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Content Benchmark E.8.B.1
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Students know the universe contains many billions of galaxies, and each galaxy contains many billions of stars.  W/L

From the earliest of time man has always looked to the sky in awe and wonder.  Man realized we are part of something larger, the universe. The universe is comprised of all of the matter and energy in existence.  The universe contains planets, the sun, stars, the Milky Way, and other galaxies.  Away from city lights, on a clear night one can see a hazy, luminous band spread out across the sky.  Many myths and stories were created to describe the “milky way”.  Galileo was the first to look at this haze with a telescope discovering it contained countless stars.  Now we know we are viewing a part of a vast disk that contains billions of stars, that includes our sun, and immense amounts of interstellar dust.  This is our galaxy, which is called the Milky Way.

Figure 1. Our Place in Space

The Milky Way includes several hundred billion stars in a spiral disk shape. The Milky Way is ~100,000 light-years across and ~10,000 light-years thick at its center.  The closest major, spiral galaxy to the Milky Way is Andromeda, which is 2.2 million light years away from us.  While the most common shape of galaxies is either spiral or irregular, there are also elliptical, dwarf, and lenticular galaxies in the universe. The observable universe is composed of up to 100 billion galaxies; many are larger than our own.  To better understand galaxies, this document will start with the parts of a galaxy and how galaxies are classified.  Finally the amount of galaxies in the universe and how many stars are contained within a galaxy will be probed.

What is a galaxy?
Galaxies are immense systems of stars, dust, and gas clouds held together by gravity.  They vary to a great extent in color, size and shape. A galaxy contains several million to more than several billion stars. The size of a galaxy is usually measured in light years.  A light year is equal to the distance traveled by light in one year, which is 9.46 x1015 meters. Galaxies can be several thousands to hundreds of thousands of light-years across.  However, to the naked eye the closest galaxy is no more than a dim, fuzzy spot.   

What are the parts of a galaxy?
A galaxy includes stars, gases, and dust.  In a spiral galaxy the stars, gas, and dust are organized into a center bulge, a disk containing spiral arms, and a halo.  Elliptical galaxies have a bulge-shape, and a halo, but do not have a disk. The shapes of galaxies will be discussed later.  The following illustration demonstrates the parts of a spiral galaxy.

Figure 2. 
Structure of a Galaxy

Bulge- The bulge is a round structure made primarily of old stars, gas, and dust.  The outer part of the bulge is difficult to tell apart from the halo.

Disk- The disk is a flattened region that surrounds the bulge in a spiral galaxy.  It includes mostly young stars, gas, and dust, which are concentrated in spiral arms.  Some old stars are present.

Spiral Arms- The spiral arms are curved extensions beginning at the bulge of a spiral galaxy, giving it a "pinwheel" appearance. Spiral arms contain a lot of gas and dust as well as young blue stars. Spiral arms are found only in spiral galaxies.  A spiral galaxy is composed of at least two arms with some galaxies having multiple arms.

Halo- The halo contains individual old stars and clusters of old stars, which are referred to as “globular clusters”. The halo also contains "dark matter," which is material that we cannot see but whose gravitational force can be measured.

Stars, gas, and dust- Stars come in a variety of types. Blue stars, which are very hot, tend to have shorter lifetimes than red stars, which are cooler. Regions of galaxies where stars are currently forming are therefore bluer than regions where there has been no recent star formation. Spiral galaxies seem to have a lot of gas and dust with new stars, while elliptical galaxies contain old stars and have very little gas or dust.

Further detail on stars can be found in the HS TIPS Benchmark E.12.B.1.

Galaxy Classification
Many astronomers use shape to classify galaxies.  Hubble classified galaxies into four major types:  spiral, barred spiral, elliptical, and irregular.  Galaxies can be classified using Edwin Hubble’s Tuning Fork Diagram describing spiral, elliptical, and irregular shapes.  Most galaxies are spirals, barred spirals, or ellipticals.


For an interactive galaxy classification tour, go to

Spiral Galaxies: Spiral galaxies have two or more arms winding out from a central disk. Spiral galaxies have a variety of shapes and are classified according to the size of the bulge and the tautness and appearance of the arms. The spiral arms hold numerous young blue stars and lots of gas and dust. Stars in the bulge tend to be older and redder.  When viewed from the side, a spiral galaxy resembles a fried egg. These galaxies rotate somewhat reminiscent of a hurricane or a whirlpool.  Spiral galaxies are classified with an “S” and sub-classified as type “a” through “d”.  Type Sa galaxies have large central bulges and tightly wound arms, while an Sc has a smaller bulge and loosely wound arms.

Barred Spiral Galaxies: Barred spiral galaxies are spirals that have a bar running across the center of the galaxyThese galaxies are classified in the same way as spiral galaxies, except the capital “B” designates barred or the bar in the center.

Lenticular Galaxies:  Lenticular galaxies are referred to as armless spiral galaxies.  Lenticular galaxies have a bulge and no spiral arms.  Some lenticular galaxies have a bar and are called “barred lenticular galaxies”.  Lenticular galaxies are classified as “SO” and a barred lenticular galaxy would be classified as a “SBO”.  

