National Aeronautics and Space Administration

The Hidden Lives of

Galaxies

Educational Product
Educators &
Grades 9-12
Students

An Information & Activity Booklet

Grades 9-12
2000-2001
(Updated 2005)

Imagine the Universe!

http://imagine.gsfc.nasa.gov/
Explore. Discover. Understand.
EG-2000-08-003-GSFC
 Imagine the Universe!
Presents

Written by

Dr. James C. Lochner

NASA/GSFC
Greenbelt, MD

Lisa Williamson Ethel Fitzhugh

Drew Freeman Middle School Drew Freeman Middle School
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

Foreword – The Story of Andromeda................................................................................2

Introduction . .....................................................................................................................3

I. The Visible Lives of Galaxies........................................................................................4

A. The Characteristics of Galaxies........................................................................4
B. How Galaxies Get Their Names........................................................................6
C. The Components of a Galaxy............................................................................6
D. The Clustering of Galaxies...............................................................................7

II. The Hidden Lives of Galaxies......................................................................................8

A. Hidden Objects..................................................................................................8
B. Hidden Mass......................................................................................................9
C. Possibilities for Dark Matter...........................................................................10
D. Formation of Galaxies.....................................................................................13

III. Classroom Activities

Activity #1 – How Big is the Universe................................................................14
Activity #2 – Identifying Galaxies.......................................................................14
Activity #3 – Classifying Galaxies Using Hubble’s Fork Diagram.....................15
Activity #4 – Identifying Unusual Galaxies........................................................16
Activity #5 – Open Clusters versus Globular Clusters........................................16
Activity #6a – Modeling Mass in the Solar System and a Galaxy......................17
Activity #6b – Evidence for Hidden Mass...........................................................18
Activity #6c – Getting a Feel for Rotation Curves..............................................19
Activity #6d – Weighing a Galaxy.......................................................................19
Activity #7 – Dark Matter Possibilities...............................................................20
Activity #8 – The Universe as Scientists Know It...............................................21
Activity #9 – Seeing as Far as You Can See........................................................22

IV. Student Worksheets....................................................................................................24

V. Glossary ......................................................................................................................47

VI. About the Poster.........................................................................................................49

Poster Credits.......................................................................................................49

VII. References and Other Resources..............................................................................50


National Mathematics and Science Content Standards
for the Activities in this Booklet
All Standards are for Grades 9-12
Science Standards Math Standards
Science as Inquiry

Nature of Science

Problem Solving
Physical Science

Communication
Evolution of the

Data Analysis
Connections
Structure &

Reasoning
History &

Functions
Universe

Patterns
Classroom
Activity

How Big is the

Universe? ✔ ✔ ✔ ✔ ✔ ✔ ✔

Identifying
Galaxies ✔ ✔ ✔ ✔

Classifying
Galaxies
Using Hubble’s ✔ ✔ ✔ ✔
Fork Diagram
Identifying
Unusual ✔ ✔ ✔ ✔
Galaxies
Open Clusters
Versus
Globular ✔ ✔ ✔ ✔
Clusters
Modeling Mass
in the Solar
System and a ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔
Galaxy
NSES Content Standards are from Chapter 6 of National Science Education Standards,
1996, National Research Council, National Academy Press, Washing DC.

NCTM Math Standards are from Chapter 7 of Principles and Standards for School
Mathematics, 2000, National Council of Teachers of Mathematics.
ii
National Mathematics and Science Content Standards
for the Activities in this Booklet
All Standards are for Grades 9-12
Science Standards Math Standards
Science as Inquiry

Nature of Science

Problem Solving
Physical Science

Communication
Evolution of the

Data Analysis
Connections
Structure &

Reasoning
History &

Functions
Universe

Patterns
Classroom
Activity

Evidence for
Hidden Mass ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔

Getting a Feel
for
Rotation ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔
Curves

Weighing a
Galaxy ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔

Dark Matter
Possibilities ✔ ✔ ✔ ✔

The Universe as
Scientists Know It ✔ ✔ ✔ ✔

Seeing as Far as
You Can See ✔ ✔ ✔ ✔

NSES Content Standards are from Chapter 6 of National Science Education Standards,
1996, National Research Council, National Academy Press, Washing DC.

NCTM Math Standards are from Chapter 7 of Principles and Standards for School
Mathematics, 2000, National Council of Teachers of Mathematics.
iii
iv
 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.

This booklet is intended to be used with the poster, “The Hidden Lives of Galaxies” (NASA #
EW-2000-07-002-GSFC). To request a copy of the poster, write to us at
itu@athena.gsfc.nasa.gov.

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 hard copies 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.

We thank Cheryl Niemela (Puyallup School District, WA) for valuable contributions and input to
the revision 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


 Foreword
The Story of Andromeda
The constellation Andromeda contains our Galaxy’s companion, the Andromeda Galaxy. Under
clear skies on a dark night, it can be seen with the naked eye. At a distance of 2.2 million light
years, it is the farthest object we can see without a telescope, and yet it is but the first stop in the
vastness of the universe outside our Galaxy.

As a tribute to our search for knowledge about these objects in the universe, we recount an early
story to explain what we see in the sky.

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


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. Over very long periods of
time, through the action of gravity, these clumps acquired more surrounding matter and grew.
Galaxies began to form when large clouds of gas and dust condensed. When very large clouds of
gas condense through gravity, stars formed.

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 our Galaxy, the Milky Way, and thus believed to be relatively
Edwin Hubble 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 became whether these objects
were within the Milky Way Galaxy, or whether they were stellar communities distinct from
our Galaxy. It wasn’t until the 1920’s 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. Thus,
these were found to be individual galaxies. Scientists now estimate that there are about 200
billion galaxies of various types in the universe.

