Introduction to Galaxies
Overview: Students examine visible and X-ray light images of galaxies to motivate questions about them.
Physical resources: None
Electronic resources: Galaxy Zoo Web site, images of spiral and elliptical galaxies.
Instructor gives general introduction to galaxies:
- Typical radius = 50,000 light years = 4.5 x 1020 meters
- If a galaxy was shrunk to the size of a CD (which is approximately the right shape, 100 times wider than it is thick), where would the next galaxy be located? 20 "galaxy diameters" away
- Typically, galaxies contain about 200 billion (2 x 1011 stars)
Comparing and contrasting properties of galaxies
- Challenge: Students complete tutorial and classify galaxies using "citizen science" Web site Galaxy Zoo (Galaxy Zoo, click "How to Take Part" to enter the tutorial).
- Focus question: Describe differences between elliptical and spiral galaxies in visible light, using the words flux, color, angular width, angular separation.
- Challenge: Make true color X-ray and false color visible and X-ray light images of an elliptical and spiral galaxy (half the class each).
- Spiral galaxy: M74 (obsid 2057), distance ~ 32 x 106 light years (X-rays Signal Presence of Elusive Intermediate-Mass Black Hole).
- Alternate spiral: M81 (obsid 5943, examined as AGN example in investigation 2), distance ~ 11 x 106 light years (Black Holes have Simple Feeding Habits).
- Elliptical galaxy: NGC 0507 (obsid 317), distance ~ 234 x 106 light years (NASA's Chandra Finds Black Holes Stirring up Galaxies).
- Students should make complete descriptions of the differences between their galaxy, as seen in X-ray and visible light, using the same words as above.
- Students should switch notebooks with a partner who examined the other kind of galaxy, and then discuss the important differences, to be shared on the whiteboard with the entire class.
Wrap up discussion:
- Discuss possible models that could explain these observations (colorful XRB point sources in spiral, no AGN at center of M74, but strong AGN possibility in elliptical NGC 0507, smoothly distributed X-ray light indicates hot gas inside and surrounding the elliptical galaxy).
- Students generate questions they have about galaxies, and share on the whiteboard.
- Could have students examine other ranges of light energy for a single galaxy, beyond visible and X-ray light. Link to Chandra information about Sombrero galaxy in IR, Vis and X-ray.
- If crunched for time, students could also investigate the difference between X-ray and visible light images of galaxy clusters, as outlined in activity 4 below.
- Which would you say is "closer together," a galaxy and 3 nearest neighbor galaxies, or a star and its 3 nearest neighbor stars? (Absolute distance is smaller for stars, but relative distance (i.e. compared to the size of the object) is smaller for galaxies.)
- Some astronomers say "looking at only visible light from an object is like listening to just the string section playing a piece of orchestra music." Why would they say this?
- If galaxies are made of stars, why do they look fuzzy and not like a bunch of dots? (issues of resolution–could have them calculate the angular size of one star at the distance to one of these galaxies).