Imagine the Universe!

Presents

Written by

Dr. James C. Lochner Lisa Williamson Ethel Fitzhugh

NASA/GSFC Drew Freeman Middle School Drew Freeman Middle School
Greenbelt, MD Suitland, MD Suitland, MD

This booklet, along with its matching poster, is meant to be used in conjunction with the
Imagine the Universe! Web site or CD-ROM.
http://imagine.gsfc.nasa.gov/
 Table of Contents
Table of Contents...........................................................................................................i

Association with National Mathematics and Science Standards.................................... ii

Preface..........................................................................................................................1

Foreward – The Story of Andromeda ............................................................................2

Introduction .................................................................................................................3
Activity #1 – How Big is the Universe ..............................................................4

I. The Lives of Galaxies...............................................................................................5

A. The Characteristics of Galaxies ....................................................................5
Activity #2 – Identifying Galaxies ............................................................6
Activity #3 – Classifying Galaxies Using Hubble’s Fork Diagram............7
B. How Galaxies Get Their Names....................................................................9
Activity #4 – Identifying Unusual Galaxies ............................................10
C. The Components of a Galaxy......................................................................11
Activity #5 – Open Clusters versus Globular Clusters.............................12
D. The Clustering of Galaxies .........................................................................13

II. The Hidden Lives of Galaxies ...............................................................................13

A. Hidden Objects...........................................................................................13
B. Hidden Mass...............................................................................................14
Activity #6a – Evidence for Hidden Mass...............................................16
Activity #6b – Extension: Weighing a Galaxy ........................................17
C. Possibilities for Dark Matter .......................................................................18
Activity #7 – Dark Matter Possibilities ...................................................20
D. Formation of Galaxies ................................................................................22
Activity #8 – The Universe as Scientists Know It ...................................23
Activity #9 – Scavenger Hunt in the Night Sky ...............................................24

Answer Key for Activities ..........................................................................................31

Glossary ....................................................................................................................34

About the Poster .........................................................................................................36

Poster Credits .............................................................................................................36

References and Other Resources .................................................................................37

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 National Mathematics and Science Content Standards
for the Activities in this Booklet

All Standards are for Grades 9-12

NSES NCTM

1. How Big is the Universe?

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard 4: Connections

2. Identifying Galaxies

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard G: History & Nature of Science Standard 4: Connections

3. Classifying Galaxies Using Hubble’s Fork Diagram

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard G: History & Nature of Science Standard 4: Connections
Standard 6: Functions
Standard 8: Patterns

4. Identifying Unusual Galaxies

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard G: History & Nature of Science Standard 4: Connections
Standard 8: Patterns

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5. Open Clusters versus Globular Clusters

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard G: History & Nature of Science Standard 4: Connections
Standard 6: Functions
Standard 8: Patterns

6a. Evidence for Hidden Mass

6b. Extension: Weighing a Galaxy

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard G: History & Nature of Science Standard 4: Connections
Standard 6: Functions
Standard 8: Patterns
Standard 10: Statistics
Standard 11: Probability

7. Dark Matter Possibilities

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard G: History & Nature of Science Standard 4: Connections

8. The Universe as Scientists Know It

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard G: History & Nature of Science Standard 4: Connections

9. Scavenger Hunt

Standard A: Science as Inquiry Standard 1: Problem Solving

Standard B: Physical Science Standard 2: Communication
Standard D: Structure & Evolution of the Universe Standard 3: Reasoning
Standard G: History & Nature of Science Standard 4: Connections
Standard 8: Patterns

iii
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 Preface

WELCOME to the fourth in a series of posters and activity booklets produced in conjunction
with the Imagine the Universe! Web site. The poster/booklet sets are intended to provide
additional curriculum support materials for some of the subjects presented in the Web site. The
information provided for the educator in the booklet is meant to give the necessary background
information so that the topic can be taught confidently to the students. The activities can be used
to engage and excite students about the topic of galaxies in a number of disciplines and ways.
The booklet and all activities can be photocopied and distributed for educational, non
commercial purposes.

New to this poster/booklet set is the availability of color images for some of the activities. These
are available for download from the Imagine the Universe Web site at
http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/transparencies/
At the time of printing this booklet, a limited number of hardcopies of the transparencies are
available upon request by writing to us at itu@heasarc.gsfc.nasa.gov.

Also note that words in boldface are found in the glossary near the end of this booklet.

For additional materials and information, visit the Imagine the Universe! Web site at
http://imagine.gsfc.nasa.gov/. We also look forward to hearing your opinions about this
poster/booklet set. Our email address is itu@heasarc.gsfc.nasa.gov

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 Foreword
The Story of Andromeda
In Greek mythology, Andromeda was the daughter of Queen Cassiopeia and
King Cepheus of Ethiopia. Andromeda’s mother claimed she was more beautiful
than the sea nymphs, the Nereids. The Nereids felt insulted by this and
complained to the sea god Poseidon.
Poseidon threatened to send a flood and a sea monster, Cetus, to destroy the
kingdom of Ethiopia. The king consulted the oracle of Ammon who advised him
to sacrifice his daughter. Andromeda, dressed only in jewels, was chained to a sea-
cliff. At this time, Perseus, a Greek hero was traveling along the coast of Africa to
the north. He noticed the beautiful woman chained to a rock and instantly fell in
love with her.
Perseus offered to rescue Andromeda in return for her hand in marriage.
Andromeda had already been promised to a man named Agenor. However, hoping
to save their daughter from the approaching sea monster, King Cepheus and Queen
Cassiopeia consented in bad faith to Perseus’ request.
Perseus was a valiant warrior and possessed some powerful weapons, including
the head of the Gorgon Medusa, which had the capability to turn everything into
stone. With the aid of the Gorgon’s head, Perseus slew Cetus and freed
Andromeda. On Andromeda’s insistence, the wedding was then celebrated. Her
parents, who had forgotten their promise to Perseus, informed Agenor of the
wedding. He interrupted the ceremony with an armed
party.
A violent fight took place with King Cepheus and
Queen Cassiopeia siding with Agenor. Perseus
prevailed, using the Gorgon’s head to petrify his
opponents. Finally, Andromeda left her country to live
with Perseus, who later became the king of Tiryns and
Mycenae. The goddess Athena placed the figure of
Andromeda among the stars as a reward for keeping
her parents’ promise.
The constellation Andromeda

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 Looking up at the sky at night, we see a small display of stars which are a part
of our Milky Way Galaxy. A galaxy consists of a multitude of billions of stars.
Our sun is part of a solar system, which belongs to a large family of more than 100
billion stars. According to the Big Bang theory, when the universe was created, all
the matter in the universe was distributed uniformly, but within that uniformity,
there existed clumps of matter. Through the action of gravity these clumps grew.
Galaxies began when large clouds of gas and dust started to shrink as a result of
their gravity. As the cloud shrinks, stars form from the gases.

Introduction
In the 19th century, astronomers thought that the Milky Way was the only galaxy
in the universe. The introduction of telescopes to the study of
astronomy opened up the universe, but it took some time for
astronomers to realize the vastness of the universe.
Telescopes were used to make dim objects in the sky look
brighter and small objects look larger. There are two types of
telescopes: a refractor, which uses a lens to collect light, and a
reflector, which uses a mirror to collect light. Telescopes
revealed that our night sky was not only populated with stars,
but with other objects which appeared like faint, patchy
clouds. These objects were nebulae that seemed to be within
Edwin Hubble our Galaxy, the Milky Way, and thus believed to be relatively
close.