Recommended Activity: How Big is the Universe? (see p. 14)


I. The Visible 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 1920’s 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 (or “classified”) their shapes as spiral, barred spiral, elliptical, irregular, and peculiar.
This system was known as the Hubble morphological sequence of galaxy types.

Hubble 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.

Hubble also noted that some spirals had a bright bar of gas through the center, and called these
barred spirals. Hubble also 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.

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.


 Hubble’s Fork Diagram of Galaxy Classification

Astronomers now have recognized that the morphology classification consists of two basic 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”.

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
Seyfert Galaxy NGC 1275 in Persus galaxies that have been studied at all wavelengths since
then.

Recommended Activities: Identifying Galaxies (see p. 14), and Classifying Galaxies Using
Hubble’s Fork Diagram (see p. 15)


B. How Galaxies Get Their Names

Some galaxies are given descriptive names (e.g. ”Andromeda”, “Whirlpool”) if they are
particularly distinctive in location or appearance. But most galaxies are known from their
designation in a catalogue. One of the earliest catalogues of objects in the sky was made by
Charles Messier. Messier was looking for comets in the 1700’s, but kept finding objects that
looked fuzzy, like comets, but didn’t move. Eventually, he created a catalogue of these objects,
listing their positions so he wouldn’t be fooled again into thinking they were comets. Later,
a number of them were identified as galaxies. Although he categorized many brilliant objects
in the night sky, his cataloguing system was completed in a random manner. So M1 (the Crab
Nebula in the constellation Taurus) is nowhere near M2 (a globular cluster in Aquarius).

As the capability of telescopes grew, larger catalogues were created. One of the oldest, but
still widely used, is the A New General Catalogue of Nebulae and Star Clusters, or NGC for
short, published by J. L. E. Dreyer in 1888. The NGC numbers objects from west to east
across the sky, so that all objects in the same area of the sky have similar NGC numbers. (The
Andromeda Galaxy is NGC 224, and the Whirlpool Galaxy is NGC 5194). Other catalogues
have been created from specific ground based observatories (e.g. ESO, the European Southern
Observatory, and the Uppsala General Catalogue (UGC) from the Palomar Observatory), orbiting
observatories (e.g. IR, for the Infrared Astronomical Satellite), or for specific objects with certain
properties (e.g. The Markarian catalogue lists galaxies with bright ultraviolet emission). The
numbers following the letter designation may indicate either the order in the list or the location
of the galaxy in the sky.

Recommended Activity: Identifying Unusual Galaxies (see p. 14)

C. The Components of a Galaxy

A galaxy 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. Elliptical galaxies do not have a disk.
The 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.

A Near-infrared image of the Milky Way Galaxy


 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, clusters,
The Pleiades
and nebulae in the disk rotate around the center of the
galaxy. In our Galaxy, it takes 200 million years for our sun to make a full revolution around the
center. 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 near their hive. The Globular Cluster M80

Elliptical galaxies consist of just one visible component, the bulge. A good example of this is
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.

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.

Recommended Activity: Open Clusters vs. Globular Clusters (see p. 16)

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.

The Milky Way and our nearest neighbor galaxies form a collection of galaxies called 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 asymmetric
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.
Superclusters are connected by lines (also referred to as filaments) of galaxies or clusters that run
outside regions that do not have any galaxies (called voids). The study of these large structures
in the Universe provides astronomers with observations that can be used to test their models in
understanding how these structures form. Different models of how structure arises give different
maps of the Universe.

II. The Hidden Lives of Galaxies

A. Hidden Objects

Observations of galaxies at wavelengths other than optical light reveal 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.

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.

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. 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.

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. Often, this
central part of the galaxy is very bright in x-rays gamma rays and radio, because of the large
amount of material interacting near the very massive black hole. Such galaxies are said to have
an Active Galactic Nucleus, and are often referred to as AGNs. The central black hole can have
a mass of millions (or even billions) times the mass of our sun.

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 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 what would be expected from the gravitation force of the visible mass (stars,
gas, dust) in the Galaxy. In reality, the stars appeared to be moving faster than this - 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. Scientists still know little about dark matter. 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. (See section C below for further discussion on the possible sources of dark
matter.)

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.

Recommended Activities: Modeling Mass in the Solar System and a Galaxy (see p. 17),
Evidence for Hidden Mass (see p. 18), Getting a Feel for Rotation Curves (see p. 19),
and Weighing a Galaxy (see p. 19)

C. Possibilities for Dark Matter

The search for the nature of dark matter is a very active field in astronomy and physics.
Scientists do not know what it is made of, but they are investigating a number of possibilities.

The chief property of dark matter is that it is “dark”, i.e. that it emits no light. Not visible,
not x-ray, not infrared. So it is not large clouds of hydrogen gas, since we can usually detect
such clouds in the infrared or radio. In addition, dark matter must interact with visible matter
gravitationally. So the dark matter must be massive enough to cause the gravitational effects that
we see in galaxies and clusters of galaxies. Large clouds of hydrogen gas don’t have enough
mass to do what the dark matter does.

The two main categories of objects that scientists consider as possibilities for dark matter include
MACHOs, and WIMPs. These are acronyms which help us to remember what they represent.
Listed below are some of the pros and cons for the likelihood that they might be a component of
dark matter.
10
MACHOs (MAssive Compact Halo Objects): MACHOs are objects ranging in size from
small stars to super massive black holes. MACHOS are made of ordinary matter (like protons,
neutrons and electrons). They may be black holes, neutron stars, or brown dwarfs.

Neutron Stars and Black Holes are the final result of a supernova of a massive star. They are
both compact objects resulting from the supernovae of 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. Because
a supernova usually leaves behind a remnant cloud of gas, these objects must travel far from the
remnant to be “hidden.”

