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Chapter 16
The Milky Way Galaxy
16.1 Overview

How many stars are in the Milky Way?
–

How many galaxies are there?
–
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billions and billions
How old is the Milky Way Galaxy?
–

About 200 billion
It is 15 billion years old and will remain active
for a another 10 billion years.
Are all stars members of the Milky Way
Galaxy?
What are some of the major
features of our galaxy?
Disk
 Nuclear Bulge
 Halo
 Spiral Arms
 Sun
 Globular Clusters
 Open Clusters and Nebula

Where in the Milky Way is our
solar system located?
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The solar system is located in a spiral arm
about 28,000 light years from the center of
the Milky Way.
How do we know that we are
located in the spiral arm?

In 1917 Harlow Shapley discovered that the
globular clusters form a huge spherical
system that is not centered on the Earth.
Finding Our Place
Globular Clusters
evenly distributed
Early view
Sun at center
Sun
Globular Clusters
unevenly distributed
Harlow
Shapley
Zone of Avoidance
a band running around the sky in which few
galaxies are visible
 caused by dust within the Milky Way
Galaxy

16.2 Differential Galactic Rotation
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Material closer to the galactic center orbits
with a shorter period than the material
farther from the galactic center.
16.2.1 Rotation and Mass Distribution

To the chalk board...
Chapter 17
Normal Galaxies
The Great Debate
April 26, 1920
A galaxy is a nebula
with the Milky Way!
A galaxy is an
island universes!
Harlow Shapley
Heber Curtis
M51
M104
M81
Spiral Galaxies

galaxies like the Milky Way with arcing
structures lying in a plane and emanating
from the nuclear bulge
M32 - E2
NGC 4125
M87 - E1
Elliptical Galaxies

galaxies with an elliptical shape, no spiral
arms, and little interstellar matter
NGC 1097
NGC 4123
M91
Barred Spiral Galaxies
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galaxies with a bar of stars running through
the nuclear bulge
Hubble Classification of Galaxies

Ellipticals
–
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Spirals
–
–
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From E0 (round) to E7 (oblate)
SO have nuclei but no spiral arms
From Sa (large nuclei and tight arms) to Sc
(small nuclei and the most open arms)
Barred Spirals
–
From SBa to SBc
Centaurus A
M87
Irregular Galaxies

galaxies that are asymmetrical and are
sometimes just two or more galaxies
colliding
Galactic Collisions
Chapter 18
Clusters of Galaxies
Edwin
Hubble
Galaxy Observations

During the 1920's Edwin Hubble and
Milton Humason photographed the spectra
of many galaxies with the 100 inch
telescope at Mount Wilson.

They found that most of the spectra
contained absorption lines with a large
redshift.
Red Shift and Distance
24 Mpc
1200 km/s
300 Mpc
15,000 km/s
780 Mpc
39,000 km/s
1220 Mpc
61,000 km/s
Galaxy Observations

Using the Doppler effect, Hubble calculated
the velocity at which each galaxy is
receding from us.

Using the period and brightness of Cepheid
variables in distant galaxies, Hubble
estimated to distances to each of the
galaxies.
Hubble’s Law

Hubble noticed that there was a linear
relationship between the recessional velocity
and the distance to the galaxies.

This relationship is know as Hubble’s Law:
v = Ho d
recessional velocity = Hubble’s Constant  Distance
Hubble’s Law

Ho is known as the Hubble constant and is
about 75km/s/Mpc.

This means that a galaxy that is 1
megaparsec from Earth will be moving
away from us at a speed of 75km/s.
Edwin Hubble
Cosmology

The study of the origin, structure and
evolution of the universe.

What does the Hubble law tells us about our
universe?
Our universe is expanding.
Raisin Cake Model
Like raisins in rising raisin cake, galaxies move away
away from each other in our expanding universe.
Cosmology

Hubble Time
 The
age of the universe if the expansion has been
constant.
 t = 1/Ho = ?
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The expanding universe probably originated
in an explosion called the Big Bang
between 12 and 18 billion years ago.
Cosmology

Will the universe end?
–
Present observations suggest that it will expand
forever.
What caused the Big Bang?
 Where did the energy come from?
 Why did it happen?

Unknown…
Chapter 19
Active Galaxies and
Quasars
Structure of Our Universe

Universe - all space
–
Galaxies and Quasars
Virgo Cluster of Galaxies
 Milky Way Galaxy
 Local Star Cluster
 Our Solar System
 The Earth
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