But as telescopes became more powerful, it was possible to see different
structures in the nebulae.
Astronomers debated the nature of these nebulae. The question arose whether
these objects were within the Milky Way, or were they communities distinct from
our Galaxy. It wasn’t until the 1920s that the American astronomer Edwin Hubble
ended the debate by discovering that some of the nebulae were composed of stars.
Hubble also determined the distances to these particular nebulae, and found that
they were far outside our Galaxy. These were thus found to be individual galaxies.
Scientists now estimate that there are about 200 billion galaxies of various shapes
in the universe.

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Activity #1 – How Big is the Universe?

Objectives:
• Student will be able to estimate the size of the visible universe.
• Student will be able to convert from centimeters to kilometers.

Directions:
Calculate the approximate size of the universe given the following scenario: The
Milky Way has a radius of approximately 50,000 light years. The visible universe
has a radius of approximately 15 billion light years or 300,000 times the size of the
Milky Way. If the Milky Way is an 8 centimeter wide coffee cup, how big would
the rest of the universe be in kilometers?

A B C

50,000 light years

8 cm
The Milky Way has a radius of The visible universe has So if an 8 cm wide
about 50,000 light years. a radius approximately coffee cup represents
15 billion light years or the Milky Way, the
What is the total size of the 300,000 times the size visible universe
Milky Way Galaxy?________ of the Milky Way. would be a sphere
approx.________km
in radius.

Show your calculations.

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I. The Lives of Galaxies

A. The Characteristics of Galaxies

Like all galaxies, the Milky Way is held together by gravity. Gravity also holds
the stars, planetary bodies, gas, and dust in orbit around the center of the galaxy.
Just as the planets orbit around the sun, the sun orbits around the center of the
Milky Way.
Galaxies come in a variety of shapes.

In the 1920s Edwin Hubble was the first to
study the morphology of galaxies. Using the 100-inch Hooker reflector telescope
at Mount Wilson Observatory in California between 1922-1926, Hubble
photographed numerous galaxies. He categorized these shapes or basic schemes as
spiral, barred spiral, elliptical, irregular, and peculiar.

This system was known as
the Hubble morphological sequence of galaxy types. First, he noted that some
galaxies, like the M31- Andromeda Galaxy, appeared as disks and had arms of
stars and dust which appeared in a spiral pattern. Like M31, these galaxies
appeared nearly uniform in brightness. In addition, Hubble observed that in some
of these types of galaxies the arms were more tightly wound around the galaxy.
He called these spiral galaxies. Our Galaxy, the Milky Way, is an example of a
spiral galaxy. Secondly, he noted that some spirals had a bright bar of gas through
the center, and called these barred spirals. Thirdly, Hubble discovered galaxies
that were slightly elliptical in shape, while others were nearly circular, such as
M32. He called these elliptical galaxies. The fourth type of galaxy observed was
neither spiral nor elliptical, but was irregular in shape. These galaxies were called
irregular. An example of this is the Magellanic Clouds. Finally, there were some
galaxies that fit none of these descriptions. These were called peculiar galaxies,
one example of which is Centaurus A.
Astronomers now have decided that the morphology classification should
consist of only two types of galaxies: the spirals and the ellipticals. Barred spirals
are a subclass of spirals. Irregulars may be either spiral or barred spiral. Peculiars
are not fundamentally a different type. They are simply galaxies in the act of
colliding; the collision distorts their shape and makes them appear “peculiar”.

5
Activity #2 – Identifying Galaxies

Objectives:
1. Using the “Hidden Lives of Galaxies” poster, the student will gain knowledge
of
galaxymorphology.


2. Student will observe different images of galaxies from the Hubble Space
Telescope deep survey image.
3. Student will be able to identify the basic types of galaxies (spiral, elliptical,
barred spiral, peculiar, or irregular) from the Hubble Space Telescope deep
survey image.

Directions:
1. List the five types of galaxies shown on the “The Hidden Lives of Galaxies”
poster and write a brief description of each. (see Transparency #1: Types of
Galaxies, http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/transparencies/)

Title___________________________

A.______________________________________________________
B.______________________________________________________
C.______________________________________________________
D.______________________________________________________
E.______________________________________________________

2. Observe the Deep Survey Image by the Hubble Space Telescope taken between
December 18 – 28, 1995. (See Transparency # 2: Deep Survey Image,
http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/transparencies/).
Identify the types of the ten galaxies labeled on the Deep Survey Image.

A.______________________ F._____________________
B.______________________ G._____________________
C.______________________ H._____________________
D.______________________ I.______________________
E.______________________ J.______________________

Assessment:
What inferences can you make from observing the Deep Survey Image?

6
 This classification sequence has become so widely used that the basic types,
spiral, barred spiral, elliptical, irregular, and peculiar, are still used by astronomers
today to classify galaxies according to their visible appearance. Spirals are
denoted by “S”, and barred spirals by “SB”. Letters “a”, “b”, “c” denote how
tightly the spiral arms are wound, with “a” being most tightly wound. The
Andromeda Galaxy is an Sb. Elliptical galaxies are denoted by “E”, with a number
from 0-7 indicating how circular it appears (0 being most circular, 7 being more
elongated). An example of this would be M87, which is an E0 galaxy. Irregulars,
such as the Small Magellanic Cloud, are denoted by “Irr”. Peculiar galaxies, such
as Centaurus A, are denoted by “P”.
To show how the various classes relate to each other, Hubble organized them
into a diagram. A simplified version of Hubble’s Fork Diagram is shown below.
Note that this diagram does not represent how galaxies form.
Note that this
diagram does not represent how galaxies form.

Sa Sb Sc

EO E7 S0

SBa SBb SBc

Hubble’s Fork Diagram of Galaxy Classification

Activity #3 – Classifying Galaxies Using Hubble’s Fork Diagram

Objectives:
• Student will gain knowledge of Hubble’s Fork Diagram of Galaxy
Classification.
• Student will be able to use his/her knowledge of the Hubble Fork Diagram to
explain how the five galaxies in the “Types of Galaxies” section were
classified.
• Student will be able to create a chart that displays the basic scheme and
classification of galaxies using the Hubble Fork Diagram.

7
Directions:
1. Review Hubble’s Fork Diagram of Galaxy Classification (see Transparency #3
– Hubble Fork Diagram).
2. Using the Galaxy Classification Chart, observe the images of each of the
galaxies. Determine the scheme and classification of each galaxy. (see
Transparency #4 – Galaxy Classification Images).
Transparencies available at http://imagine.gsfc.nasa.gov/docs/teachers/transparencies/.

Galaxy Classification Chart

Galaxy Image Scheme Classification
Andromeda Spiral Sb

M84, NGC4374

NGC 2997

NGC5383

Large Magellanic
NGC4622

M83

Centaurus A
M59, NGC4621

NGC1365

Assessment:
1. With a partner, brainstorm ideas of how you classify objects in your everyday
life (Pair/Share).