Pros: Neutron stars are very massive, and if they are isolated, they both can be dark.

Cons: Because they result from supernovae, they are not necessarily common objects. 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.

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.

Astronomers have been detecting MACHOs using their gravitational effects on the light from
distant objects. In formulating his theory of gravity, Einstein discovered that the gravitational
attraction of a massive object can bend the path of a light ray, much like a lens does. So when a
massive object passes in front of a distant object (e.g. a star or another galaxy), the light bends
is “focused” and the object appears brighter for a short time. Astronomers search for MACHOs
(usually brown dwarfs) in the halo of our galaxy by monitoring the brightness of stars near the
center of our galaxy and of stars in the Large Magellanic Cloud.

The MACHO Project, one of the groups using this “gravitational lens” technique, observed about
15 lensing events toward the LMC over a span of 6 years of observations. They set a limit of
20% as the contribution to the dark matter in our Galaxy due to objects with mass less than 0.5
that of the sun.

Pros: Astronomers have observed objects that are either brown dwarfs or large planets
around other stars using the properties of gravitational lenses.

Cons: While they have been observed, astronomers have found no evidence of a large
enough population of brown dwarfs that would account for all the dark matter in our
Galaxy.
11
WIMPs (Weakly Interacting Massive Particles): WIMPs are the subatomic particles which are
not made up of ordinary matter. They are “weakly interacting” because they can pass through
ordinary matter without any effects. They are “massive” in the sense of having mass (whether
they are light or heavy depends on the particle). The prime candidates include neutrinos, axions,
and neutralinos.

Neutrinos were first “invented” by physicists in the early 20th century to help make particle
physics interactions work properly. They were later discovered, and physicists and astronomers
had a good idea how many neutrinos there are in the universe. But they were thought to be
without mass. However, in 1998 one type of neutrino was discovered to have a mass, albeit very
small. This mass is too small for the neutrino to contribute significantly to the dark matter.

Axions are particles which have been proposed to explain the absence of an electrical dipole
moment for the neutron. They thus serve a purpose for both particle physics and for astronomy.
Although axions may not have much mass, they would have been produced abundantly in the
Big Bang. Current searches for axions include laboratory experiments, and searches in the halo
of our Galaxy and in the Sun.

Neutralinos are members of another set of particles which has been proposed as part of a physics
theory known as supersymmetry. This theory is one that attempts to unify all the known forces
in physics. Neutralinos are massive particles (they may be 30x to 5000x the mass of the proton),
but they are the lightest of the electrically neutral supersymmetric particles. Astronomers and
physicists are developing ways of detecting the neutralino either underground or searching the
universe for signs of their interactions.

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. These particles are also important to physicist
who seek to understand the nature of sub-atomic physics.

Cons: The neutrino does not have enough mass to be a major component of Dark Matter.
Observations have so far not detected axions or neutralinos.

There are other factors which help scientists determine the mix between MACHOs and WIMPs
as components of the dark matter. Recent results by the WMAP satellite show that our universe
is made up of only 4% ordinary matter. This seems to exclude a large component of MACHOs.
About 23% of our universe is dark matter. This favors the dark matter being made up mostly
of some type of WIMP. However, the evolution of structure in the universe indicates that the
dark matter must not be fast moving, since fast moving particles prevent the clumping of matter
in the universe. So while neutrinos may make up part of the dark matter, they are not a major
component. Particles such as the axion and neutralino appear to have the appropriate properties
to be dark matter. However, they have yet to be detected.

Recommended Activity: Dark Matter Possibilities (see p. 20).

12
D. Formation of Galaxies

How galaxies formed after the Big Bang is a question still being studied by astronomers.
Astronomers hypothesize that within the first few hundred thousand 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. These first galaxies
appeared 12.5 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.

Recommended Summary Activities: The Universe as Scientists Know It (see p. 21), and Seeing as
Far as You Can See (see p. 22).

13
III. Classroom Activities
Most of the classroom activities have a Student Worksheet which accompanies them.
These worksheets appear on pages 24-46 and are formatted for easy copying. Some
activities also use transparencies that are available at http://imagine.gsfc.nasa.gov/
docs/teachers/galaxies/transparencies/.

Activity #1 – How Big is the Universe?

In this activity, students estimate the size of the visible universe in relation to the size of
the Milky Way Galaxy. To do so, students will get a sense of scale and will convert from
centimeters to kilometers.

See Student Worksheet, page 24

Worksheet Answers
1. What is the total size of the Milky Way Galaxy? 100,000 light years
2. If the Milkyway Galaxy is represented by an 8 cm wide coffee mug, the visible universe
would approximately 12 km in radius.

Activity #2 – Identifying Galaxies

In this activity, students describe the characteristics of the different types of galaxies (spiral,
elliptical, barred spiral, peculiar, or irregular) in their own words. They also classify galaxies
seen in the Hubble Deep Field. Note that this activity uses the transparencies that accompany
this booklet. The transparencies are also available at http://imagine.gsfc.nasa.gov/docs/
teachers/galaxies/transparencies/

See Student Worksheet, page 25.

Worksheet Answers/Assessment Guide

1. Show the students either “The Hidden Lives of Galaxies” poster or Transparency #1: Types
of Galaxies. Students write a brief description of each galaxy type.
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. Show Transparency #2: Deep Survey Image. Students 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.

14
Activity #3 – Classifying Galaxies Using Hubble’s Fork Diagram
In this activity, students explore the idea of classifying objects. They start by giving examples of
objects that can be classified in everyday life and in science. They then characterize and classify
a set of galaxies using their own scheme, and using Hubble’s classification scheme.