Elliptical Galaxies: Elliptical galaxies do not have a disk or arms.  Elliptical galaxies come in a variety of shapes ranging from round to flattened. Ellipticals contain old stars, and possess little gas or dust. Most elliptical galaxies are yellow and red because they do not contain young stars. In contrast to spirals, the stars in ellipticals do not revolve around the center in an ordered way. The stars move on randomly oriented orbits within the galaxy similar to a swarm of bees.  Elliptical galaxies are classified with the capital letter “E” with a number following indicating how round or flattened the galaxy is.  An E0 is nearly round and an E5 is more elliptically shaped or flattened as indicated in the diagram above.

Irregular Galaxies: Irregular galaxies are devoid of a definite shape.  They have stars, gas, and dust scattered in random patches. These galaxies often have active regions of star formation. Sometimes the irregular shape of these galaxies results from interactions or collisions between galaxies.

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Content Benchmark E.8.B.1

Students know the universe contains many billions of galaxies, and each galaxy contains many billions of stars.  W/L

Common misconceptions associated with this benchmark

1. Students have difficulty understanding that galaxies are groups of stars not just single stars.

Students should be introduced to galaxies and shown that they are large collections of stars, gas, and dust held together by gravity.  Students could begin with general information on stars such as their characteristics.  Understanding that stars have different masses, colors, and brightness can aid in realizing these characteristics influence a galaxy’s appearance.  By viewing images of galaxies students will realize galaxies are made of billions of stars.

For information on this misconception and strategies to address it NASA has two lessons in PDF that tackle this misconception. 

Visit the following M104:  The Sombrero Galaxy print/lithos/sombrero_litho.pdf

M101:  The Pinwheel Galaxy print/lithos/pinwheel_litho.pdf

2. Students do not understand the concept of size, scale, and distance in our solar system, the galaxy, and the universe.

Students may be confused by the terminology or may not understand the actual words.  The words represent a large system and the sense of their relationships is misunderstood.  Students focus on the solar system in middle school and do not fully grasp the expanse beyond our solar system.

To view an article about student misconceptions go to Lunar and Planetary Science XXXVI (2005) 1954

For learning experiences, lessons, and ideas to address this misconception visit
Universe Forum

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Content Benchmark E.8.B.1

Students know the universe contains many billions of galaxies, and each galaxy contains many billions of stars.  W/L

Sample Test Questions

Depth of Knowledge Level 1

  1. What type of galaxy is the Milky Way?
    1. Elliptical
    2. Irregular
    3. Lenticular
    4. Spiral
  1. How many galaxies are in our solar system?
    1. Zero
    2. Two
    3. Hundreds
    4. Millions
  1. A galaxy consisting of large clouds of gas and dust with active star formation is most likely what type of galaxy?
    1. Elliptical
    2. Irregular
    3. Lenticular
    4. Spiral
  1. A galaxy is composed of old stars and has no active star formation occurring.  Which type of galaxy would this indicate?
    1. Elliptical
    2. Irregular
    3. Lenticular
    4. Spiral
  1. How many stars are in our Solar System?
    1. One
    2. Two
    3. Hundreds
    4. Millions
  1. The majority of stars in the Milky Way are
    1. blue stars.
    2. red giant stars.
    3. main-sequence stars.
    4. white dwarf stars.

Depth of Knowledge Level 2

  1. Use the diagrams below to answer the question.
Figure 1
Figure 2
Figure 3
Figure 4
  1. Which illustration indicates a galaxy with mostly older stars that contains little gas or  dust?
    1. Diagram 1
    2. Diagram 2
    3. Diagram 3
    4. Diagram 4
  1. Use the graph showing the Hubble law relating the distances of galaxies and speed moving away from the Milky Way to answer the question.

  1. What is the speed of the galaxy marked A in the graph?
    1. 5,000 km/s
    2. 10,000 km/s
    3. 15,000 km/s
    4. 20,000 km/s
  1. Use the graph showing the Hubble law relating the distances of galaxies and speed moving away from the Milky Way to answer the question.

  1. What is the approximate distance in millions of light-years of galaxy A in the graph?
    1. 30 mly
    2. 60 mly
    3. 90 mly
    4. 120 mly
  1. Use the graph showing the Hubble law relating the distances of galaxies and speed moving away from the Milky Way to answer the question.

  1. If a new galaxy with a speed of 15,000 km/s were discovered, it would be found at an approximate distance of
    1. 30 mly.
    2. 60 mly.
    3. 90 mly.
    4. 120 mly.

Constructed Response Question

Content Benchmark E.8.B.1

1.  Use the following diagram and information to answer the questions below:

Figure 1.  Hubble Tuning Fork Diagram

  1. Describe the three general types of galaxies originally identified by Edwin Hubble.
  2. Hubble used to believe this diagram was the evolutionary path of galaxies. Describe why he might have inferred this.
  3. What is the evidence that changed this idea?

