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Visions of the Universe
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~ Four Centuries of Discovery
G alaxies
Revealed
supplemental materials
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Galaxies Revealed
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Table of Contents
Exhibit Overview � � � � � � � � � � � � � � � � � � � � � � � � 3
Science Background � � � � � � � � � � � � � � � � � � � � 4
Science Misconceptions� � � � � � � � � � � � � � � � 6
Glossary� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 7
Related Educational Resources� � � � � � 9
Exhibit Overview
I
n turning his telescope to the heavens in 1609, Galileo embarked upon a journey
that would revolutionize science and culture alike, profoundly changing our
view of our place in the universe. Our views of the universe, and how they have
evolved over time, are portrayed in the images and text of the “Visions of the
Universe: Four Centuries of Discovery” exhibit.
In recognition of the International Year of Astronomy, this exhibit includes six twosided panels that feature key astronomical discoveries from the past 400 years. The
exhibit also highlights the technological advancements that made these discoveries
possible. Exhibit topics range from celestial objects within our own “cosmic
backyard” to those beyond the realm of our solar system. Featured objects include
the Sun, the Moon, Saturn, Mars, comets, stars, nebulae, and galaxies. Images are
accompanied by captions that highlight relevant, historical discoveries.
The “Visions of the Universe: Four Centuries of Discovery” exhibit is supported
by supplemental resource documents available online, in PDF format, for each
panel. These twelve documents include science background information in the
form of Q&As, related science misconceptions, a glossary, and links to additional
resources on NASA’s Amazing Space Web site. In addition, each exhibit panel is
available as a downloadable, poster-size file. These materials are available from:
http://amazing-space.stsci.edu/visions
The “Visions of the Universe: Four Centuries of Discovery” exhibit was produced
in December 2008 by the Space Telescope Science Institute, the American Library
Association, and the Smithsonian Astrophysical Observatory, through funding
from the National Aeronautics and Space Administration. •
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Science Background
NOTE: Words in boldface italics are defined in the glossary.
Q1:
Who was Charles Messier?
ANSWER:
Charles Messier (1730–1817) was a French astronomer who hunted for comets. He is credited with the
discovery of 13 comets, but is most noted for publishing a catalog of faint, deep-sky objects, such as galaxies
and open star clusters. Messier used a small telescope to search the sky for faraway, fuzzy comets. To determine
whether an object was a comet or something else, Messier would observe it over a period of time to see if it
moved with respect to background stars. Messier kept a record of the faint objects he found and published a
catalog of them. The catalog entries became known as Messier objects.
Q2:
Why did Messier compile a catalog of faint objects?
ANSWER:
Messier created his catalog to help other comet hunters avoid objects that were not comets. Messier simply
listed these objects as he found them, assigning them numbers in chronological order. Today, Messier’s catalog
includes 110 objects that are still referred to by the number he assigned to them. For example, the Whirlpool
Galaxy is listed as number 51 in Messier’s catalog. It is therefore known as Messier 51, or just M51.
Q3:
Who was William Parsons?
ANSWER:
William Parsons (1800–1867), the third Earl of Rosse, was a telescope builder and an astronomer. Between 1843
and 1845, he built a 72-inch (1.8-meter) telescope that remained the largest reflector for seven decades. Designing
and building such a large telescope shows how skillful the Earl was in engineering and optics.
Q4:
How did William Parsons find the first spiral nebulae?
ANSWER:
In 1845 William Parsons used his reflector to discover a spiral pattern in the object known as Messier 5 (later
called the Whirlpool Galaxy). He carefully recorded the pattern by drawing dark lines on white paper. In
1848 Parsons observed a similar spiral pattern in Messier 99. He ultimately recorded 14 spiral-shaped objects.
These objects became known as spiral nebulae. Decades later, astronomers discovered that spiral nebulae were
actually galaxies located outside our Milky Way Galaxy.
Continued …
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Science Background, continued …
Q5:
What was the “Great Debate” and why was it important?
ANSWER:
The “Great Debate,” also called the Shapley-Curtis Debate, was a 1920 debate between astronomers Harlow
Shapley and Heber Curtis. Shapley contended that the universe was composed of only one big galaxy, the
Milky Way. Spiral nebulae, such as Andromeda, were just nearby gas clouds. Curtis, on the other hand, argued
that Andromeda and other spiral nebulae were separate, independent galaxies outside the Milky Way. The Great
Debate is not only a significant historical event; it also provides insight into the reasoning processes of wellknown scientists of that time. At the time of the debate, astronomers had not yet uncovered key information
about our universe.
Q6:
Who was Edwin Hubble?