See Student Worksheet, page 26.

Note that the Galaxy Classification Chart is on the “Hidden Lives of Galaxy” poster and is
available as Transparency #4 – Galaxy Classification Images. Transparencies are available at
http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/transparencies/.

Worksheet Answers/Assessment Guide

Part 1
1. We put objects into categories to help us organize the items, and to identify similarities. By
putting items in different categories, we can identify their differences.
2. Examples of items we categorize in everyday life may include a CD collection, clothes,
magazines, books. 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.
Classifying objects into categories helps us identify similarities in their properties and/or
their functions. Examining a group of similar objects together helps scientists determine the
reasons or causes behind their properties or behavior.
3. Scientists classify the different elements (through the periodic table), different forms of life
(through kingdoms, species, etc.), different types of stars, as just a few examples.
Part 2
3. Below are the names given to these galaxies by astronomers (hopefully your students came
up with better names!), the galaxy type, and galaxy classification using the Hubble Fork
Diagram.
Galaxy Classification Chart

M84 NGC 2997 NGC 5383

Elliptical, E1 Spiral, Sb Barred Spiral, SBb

Large Magellanic Cloud NGC 4622 M83

Irregular, Ir Spiral, Sb Spiral, Sc

Centaurus A M59 NGC 1365

Peculiar, P Elliptical, E5 Barred Spiral, SBc
15
Activity #4 – Identifying Unusual Galaxies
In this activity, students will identify galaxies by their distinctive appearance.

See Student Worksheet, page 29.

Worksheet Answers
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

In this activity, students will describe similarities and differences between galactic star
clusters and globular clusters.

Show students Transparency #5: M37/M80, compare the open cluster M37 to the globular
cluster M80 (See http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/transparencies/ for
transparency.) Also review the material in section C on the components of a galaxy.

See Student Worksheet, page 30.

Worksheet Answers/Assessment Guide

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 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.

The analogy between a globular cluster and bees swarming about a beehive can be appropriate in
the depiction of many stars moving about a common center. The analogy can be inappropriate in
that the stars move slowly, and in paths determined by the gravitational forces of the stars in the
cluster. The bees, on the other hand, are “self-propelled” and move more randomly.

16
Activity #6
This is a series of activities which illustrate the principles behind hidden mass, rotation curves
and the evidence for hidden mass.

Activity #6a: Modeling Mass in the Solar System and a Galaxy

In this activity, students will discover how mass is distributed in the solar system and a galaxy.

Materials:
1 10-lb (4.54 kg) bag of kitty litter
Material such as sand or small pebbles
large paper
A balance capable of measuring a few grams
A balance capable of measuring a few kilograms.

The Student Worksheet (page 31) gives the instructions for the activity.

Worksheet Answers
Part A
1. To get the “kitty litter” equivalent masses of the planets, use ratios. For example, for Jupiter,
use x/4536 = (1.90 x 1027)/(2 x 1030). Here are the masses of the planets, and those masses as
fractions of the sun’s mass:

Kitty Litter Mass Inside

Object Mass (kg) Equiv. (g) Distance (km) Orbit (g)
Sun 2.00 x 1030 4536 0 4536

Mercury 3.30 x 1023 7.5 x 10-4 57.9 x 106 4536

Venus 4.87 x 1024 1.1 x 10-2 108.2 x 106 4536.01

Earth 5.97 x 1024 1.4 x 10-2 149.6 x 106 4540.02

Mars 7.35 x 1022 1.7 x 10-4 227.9 x 106 4540.02

Jupiter 1.90 x 1027 4.31 778.4 x 106 4540.34

Saturn 5.69 x 1026 1.29 1427 x 106 4541.63

Uranus 8.68 x 1025 0.20 2871 x 106 4541.83

Neptune 1.02 x 1026 0.23 4498 x 106 4542.06

Pluto 1.3 x 1022 2.9 x 10-5 5906 x 106 4542.06

17
 So if the sun is represented by 4.55 kg of kitty litter, then Jupiter is 4.3 gms, Saturn is 1.3
gms, Uranus and Neptune are each about 0.2 gms, and all the terrestrial planets combined
are about 0.025 gms. The “kitty litter” equivalents of the terrestrial planets individually are
impossible to measure out; even their combined amount is most likely difficult. Computing
these “kitty litter” equivalents provides the students a sense of how much more massive the
sun is.

4. Note that in this exercise, the students are asked to imagine passing the orbits of the planets.
The masses they are adding up is the mass within that distance to the sun. So it doesn’t
matter if the planet is actually on the opposite side of the sun, just as long as its closer to the
sun than the distance being considered. Alternatively, you may ask students to imagine that
the planets are lined up.

The mass within the distance between you and the sun is the sun’s mass plus whatever planet
orbits haven’t been passed. For the most part, it’s 4.536 kg. Details are in the chart above.
Students should not record values less than one hundredth of a gram.

Part B
1. You expect the matter in the galaxy to be where the light is. So a good deal of the mass should
be in the central bulge, but a fair amount should be spread around in the spiral arms.
2. Using the fact that the area of a pie segment is 0.5 R2 θ (where θ is in radians), advanced
students might derive the appropriate radii for the portions.
4. Now the students should note that the mass within their distance to the center will significantly
decrease as they move toward the center.
5. Distribute the new material uniformly throughout the model galaxy.
6. Each third of the slice will be more massive. What the students should note is that the fraction
by which the mass increases is larger for the outer thirds of the pie segment. This is because
we distributed the “dark matter” uniformly, but the original visible matter is not. So the mass
in the outer portion has more of an effect.
7. Now the students will note that the mass does not change as much. If they could “see” the
dark matter, the distribution of mass in the galaxy would look more uniform.