2. How does classifying help organize your life? List five examples.
___________________________________________________________
___________________________________________________________
___________________________________________________________

8
 Later, astronomers added other classifications. One
of these astronomers was Carl Seyfert. In 1943, he
discovered galaxies with very bright central regions.
Seyfert studied the spectra of these galaxies. The
spectra indicated that the central region was bright at
all wavelengths. This indicated some enhanced
activity, and “Seyfert” galaxies became the first of a
range of active galaxies that have been studied at all
Seyfert Galaxy NGC 1275 in Perseus
wavelengths since then.

B. How Galaxies Get Their Names

Catalogues are used to list galaxies. One of the earliest catalogues of objects in
the sky was made by Charles Messier, who denoted objects
by using the letter “M”.
Messier, a comet-hunter in the 1700s, kept finding galaxies and nebulae in the sky
because many of them looked like comets. Eventually, he created a catalogue of
these objects, listing their positions so he wouldn’t be fooled again into thinking
they were comets. Although he categorized many brilliant objects in the night sky,
his cataloguing system was completed in a random manner.
Another common cataloguing system is the NGC (New General Catalogue)
which dates from the 19th century. The NGC numbers objects from west to east
across the sky. All objects in the same area of the sky have similar NGC numbers.
Several other cataloguing systems are: ESO (European Southern Observatory), IR
(Infrared Astronomical Satellite), Mrk (Markarian), and UGC (Uppsala General
Catalog). The numbers following the letter designation may indicate either the
order in the list or the location of the galaxy in the sky. Some galaxies are given
descriptive names (e.g. ”Andromeda”, “Whirlpool”) if they are particularly
distinctive in location or appearance.

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Activity #4 – Identifying Unusual Galaxies

Objective:
• Student will be able to identify different types of galaxies by their distinctive
appearance.

Directions:
Match the unusual galaxy on the left with its distinctive name on the right. Justify
your reasoning.

1._____ a. Polar Ring Galaxy

Reason_____________________________
___________________________________

2. _____ b. Siamese Twins Galaxy

Reason_____________________________
___________________________________

3. _____ c. Sombrero Galaxy

Reason_____________________________
___________________________________

4. _____ d. Whirlpool Galaxy

Reason____________________________
__________________________________

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C. The Components of a Galaxy
A galaxy is may be made up of two visible components. The two components
are the disk and the bulge. Spiral galaxies have most of their stars in a disk. The

A Near-infrared image of the Milky Way Galaxy

stars may be single stars, double, or multiple stars, or may be part of clusters. In
the disk, stars cluster into open clusters (also called “galactic clusters”) which are
asymmetric group of stars. There may be as few as ten or as many as 2000 stars in
an
open cluster.
In our Galaxy, the Pleiades is the well known example of an open cluster (see
image left), and it contains a few hundred stars. Open
clusters tend to be the younger of the type of clusters
which appear in a galaxy. The galaxy disk also
contains clouds of gas and dust, called nebulae. Some
nebulae result from the death of stars, while others are
the place where stars are being created. Some nebulae
emit light, while others absorb light. The stars,
The Pleiades
clusters, and nebulae in the disk rotate around the
center of the galaxy. In our Galaxy, it takes 200 million years for our sun to go
around once. In addition to the disk, spiral galaxies also have a “bulge”, which is a
large, squashed sphere surrounding the galaxy’s center. This region is composed
of stars, dust, and gas. In the Milky Way Galaxy, the bulge contributes about 1/5
of the total light of the galaxy. The bulge consists of older stars and not very much
gas or dust. Above and surrounding the bulge, stars cluster into
globular clusters (see image right), which are collections of up
to hundreds of thousands of stars bound together in a tight
spherical swarm. Since they consist of old stars, globular
clusters can be used to determine the age of the galaxy. In
globular clusters, the stars move about just as bees swarm
The Globular Cluster M80
near their hive.
Elliptical galaxies consist of just one visible component, the bulge. A good
example of this would be M87. Elliptical galaxies contain old stars and a small
amount of gas and dust. Stars in these galaxies collect into globular clusters, but
not open clusters.

11
 In some irregular galaxies, one can see the individual stars, nebula and clusters,
while in other irregular galaxies we cannot see these same objects. Irregular
galaxies have a disk, but no spiral arms. However, these galaxies do have a
mixture of old and young stars combined with a large amount of gas and dust.

Activity # 5– Open Clusters versus Globular Clusters

Objective:
• Student will be able to show similarities and differences between galactic
clusters and globular clusters.

Directions:
Complete the Venn diagram, using Transparency #5: M37/M80 and Section C:
The Components of a Galaxy. (See http://imagine.gsfc.nasa.gov/docs/teachers/
galaxies/transparencies/ for transparency.)

Assessment:
Compare a swarming beehive to a globular cluster.

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D. The Clustering of Galaxies
Like stars, galaxies often appear together in groups and clusters. Groups may
consist of a few galaxies and are often a part of larger galaxy clusters. Galaxies
also often have small companion galaxies. Our Milky Way Galaxy is accompanied
by the Large and Small Magellanic Clouds, which are both irregular galaxies
visible from the southern hemisphere. The Andromeda Galaxy has two small
companion elliptical galaxies, M32 and M110.
Our nearest neighbor galaxies form the Local Group, which consists of about
two dozen galaxies of various types - spiral, elliptical, and irregular. The nearest
large cluster of galaxies is the Virgo Cluster. It covers a region in the sky about six
degrees across in the constellation Virgo. It consists of over one hundred galaxies
of many types, including spiral, elliptical, and irregular galaxies. The center of the
Virgo Cluster is twenty million parsecs from Earth. Other clusters are farther, and
some have a
symmetric distribution of galaxies.
Some clusters are members of superclusters. The Local Group and Virgo
Cluster are part of a supercluster that contains one hundred other clusters and is
one hundred megaparsecs across. The study of these superclusters leads to under
standing the very structure and evolution of the Universe.

II. The Hidden Lives of Galaxies

A. Hidden Objects
Observation of galaxies at wavelengths other than optical light reveals other
objects and components. Some are also seen in optical wavelengths, but are
brighter in other parts of the spectrum.
For example, at radio wavelengths astronomers can detect much of the
hydrogen that lies between the stars. These hydrogen atoms emit radio waves
having a frequency of 1420 MHz (= 1420 x 106 Hertz), or as it is more commonly
referred to, a wavelength of 21 cm. Astronomers use the detection of this gas to
map out the location of hydrogen in our Galaxy. Astronomers can also determine
the velocity at which the gas is moving, and whether it is moving toward or away
from us. In this manner, the general motion of gas, and presumably the stars
formed from the gas, can be determined.
In X-ray wavelengths, we see individual stars, supernova remnants, binary star
systems, and globular clusters. All of these occur in our own Galaxy, and we can
see other galaxies which also contain these objects.

