Galaxies Galore,
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Teacher Page: Science Background



Science Background
Words from the Scientist


Science Background:

The following information is provided to give the teacher some additional information about galaxies. You can use this information as a teacher reference or use the question headings as a form of review for class discussion. This science background is organized to provide information as it relates to each of the lesson's activities.

Questions related to the activity Build Our Milky Way

1. What is a galaxy?

A galaxy is an enormous collection of a few million to trillions of stars, gas, and dust held together by gravity. They can be several thousand to hundreds of thousands of light-years across.

2. What is the name of our galaxy?

The name of our galaxy is the Milky Way. All of the stars that you see at night and our Sun belong to the Milky Way. When you go outside in the country on a dark night and look up, you will see a milky, misty-looking band stretching across the sky. When you look at this band, you are looking into the densest parts of the Milky Way: the "disk" and the "bulge."

3. Where is the Earth in the Milky Way galaxy?

Our solar system is in a spiral arm called the Orion Arm, and is about two-thirds of the way from the center of our galaxy to the edge of the starlight. The Earth is the third planet from the sun in our solar system of nine planets.

4. What is the closest galaxy like our own, and how far away is it?

The closest spiral galaxy is Andromeda, a galaxy much like our own Milky Way. It is 2.2 million light years away from us. Andromeda is approaching our galaxy at a rate of 670,000 miles per hour. Five billion years from now it may even collide with our Milky Way galaxy.

5. What are the parts of a galaxy?

A galaxy contains stars, gas, and dust. In a spiral galaxy like the Milky Way, the stars, gas, and dust are organized into a "bulge," a "disk" containing "spiral arms," and a "halo." Elliptical galaxies have a "bulge-shape" and a "halo," but do not have a "disk."

Bulge- The bulge is a round structure made primarily of old stars, gas, and dust. The bulge of the Milky Way is roughly 10,000 light years across. The outer parts of the bulge are difficult to distinguish from the halo.

Disk- The disk is a flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The disk of the Milky Way is 100,000 light years across and 2,000 light years thick. It contains mostly young stars, gas, and dust, which are concentrated in spiral arms. Some old stars are also 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.

Halo- The halo primarily contains individual old stars and clusters of old stars ("globular clusters"). It may be over 130,000 light years across. 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, while elliptical galaxies have very little gas or dust.

Questions related to how galaxies are classified in activities Spiral Shapes, Elliptical Slide, and Imagine Irregulars.

6. How are galaxies classified? What do they look like?

Edwin Hubble classified galaxies into four major types: spiral, barred spiral, elliptical, and irregular (see also question # 8 and question #9). Most galaxies are spirals, barred spirals, or ellipticals.

Spiral galaxies are made up of a flattened disk containing spiral (pinwheel-shaped) arms, a bulge at its center, and a halo.. Spiral galaxies have a variety of shapes and are classified according to the size of the bulge and the tightness and appearance of the arms. The spiral arms, which wrap around the bulge, contain numerous young blue stars and lots of gas and dust. Stars in the bulge tend to be older and redder. Yellow stars like our Sun are found throughout the disk of a spiral galaxy. These galaxies rotate somewhat like a hurricane or a whirlpool.

Barred spiral galaxies are spirals that have a bar running across the center of the galaxy.

Elliptical galaxies do not have a disk or arms. Instead, they are characterized by a smooth, ball-shaped appearance. Ellipticals contain old stars, and possess little gas or dust. They are classified by the shape of the ball, which can range from round to oval (baseball-shaped to football-shaped). The smallest elliptical galaxies (called "dwarf ellipticals") are probably the most common type of galaxy in the nearby universe. In contrast to spirals, the stars in ellipticals do not revolve around the center in an organized way. The stars move on randomly oriented orbits within the galaxy like a swarm of bees.

Irregular galaxies are galaxies that are neither spiral or elliptical. They tend to be smaller objects that are without definite shape and tend to have very hot newer stars mixed in with lots of gas and dust. These galaxies often have active regions of star formation. Sometimes the irregular shape of these galaxies results from interactions or collisions between galaxies. Observations such as the Hubble Deep Fields show that irregular galaxies were more common in the distant (early) universe.