ANSWER:
Edwin Hubble (1889–1953) was an American astronomer who studied galaxies. He is most noted for
demonstrating the existence of other galaxies outside the Milky Way Galaxy, establishing the expansion of the
universe, and creating a model for classifying galaxies.
Q7:
How did Hubble show that some nebulae are galaxies?
ANSWER:
Using the 100-inch Hooker Telescope at Mount Wilson, then the largest telescope in the world, Hubble
identified special stars in what was then known as the Andromeda Nebula (M31). These stars, called variable
stars, allowed Hubble to show that the distance to M31 was so great that it had to be outside the Milky Way
Galaxy. M31, therefore, was a separate galaxy much like our own Milky Way. •
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Science Misconceptions
MYTH 1:
Most galaxies are easily viewed with the naked eye.
F
ACT: Most galaxies are so far away that people can view them only with the help of telescopes.
Only three galaxies can be seen with the unaided eye: the Andromeda Galaxy and the Large and Small
Magellanic Clouds. These galaxies appear as cloudy patches in the sky. Other galaxies appear as fuzzy spots
in the sky when viewed with small telescopes.
MYTH 2:
All galaxies are the same.
F
ACT: The shapes of galaxies vary — some are elliptical, others are spiral, and still others have no
definite shape. Galaxies differ in color, composition, orientation, age, size, the number of stars within
them, and their distance from Earth.
MYTH 3:
Galaxies are composed of material that is different from stars.
FACT: Galaxies are large collections of stars, gas, and dust held together by their gravitational attraction.
MYTH 4:
Galaxies are static, remaining unchanged with time.
F
ACT: In fact, galaxies are dynamic and change over millions of years. Stars are born and die in galaxies.
A galaxy also can interact with another galaxy, which alters both galaxies’ shapes.
MYTH 5:
You can judge the distance of a galaxy based on its size.
FACT: The size a galaxy appears to be to an observer depends on how many stars are in it and how far
away it is. Two galaxies may appear to be the same size. One of them, however, may be a small galaxy close
to Earth and the other a large galaxy that is much farther away. •
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Glossary
Andromeda Galaxy
A spiral galaxy approximately 2.5 million light-years away in the constellation Andromeda. It is the
nearest spiral galaxy to our own, the Milky Way. It is also known as Messier 31, M31, or NGC 224, and
was originally known as the Andromeda Nebula.
Barred-spiral galaxy
A galaxy with a “bar” of stars and interstellar matter, such as dust and gas, slicing across its center. The
Milky Way is thought to be a barred-spiral galaxy.
Elliptical galaxy
A galaxy that appears spherical or football-shaped. Elliptical galaxies comprise mostly old stars and
contain very little dust and “cool” gas that can form stars.
Galaxy
A collection of stars, gas, and dust bound together by gravity. The smallest galaxies may contain only a
few million stars, and the largest galaxies have thousands of billions of stars. Our Sun is one star in the
Milky Way Galaxy. Galaxies are classified or grouped by their shapes. Round or oval galaxies are elliptical
galaxies, and those showing a pinwheel structure are spiral galaxies. Others are called irregular because
they do not resemble elliptical or spiral galaxies.
Irregular galaxy
A galaxy that appears disorganized and disordered, without a distinct spiral or elliptical shape. Irregular
galaxies are usually rich in interstellar matter, such as dust and gas. The Large and Small Magellanic
Clouds are examples of nearby irregular galaxies.
Lenticular galaxy
A type of galaxy that shares the characteristics of an elliptical galaxy and a spiral galaxy. Lenticular galaxies
have disks like spiral galaxies, as well as large rounded bulges of stars that resemble elliptical galaxies.
Lenticulars also have more gas and dust than ellipticals, but considerably less star formation than spirals.
Continued …
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Glossary, continued …
Milky Way Galaxy
The Milky Way, a spiral galaxy, is the home of Earth. The Milky Way contains more than 100 billion
stars and has a diameter of about 100,000 light-years.
Nebula (pl., nebulae)
A cloud of gas and dust located between stars and/or surrounding stars. Nebulae are often places where
stars form.
Reflector (Reflecting telescope)
A type of telescope, also known as a reflecting telescope, which uses one or more polished, curved mirrors
to gather light and reflect it to a focal point.
Spiral galaxy
A spiral-shaped system of stars, dust, and gas clouds. A typical spiral galaxy has a spherical central bulge
of older stars surrounded by a flattened galactic disk that contains a spiral pattern of young, hot stars, as
well as interstellar matter.
