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PENN STATE
ASTRONOMY LABORATORY
# 11
THE STRUCTURE OF THE MILKY WAY GALAXY
I. Objective
In this lab, you will learn that we live in the Milky Way Galaxy. Our solar system and all the
stars you can see with your own eyes are in this galaxy. You will learn where we are in the galaxy,
what it looks like, and how old it is.
II. Exercises
Our Milky Way Galaxy is made up of a disk (which you see in the sky as the band of the
Milky Way), a bulge (a large clump of stars surrounding the galactic center), and a halo (a larger,
spherical cloud of stars that surrounds the entire galaxy). The halo is much larger than the bulge.
Our Milky Way Galaxy is made up of mostly stars, gas, and dust. The dust blocks out light from
distant stars, and makes it hard to see a lot of the galaxy, especially the bulge and parts of the
disk.
Now, we know that we live in a galaxy that has a disk-like shape. But, where in the disk do we
live? Are we at the center, or o on one side? In the early 1920's, an American astronomer named
Harlow Shapley studied this question. He found that there were clusters of stars, called globular
clusters, that were spread around the center of the galaxy evenly in a spherical distribution in
the halo. Harlow Shapley realized that depending on where you are in the galaxy, the pattern of
globular clusters will look dierent.
Let's try to perform Harlow Shapley's experiment and nd the center of the galaxy from the
globular clusters. We must start, however, with a discussion of the coordinates involved. Skip the
next text section if you are already familiar with right ascension and declination.
SKY COORDINATES
The celestial sphere is divided up in a system like the \longitude-latitude" system on the Earth's
surface. The latitude of an object in the sky is called the declination; an object (like Polaris) whose
position is over the Earth's north pole has a declination of +90 degrees; an object over the south
pole is at ,90 degrees. Objects on the celestial equator have a declination of zero.
Longitude on the sky is called right ascension. On Earth, the line dening zero degrees longitude
is fairly arbitrary | it's the circle that goes from the north pole to the south pole which passes
through Greenwich, England. Right ascension (RA) on the sky also has an arbitrary zero point |
it's a circle from the north celestial pole (Polaris) to the south celestial pole that passes through
one of the points where the ecliptic crosses the celestial equator (in the constellation of Pisces).
The only dierence between longitude on earth and right ascension on the sky is that longitude is
usually measured in degrees (from 0 to 360 degrees), while right ascension is measured in HOURS
(from 0 to 24 hours).
The point to the above discussion is that the coordinate system astronomers have for objects in
the sky is similar to the coordinate system that map makers have on earth. Just as every location
on earth has a longitude and latitude, every object in the sky has a right ascension and declination.
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The diagram below represents a view of Earth from outside the Milky Way, looking down at the
Earth's North Pole. Earth is at the center, the lines show the various directions of right ascension,
and each circle represents two kiloparsecs of distance from the earth. (On this scale, the Earth
would be a VERY tiny point at the center.) The locations and distances for a sample of globular
clusters are plotted.
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c. With an uncertainty of 12 hour, what is the right ascension of the galactic center?
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Below is a diagram which views the Earth from above its equator. Again, Earth is a tiny dot
at the center, and each circle represents two kiloparsecs in distance, but this time, directions are
degrees of declination. The same sample of globular clusters is plotted again with declination
against their distance in this diagram.
d. With an uncertainty of 10 degrees, what is the declination of the galactic center?
e. From both diagrams, what is the approximate distance to the galactic center?
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On page 73 is a plot comparing the distributions in the sky of two classes of objects: globular
clusters and groups of young stars. This plot represents the whole sky, plotted in a dierent
\latitude-longitude" coordinate system, where the origin of the plot is the direction to the center of
the Galaxy (which you just found the coordinates of in the RA and Dec. system). \Longitude" is
measured along the plane of the Galaxy, in degrees ranging from 0 (at the center) to 180 (opposite
from the center) to 360 (back to the center). \Latitude" is measured above and below the plane
of the Galaxy in degrees and ranges from: 0 on the plane of the galaxy to +90 directly above the
plane, and ,90 below the plane.
f. Compare the distribution of globular clusters with that of the young groups of stars.
(Are the two groups spread evenly over the sky or are they clustered in specic parts
of the sky?)
g. Which group would you study to learn about the properties of the disk of our
Galaxy and why? Which group would you study to learn about the halo of our Galaxy
and why?
h. In lab 10 you found the ages of a \young" group of stars (the Pleiades) and a
globular cluster (M15). Based on the ages you found in lab 10, and your answer to
question g., what can you conclude about the relative ages of the halo and disk of our
Galaxy?
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Distribution of Globular Clusters and Young Star Groups on the Sky
+90 o
Young Star Groups
+80 o x
x
x Globular Clusters
+60 o
x
x
+40 o
x
x
x
+20 o
x
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180o
150o
120o
90o
60o
x
x
30o
0o
x
x
x
330 o
x
270 o
300 o
x x
240 o
x
xo
−20
x
x
x
x
−40 o
−60 o
−80 o
x
−90 o
x
x
210 o
180o
Assuming that young groups of stars trace the structure of our Galaxy's disk, then by examining
their distribution, we we can study the structure of the disk of our Galaxy. The following plot shows
the spatial distribution of young star groups around the Sun. At the center of the graph is the
Sun. Distance away from the Sun is plotted radially in kiloparsecs (kpc). Around the circle are
angles measured along the plane of the Galaxy away from the galactic center (0 represents the
direction to the center of the Galaxy, 180 represents the direction exactly opposite the center of
the Galaxy). Think of this plot as a \picture" looking down on a little piece of the disk of our
Galaxy from far above the Sun.
i. Describe the distribution of young star groups near the Sun. (Are the young groups
of stars evenly distributed around the Sun or are they clumped? Are there any trends
in their arrangement?)
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Our Galaxy is classied as a spiral galaxy. This means that the disk of the galaxy is in the shape
of a \spiral," \vortex," or \whirlpool." So within the disk of our Galaxy there are spiral arms,
which are long streams of denser concentrations of stars, gas, and dust.
j. In the diagram of young star groups near the sun, on page 74, there are stars from
parts of three dierent spiral arms near the sun. Mark the locations of the three spiral
arms in that diagram.
k. The circle below represents the entire disk of our Galaxy viewed from above. The
actual diameter of this disk is about 30 kpc. To scale, add and label the following
structures to complete this scale model of the Galaxy: 1. the central spherical bulge
(diameter = 2 kpc), 2. the position of the Sun (you found this earlier), 3. a circle
representing the extent of the young star groups near the Sun from the gure on
page 74 (NOTE: all of the stars we can see in the sky are contained within this circle).
l. Create and label an edge-on view of this model in the right-hand margin, next to
the face-on view above. Include: the thickness of the disk and bulge (the disk is about
0.6 kpc thick and the bulge is about 0.8 kpc thick), and the location of the Sun.
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m. Why can't we see stars on the opposite side of the Galaxy from the Sun in visible
light? (Remember that all the stars in our galaxy's disk that we can see in the sky
are contained within the circle dened by the young star groups that you drew on the
model of the Galaxy.)
n. Summarize the facts and ideas presented in this lab. Also include any additional
questions that you may have.
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