7. How are galaxies classified today?

Hubble Tuning Fork Diagram

Today we classify galaxies mainly into two major groups following Hubble's examples. Elliptical galaxies range from round shapes (E0) to oval shapes (E7). Spiral galaxies have a pinwheel shape and are classified according to their bulge, as well as how tightly their arms are wrapped around the bulge. They range from Sa, which has a large bulge and tight, smooth arms, to Sc, which has a small bulge and loose, lumpy arms. Barred spiral galaxies classified as SB are pinwheel-shaped and have a distinct "bar" of stars, dust and gas across their bulge. They range from an SBa, which has a bar across its large bulge and tight, smooth arms, to an SBc, which has a bar across its small bulge and loose, lumpy arms. Irregular galaxies have no definite shape but still contain new stars, gas, and dust. The chart below summarizes the properties of the main classes of galaxies.


Galaxy Chart
Spiral/Barred Spiral
Shape and Structural Properties
Disks of stars, gas and dust containing spiral arms that attach to central bulge. Sa and SBa have largest bulge. SB galaxies have central bar. No disk and no arms. Stars distributed evenly from near circular to oval (football). No definite structure.
Stellar Content
Have both young and old stars. Halos consist of old stars only. Contain mostly old stars. Contain both young and old stars.
Gas and Dust
Disks contain gas and dust. Halos contain little gas or dust. Little or no gas or dust. A lot of gas and dust.
Star Formation
Stars form largely in spiral arms. Little or no formation seen. A lot of star formation.
Stellar Motion
Gas and stars rotate around the center of the galaxy. Stars move on randomly oriented orbits like a swarm of bees. Stars and gas have irregular orbits.


8. Galaxy names are identified by a group of letters and numbers. What do they stand for?

Scientists classify galaxies in different catalogs. The most common catalog is NGC, which stands for New General Catalog. Other catalogs include M (Messier), ESO (European Southern Observatory), IR (Infrared Astronomical Satellite), Mrk (Markarian), and UGC (Uppsala General Catalog).

The numbers following the letters help scientists to locate the galaxy in its relative position in the sky, such as Mrk 917 (Sc) or NGC1433 (SB).

Sometimes a galaxy appears in more than one catalog and can have more than one name.

9. What are colliding galaxies?

When two or more galaxies are close enough to each other, gravitational forces will pull the galaxies toward each other. This gravitational attraction increases as the galaxies travel toward each other. The galaxies may pass by each other or collide. Two galaxies that are interacting or colliding may be referred to as a pair, or one galaxy may be referred to as a companion of the other. These HST images show how different colliding galaxies can look. The appearance of an interacting system of galaxies depends on many factors, including the number of galaxies involved in the interaction, their masses and types, how close they are, and how they approach each other. The Antennae galaxies are an example of two spirals that are in the process of colliding. We will not see the end result during our lifetimes because this process takes hundreds of millions of years. Sometimes smaller galaxies plunge into larger galaxies. This type of collision produces a ripple effect, like a rock thrown into a pond. The Cartwheel galaxy is an example of this type of collision. The outer ring of blue stars in this galaxy indicates a ripple of star formation resulting from the collision. The Milky Way and Andromeda are examples of two spiral galaxies that may eventually collide (about 5 billion years in the future).

Questions related to the activity Galaxy Hunt

10. Who is Edwin P. Hubble and what has he to do with galaxies?

Edwin P. Hubble revolutionized cosmology by proving that galaxies are indeed "island universes" beyond our Milky Way galaxy. His greatest discovery was in 1929, when he identified the relationship between a galaxy's distance and the speed with which it is moving. The farther a galaxy is from Earth, the faster it is moving away from us. This is known as Hubble's Law. He also constructed a method of classifying the different shapes of galaxies with the Hubble Tuning Fork. (See #9)

Edwin Powell Hubble was born in Kentucky where he grew up observing the habits of birds and animals. In 1910 he received his undergraduate degree from the University of Chicago and studied law under a Rhodes Scholarship at Oxford University. Later he changed his mind and completed his Ph.D. in astronomy at Chicago's Yerkes Observatory in 1917. He had several other interests, and for a while he thought of becoming a professional boxer. He also enjoyed basketball, and even answered a call in World War I to serve in the infantry.