13
 Some stars have a hot corona composed of gas at a very high temperature.
This gas emits X-rays. In external galaxies, the individual stars must be very
bright X-ray emitters for us to see them. Thus, most individual stars we see in
other galaxies are “O” type stars, which are very massive and very hot. “O” type
stars don’t live very long and are, in fact, rarely seen in galaxies.
X-rays are also emitted by supernova remnants. These are shrouds of gas and
dust left behind after a massive star has exploded at the end of its life. The hot
ejecta from the exploded star runs into the gas and dust lying in the region around
the star, emitting X-rays. Some massive stars leave behind a dense neutron star
after the supernova. Neutron stars have a strong magnetic field, which can also
feed energy into the remnant.
In addition, observations at X-ray wavelengths show that other galaxies contain
binary star systems that emit X-rays. These X-ray binary systems consist of a
normal star and a “compact object”. This compact object may be a black hole,
neutron star, or white dwarf. These objects are
formed from normal stars which
have used up their nuclear fuel. In the binary system, material from the companion
star is funneled into the compact object. This material is heated as it spirals in and
emits X-rays as it is heated.
X-rays may also come from globular clusters. In these dense clusters of stars,
the most massive members quickly exhaust their nuclear fuel and become neutron
stars (or sometimes black holes). Through motions and gravitational interactions
within the cluster, these neutron stars can join with a normal star to become an X-
ray binary system. In our Galaxy, some globular clusters are observed to have a
number of individual X-ray sources, all of which are believed to be X-ray binaries.
Because other galaxies are far away, we see individual globular clusters as a point-
like X-ray source.
Finally, it is common for a galaxy to harbor a massive black hole near its center.
Observations by the Chandra X-ray Observatory of the central region of the
Andromeda Galaxy reveal more than 100 X-ray sources. Many of them are likely
X-ray binaries. One of them was at the previously determined position of a super-
massive black hole, which has a mass of 30 million times that of our sun. The
Chandra observation also showed a diffused glow surrounding the central region of
the galaxy. It is not known whether the glow is from many faint individual sources
or from a diffuse, hot gas.

B. Hidden Mass
Stars move about in galaxies under the influence of gravity in different ways,
depending on the type of galaxy. The stars in elliptical galaxies move in all
directions. The stars in the arms of spiral galaxies move in more orderly fashion

14
around the center of the galaxy. Stars in irregular galaxies move more or less in
random fashion.
The presence of dark matter was first discovered in 1932 by the Dutch
astronomer Jan Oort. By examining the Doppler shifts in the spectra of stars in
the Milky Way Galaxy, Oort measured their velocities. He found that the stars
moved faster than expected. Oort expected the stars to move only as fast as the
visible mass in the Galaxy (stars, gas, dust) would allow. In reality, the stars
appeared to be moving fast enough to escape the galaxy. Since Oort knew that this
couldn’t be the case, he hypothesized that there must be additional mass in the
galaxy that wasn’t visible and would keep the stars bound to the galaxy. A year
later, an American astronomer, Fritz Zwicky, came to the same conclusion while
measuring the velocities of galaxies in the Coma Cluster. Scientists now know that
this “hidden mass”, also known as dark matter, can account for nearly 90% of the
total mass of a galaxy.
In spiral galaxies, stars located at greater distances from the center of the galaxy
are expected to have smaller velocities than stars that are close to the center of the
galaxy. What scientists observe is that velocities are constant in the arms, and rise
very quickly in the bulge. Scientists can account for this if there is a very massive
object at the center of the galaxy, and a large halo of invisible matter surrounding
the entire galaxy. X-ray observations confirm that massive black holes lie at the
center of many galaxies. In the dark halo, the amount of mass increases linearly
with radius. Spiral galaxies have extended dark halos that account for 90% of the
total mass of the galaxy. It is not known what the dark matter might be made of. It
might be objects too small to become stars and hence too small to give off their
own light (e.g., planets and brown dwarfs). Or it might be an entirely new type of
matter made of particles which interact only gravitationally and do not give off
light.
Measuring rotation curves in an elliptical galaxy is difficult because of the
nature of the orbits of stars in this type of galaxy and because the spectral lines are
too weak
to be used to measure the velocity. However, X-ray observations show
that elliptical galaxies have a halo of hot gas extending well outside the optical
limits of the galaxy. As an example, in one of the elliptical galaxies in the Virgo
Cluster, the total mass of this hot gas can be 1010 times the mass of the sun. This is
small compared to the total mass of all the stars in the galaxy - 1012 times the mass
of the sun. However, in order for the gas to be bound to the galaxy, the galaxy
must have a mass of 5x1012 times the mass of the sun. Because this is more than
what is seen, astronomers conclude that elliptical galaxies also have halos of
hidden mass. As in the case of spiral galaxies, the “dark matter” halos in elliptical
galaxies may contain up to 90% of the total mass of the galaxy.

15
Activity #6a - Evidence for Hidden Mass

Objectives:
1. Student will be able to interpret and analyze the information in the data
presented on the “Evidence for Hidden Mass” graph.
2. Student will be able to determine why the data looks the way it does on the
graph.
3. Student will be able to observe trends to determine if there is a pattern to the
way the data is shown.

Directions:
1. Finish writing the paragraph by interpreting the data about Solar System planets
on the “Evidence for Hidden Mass” graph on the “Hidden Lives of Galaxies”.
(See Transparency #6: Evidence for Hidden Mass, http://imagine.gsfc.nasa.gov/
docs/teachers/galaxies/transparencies/)

There are ________ solar system planets presented on the graph. The
planets, from the closest to the sun to the furthest from the sun, are _______
_____________________________________________________________
_____________________________________________________________.
Using the graph, the velocities of the solar system planets, from the lowest value to
the highest value, are _________________________________________________
__________________________________________________________________.
Using the graph, the distances of the planets from the Sun are, from least to
greatest, ___________________________________________________________
__________________________________________________________________.
In general, the further the planet is away from the sun the ________________its
velocity. The closer the planet is to the sun the __________________its velocity.

2. Write a paragraph interpreting the data for Galaxy F563-1. Include information
about distance from the center, velocity, and trends.
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________

16
Activity #6b - Extension: Weighing a Galaxy

Objectives:
• Students will be able to use Newton’s Laws of Motion to determine the mass of
the sun from the motions of the planets.
• Students will be able to estimate the mass of a galaxy by applying Newton’s
Laws to the motion of stars in the galaxy.
• Students will be able to convert among different units used in astronomy.

Directions:
Using Newton’s Law for gravity, we can determine the mass of an object by
measuring the motion of other bodies around it. We can show this by applying
Newton’s Law of motion to bodies orbiting around another body.
We start with Newton’s Second Law
F = ma,
where F is the force exerted on the orbiting body, m is its mass, and a is its
acceleration. The force is the gravitational force exerted by the central object, and
the acceleration is due to circular motion. So we now have
GMm/r2 = mv2/r,
where G is the gravitational constant, M is the mass of the central object, r is the
distance of mass m from M, and v is the velocity of m. Simplifying gives
GM/r = v2.
Solving for M gives
M = v2r/G.

Note that G = 6.67 x 10-11 m3/kg-s2.

