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Where we live:
Milkyway Galaxy
Orion Arm
System of Sol
Third Planet
Galaxies
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Herschel
Band of light
stretching in the
night sky was known
to the ancients.
Methodical study of
the sky (1785).
Divide into little
squares and count
the number of stars.
More stars on the
diagonal.
Sun at the center.
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Milkyway Galaxy
50,000 ly radius halo (nonluminous matter)
1,000 ly nuclear bulge
Average size galaxy
Spiral galaxy with 3 ½ arms.
Perseus, Cygnus, Carina, Orion
80,000 lightyears diameter
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Center of our galaxy
At the center of our galaxy there
is a super massive blackhole.
The size of this blackhole is
estimated to be 4 million Solar
masses.
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Size of the galaxies
Copernicus  Earth is not the center of the Solar system
Harlow Shapley  Sun is not the center of the Milkyway
galaxy.
Cepheids and RR Lyrae are at
the end of their lives and they
change their brightness in a well
known way.
As their brightness is known
they can be used as standard
candles to determine distances.
Shapley used 93 variable stars
to measure the distances in our
galaxy.
And proved that we are not at the
center of our galaxy.
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Rotation Speed
Our galaxy rotates around its center.
The sun revolves around the galactic
center every 200,000,000 years.
The density of stars increase when
we go closer to the galactic center.
The mass of the stars in a galaxy can
be calculated by the revolution speed
of the stars around the galactic
center.
The more mass, the higher should be
the revolution speed.
However…
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Dark Matter
Sun revolves around the
galactic center much
faster than the visible
matter of the galaxy
suggests 
There is more mass in the
galaxy than we can see
DARK MATTER
Not just a little to make
the calculations correct,
but around 80-90% of
Milkyway’s mass in dark
matter.
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Types of Galaxies
Classification made by Edwin Hubble in the 1920s.
Three types of galaxies: Elliptical, Spiral and Irregular
Spirals are further divided into regular spirals and barred spirals.
These groups are further divided among themselves depending on how
large the nucleus is (spirals) or how spherical they are (elliptical).
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Other Galaxies
10th century astronomer Al-Sufi  M31 – Andromeda galaxy, Little Cloud
Magellan  Clouds in the Southern Sky  Magellanic Clouds
Messier  1781 Catalog of nebulous objects (M31)
Herschel  New General Catalog (NGC3034 = M82)
Big question: Where are they located??
Shapley (no galaxies) – Curtis (galaxies) debate (1920)
Hubble and Humason resolved in 1932.
 discovered many Cepheid variables in the Andromeda galaxy
 they are not in this galaxy, far far away.
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Elliptical Galaxies
M32: Satellite of
Andromeda
M87: Virgo cluster
M110: Satellite of
Andromeda
Leo I: Local group
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Spiral Galaxies
NGC 2997
NGC 3351
= M95
Triangulum
Galaxy (M33) in
our local cluster
NGC 1365
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Irregulars
Small Magellanic Cloud
Large Magellanic Cloud
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Clusters
Galaxies love to cluster together.
Hercules cluster: A poor cluster
Coma cluster: A rich cluster
Milkway is part of a poor cluster
called the Local Group.
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Galaxy Collisions
If there is clustering because of gravitational
interactions, there is bound to be some collisions.
BTW, we are going to collide with the Andromeda
galaxy in a few hundred million years.
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Hubble’s Law
In 1931, Hubble and Humason discovered that more distant galaxies are
moving faster away from us.
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Quasars
Very difficult to distinguish from regular
stars in the sky.
However, they have a very large red-shift
meaning that they are very far away
(Hubble’s Law).
They are also changing their luminosity very
rapidly, we can estimate their sizes.
Optical observations confirm that they are
at the center of galaxies far away.
The combination of all this information tells us that:
The size of a quasar is less than a lightyear in diameter.
They are about 1000 times brighter than our galaxy.
They can be at distances about 10 billion lightyears away, but there are no
quasar located farther away than that.