He once said that he "chucked the law for astronomy," knowing that even if he was second-rate or third-rate, it was astronomy that mattered.

11. What is the Hubble Space Telescope?

The Hubble Space Telescope (HST) is a space-based telescope that was launched in 1990 from the space shuttle. From its position 380 miles above the Earth's surface, the HST has expanded our understanding of the universe, in particular star birth, star death, galaxy evolution, and black holes.

The telescope's instruments are the astronomer's eyes to the universe. Its instruments include the Wide Field Planetary Camera 2, Space Telescope Imaging Spectrograph, Near Infrared Camera and Multi-Object Spectrometer, Faint Object Camera, andthe Advanced Camera for Surveys.

When first launched, the HST's lens was out of shape on the edges by 1/50 of a human hair. This very small defect made it difficult to focus faint objects being viewed by Hubble. Because the HST is in low Earth orbit, it can be serviced by a shuttle. The defect was corrected in the first servicing mission. The Hubble Space Telescope is scheduled for one more servicing mission before its planned retrieval in 2010. You may one day be able to visit the Hubble Space Telescope at the National Air and Space Museum.

12. What are the Hubble Deep Fields?

The Hubble Deep Field project was inspired by some of the first deep images to return from the telescope after the 1993 Hubble Space Telescope servicing mission. These images showed that the early universe contained galaxies in a bewildering variety of shapes and sizes. Some had the familiar elliptical and spiral shapes seen among normal galaxies, but there were many peculiar shapes as well. Such images of the early universe are likely to be one of the enduring legacies of the Hubble Space Telescope. Few astronomers had expected to see this activity presented in such amazing detail.

Impressed by the results of earlier observations such as the Hubble Medium Deep Survey, a special advisory committee convened by Robert Williams, then Director of the Space Telescope Science Institute (STScI), recommended that he use a significant fraction of his annual director's discretionary time to take the deepest optical picture of the universe, by aiming Hubble for 150 consecutive orbits on a single piece of sky. The research was done by pointing the telescope at one spot in the northern sky for 10 days in December of 1995 as a service to the entire astronomical community. Images from the Hubble Deep Field project were made available to the astronomers around the world shortly after completion of the observation.

Few thousand never before seen galaxies are visible in this "deepest-ever" view of the universe, called the "Hubble Deep Field" (later named the HDF-North). Besides the classical, the variety of other galaxy shapes and colors are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang.

Hubble took a second deep look in the southern hemisphere in October of 1998, the HDF-South, to see if a similar result would be obtained. Each of the Hubble Deep Fields shows hundreds of galaxies in an area of the sky that is as small as the size of President Roosevelt's eye on a dime held at arm's length.


Words from the Scientist:

Our Sun is one of hundreds of billions of stars that make up the Milky Way galaxy. By studying other galaxies, astronomers learn more about the galaxy that we live in. Do all galaxies have the same shape? Are all galaxies the same size? Do they all have the same number of stars? How and when did galaxies form?

"Galaxies Galore: Games and More" uses pictures obtained by the Hubble Space Telescope and by telescopes on Earth to illustrate the spectacular variety of galaxies that exist in our universe. You will see how scientists classify different types of galaxies and have a chance to try it yourself. Occasionally, two galaxies may pass close enough to each other that they collide. Such collisions can result in a tremendous burst of star formation. Some scientists, such as myself, study the behavior of these "starbursts" to understand their role in galaxy evolution. You will see an example of this spectacular phenomenon in the Hubble image of the Antennae galaxies.

In "Galaxy Hunt," you will look back in time with astronomers to see how galaxies looked when the universe was younger. Astronomers are using the Hubble Deep Field image to help us understand how galaxies form and evolve. Do these galaxies look the same as galaxies we see today?

We are still in the process of learning about galaxies. Each new observation answers some questions and raises others. I hope that this activity will show you how intriguing galaxies can be!

Denise A. Smith
Space Telescope Science Institute
3700 San Martin Dr.
Baltimore, MD 21218

Specialties: starburst galaxies and extragalactic star formation processes



See Grab Bag page for a complete list of Web sites, books, and other related materials that can be used as references for galaxies.