1. Apply this equation to three of the planets in our solar system, given in the table
below.

Planet Distance from Velocity (km/s) Mass (kg)

Sun (km)
Earth 1.5 x 108 29.8
8
Jupiter 7.8 x 10 13.1
9
Neptune 4.5 x 10 5.4
What do you notice about the values of the Mass ? ___________________
What would you conclude the mass of the sun to be ? _________________

17
2. Now apply this equation to the galaxy F563-1. Determine the mass M using the
equation and the velocity at various distances from the center of the galaxy
given in the table below. Each of these resulting mass values gives mass
enclosed within that distance. [Note that 1 kiloparsec (kpc) = 3.1 x 1019 meters]

Distance (kpc) Velocity (km/s) Mass (kg)

5.0 95.0
10.0 110.0
15.0 110.0

What do you notice about the values of the mass as the distance increases?
___________________________________________________________________.
What would you conclude the mass of the galaxy to be ? _____________________.
How much more massive is this galaxy than our sun ? _______________________.

C. Possibilities for Dark Matter

The three main categories of objects that scientists consider as possibilities for
dark matter include MACHOs, WIMPs, and gas. The first two are acronyms
which help us to remember what they represent. Listed below are the pros and
cons for the likelihood that they might be a component of dark matter.

MACHOs (Massive Compact Halo Objects): MACHOs are the big, strong, dark
matter objects ranging in size from small stars to super massive black holes.
MACHOS are made of ordinary matter, which is called baryonic matter.
Astronomers search for MACHOs. Examples: black holes, neutron stars, white
dwarfs, brown dwarfs.

Neutron Stars and Black Holes are the final result of a supernova. They are both
very massive stars. Neutron stars are 1.4 to 3 times the mass of the sun. Black
holes are greater than 3 times the mass of the sun.

Pros: They both can be dark. However, black holes emit no light; they are truly
black.

Cons: These objects occur less frequently than white dwarfs. As a result of a
supernova, a release of a massive amount of energy and heavy elements
should occur. However, there is no such evidence that they occur in sufficient
numbers in the halo of galaxies.

18
White Dwarfs are what remain of a small to medium sized star after it has passed
through the red giant phase.

Pros: There is an abundance of white dwarfs in the universe. If young galaxies

produced white dwarfs that cool more rapidly and become undetectable,
maybe they could be abundant enough to explain dark matter.

Cons: With the production of huge numbers of white dwarfs, in theory, one
would expect to see the production of massive amounts of helium.
However, this is not observed.

Brown Dwarfs have a mass that is less than eight percent of the mass of the sun,
resulting in a mass too small to produce the nuclear reactions that make stars shine.
The signature of these objects is an occasional brightening.

Pros: Astronomers have observed distant objects that are either brown dwarf
stars or large planets around other stars. Astronomers believe that the
brightening and dimming of brown dwarfs are due to the gravitational lens
effect of a foreground star. They also believe the brightening and
dimming may provide further evidence for a large population of brown
dwarfs in our Galaxy.

Cons: While they have been observed, astronomers have found no evidence of a large
population of brown dwarfs that would account for the dark matter in our Galaxy.

WIMPs (Weakly Interacting Massive Particles): WIMPs are the little, weak,
subatomic dark matter candidates, which are thought to be made of stuff other than
ordinary matter, called non-baryonic matter. Particle physicists search for WIMPs.
Examples: Exotic subatomic particles such as axions, heavy neutrinos, and
photinos.

Pros: Theoretically, there is the possibility that very massive subatomic particles,
created in the right amounts, and with the right properties in the first
moments of time after the
Big Bang, are the dark matter of the universe.

Cons: Observations have been fruitless. No one has observed even one of these
particles.

19
Hydrogen Gas

Pros: Hydrogen gas, one of the most basic elements, is 70-75% of the visible
matter in the universe. Most of the dark matter may exist as small clouds of
hydrogen gas.

Cons: Hydrogen is easily detected by radio, infrared, optical, ultraviolet,

and X-ray telescopes. The necessary amount of hydrogen hasn’t been seen.

Activity #7 – Dark Matter Possibilities

Objectives:
1. The student will be able to complete the graphic organizer using the knowledge
gained from reading section “C- Possibilities for Dark Matter”.
2. The student will gain a greater understanding of the three possibilities for the
hidden mass in galaxies.

Directions:
1. Reread the Possibilities for Dark Matter. Complete the graphic organizer,
“Decision-Making”, stating the pros and cons of the possibilities.
2. To complete the assessment, read the definition of Einstein’s Theory of Gravity
given after the Assessment.

Decision Making
Problem Goals
What can the dark matter be? 3. Provide a conceptual framework for
integrating new information.

4. Process and reorganize information.

5. Select important ideas and details.

20
Possibilities
Pros and Cons
MACHOs (Massive Compact
Halo Objects)
• Neutron Stars & Black Holes
• White Dwarfs
• Brown Dwarfs

WIMPs (Weakly Interacting

Massive Particles)
1. Exotic subatomic particles,
such as axions, massive
neutrinos, photinos

Hydrogen Gas

Assessment: Could Einstein’s Theory of Gravity, which has proved to be correct in all
cases so far, be somehow wrong? Justify your answer.
Einstein’s Theory in a Nutshell: Space-time tells matter how to move. Matter tells
space-time how to bend.

Decision(s) Reason(s)

21
D. Formation of Galaxies

How galaxies formed after the Big Bang is a question still being studied by
astronomers. Astronomers hypothesize that approximately a billion years after the
Big Bang, there were clumps of matter scattered throughout the universe. Some of
these clumps were dispersed by their internal motions, while others grew by
attracting other nearby matter. These surviving clumps became the beginnings of
the galaxies we see today. They first appeared about 14 billion years ago.
When a clump becomes massive enough, it starts to collapse under its own
gravity. At this point, the clump becomes a protogalaxy. Astronomers
hypothesize that protogalaxies consist of both dark matter and normal hydrogen
gas. Due to collisions within the gas, the hydrogen loses energy and falls to the
central region of the protogalaxy. Because of the collisions of the gas,
protogalaxies should emit infrared light. The dark matter remains as a halo
surrounding the protogalaxy.
Astronomers think that the difference in appearance between elliptical and
spiral galaxies is related to how quickly stars were made. Stars form when gas
clouds in the protogalaxy collide. If the stars are formed over a long period of
time, while some stars are forming, the remaining gas between the stars continues
to collapse. Due to the overall motion of matter in the protogalaxy, this gas settles
into a disk. Further variations in the density of the gas result in the establishment
of “arms” in the disk. The result is a spiral galaxy. If, on the other hand, stars are
made all at once, then the stars remain in the initial spherical distribution that the
gas had in the protogalaxy. These form an elliptical galaxy.
Astronomers also think that collisions between galaxies play a role in
establishing the different types of galaxies. When two galaxies come close to each
other, they may merge, throw out matter and stars from one galaxy, and/or induce
new star formation. Astronomers now think that many ellipticals result from the
collision of galaxies. We now know that giant ellipticals found in the center of
galaxy clusters are due to multiple galaxy collisions.

22
Activity # 8 – The Universe as Scientists Know It

Objective:
Student will be able to assess his/her degree of understanding of what makes up the
universe as scientists know it.
Directions:
Using knowledge gained, fill in the concept map with the following terms:
Planetary Systems, Galaxies, Planets, Sun, Venus, Moon, Stars, Sirius, Solar
System, Comet, Meteor, Open Clusters, Stellar Regions, Jupiter, Titan, Solar
Neighborhood, M80.

