Lab 10 - Mike Jacobs
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1403
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Astronomy
Date
Dec 6, 2023
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© 1997 MKS Publishing, Inc.
99
PHYS 1403 Lab 10: CENTER OF THE GALAXY
Worksheet
Name:
_________________________________________________________
CWID:
_________________________________________________________
INTRODUCTION
For centuries it was believed that the Earth was the center of the Universe. This
belief, known as the geocentric model, held that the Earth was stationary while the sky
revolved above it on a set of celestial spheres. Aristotle eloquently argued that the
Earth must be stationary and thus the center of the Universe because:
1) you could not feel the Earth move
2) there was no wind due to a moving Earth
3) the birds and the clouds would be left behind if the Earth was moving
4) if the Earth was moving the stars would display a parallax effect
This was to be the common belief for over 1800 years. Not only because of the
persuasive arguments of Aristotle, but also because Greek philosophy declared that the
Universe was perfect. So, they believed that the celestial spheres the stars moved on
were perfect, and everything in the sky was perfect. Later, as the Catholic Church rose
in prominence, this would be interpreted as a philosophy of a perfect sky as an
indication of a perfect God. Therefore, any philosophy that did not agree with the
geocentric belief was considered heretical. Anyone professing such beliefs risked facing
the wrath of the Church. This understandably made people reluctant to present new
ideas.
In 1543, Nicolaus Copernicus published
Concerning the Revolutions of Celestial
Spheres
. He was very elderly, and in fact received the first copy of his book the day he
died. What Copernicus claimed was that the Sun, not the Earth, was the center of the
Universe and the Earth, stars and planets all revolved about the Sun. It is this book that
marks the beginning of modern astronomy and modern science. Because of the
enormous importance of the work, the heliocentric model is commonly called the
Copernican System.
Copernicus’ work was carried forward by Johannes Kepler, who devoted himself
to the task of mathematically describing the orbits of the planets about the Sun. He was
able to solve this problem by 1619 with Kepler’s Three Laws of Planetary Motion.
100
Galileo declared in the early 1600's that he supported the Copernican System,
but he didn’t have the necessary tools to prove it. However, by the end of 1610 Galileo
had a new tool, the telescope.
In that year, or early 1611, Galileo became the first
person to observe the heavens through a telescope. He immediately made several
amazing discoveries, including mountains on the Moon, moons orbiting Jupiter (now
known as the Galilean moons), that the Milky Way was made of thousands of small
stars, and Venus went through a whole set of phases like the Moon’s. He presented his
findings in a book called
Sidereus Nuncias
(the Messenger of the Stars) in 1611.
The popularity of the Copernican System grew until it was the accepted model by
much of society. Does this mean that it was no longer believed that we were the center
of the Universe? No. The belief was that the Sun, not the Earth, was at the center of the
Universe. We had simply moved that universal center from our planet to our star.
One of the reasons for this is because astronomers saw basically the same
numbers of stars in all directions. If we were not the center, they reasoned, then we
would see more stars in one direction than another. Since this was not the case, it must
be because we are at the center.
By the latter part of the 19
th
century a number of star groups called globular
clusters had been identified. Globular clusters are large, spherical groups of stars,
numbering in the hundreds of thousands, or even millions of stars, and orbit the Milky
Way. Distances to these globular clusters could be calculated and the astronomer
Harlow Shapley reasoned that if we were at the center of the Universe, then there
should be an even distribution of globular clusters in all directions. However, when he
plotted the locations of the globular clusters, he found we were not at the center of the
Milky Way, and he was able to make a good estimate of the distance to the true center.
Now it was established that the Sun was not the center of the Universe, but it
was still thought that the center of the Milky Way was the center of the Universe. It was
not until the twentieth century that it was finally established that we, or our galaxy, are
not at the center of the Universe.
101
PRELAB QUESTIONS
1.
What is meant by a “geocentric” model?
2. What is meant by a “heliocentric” model?
3. What were the main arguments made (by Aristotle) against the heliocentric model?
4. What is the difference between the “Galaxy” and the “Universe”?
5. How will we determine the location of the center of our galaxy in today’s lab?
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GRAPH #1
LOCATION OF GLOBULAR CLUSTERS RELATIVE TO THE EARTH
102
EXERCISE
1.
Use the data listed in Table #1 to plot the location of the globular clusters on
Graph #1. This graph can be found in the file labeled “Lab10_Graph.pdf” found
in the Lab Materials page in Canvas. Right Ascension (RA) goes in a counter-
clockwise direction. That is, RA 0
h
is at the top, 6
h
is directly to the left, 12
h
is
straight down, and 18
h
is directly to the right. Distance increases outward from
the center. The rings represent increments of 2 kpc, such that the inner most ring
is 2 kpc and the outer most ring is 12 kpc. You don’t have to label each point…
just make a dot.
Take a picture of your graph with a digital camera or cell
phone camera, and upload the picture as a pdf at the same time you upload
this worksheet using the Lab 10 submission link in Canvas
.
2.
From Graph #1, determine the direction that you believe to be towards the center
of the Galaxy. What is the RA of that direction? How did you determine that
direction from the graph?
3.
The center of the Milky Way actually lies in the direction of the constellation
Sagittarius, at a Right Ascension of 17.75 hours. How does this compare with
your results? Based on this comparison, do you feel that using globular clusters
to locate the direction of the galactic center is accurate? Why or why not?
4.
From Graph #1, estimate the distance to the center of the Galaxy? What is the
distance? How did you determine the distance from the graph?
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5.
The actual distance to the center of the galaxy is 7.6 kpc. How does this
compare with your results? Based on this comparison, do you feel that using
globular clusters to locate the distance to the galactic center is accurate? Why or
why not?
Table #1
Globular Cluster
Right Ascension (Hours)
Distance (kpc)
NGC 362
01.1
9.0
NGC 1851
05.25
10.8
NGC 2802
09.2
9.2
NGC 3201
10.3
5.0
NGC 4372
12.4
4.9
NGC 6352
17.4
5.4
NGC 6362
17.5
7.1
NGC 6440
17.8
3.7
NGC 6496
18.0
9.0
NGC 6528
18.1
7.3
NGC 6553
18.15
5.9
NGC 6624
18.4
8.5
NGC 6642
18.5
6.2
NGC 6712
18.9
7.6
NGC 6752
19.15
4.2
M 4
16.4
2.1
M 5
15.3
7.6
M 10
16.9
4.4
M 13
16.7
7.2
M 14
17.6
10.2
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104
M 15
21.5
9.4
M 19
17.0
10.6
M 30
21.7
8.2
M 55
19.7
5.2
M 80
16.3
8.3
M 92
17.4
7.8
M 107
16.5
5.9
47 Tuc
00.4
4.6
T Cen
13.4
5.2