visible part to
reproduce the observed curve. This mass is called
Dark Matter
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There is much more dark matter in the Universe
than bright. Some scientists think 90 percent of
matter is dark.
Astronomers know about dark matter because its
gravity pulls on stars and galaxies, changing their
orbits and the way they rotate.
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Matter is space matter we cannot see because,
unlike stars and galaxies, it does not give off light.
This is a view towards the
Galactic Center, in visible light.
The two arrows in the inset
indicate the location of the
center; it is entirely obscured by
dust.
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Because of interstellar dust along the
line of sight, the available information
about the Galactic Center comes from
observations at gamma ray, hard X-ray,
infrared and radio wavelengths.
Coordinates of the Galactic Center were
first found by Harlow Shapely in his
1918 study of the distribution of the
globular clusters.
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Direct study of a black hole such as the one widely
suspected to exist at the center of our galaxy is
tricky, as black holes swallow up nearby light,
rendering themselves virtually invisible. But
researchers can infer properties of a black hole from
its hearty gravitational influence on nearby stars. A
large star S2 in that region was found to orbit a dark
concentration of mass, estimated at 3.7 million
times the mass of our Sun The laws of physics have
ruled out any explanation but one--that this is
indeed an enormous black hole.
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Direct study of a black hole such as the one widely suspected to
exist at the center of our galaxy is tricky, as black holes swallow
up nearby light, rendering themselves virtually invisible. But
researchers can infer properties of a black hole from its hearty
gravitational influence on nearby stars. A large star S2 in that
region was found to orbit a dark concentration of mass,
estimated at 3.7 million
times the mass of our
Sun The laws of physics
have ruled out any
explanation but one—
that this is indeed an
enormous black hole.
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End of Chapter 20
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Chapter 20
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