The Universe as Scientists Know it

The Universe

Galaxy Clusters
Example: Virgo Cluster

Example: Milky Way

Globular Clusters
Example: ______________ Example: Solar Neighborhood Example: Pleiades

Example:__________

Small Bodies

Examples: Examples: Examples:

Earth

Asteroid

Sirius
Saturn

Io Callisto

Ganymeade Europa

23
Activity # 9 – Scavenger Hunt in the Night Sky

Objectives:
• Student will become familiar with how objects appear using the naked eye,
binoculars, finderscope, and telescope.
• Student will be able to identify the major uses of a telescope and identify the
two main types of telescopes.
• Student will be able to identify the galaxy in which our Solar System is
located
and distinguish the morphology of the five basic kinds of galaxies.
• Student will be able to identify five common constellations.

Materials Needed
Flashlight
Piece of red cellophane
Rubberband
Pencil
Planisphere
Observation Log (included)
Seasonal Guide Posts (included)
Scavenger Hunt in the Night Sky Tip Sheet (included)
Blanket/Lawn Chair
Telescope
Binoculars

Suggestions for locating telescopes in your community

• School
• Parent in the school
• Other local schools
• Local astronomy clubs
• Local science museum/planetarium
• Local universities

24
Directions:
1. Orient yourself to the objects in the night sky by looking at Directional Chart.
2. Read Seasonal Guide Posts and “Scavenger Hunt in the Night Sky” Tip Sheet.
3. Fill out Observation Log (p. 29). Make additional copies for additional objects
4. Complete assessment.

Directional Chart

Below are three orientations of a star field: as it appears to the naked eye; as it
appears in the finderscope (upside down and backwards); and as it appears in most
telescopes with a star diagonal (mirror image). Newtonian Reflectors will not
have the mirror image effect. The arrows show the directions that stars appear to
drift, moving east to west, across the field of view.

Naked Eye Finderscope Most Telescopes

with star diagonal
Binoculars Most Telescopes
without star diagonal

25
 Autumn Seasonal Guide Posts: For Naked Eye/Binoculars (~ 9 p.m.)
Object Constellation Type
Polaris Ursa Minor Star
Big Dipper Ursa Major Asterism
Altair Aquila Star
Vega Lyra Star
Deneb Cygnus Star
Great Square Pegasus Asterism
Cassiopeia Cassiopeia Constellation
Pleiades Taurus Open cluster
Capella Auriga Star
Andromeda (M31) Andromeda Spiral Galaxy
Looking due north, about half-way up from the horizon will be a bright star, Polaris, the North
Star. The Big Dipper can be hard to find in Autumn because it lies along the northern horizon.
Now look to the south and west of Polaris. There you will see the 3 bright stars of the summer
triangle slowly setting. Altair is to the south; the brilliant star nearest to the horizon is Vega;
and a bit higher overhead is Deneb. Deneb is at the top of a collection of stars in the form of a
cross. The cross is between Vega and Altair, standing almost upright this time of year. High
overhead are the 4 stars of the Great Square. Although they’re not particularly brilliant, they
stand out because they are brighter than any other stars near them. After you have found the
Great Square, look north. You’ll see the 5 main stars of Cassiopeia making a bright “W” shape
or an “M” depending on the way that you’re turned around. To the east you’ll see a small cluster
of stars called the Pleiades. North of the Pleiades, the brilliant Cappella rises. --- Now to find
the Andromeda Galaxy, locate the Great Square overhead. Between Cassiopeia and the Great
Square is the constellation Andromeda. From the northeast corner, find 3 bright stars in a long
line, arcing across the sky west to east, just south of Cassiopeia. From the middle of these 3
stars go north towards Cassiopeia past 1 star to a second star, in a slightly curving line. The
galaxy is just barely visible to the naked eye.

26
 Seasonal Guide Posts for Galaxies: For the Telescope
Season Object Name Constellation Morphology Rating
Autumn M31 Andromeda Galaxy Andromeda Spiral 4
M32 Companion to M31 Andromeda Elliptical 2

Winter M33 Triangulum Galaxy Triangulum Spiral 3

Spring M81 Ursa Major Spiral 3

M82 Ursa Major Peculiar 2
Now to find the Andromeda Galaxy, locate the Great Square overhead. From the northeast corner,
find 3 bright stars in a long line, arcing across the sky west to east, just south of Cassiopeia. From the
middle of these 3 stars go north towards Cassiopeia past 1 star to a second star, in a slightly curving
line. Through a telescope, the galaxy looks like a bright oval embedded in the center of a long swath
of light, which extends across the field of view. Off to the south, and a bit east, is what looks like an
oversized star making a right triangle with 2 faint stars. This is the companion galaxy, M32.
Increasing magnification, you can see it is an egg-shaped cloud of light. Next, to locate the
Triangulum Galaxy, find the Great Square high in the east. From the northeast corner, find 3 bright
stars in a long line arcing across the sky west to east just below Cassiopeia. Down and to the left of
the second and third stars you’ll find 3 stars forming a narrow triangle pointing towards the
southwest. This is the constellation Triangulum. Use the distance from the northernmost star of this
triangle to the point of the triangle as a yardstick. Half this distance up and to the right from the point
is a very faint star. Past this star half as far is M33. Four stars in the shape of a “kite” should be
visible in the telescope. The galaxy will look like a large, but very faint patch of light in this kite. Be
sure to use your lowest power. Finally, to locate M81 and M82, look for the Big Dipper, and locate
the 4 stars which make up the bowl of the Dipper. Start from the lower handle end of the bowl, and
imagine a line running diagonally upward across the bowl to the opposite side of the bowl. Continue
on this path until you reach M81 and M82. The 2 galaxies look like 2 fuzzy spots of light. The one
to the south, away from Polaris, is M81. It has an obvious oval shape. M82 is thin and pencil-
shaped, looking like a string of pearls.

27
Scavenger Hunt in the Night Sky Tip Sheet

It’s sometimes difficult to identify objects in the night sky. Here are some hints to
help you determine what you’re looking at.

Jets, Planes, Earth-orbiting Satellites

These objects move extremely fast. Blinking lights and loud noises reveal a jet
plane. Satellites travel in a straight line across the sky.

Planets
Some planets have a distinct appearance, others do not. To the naked eye, the
planets do not twinkle as the stars do. The disk of the brighter planets can be seen
with a telescope.

Meteors
Meteors are small pieces of rock that blaze across the sky appearing to leave a trail.
They are often called “shooting stars”.

Comets
These objects come into sight over a course of several weeks. They usually appear
with a long tail and a somewhat fuzzy head.

Stars
The majority of the objects that we see in the night sky are stars. They appear to
be moving slowly because the Earth is turning underneath them.

Galaxies and Nebulae

Most galaxies and nebulae are too faint to see with the naked eye. Therefore, you
will need to use binoculars or a telescope. The two exceptions are the Andromeda
Galaxy and the Orion Nebulae.

Observation Tips
• Choose a safe location on a clear night. Be patient and let your eyes adjust to
the darkness for 30-45 minutes.
• Allow telescopes and binoculars to adjust to the air temperature. Let
condensation on lenses or mirrors evaporate on its own.
• Attach red cellophane to the flashlight using the rubberband. Red light
interferes the least with night vision.
• Take along a pencil, observation log, and your planisphere.

