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An Introduction to Astronomy
Part XII: The Milky Way and other
Galaxies
Lambert E. Murray, Ph.D.
Professor of Physics
Fish-Eye
View of the
Night Sky
The Nature of the Milky Way
• The Milky Way, as the bright central region in the
last picture is known, has been visible to
observers from ancient times.
• The nature of the milky way, however, way was
not know until Galileo began to make
observations with his telescope. He observed that
the Milky Way was a large collection of stars
concentrated in a certain region of the sky.
• Star counts by Sir William Herschel in the 1800s
confirmed the “disk” nature of the Milky Way.
Based on his data, Herschel concluded,
incorrectly, that the sun was at the center of the
disk.
Wide Angle View of Milky Way
Computer Generated Picture of
Milky Way Galaxy
But What is the Milky Way
Is it the sum total of all the stars in our universe –
are these all distributed in a disk?
 Or is the Milky Way but one member of a large
number of “island universes” as was proposed by
Immanuel Kant?
 After 1845 Lord Rosse examined many of the
“nebulae” which Hershell had observed and found
that some of these has a distinctly spiral shape.
 The lack of resolution in the telescopes of the day
made it difficult to determine if these “nebulae”
were gas clouds are distant groups of stars.

Higher Resolution:
Gas Cloud or Island Universe?
The Answer:
A New Standard Candle

The term “standard candle” is the name given to a
stellar distance measuring devise.
– Remember that stellar distances can be determined if
the luminosity of a star is known.
– A “standard candle” is a star with known luminosity

Henrietta Leavitt in 1912 published her study of
Class I Cephied variables found in the Small
Magellanic Cloud.
– At the time it was not known that the Small Magellanic
Cloud was a galaxy apart from the Milky Way. It was
believed to be just another grouping of stars within the
Milky Way like other open- and globular-clusters.
The Period Luminosity Curve for
Cephied Variable Stars
Leavitt assumed that all the Cepheid variables
found within the Small Magellanic Cloud were
roughly the same distance from us.
 She found that those Cephied variables with
greater apparent magnitude had longer periods
than those with less apparent magnitude.
 Using the assumption that all the stars are equal
distant from us, and that their apparent magnitudes
are related to their period, we can derive a
luminosity – period relationship if we can
determine that absolute magnitude of a single
Cephied variable.

Period – Luminosity Relationship for Cepheid
Variables
Note on This Distance Scale



When Cepheid variables were first utilized, there were no
Cepheid variables close enough to measure their distance
using direct parallax measurements.
Distances to the Cepheid variables in our own galaxy were
determined using statistical techniques, and were
dependent upon these theoretical calculations.
This situation recently changed with the Hipparcos
satellite, and we were able to get a more accurate measure
of this important “standard candle”. It turns out that the
Cepheid variable stars are actually brighter than was
initially guessed using the statistical arguments. As a
result, we now know that objects are actually about 10%
farther away than we previously had thought, i.e., the
universe is about 10% larger than we thought.
Hubble’s Measurement for M31
Making use of a Cepheid variable star he observed
in M31 (a spiral-shaped nebula), Hubble
concluded that this nebula was 2.2 million light
years beyond the Milky Way, thus establishing the
existence of “island universes”, or what we today
call galaxies.
 Today we recognize M31 as the Andromeda
Galaxy, the only object not part of the Milky Way
that can be seen with the naked eye from the
Earth’s northern hemisphere.

M31 – The Andromeda Galaxy
Star Clusters Found in the Milky
Way
There are basically two types of star clusters found
in the Milky Way:
 Globular Clusters – Spherical clusters of a large
number of stars. These star clusters are found to
be distributed approximately spherically about the
center of the galaxy.
 Open Clusters – These clusters are relatively open
in structure and are found more in the spiral arms
of the galaxy, often associated with areas rich in
interstellar gases and dust.

Globular Clusters
 A globular cluster
is a spherical group of
up to hundreds of thousands of stars, found
primarily in the halo of the Galaxy.
 The average separation of stars near the
center of a globular cluster is 0.5 light-year.
Stars in the region of our Sun average 4–5
light-years apart.
Globular Cluster M13
An Open
Cluster
Shapley Uses RR Lyra Variables to
Determine our Place in the Milky Way
Shapley made use of RR Lyra variable stars found
in many globular clusters to determine the
distances to these clusters.
 RR Lyra variables, like the Cepheid variables also
have a period – luminosity relationship.
 By measuring the distances to, and the relative
locations of the globular clusters, Shapley
determined that these clusters were spherically
distributed about a point not centered on the Earth.
 He made the correct assumption that these clusters
are clustered around the center of the galaxy.