Activity #6b - Evidence for Hidden Mass

In this activity, students will interpret and analyze the information presented on the “Evidence for
Hidden Mass” graph. They will observe trends in the graphs, and use it to determine if there is
evidence for hidden mass.

The “Evidence for Hidden Mass” graph is on the “Hidden Lives of Galaxies” poster. It is also
available as Transparency #6 , as part of the transparency set. The transparencies are available at
http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/.

See Student Worksheet, page 33.

18
Worksheet Answers
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 the distance ranges from 0 to about 19 kpc, and the velocity ranges
from 0 to 100 km/s. The velocities first increase with increasing distance. But at distances
larger than 10 kpc, the velocity becomes constant with increasing distance.

Activity #6c – Getting a Feel for Rotation Curves

This activity is a kinesthetic exercise for students to experience rotation curves for themselves.
Note that there is no student worksheet for this activity.

Divide the students into two groups. One group will participate in the activity while the other
observes. The groups should switch for different parts of the activity.

1. Have students stand in a line, linking arms. Have the person at one end start to turn slowly.
Ask the Observers, “How would you describe the motion of the other students? Can you plot
their positions as they change with time? What would its rotation curve look like?”
• The students represent objects that are tightly bound together, e.g like a rod, or a CD
or a vinyl record. So the students will likely move mostly in unison, but the inner
students cover shorter distances than the outer students do in the same amount of
time. The rotation curve would show a linear increase in student velocity as distance
from the center increases. This mimics stars in the bulge of a galaxy.

2. Now have the students hold hands. Again have the person at one end turn slowly in place.
Ask the Observers, “Now how would you describe the motion of the other students? What
does the rotation curve look like now?”
• Now the students are not so tightly bound together, but they still move in a circle
around the central point. The students will not move so much in unison. The rotation
curve will initially rise but then level off as distance increases. This mimics stars in
the spiral arms of a galaxy.

3. Now let the first half of the line links arms, and the rest of the students hold hands. Ask the
Observers to describe the motion.
• The students linking arms will move mostly in unison, while velocity of the outer
students will approach a constant value. Together, this mimics motions of stars in the
bulge and spiral arms.
19
4. Finally, let the students walk freely in circles (i.e. as in orbits) around the student in the
center.
• Here the rotation curve will vary depending on the creativity of the students. See if
they can imitate the rotation curve of the solar system by having the inner students
move more quickly than the outer students.

Activity #6d - Weighing a Galaxy

Students will use Newton’s Laws of Motion to determine the mass of the sun from the motions
of the planets. They will then use the same techniques to determine the mass of a galaxy. In
doing so, students will convert among different measurement units used in astronomy.

See Student Worksheet, page 35.

Worksheet Answers
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.

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 10 10 times more massive than the sun.

20
Activity #7 – Dark Matter Possibilities
In this activity, students investigate one specific topic related to dark matter using available
resources. Students will organize their findings and present this information in a creative and
engaging fashion.

See student worksheet, page 37.

Directions:
1. Organize students into groups of about four students each. Assign each group one of the
following topics related to dark matter (you may need to assign several groups to the same
topic).

• Massive Compact Halo Objects (MACHOs)

• Weakly Interacting Massive Particles (WIMPs)
• Hydrogen Gas

2. The Student Instruction Sheet (see page 37) gives students a series of questions to guide the
investigation of their topic. It also lists suggested resources to use. Ask students to gather
their information, citing their sources in a bibliography. Allow one to two days for the
students to carry out their research.

3. Ask students to prepare a presentation to the class. Some possible presentation formats are
on the Student Instruction Sheet. Feel free, of course to suggest and/or encourage others.

A suggested rubric for assessment:

Criteria 1 point 2 points 3 points 4 points

Half of the questions Most questions are All questions are All questions are
Answering are answered with little answered; most in answered; most in answered completely, in
Questions attempt for complete complete sentences with complete sentences with complete sentences with
sentences and spelling correct spelling correct spelling correct spelling

At least one source is At least two sources At least three sources At least four sources
Citing
listed, using correct are listed, using correct are listed, using correct are listed, using correct
Resources bibliography formatting bibliography formatting bibliography formatting bibliography formatting

Group members are Group members attempt

Group members work Group members work
Working in able to complete their working together
effectively together with effectively together with
Groups research and complete with little teacher
only one warning no warnings
the project intervention

One to two minutes One to two minutes 2 to 3 minutes long; 3 to 5 minutes long;
Presentation long; half of the long; includes most of includes all of the includes all of the
research information the research information research information research information

Audience is engaged; Audience is engaged;

No attempt to engage Audience is engaged; no
Creativity audience is made props are used
one or two props are several props are
created/used created/used

21
Activity #8 - The Universe as Scientists Know It
The student will be able to assess his/her degree of understanding of what makes up the universe
as scientists know it.

See Student Worksheet, page 39.

Worksheet answer

The Universe As Scientists Know It

The Universe

Galaxy Clusters
Example: Virgo Cluster

Galaxies
Example: Milkyway

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

Alpha Centauri
Asteroid
Moon Sirius
Meteor

Jupiter Saturn

Io Callisto Titan

Ganymeade Europa

22
Activity #9 – Seeing as Far as You Can See
In this activity, students become familiar with using binoculars and/or a telescope. They locate
and identify constellations, and the Andromeda Galaxy.

This is a night-time activity that may be best done with the help of parents, a local astronomy
club, museum, planetarium, or university. It offers the opportunity for students to experience
the night sky and to see a galaxy. This activity is designed to be done in the fall, when the
Andromeda Galaxy is most easily visible in the early evening.

See Student Worksheet, pp 40-46.