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Content Benchmark E.8.B.1

Students know the universe contains many billions of galaxies, and each galaxy contains many billions of stars.  W/L

Answers to Sample Test Questions

  1. D, DOK Level 1
  2. A, DOK Level 1
  3. D, DOK Level 1
  4. A, DOK Level 1
  5. A, DOK Level 1
  6. C, DOK Level 1
  7. B, DOK Level 2
  8. A, DOK Level 2
  9. A, DOK Level 2
  10. C, DOK Level 2

Constructed Response E.8.B.1 Score Rubric:

3 points

Response addresses all parts of the question clearly and correctly.

  1. Most galaxies are spiral galaxies, which have a nuclear bulge in the center and a few spiral arms. About one-third of all galaxies are elliptical galaxies. They have very bright centers and very little dust and gas. New stars do not form in elliptical galaxies. Irregular galaxies are those that don't fit into any other class. Their irregular shape may be the result of distortion caused by the gravitational attraction of spiral galaxies nearby. 
  2. Hubble thought elliptical galaxies were “early galaxies” and spiral galaxies were “late galaxies”.  He thought elliptical galaxies evolved into spiral galaxies. Scientists now know spiral galaxies rotate quickly, while elliptical galaxies do not. 
  3. There is no way an elliptical galaxy could spontaneously begin rotating, as a result an elliptical galaxy could not turn into a spiral galaxy.    

2 points

 Response addresses all parts of the question and includes only minor errors.

1 points

 Response does not address all parts of the question. 

0 points

 The response is totally incorrect or no response provided. 

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Content Benchmark E.8.B.1

Students know the universe contains many billions of galaxies, and each galaxy contains many billions of stars.  W/L

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Galaxies

This site teaches about different galaxies and how to classify them.  There are lessons on classification and some history is included.  Students work through the pages to learn about galaxy classification.

To access this interactive web site, go to

2. Capture the Cosmos

Explore astronomy topics such as black holes, galaxies, and the solar system.  Online adventure, exploration and interactive pages loaded with pictures, downloadable PDF lithographs, and answers to common questions.  Teacher materials are available.

To access this Internet resource, go to

3. Classifying Galaxies

This site presents an online, interactive lesson on the Hubble System of Classifying Galaxies.  Students become familiar with the Hubble Tuning Fork Diagram developed in the 1920’s by astronomer Edwin Hubble.  There are teacher and student pages, which are thorough and complete.  Background information is provided and loads of information via links.

To access the web resources, go to

4. Fun and Games-Galaxies and Comets

This is a terrific site with pod casts, interactive online activities, and tremendous amounts of current information.  There are detailed lessons for teaching about galaxies including glossaries and background information.  This site also shows you how to scan the night sky with Google Earth.

To access this web resource, go to

5.Fall Into A Blackhole

There is a downloadable board game where students take an intergalactic adventure to explore a black hole.

To access the teacher resources, go to

6. Hubble Deep Field Academy

A site that is set up for students to learn about Hubble Deep Field in an interactive format.  Students learn as cadets as they go through the pages and they have four activities to complete.

To access this interactive resource, go to

7. Cosmic Collision

A movie shows what happens when two galaxies collide.  There are wonderful pictures of actual galaxies and the documentary is informative.  The resource page provides more animations, movies, pictures, and press releases.

To access the pictorial resource, go to

8. Micro-Observatory Online Telescopes

Absolutely a must see web site, which allows you to explore the universe with telescopes that you control over the Internet.  All students should look at the online telescopes, after choosing a target an email link is sent to the observer and then you view your image.

To access the web site, go to

9. Galaxies Galore

Students learn about different types of galaxies through games and interactive simulations.

To access this Internet resource, go to

10. Average Galaxies

This site provides an informative news article for students from Science News for Kids. The article describes the discovery of 27 smaller, distant galaxies and their importance to scientists. Some information is given on the electromagnetic spectrum and how scientists figure the age of a galaxy.

To access the article, go to

11. A Whole Lot of Nothing

This site provides an informative news article for students from Science News for Kids.  The article describes the discovery of a vast void and explains why this is important to scientists.  Also, cosmic background radiation and the big bang are discussed.

To access the article, go to

12. Astronomy Picture of the Day Archive

This site links to hundreds of photographs and images of objects in the universe. There is a brief description of each image with links to more information. 

To access the photographs, go to

13. Galaxy Hunters

This site provides the student an interactive photo safari from Hubble Deep Fields.  The student learns about galaxies as they navigate through the site.

To access this interactive resource, go to explorations/ghunter/home.html

14. How Fast Do Galaxies Move?

This is an interactive lab that combines real data with a virtual spectroscope to introduce students to the concept of spectra and galactic motion in an expanding universe.  This site also goes into the Doppler Effect and introduces students to red shift.

To access the web resource, go to

15. Classifying Galaxies

This is an interactive site where students explore galaxy classification.  Good information is provided on the main types of galaxies.

To access the resources, go to classifying_galaxies/student1.htm

16. Galaxy Zoo

The student learns the different types of galaxies by actually having to identify different types of galaxies.  If enough are answered correctly the student moves on to galaxy analysis, which is actually used by scientists to classify galaxies.

To access the Internet resource, go to

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Support Pages

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Benchmark Related Vocabulary