Download The Milky Way Galaxy

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The Milky Way Galaxy
The Milky Way appears as a diffuse band of light
encircling the celestial sphere.
Early attempts to probe the size of the galaxy were
unsuccessful due to intervening dust.
In the early 1900s, Harlow Shapley devised a method to
determine the distance to the galactic center.
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The Milky Way galaxy can be described by
Disk:
Halo:
Nuclear Bulge:
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Determining mass of Milky Way galaxy:
Distance between Sun and galactic center ≈ 28,000 lt yrs
≈ 1.8x109 AUs
 V ≈ 220 km/s
 Periodorbit ≈ 2.4 x 108 years
Apply Newton’s version of Kepler’s third law:
Minner galaxy + Mʘ = a3/p2
 a = distance to galactic center in AUs
 p = orbital period of Sun in years
Minner galaxy ≈ 1011 Mʘ
Assuming the average star has mass of 0.5 Mʘ, ≈ 200
billion stars in galaxy (interior to Sun’s orbit)
The outer disk of our galaxy rotates much faster than can
be accounted for by the gravity of the visible mass.
Dark Matter:
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Galaxies
Early astronomers often referred to these fuzzy patches of
light as spiral nebulae. They were believed to be either
 small, nearby objects within the Milky Way
(Shapley)
or
 large objects like the MW that are very far away and
outside our galaxy—“Island Universes” (Curtis)
Controversy reached its peak in 1920 with the CurtisShapley debate. Issues to be resolved:
 What are the distances to the spirals?
 Are spirals composed of stars or gas?
 Why do spirals “avoid” the plane of the MW?
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1923: controversy finally resolved by Edwin Hubble
 Used Cepheid variable stars to determine the distance
to the “Great Spiral in Andromeda”
There are millions of galaxies seen in all directions, and
density counts indicate there are billions in the observable
universe.
Clusters: just as stars form in groups, so do galaxies
The Milky Way and Andromeda galaxies are part of a
“poor” cluster called The Local Group
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A typical “rich” cluster contains thousands of galaxies
and is tens of millions of light years in diameter.
Clusters are also members of even larger associations
called Superclusters.
 Represent largest scale of structure in the universe
 Typically hundreds of millions of light years across
containing million of galaxies
Galaxies are not evenly distributed throughout the
universe.
 Distribution best described as “frothy” with galaxies
on the surfaces of enormous bubbles
The velocities of galaxies within clusters often exceed the
escape velocities calculated from the observed mass
→ more evidence of dark matter
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Cosmology
Cosmology: the study of the origin, evolution, and
ultimate fate of the universe.
Cosmological principle: on a large scale, the universe
should look essentially the same from any vantage point.
 No edge and no center!
There are two testable predictions of the cosmological
principle. The universe should be:
1) Homogeneous: at the largest scale, matter is spread
uniformly throughout the universe.
2) Isotropic: In general, the universe looks the same in
every direction (applies to expansion).
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These two predictions (and the assumption that the
universe was infinitely old) originally lead to a problem.
Olber’s Paradox: Why does the sky get dark at night?
Resolution to Olber’s paradox: universe is not static in
time or space.
 The universe is expanding, which implies it had a
beginning
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Expansion of the universe: most profound cosmological
discovery of the 20th century
 actually predicted in Einstein’s original equations for
General Relativity
Expansion discovered by Edwin Hubble in 1929
 By measuring the redshift of a galaxy, can determine
its radial velocity using the Doppler equation
 direct relationship between distance and velocity
indicates expansion—Hubble’s Law:
V = H•D
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H = V km/s
D Mpc
km and Mpc are both units of distance
Can estimate the age of the universe from H
Age ∝ 1 (after canceling units of distance)
H
 Gives time elapsed since all galaxies were in the
same place at the same time
 If H = 100 → age of universe ≈ 10 billion years
 If H = 50 → age of universe ≈ 20 billion years
Recently, H determined to be ≈ 72 km/s/Mpc, which
yields an age of ≈ 14 billion years