Module 4: Milky Way Galaxy and Dark Matter

Lesson 1: The Milky Way

Overview

Since there is no corresponding extensive content in Touch the Stars book,we give here an overview of what kind of galaxy the Milky Way is, its regions, its contents, and our location in it. Students will investigate the structure of the Galaxy and then categorize objects as in the disk, bulge, or halo by putting various representative objects into their correct plate, box, or bowl.

Learning Outcomes

Materials

Spiral galaxies created out of cut foam.

Pre-assessment Questions and Discussion

What is the Milky Way and what does it look like?
The Milky way is the galaxy in which we live, it is a spiral galaxy.
What type of things is the Milky Way made of?
Stars, gas, dust,and dark matter.
Where in the Milky Way is the Earth located? Center, edge...?
The earth is located close to the edge in one of the arms, within the Orion spiral arm.

Text

The Milky Way is the galaxy in which the solar system (and our Earth) is located. It is around 13 billion years old, which is just a little less than the age of the Universe. The Milky Way is one of billions of galaxies that astronomers are able to observe. We didn't always know this. Up until the 1920's, astronomers thought that the Milky Way was the entire universe. We have learned that the Galaxy is a barred spiral galaxy, which means it looks a lot like a pinwheel with a bar at the center and several spiral arms coming out of it (a bit like bent spokes of a bike wheel). Our Sun is located halfway to the edge of the Galaxy along the Orion spiral arm, about 2/3 of the way out from the center of the Galaxy. It takes our sun over 200 million years just to make one loop around the center of the Galaxy, and this is traveling at the incredibly fast speed of 500,000 miles per hour (800,000 km/hr).

The Milky Way is made up of hundreds of billions of stars, dust, gas, dark matter, and even a really massive black hole at its center. The Milky Way, like most spiral galaxies, can be separated into different regions: the halo, bulge and disk. Each of these regions is slightly different from the other two in where it is located, the kinds of objects it contains, and its shape.

Follow-up Questions on Reading

  1. What are the 3 components (or parts) of the Milky Way and where would you find in them?
  2. List some of the objects found in each of the main regions of the Galaxy: disk, bulge, halo.
  3. What type of galaxy is the Milky Way and what does it look like?
  4. Where is the Earth located?
  5. How long does it take the Sun (and thus us) to travel all the way around the center of the Milky Way?

Reinforcing Activity

Make a Milky Way galaxy using a large paper plate for the disk, a bowl for the halo, and a small box for the bulge (if small enough, place in the middle of the paper plate). Read off all or some of the Milky Way objects given in the table below, and have students separate the representative objects into the plate, bowl, or box. Sometimes objects are located in 2 or even all 3 regions.

Where in the Galaxy would you find:

Representative object suggestions Milky Way Object Disk Halo Bulge

Food

Non-food
       
Cotton candy Fluffy cotton balls Active star-forming regions
x
Strawberry / blackberry Ping pong ball Open clusters with young stars
x
Coconut haystacks "Wooly balls" Supernovae
x
x
Gummy worms / licorice Short lengths rope Spiral arms
x
Cotton candy Fluffy cotton balls Lots of dust and free gas
x
x
Stick of gum / brownie Erasure The Bar
x
Extra dark chocolate Silky scarf (lays over all) Dark matter
x
x
x
M&Ms Small beads Earth and Sun
x
Chocolate candy kiss Heavy ball bearing Super massive black hole
x
Donut holes Large foam ball Globular clusters with extremely old stars
x
Life Savers / donut Plastic bracelets Planetary nebulae
x
x
x

Summary and Post-Assessment

Discuss your view of the Milky Way galaxy now that you have gone through this lesson. What did you know before, and what did you learn that impressed you the most? Compare your thoughts with those of another classmate. What differences did you find? What similarities? Is there any easy way to picture just how large the Galaxy is and the various objects that it contains?

Relevant Information and Links

Needed.


Lesson 2: Dark Matter

Overview

The detection of something in the Universe that is totally invisible at all wavelengths of light is demonstrated through the detection of magnets hidden between two smooth foam boards. This we call dark matter. Students will explore invisible "detection" after reading a segment about dark matter and our current understanding of its nature based on objects that dark matter is affecting through its gravitational force.