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. Answers may vary, but The Big Dipper (part of Ursa Major), Pegasus, Cassiopeia, Cygnus,
Andromeda are among the possibilities
3. The Solar System is located in the Milkyway galaxy. The morphology of the Andromeda
Galaxies is spiral; the Triangulum Galaxy is also spiral; M33, the companion to Andromeda
is elliptical.

23
 Activity #1
How Big is the Universe?

The Milky Way has a radius of approximately 50,000 light years. The visible
universe has a size of approximately 15 billion light years. If the Milky Way is
represented by 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 The visible So if an 8 cm

a radius of about universe has wide coffee cup
50,000 light years. a radius of represents the
approximately 15 Milky the visible
What is the total billion light years. universe would be
size of the Milky approx. ________
Way Galaxy? km in radius.
________

Show your calculations

What other objects can you use to compare the size of the universe with the size
of the Milky Way galaxy?

24
 Activity #2
Identifying Galaxies

1. Your teacher will show you an image of different types of galaxies. List the five
types of galaxies and write a brief description of each.

Title___________________________

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

2. The “Hubble Deep Field” image was taken by the Hubble Space Telescope
December 18 – 28, 1995. It was taken of a region near the handle of the Big
Dipper, and covers a patch of sky about only 0.05 degrees across (equivalent
to the width of a dime viewed 75 feet away). This region was chosen because
there are very few stars there. So nearly every object in the image is a galaxy.

Your teacher will show you a portion of the Hubble Deep Field image. Identify
the types of the ten galaxies labeled on the Deep Survey Image.

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

Assessment:
List at least five specific observations from Deep Survey Image.

25
 Activity #3

Classifying Galaxies with Hubble’s Fork Diagram

Part I
Brainstorm answers to the following questions in your group.

1. Why do people put things into classifications or categories? How does this help
us? ________________________________________________________
___________________________________________________________
__________________________________________________
2. What are some things we categorize in our daily lives? Why? _____________
___________________________________________________________
___________________________________________________________
__________________________________
3. What types of objects do scientists classify? Name five different areas of
science that classify objects and identify them. Tell what they classify.
q __________________________________________________
q __________________________________________________
q __________________________________________________
q __________________________________________________
q __________________________________________________

Part II
In your groups, look at the images of actual galaxies on the page 2 and suggest
answers to the following questions.

1. Pretend that a NASA astronomer comes to your school and asks you to name
the galaxies pictured in the chart based upon their resemblance to common
objects. What would you name them? Write your suggestions underneath
each picture.

26
 Galaxy Classification Chart

--------------------- --------------------- ---------------------

--------------------- --------------------- ---------------------

--------------------- --------------------- ---------------------

--------------------- --------------------- ---------------------

--------------------- --------------------- ---------------------

--------------------- --------------------- ---------------------

2. Without using any prior information, how many different types of galaxies are
represented in these pictures? Decide on how many groups or classifications
you would have and give each group a name. Then, underneath, include the
criteria you would use to include a galaxy in this group.

q Group Name : _______________________________________

Criteria : __________________________________________
_________________________________________________
q Group Name : _______________________________________
Criteria : __________________________________________
_________________________________________________
q Group Name : _______________________________________
Criteria : __________________________________________
_________________________________________________
q Group Name : _______________________________________
Criteria : __________________________________________
_________________________________________________
q Group Name : _______________________________________
Criteria : __________________________________________
_________________________________________________

27
3. Now imagine that the NASA astronomer needs your help to classify these
newly discovered galaxies based upon your knowledge of the Hubble Fork
Diagram. Classify each galaxy according to that scheme. Write the galaxy
type and classification below your name of each image. (For example, the
Andromeda Galaxy is Spiral, Sb.)

Part III – Optional

In your groups, research one of the different types of galaxies in the Hubble
Fork Diagram. Using the resources provided by your teacher, identify the
following information about your galaxy type and present this information to the
class.
q Type and classification
q Shape
q Examples of this galaxy type
q How this galaxy forms
q How stars move in this galaxy type

You must use at least one of the following methods to present this information:
create one large 3-D poster, write and perform a skit, write and perform a song or
rap, or create a 3-D model.

28
 Activity #4
Identifying Unusual Galaxies

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____________________________
__________________________________

29
 Activity #5
Open Clusters vs. Globular Clusters

Using the image provided by your teacher, compare the open cluster M37 to the
globular cluster M80. Complete the Venn diagram below to compare and contrast
the properties of open clusters with globular clusters.

Assessment:
A globular cluster is sometimes compared to bees swarming around a beehive. How
might this analogy be appropriate? How might this analogy be inappropriate?

30
 Activity #6a
Modeling Mass in the Solar System and a Galaxy

Part A: Create a mass model of the solar system

1. Using the chart below, answer the following questions
a. If the mass of the sun is represented by 4.55 kg of kitty liter, how much
kitty litter would represent the mass of Jupiter?
b. Of Saturn?
c. Of Uranus or Neptune?
d. All the other planets (Mercury, Venus, Earth, Mars, Pluto) combined?

Object Mass (kg) Kitty Litter Avg. Distance Mass Inside

Equivalent (g) from Sun (km) Orbit (g)
Sun 2.00 x 1030 4550 0
Mercury 3.30 x 10 23
57.9 x 106
Venus 4.87 x 1024 108.2 x 106
Earth 5.97 x 1024 149.6 x 106
Mars 7.35 x 1022 227.9 x 106
Jupiter 1.90 x 1027 778.4 x 106
Saturn 5.69 x 1026 1427 x 106
Uranus 8.68 x 1025 2871 x 106
Neptune 1.02 x 1026 4498 x 106
Pluto 1.3 x 1022 5906 x 106

2. Measure out these masses from the kitty litter for each of the gas planets and
for the combination of the terrestrial planets.
3. Optional – Pour out the rest of the kitty litter to represent the mass of the sun.
4. If you were a space alien entering the solar system, as you approach the sun you
would pass the orbits of the planets. As you passed by the orbits of each of
the planets, how much mass (using the “kitty litter equivalents”) is left within
the distance between you and the sun? Fill in this information in the fourth
column of the chart. Make a graph of this mass vs. distance.