28
Scavenger Hunt: Observation Log Observation Date:_________

Observer’s Name:______________________________________________

Location (City, Country):_______________________________________

Temperature:______________Cloud Cover (% estimated):___________

Object Name:_________________________________________________

Description:________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
_________________________________________
Observing Start Time:___________Observing End Time:_____________

Explanation of Rating Scale based on a Rating:______

small telescope Sketch:

Rating Description
Breathtaking, easily
seen even on a hazy
5 night, the best
example of its type,
not to be missed.
Impressive, easily
4 seen even on a hazy
night, a good
example of its type.
Also impressive, but
3 not the best of its
class.
May or may not be
2 easy to find, but not
exciting to observe,
lacks color
Not spectacular at
first sight. Difficult
1 to see. Pushes the
small telescope to its
limits.

29
Assessment:
• Name the major uses of a telescope and identify the two main types of
telescopes.
________________________________________________________________
________________________________________________________________
_______________________________________________________________.

• Identify the galaxy in which our solar system is located and list the morphology
of the galaxies you observed in the night sky.
________________________________________________________________
________________________________________________________________
_______________________________________________________________.

• Identify five common constellations you observed in the night sky.

________________________________________________________________
________________________________________________________________
_______________________________________________________________.

30
Answer Key for Activities

Activity #1 - How Big is the Universe

1. What is the total size of the Milky Way Galaxy? 100,000 light years
2. The visible universe will be a sphere approximately 12 km in radius.

Activity #2 – Identifying Galaxies

1.Titles will vary, but may be, e.g. “The Galaxy Types”, or “Five Types of Galaxies”
A. Spiral – galaxy with tightly wound spiral arms
B. Elliptical – slightly elliptical to nearly circular
C. Barred Spiral – spiral with a bright bar of gas through the center
D. Peculiar – fits none of the descriptions
E. Irregular – small, patchy, irregularly shaped galaxy

2. Identify the types of the ten galaxies labeled on the Deep Survey Image.
A. Irregular F. Barred spiral
B. Spiral G. Elliptical
C. Spiral H. Spiral
D. Elliptical I. Irregular (or Peculiar)
E. Barred spiral J. Elliptical
Assessment
Inferences that can be made from observing the Hubble Deep Survey Image should include that
the objects are galaxies and not stars. These galaxies are of different sizes, shapes, brightnesses,
distances, and color.

Activity #3 – Classifying Galaxies Using Hubble’s Fork Diagram

Galaxy Classification Chart

Galaxy Scheme Classification
Andromeda Spiral Sb
M84, NGC4374 Elliptical E1
NGC 2997 Spiral Sb
NGC5383 Barred Spiral SBb
Large Magellanic Irregular Ir
NGC4622 Spiral Sb
M83 Spiral Sc
Centaurus A Peculiar P
M59, NGC4621 Elliptical E5
NGC1365 Barred Spiral SBc

Assessment
1. Examples of how to classify objects in everyday life may include organizing a CD collection,
sorting clothes, organizing magazines.
2. An example of how classifying helps organize one’s life may be that an organized CD
collection makes it easier to locate music by artist name or music type.

31
Activity #4 – Identifying Unusual Galaxies
1. c. {Sombrero Galaxy} With a bright halo of stars and a large central bulge of stars,
it looks like a hat.
2. d. {Whirlpool Galaxy} It looks like a whirlpool in the ocean or water going down a
drain.
3. a. {Polar Ring Galaxy} It contains an inner central disk of old stars and an outer
ring of younger stars giving it the appearance of a ring on a ringer.
4. b. {Siamese Twins Galaxy} It shows how gravitational pull sometimes causes
two galaxies to collide or brush against each other, giving the appearance of
two joined bodies.

Activity #5 - Open Clusters versus Globular Clusters

Answers should include the following similarities: both contain numerous red stars, and colored
stars. Globular clusters contain older stars. Stars near the center tend to be brighter. The stars
appear to be gravitating towards the center of the cluster, like bees swarming around the hive.
The cluster contains hundreds to thousands of stars. The stars appear closer together. Open
cluster stars appear scattered. The blue stars are more visible. Open clusters contain hundreds
of stars, many which are bright, young, and blue.

Activity #6a - Evidence for Hidden Mass

1. There are 9 Solar System planets presented on the graph. The planets from the closest to sun
to the furthest from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
Neptune, and Pluto.
Using the graph, the velocities of the solar system planets, from the lowest value to the
highest value, are approximately 48, 35, 30, 24, 13, 10, 7, 5, 4 km s-1.
Using the graph, the distances of the planets from the Sun are, from least to greatest, 0, 110,
150, 250, 800, 1500, 2800, 4500, 6000 million km.
In general, the further the planet is away from the sun the slower the velocity. The closer the
planet is to the sun the faster the velocity.
2. Answers should include fact that velocities first increase with increasing distance. At
distances larger than 10 kpc, the velocity becomes constant with increasing distance.

Activty #6b - Extension: Weighing a Galaxy

1. The calculation for each of the planets should result in a value for M of 2.0 x 1030 kg, which is
the mass of the Sun. It is the same for each because the central mass for the solar system is
concentrated in the Sun.
2.
Distance (kpc) Velocity (km/s) Mass (kg)
5.0 95.0 2.1 x 1040
10.0 110.0 5.6 x 1040
15.0 110.0 8.4 x 1040

The mass increases as the distance from the center of the galaxy increases. This is because
stars move under the gravitational influence of all the matter within their orbit.
So stars at greater distances move under the influence of more mass than stars closer to the
center.

32
 The best estimate for the mass of the galaxy is the one which includes the most amount of
mass. From this calculation, the largest value is 8.4 x 1040 kg. The actual mass of the galaxy is
likely to be more than this. From Part 1, we know the mass of the sun is 2.0 x 1030 kg. So this
galaxy is at least 4.2 x 1010 times more massive than the sun.

Activity #7 – Dark Matter Possibilities

Students should restate the material presented in “Possibilities for Dark Matter”.
Assessment – Student’s decisions may vary, but they should give reasons for their decision.

Activity #8 - The Universe as Scientists Know It

The Universe as Scientists Know it

The Universe

Galaxy Clusters
Example: Virgo Cluster

Galaxies
Example: Milky Way

Globular Clusters Stellar Regions Open Clusters

Example: M80 Example: Solar Neighborhood Example: Pleiades

Planetary Systems
Example: Solar System

Small Bodies Planets Stars

Examples: Examples: Examples:

Comet Earth Venus The Sun

Asteroid Alpha Centauri

Moon
Meteor Sirius
Jupiter Saturn

Io Callisto Titan

Ganymeade Europa

Activity #9 – Scavanter Hunt in the Night Sky

Assessment:
1. Telescopes are used to make small objects appear larger and dim objects appear brighter.
The two main types of telescopes are refractors and reflectors.
2. The Solar System is located in the Milkyway galaxy. Morphology of galaxies will usually be
spiral and elliptical.
3. Answers will vary depending on the season.

33
Glossary

ACTIVE GALAXY – A galaxy whose center emits large amounts of excess energy, often in the
form of radio emissions. Active galaxies are suspected of having massive black holes in their
centers into which matter is flowing.

BIG BANG THEORY – A theory in which the expansion of the universe is presumed to have
begun with an explosion (referred to as the “Big Bang”).