Shapley’s Conclusions
• Shapley correctly concluded that the sun lay about 2/3 of the
way out from the center of the galactic disk.
• Shapley’s distance measurements, however, were wrong, for
like many early astronomers, he was unaware of the
importance of the Interstellar Medium (or ISM) and the
dimming produced by the interstellar dust.
• This same mistake had earlier convinced William Herschel
that the earth was at the center of the galaxy. He had argued
that the center of the galaxy should have more stars. But
when he counted the stars is the Milky Way he found that
there were about as many stars is one direction as the other.
He did not know about the obscuring gas and dust in the
center of our field of view.
Our
Galaxy
Radar Mapping of the Milky Way
 Radar
can penetrate dust and clouds, unlike
the visible region of the electromagnetic
spectrum.
 Radar was used to map the surface of Venus
through its constant cloud cover.
 Radar has been used to map the location
and relative motion of different parts of the
Milky Way Galaxy because it can penetrate
the gas and dust clouds in the plane of the
galaxy.
Our Galaxy
as Mapped
by Radar
What it Might Look Like in
Visible Light
Galactic Rotation
 Measurments
of the velocities of large
numbers of stars in the Milky Way allow
astronomers to determine the speed of the sun
as it orbits the center of the galaxy.
 The sun, and most of its neighboring stars are
found to move at ~ 220 km/sec.
 Thus, it takes about 240 million years for the
solar system to make one complete orbit.
Knowing the orbital speed, one can calculate the force of
gravity necessary...which in turns tells how much mass is
necessary to keep the sun in orbit
This mass is about 1011 M0
Assuming most stars are
like the sun, gives about
100 billion stars in the
galaxy.
force of gravity
solar system
Consider a galaxy like our own.......
Rotation Curves
For Theoretical Galaxies
Rotation curve if all the
known mass were located at
the center
Rotation curve if most mass
were near the center tapering
off near the edges
Actual rotation curve for the
Milky Way galaxy
Rotation Curves for Other Spiral
Galaxies
An Interesting Dilemma
The rotation rate of a star in the galaxy depends
upon the mass distribution in the galaxy.
 A number of spiral galaxies have rotation curves
that are similar to one another.
 These rotation curves are inconsistent with the
assumption that most of the mass of the galaxy is
located toward the center of the galaxy – although
that is the way things appear in the regions of the
spectrum that we can detect.

Dark Matter
This would seem to indicate that there is a large
amount of matter which is not “visible” in the
outer arms of the spiral galaxies.
 We have already noticed the existence of large
dark nebulae associated with cool gases and dust.
 Much of the mass of a galaxy must be “dark
matter”.
 The nature of this “dark matter” is an active topic
of discussion in the astronomical community
today.

Galaxies, Galaxies, Everywhere
The Universe if Full of Them
We now know that there are galaxies in all
directions – thousands of them (perhaps millions)
that we can see.
 These galaxies are distributed throughout space,
but do seem to show some groupings indicating
mutual gravitational interactions
 The following diagram is a plot of the location of
some 9325 galaxies in two wedges of the universe

A Plot of
9325
Galaxies
Our Local
Group
Classification of Galaxies
 Galaxies
are typically classified by their
shape.
 The three major shapes are:
– Spiral
– Elliptical
– Bar
 These
are then subdivided according to the
following scheme.
Hubble’s Observation Relating the
Nuclear Bulge to the Spiral Arms
 Hubble,
once he realized the “spiral
nebulae” were really galaxies spent much
time trying to classify these galaxies.
 He noted that the size of the nuclear bulge
in spiral galaxies is correlated to the
tightness of the spiral arms – the larger the
bulge, the tighter the spiral arms.
Classification of Spirals
Note the correlation of the relative size of the nuclear bulge to the
tightness of the spiral arms.
Determining Distances to Galaxies
Cepheid variable stars can be used to measure the
distances to galaxies that are within about 60 Mpc
(200 million ly).
 Beyond that other standard candles must be used
– these all depend upon knowing (or at least
having some estimate of) the absolute magnitude
of the standard candle.
 Other standard candles which are used.

– Red and Blue Supergiants (out to about 250 Mpc or 800
million ly)
– Bright Globular clusters (out to 400 Mpc)
– Bright HII regions (out to 900 Mpc)
– Supernova explosions (beyond 900 Mpc)
The Hubble Law
As early as 1914, Slipher, working at the Lowell
Observatory had observed that a large number of
spiral galaxies that he had been studying exhibited
a red shift in their spectra – indicating that most of
these galaxies were moving away from us.
 During the 1920’s Edwin Hubble determined the
distances to a number of these galaxies using
Cepheid variable stars and noticed a correlation
between the distance to the galaxies and the
observed red–shift of the spectrum.
 He found that the farther away a galaxy was, the
greater the red-shift.

Galaxies and
Their Red-Shift
The Hubble Law
The Hubble Law
as a Standard Candle
 The
Hubble relationship has been so well
established that it is now used to determine
the distance to far away galaxies.
 We appear to be in an ever-expanding
universe where the galaxies are moving
farther and farther away from one another
(much like raisins in raisin bread as it rises).
End of Part XII