Part B: Create a mass model of a galaxy.

1. Examine a picture of the Andromeda galaxy. Where do you expect the matter
to be in the galaxy?
_______________________________________________
_______________________________________________
Distribute the kitty litter according to where you expect the matter to be.

31
2. Select a pie section of your model galaxy.
Divide that pie section radially into thirds
(so there’s an inner portion, a middle portion,
and an outer portion). Each third should have
the same area (If R is the radius of the pie
section, the first third should extend 0.6R
from the center, and the second portion
between 0.6R and 0.8R from the center).
If you divide a slice of a galaxy into equal
thirds in this manner, would you expect to
have the same amount of mass in each? Why
or why not?
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________

3. Determine and record the mass of each third of the slice:

Outer Third ______ Middle Third ________ Inner Third _______
4. Now imagine you are a traveling to the Andromeda galaxy. As you travel through
the galaxy, how much mass remains between you and the center. Create a graph
of mass vs distance for this.
___________________________________________________________
___________________________________________________________

5. Now your teacher will distribute a new material in your galaxy.

6. Determine the mass of each of your portions in the pie section again.
Outer Third ______ Middle Third ________ Inner Third _______
Compare with your previous measurements. What do you notice?
___________________________________________________________
___________________________________________________________

7. Repeat your imaginary trip through the galaxy. Now what does the graph of mass
vs distance look like? If you could see the hidden mass, what do you think the
galaxy would look like? Draw a picture of what you think the galaxy would look
like if you could see the whole galaxy?
___________________________________________________________
___________________________________________________________
___________________________________________________________

32
 Activity #6b
Evidence for Hidden Mass

Examine the “Evidence for Hidden Mass” graph. Note that the graph has a curve
labeled “Solar System Planets” (in yellow) and a curve labeled “Galaxy F563-1” (in
white). The curve for the Solar System shows the relationship between distance
from the sun and orbital velocity for each of the planets. The curve for the galaxy
shows the relationship between distance from the galaxy center and velocity
around that center for stars in the galaxy.

1. Finish writing the paragraph below by interpreting the data about Solar System
planets on the “Evidence for Hidden Mass” graph.

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.

33
 Activity #6b
Evidence for Hidden Mass (cont.)

2. Now use your own words to describe the graph for Galaxy F563-1. Describe
the range of distances and velocities. Also describe the behavior of velocity as
distance increases. _____________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________.

34
 Activity #6d
Weighing a Galaxy

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.

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

Earth 1.5 x 108 29.8

Jupiter 7.8 x 108 13.1

Neptune 4.5 x 109 5.4

What do you notice about the values of the Mass ? ___________________

What would you conclude the mass of the sun to be ? _________________

35
1. 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? _______________________
_____________________________.

36
 Activity #7
Dark Matter Possibilities

In this activity, you will research one of the topics related to the possible
sources of dark matter.

As you research your topic, answer the following questions:

1. What are the objects that could be included into this category?
2. What are the characteristics of these object(s)? Specifically:
a. Where are these objects found in space?
b. What is their approximate size per particle or object?
c. How are these objects formed? Do they appear together or separately
in space?
d. Are these objects found in conjunction with other astronomical objects
(e.g. stars, galaxies or nebulae)?
3. How could these objects be thought to contribute to dark matter?
4. What tools are required to detect these objects by astronomers or physicists?

Here are some suggested resources for investigating your topic:

• Imagine the Universe! web site:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/dark_matter.html
• Imagine the Universe! CD-ROM
• BBC Science & Nature:
http://bbc.co.uk/science/space/deepspace/darkmatter/
• WMAP web site:
http://map.gsfc.nasa.gov/m_uni/uni_101matter.html

37
 Activity #7
Dark Matter Possibilities (continued)

Now you will present your findings to the class. Here are some suggested
ways to present your results:

• Imagine that your group is to give a press conference about your newly
discovered object(s), explaining what this is and how it may be a source for
dark matter. Prepare your 5-minute presentation for the press, covering
the information you discovered in answering the questions. Select one
person to be the news anchor, one person to be the science reporter asking
the questions, and one or two people to be the astronomers answering the
questions. Dress your part! Act your part! (There may be a Nobel Prize in
this for you!) Help the public understand your new findings. Create props, as
needed.

• Imagine that you are a pop musician asked to produce a new CD for high
school students that teaches about the subject of dark matter. This is a
little different from your regular music, but you are willing to give it a try
because you want to help students become excited about exploring space.
In your group, write and perform a song or rap to teach about dark matter
and the different possibilities for it. Give details about the topic that your
group explored in your research. Perform this for the class, using props,
backup music and costumes, as needed. Remember, you want your audience
informed and excited about dark matter!

• Imagine that your group has been asked to create a game for high school
students that will teach them about dark matter and, specifically, about
the topic you researched. You and this game are to be featured in the local
newspaper, as everyone in the community has heard how creative you are.
You may create a card game, board game or physical game (e.g. tag, relay
race) that will teach your material to the participants. When you are done,
name your game, and teach it to the class.

• Imagine that your group has been contacted by a local theater company,
specializing in interpretive drama. They have run out of fresh material and
have just learned that you have discovered new information about something
exotic called “dark matter.” Naturally they think that this would make an
excellent subject for a new play and they want you to help them write and
perform it. Your job is to write a short play that explains your information
in a creative way. Create costumes and find props that can help your play
become more dramatic. Remember, if your audience isn’t entertained, they
won’t learn as much! (There may be an Academy Award in this for you!)