BLACK HOLE – An object whose gravity is so strong that not even light can escape from it.

BULGE – The round or elliptical central region of a galaxy. It is often uniform in brightness.

CORONA – The hot, tenuous, outermost region of the sun and other stars. The sun’s corona is
visible during a total solar eclipse.

CONSTELLATION – A grouping of stars into one of the 88 areas of the sky.

DARK MATTER – A form of matter which does not emit light. Its nature is still being
investigated.

DISK – The flat, circular region of a spiral galaxy extending out from the central bulge. The
disk of a spiral galaxy often has distinct arms of stars and bright gas.

DOPPLER SHIFT – The apparent change in wavelength of sound or light caused by the motion
of the source, observer or both. Waves emitted by a moving object as received by an observer
will be blueshifted (compressed) if approaching, redshifted (elongated) if receding.

FINDERSCOPE – A small, low-power telescope with a wide field of view, attached to the main
telescope.

GALAXY CLUSTER – A group of galaxies bound together by gravity.

GRAVITY – A mutual physical force attracting two bodies.

LIGHT YEAR – The distance light travels in one year, which is approximately 9.46 x 1015
meters.

MORPHOLOGY – The study of the shape and structure of galaxies.

NEBULA – A diffuse mass of interstellar dust and gas.

NEUTRON STAR – The imploded have a mass 1.4 times the mass of the Sun, and a radius of
about 5 miles. Neutron stars can be observed core of a massive star sometimes produced by a
supernova explosion. Neutron stars are typically as pulsars.

34
PARSEC – A distance equal to 3.26 light years, or 3.1 x 1018 cm. A kiloparsec (kpc) is equal to
1000 parsecs. A megaparsec (Mpc) is equal to a million (106) parsecs.

PLANETARY SYSTEM – A star with one or more planets. This system may include moon(s),
comet(s), meteoroids, and asteroid belts in addition to planets.

PLANISPHERE – A handheld device which shows the appearance of the night sky at any
specified time of day and day of the year.

ROTATION CURVE – A graph of stellar velocity versus stellar distance from the center of a
galaxy.

SEYFERT GALAXY – A spiral galaxy whose nucleus shows bright spectral emission lines in
all wavelengths; a class of galaxies first described by C. Seyfert.

SPECTRA – A plot of the intensity of light at different frequencies.

STELLAR REGION – A region in space consisting of hundreds to thousands of stars.

SUPERCLUSTER – A collection of clusters of galaxies.

SUPERNOVA – The death explosion of a massive star, resulting in a sharp increase in

brightness followed by a gradual fading. At peak light output, supernova explosions can
outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive
cloud. This expanding cloud, visible long after the initial explosion fades from view
forms a supernova remnant (SNR).

TELESCOPE – A tool used to make dim objects look brighter and smaller objects look
larger.

X-RAY – A form of with a wavelength between that of ultraviolet radiation and gamma rays.

X-RAY BINARY SYSTEM – A binary star system contain contains a normal star and a
collapsed star. The collapsed star may be a white dwarf, neutron star, or black hole. X-rays are
emitted from the region around the collapsed star.

35
About the Poster

“The Hidden Lives of Galaxies” poster illustrates various facets of galaxies, both what is visible
at optical wavelengths and what can be seen and revealed only at X-ray wavelengths. The
central image is a composite X-ray (left-hand side) and optical (right-hand side) image of the
Andromeda Galaxy. The optical image shows the familiar dust lanes and spiral arms. The X-ray
image, taken from ROSAT data at 0.5-2.0 keV, shows individual sources and emission from gas
in the galaxy. The inset shows a portion of the observations taken by the Chandra X-ray
Observatory. The increased spatial resolution of the Chandra data now reveals many individual
X-ray sources in the central region of the galaxy, as well as diffuse X-ray emission from hot gas.

On the right hand side of the poster is an illustration of the changing size and distance scale
within the universe as we compare a planetary system to a neighborhood of stars in a stellar
region, to a galaxy, and finally to a cluster of galaxies.

The lower left box shows the different types of galaxies – spiral, barred spiral, elliptical,
irregular, and peculiar.

The lower right box illustrates and discusses the evidence for missing mass in galaxies as
revealed by optical and X-ray observations. The left-hand plot compares the rotation curve of a
galaxy to the velocities of planets in the solar system. The constant value for the velocities of
stars in the outer reaches of the galaxy shows that there must be more mass in the galaxy than
just the visible mass in order for the stars to remain bound to the galaxy. Likewise, the right-
hand plot shows hot X-ray gas extending far beyond the visible image of the elliptical galaxy.
Because this gas must be part of the galaxy, this too shows evidence for more matter than what is
visible.

Poster Credits

“The Hidden Lives of Galaxies” poster was designed and assembled by Karen Smale, with
additional artwork by Maggie Masetti. Also contributing were Gail Rohrbach, Brian Hewitt, and
Steve Fantasia. Chief scientific consultant was Dr. Greg Madjeski, with additional assistance
from Dr. Michael Lowenstein. Thanks also to Drs. Kimberly Weaver and William Pence for
additional comments. The project was supervised by Dr. James Lochner.

The Chandra image of the nucleus of M31 is courtesy of Dr. Steve Murray, NASA/CXC/SAO.
The rotation curve of the galaxy F563-1was provided by Dr. Stacy Mcgaugh, Univ. of Maryland.
The optical image of NGC 4414 is from the Hubble Space Telescope, courtesy
AURA/STScI/NASA, whereas the optical images M87, Centaurus A, Small Magellanic Cloud
and NGC 1530 are copyright AURA/NOAO/NSF, used by permission. The image of M31 is
copyright Bill Schoening, Vanessa Harvey, REU program/AURA/NOAO/NSF, used by
permission for educational purposes.

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References and Other Resources:

For additional activities using the Hubble Deep Field, see “Galaxies Galore” on Amazing Space,
http://amazing-space.stsci.edu/

For additional information on active galaxies, see the Active Galaxy articles on Imagine the
Universe!, http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html

Planishperes may be purchased at any planetarium and many science museum shops. To make
your own planisphere, see http://www.otterbein.edu/dept/PHYS/is410/plan.html.

For an excellent summary of the formation of galaxies, see

• http://blueox.uoregon.edu/~karen/astro123/lectures/lec25.html

You can construct your own galaxy rotation curves at

• http://www.astro.queensu.ca/~dursi/dm-tutorial/rot-vel.html

A detailed description of how a galaxy rotation curve is made from an observation is at

http://www.astro.ruhr-uni-bochum.de/geiers/GAL/gal_rot.htm.

For other discussions of rotation curves and the evidence for dark matter, see
• http://cfa-www.harvard.edu/~mwhite/darkmatter/rotcurve.html
• http://www.owlnet.rice.edu/~spac250/elio/spac.html

For additional discussion of dark matter, see

• http://www.physics.fsu.edu/courses/spring99/ast3033/darkmatter.htm

An excellent guide to observing the night sky with a small telescope is “Turn Left at Orion: A
Hundred Night Sky Objects to See in a Small Telescope - And How to Find Them,” by Dan M.
Davis, Guy J. Consolmagno, Daniel M. Davis, Cambridge Univ Pr (Trd); ISBN: 0521482119.

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