Spiral nebula (pl., nebulae)
Faint, cloudy objects observed by early astronomers to have a spiral pattern. Today, astronomers know
that spiral nebulae are actually spiral galaxies composed of billions of stars.
Star cluster
A group of stars born at almost the same time and place. Some clusters are capable of remaining together
for billions of years because of their mutual gravitational attraction. •
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Related Educational Resources
from NASA's “Amazing Space” Web site
http://amazing-space.stsci.edu
“Amazing Space” uses the Hubble Space Telescope’s discoveries to inspire and educate about the wonders of our
universe. The Amazing Space Web site includes curriculum support tools, a section for the general public, and a
special section for educators and developers. The curriculum support tools are developed by a team of scientists,
education specialists, and multimedia experts. They are aligned with national educational standards and are
reviewed for scientific accuracy. Find teaching tools, graphic organizers, Online Explorations, and more.
Online Explorations
“Online Explorations” are fun, interactive activities for exploring various space science topics.
• Telescopes From the Ground Up
“Telescopes From the Ground Up” traces the 400 years of telescope development from Galileo’s refractor
to NASA’s Great Observatories. The overview for this activity can be found at:
http://amazing-space.stsci.edu/eds/overviews/explorations/telehistory.php
Specific sections of “Telescopes From the Ground Up” related to this exhibit panel include:
“Lord Rosse’s Reflectors,” which describes the development and design of Lord Rosse’s (William
Parsons’) reflecting telescopes.
http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/rosse/index.php
“1845 Discovery: Cloudy objects are really spirals,” which describes how Lord Rosse used his
largest telescope to discover the spiral pattern in some nebulae.
http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/rosse/discovery.php
“Hooker 100-inch Reflector,” which describes the telescope that Edwin Hubble used to show that
galaxies exist outside our Milky Way Galaxy.
http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/mt_wilson/index.php
“1923 Discovery: Other galaxies exist,” which describes Edwin Hubble’s discovery that galaxies
exist outside our Milky Way.
http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/mt_wilson/discovery.php
“Biography: Edwin Hubble,” which introduces the American astronomer who studied galaxies.
http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/bios/hubble/
• Galaxies Galore, Games and More
In this Online Exploration, explore the shapes of galaxies and learn to classify them. The overview for
this activity can be found at:
http://amazing-space.stsci.edu/eds/overviews/explorations/galore.php
Continued …
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Educational Resources, continued …
Classroom Activities
These are activities aligned with national education standards that include student activity sheets, science
background information, and vocabulary.
Galaxy Trading Cards
This deck of eight cards features different galaxies and the constellations in which the galaxies are found.
Preview the cards and download the Galaxy Trading Cards PDF from:
http://amazing-space.stsci.edu/capture/galaxies/preview-tcards.php
Find the Right Circle
This classroom activity features the Galaxy Trading Cards. Use the Galaxy Trading Cards to observe and
record the properties of galaxies.
http://amazing-space.stsci.edu/eds/overviews/print/classroom_activities/circle.php
Group the Galaxies
This classroom activity features the Galaxy Trading Cards. Use the Galaxy Trading Cards to sort galaxies
into groups based on similar characteristics.
http://amazing-space.stsci.edu/eds/overviews/print/classroom_activities/group.php
Graphic Organizers
This is a collection of T-charts and Venn diagrams that compare and contrast various celestial objects
and phenomena. The downloadable organizers are available in teacher versions (full chart) and student
versions (blank organizer with images).
Comparison of spiral, elliptical, and irregular galaxies
This T-chart shows the similarities and differences between the three main types of galaxies: spiral,
elliptical, and irregular.
http://amazing-space.stsci.edu/eds/overviews/organizers/galaxy.php
Comparison of the Warped Galaxy and the Whirlpool Galaxy
This Venn diagram shows the similarities and differences between edge-on and face-on views of
spiral galaxies.
http://amazing-space.stsci.edu/eds/overviews/organizers/galaxy-compare.php
Lithographs
These are Hubble Space Telescope images of celestial objects that include informational text and inquirybased classroom activities.
Diverse Galaxies
This lithograph features a huge elliptical galaxy along with a variety of other galaxies.
http://amazing-space.stsci.edu/eds/overviews/print/lithos/diverse-galaxies.php
Continued …
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Educational Resources, continued …
Whirlpool Galaxy
This lithograph features the spiral arms and dust clouds of a nearby galaxy also known as M51.
http://amazing-space.stsci.edu/eds/overviews/print/lithos/whirlpool.php
Warped Galaxy
This lithograph features an image of an unusual galaxy with a warped, dusty disk, as viewed edge-on.
http://amazing-space.stsci.edu/eds/overviews/print/lithos/warped.php
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