Learning Outcomes

Materials

Image of the dark matter detection board: magnets sandwiched between 2 foam boards. The dark matter detection board consists of two rectangles of foam board (these are about 12 x 14 inches) with 2 - 3 magnets sandwiched in between. The foam boards are then taped together. Magnets such as the one shown at the left, can then be moved over the foam board to "detect" dark matter.

Pre-assessment Questions and Discussion

How would you go about finding something that appeared to be totally invisible here on Earth?
See if it bumps into something. See if it leaves footprints. Similar answers.
Have you heard of "dark matter" before?
Encourage all answers.
What do you think dark matter might be?
Lots of soccer balls? Huge number of black holes? A gazillion Jupiter-like planets?

Text

You may have heard of the mysterious term "dark matter" before. Similar to black holes, dark matter does not emit light. However, unlike black holes, dark matter does not absorb light either. Instead, dark matter lets light pass through it, so it is impossible to take pictures of it.

You might wonder how we know dark matter really exists if we can't take a picture of it. The reason we know it exists is that we can measure its influence on normal matter in space. Recall gravity is a force that exists between any two objects with mass. Because dark matter has mass it exerts a discernible gravitational force on the objects that surround it.

Dark matter was first proposed by Fritz Zwicky in 1934 to explain why galaxies seemed to move much more quickly around each other than predicted. He suggested that if there were unseen matter tugging on these galaxies, it would cause them to orbit one another at a faster pace. In the mid-1970s, astrophysicist Vera Rubin showed that the outskirts of spiral galaxies also revolve far too quickly to be explained by just the stars, gas, and dust present in the galaxy alone. In other words, if there were not something with mass that was gravitationally holding on to the galactic material, the stars, gas, dust, and other material would be flying out away from the galaxies!

Today we understand that most of the matter in the Universe must be this unseen matter; in fact, only 5% of matter in the Universe is the normal matter (atoms) that we are made of and another 23% is what we refer to as dark matter. There is thought to be something else in the Universe that makes up the remaining 72%, dark energy. We will have to leave our lessons about dark energy for the future as scientists are still trying to figure out what it is!

Originally, scientists thought that dark matter was just normal matter that was hard to photograph; dark stars and dark planets were proposed as potential candidates for dark matter. However, the results of astronomical surveys like the OGLE survey have convinced scientists that there are not possibly enough dark stars or planets in the Universe to cause the gravitational effects they have measured.

Most scientists think that dark matter is something much more bizarre than normal matter. It is likely a special type of exotic sub-atomic particle that we refer to as a WIMP. WIMP stands for Weakly Interacting Massive Particle. While an individual WIMP likely has a lot more mass than a neutrino or an electron, it is likely to be extremely small. To have such an effect on other objects, there must be a whole lot of them together in space. Dark matter/WIMPs exists everywhere in our galaxy, from the central bulge to well beyond the edge of the disk. In fact, the greatest concentration of dark matter/WIMPS is in the central bulge and there is less and less farther from the from the center you go.

Experimental physicists are currently trying to create and detect WIMPs in the lab; the investigation of dark matter is a very active area of research in both astronomy and physics.

Follow-up Questions on the Reading

  1. Why do scientists call dark matter "dark"? (Be specific.)
  2. What kind of force does dark matter have on other objects?
  3. If we cannot actually see dark matter, how do we know it exists?
  4. What observations have provided evidence that dark matter is present in spiral galaxies?
  5. What kinds of objects have scientists eliminated as possible candidates for dark matter?
  6. What are these things called WIMPs?

Reinforcing Activity

We will now do a demonstration to show how this works. Much like holding two magnets near each other exerts a tug between them, dark matter tugs on stars and gas in galaxies through the force of gravity. While we can't see the dark matter, we are reasonably sure it exists because of how the stars and gas moves as a result of its gravitational influence.

Dark detection board demonstration:

Ask the following questions

  1. Is it easy to glide the magnet over the board?
  2. Are there regions of the board that the magnet wants to be in more than others?
  3. Why might the magnet want to be one region more than another?
  4. Do you know for sure that there are magnets underneath the board?
  5. Is it likely there magnets underneath the board?

Summary and Post-Assessment

Discuss how the magnet-board or "Dark-Matter Detection Board" relates to the real dark matter and how scientists detect it even though it is invisible. Is the detection board a good analogy? Explain everything you know about dark matter to someone who has never heard about it.

Relevant Information and Links

Needed.