38
 Activity #8
The Universe as Scientists Know It

Directions:
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, Upsilon
Andromeda.

The Universe As Scientists Know It

The Universe

Galaxy Clusters
Example: Virgo Cluster

Example: Milkyway

Globular Clusters
Example:____________ Example: Solar Neighborhood Example: Pleiades

Example: ____________

Small Bodies

Examples: Examples: Examples:

Earth

Asteroid

Sirius

Saturn

Io Callisto

Ganymeade Europa

39
 Activity # 9
Seeing as Far as You Can See
Directions:
In this night-time activity, you will become familiar with some of the stars and constellations in
the autumn sky. You will also locate and see the Andromeda Galaxy.

1. Orient yourself to how objects in the night sky appear in binoculars and telescopes by
looking at Directional Chart.
2. Read “Tip Sheet” for types of objects you might see in the night sky, and for observing tips.
3. Read “Autumn Seasonal Guide Posts” and “Galaxies Visible in the Autumn Sky” to identify
constellations in the Autumn sky and directions for locating the Andromeda Galaxy.
4. Fill out “Observation Log” for each object you observe. Make additional copies for
additional objects.
5. Complete the assessment.

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

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 as it appears in most telescopes with a star diagonal (mirror
image). Reflecting telescopes 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.

40
 N S N
E Star W W Star EW ratS E
S N S
Naked Eye Finderscope Most Telescopes
with star diagonal
Binoculars Most Telescopes
without star diagonal
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. Two exceptions are the Andromeda Galaxy and the Orion Nebula.

41
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.

Autumn Seasonal Guide Posts

The following will help you become familiar with the stars and constellations of the Autumn sky.

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 Andromeda Constellation

Looking due north, about half-way up from the horizon will be a modestly 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. Between Cassiopeia and the Great
Square is the constellation Andromeda. To the east you’ll see a small cluster of stars called the
Pleiades. North of the Pleiades, the brilliant Cappella rises.

Now we locate a few galaxies in the Autumn sky. You’ll need binoculars to see the Andromeda
Galaxy, and a small telescope to see the other. It should also be a night in which the moon is not
up, and you are at a dark site.
42
Galaxies Visible in the Autumn Sky

Galaxies Visible in the Autumn Sky

Object Name Constellation Morphology Rating
M31 Andromeda Galaxy Andromeda Spiral 4
M32 Companion to M31 Andromeda Elliptical 2

M33 Triangulum Galaxy Triangulum Spiral 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. (These 3 stars are part of the constellation Andromeda). From the middle of these
3 stars go north towards Cassiopeia past one star to a second star, in a slightly curving line. The
Andromeda Galaxy is near the second star. On a moonless night in a dark sky you may be able
to see it without binoculars. If so, Congratulations! You’re seeing an object 2.2 million light
years away! Through binoculars, the galaxy looks like a bright oval embedded in the center of
a long swath of light. In a small telescope at low power, the galaxy extends across the field of
view. In the telescope, 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, locate the Triangulum Galaxy (M33), which is not far from the Andromeda Galaxy.
Locate again the Great Square, and follow the curving line of 3 stars toward Cassiopeia. We
used the middle of these to find the Andromeda galaxy. Starting at the middle star again, the
Triangulum galaxy is in the opposite direction from Andromeda. 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. On a dark night you may be able to
see it in binoculars. In a telescope, the galaxy is toward one end of 4 stars arranged as a “kite”.
The galaxy will look appear large, but very faint. Be sure to use your lowest power.

43
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 bright 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. Over the course of the night,
they move slowly across the sky. In reality, 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.

44
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 night, the best
5 example of its type,
not to be missed.
Impressive, easily
seen even on a
4 hazy night, a good
example of its type.
Also impressive, but
not the best of its
3
class.
May or may not be
easy to find, but not
2 exciting to observe,
lacks color
Not spectacular at
first sight. Difficult
to see. Pushes the
1 small telescope to its
limits.

45
Assessment:
1. Identify five constellations you observed in the night sky.
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
______________________________.

2. Identify any planets that you located.

___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
______________________________.

3. Identify the galaxy in which our solar system is located and list the morphology of the galaxies
you observed in the night sky.
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
______________________________.

46
V. Glossary
ARM – In galaxies, a structure composed of gas, dust, and stars which winds out from near the
galaxy’s center in a spiral pattern.

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. It’s 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.

GLOBULAR CLUSTER - A spherical bundle of stars that orbits a galaxy as a satellite.

Globular clusters are very tightly gravitationally bound, which gives them their spherical shape,
and extremely dense (in relative terms) towards their core.

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.

47
NEUTRON STAR – The imploded core of a massive star sometimes produced by a supernova
explosion. Neutron stars are typically have a mass 1.4 times the mass of the Sun, and a radius of
about 5 miles. Neutron stars can be observed as pulsars.

OPEN CLUSTER - A group of stars that were born at the same time from a molecular cloud,
and are still near to each other. They are also called galactic clusters since they exist within the
galaxy’s disk.

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 light 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

48
VI. 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 F563-1 was 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.

49
VII. 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.washjeff.edu/physics/plan.html.

The galaxy images used in this booklet are from the Digital Sky Survey, as catalogued on the
Interactive NGC Catalog Online
• http://www.seds.org/~spider/ngc/ngc.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

For other discussions of rotation curves and the evidence for dark matter, see
• http://astron.berkeley.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.

50
 Produced by
NASA Goddard Space Flight Center
Exploration of the Universe Division