# Astr 222 Practice Midterm #1

5 questions similar in style to the ones below
Each question is worth 5 points.
Your answer should be a a few sentences / short paragraph long for each.

• Describe what is meant by Population I objects and Population II objects. Give examples.
• Describe the Magallenic Clouds.
• Why do we believe there is a lot of dark matter in the galaxy?
• Describe what is meant by the "luminosity function" of galaxies. In this context, what is L*? Sketch what this function looks like.
• Why does the X-ray variability of the galactic center place a limit on the size of the object at the center?

## Numerical Problem

2 questions similar in style to the ones below.
Each question is worth 5 points.
Make sure to bring a stand-alone scientific calculator! No smartphones allowed.

• You are studying a distant star cluster, and find that the stars appear too red for their spectral type -- their colors are too red by 0.25 in B-V color. You also find that there is a Cepheid variable star in this cluster, with a period of 10 days and a mean apparent V magnitude of 7.0. How far away is the cluster?
• If your telescope can reliably measure the brightnesses of stars down to 20th magnitude, what is the farthest away you could detect a Cepheid variable? Remember that the most luminous Cepheids have period of about 100 days.

## Essay

One question, which will be taken from the list below.
The question is worth 15 points.

• Describe the structure of the Milky Way Galaxy. Be sure to talk about the properties and relative sizes of the different components of the Galaxy. Where is the Sun's location in the Galaxy? A sketch will probably be useful here!
• Describe the Great Debate. Make sure to explain and evaluate the arguments on both sides -- whose evidence was flawed, and why? How did Edwin Hubble resolve this issue?
• Sketch the Hubble Sequence, defining the different classification terms and how galaxies are classified. What kind of galaxy is the Milky Way? What about the Large Magellanic Cloud?
• Describe the Oort Limit and sketch the rotation curve of the Milky Way. Explain how the rotation curve shows that there is dark matter in our Galaxy. Using those two arguments alone (Oort Limit and rotation curve), also explain why we believe the dark matter is in an extended halo rather than confined in the disk or in the inner parts of the galaxy.
• Describe three different ways you could measure distances to stars or star clusters in the Milky Way galaxy. For each method, describe the data you would need, along with the advantages and disadvantages to each method.
• Imagine a galaxy forms from a single burst of star formation, after which time all star formation stops. Describe how the galaxy's luminosity, B-V color, and stellar mass-to-light ratio changes over time. Be sure to explain physically why these changes are happening, and also give some characteristic timescales for the changes. Again, a sketch might help.
• Describe three arguments that support the idea that there is a supermassive black hole in the nucleus of the Milky Way. How massive is this black hole?
• The Large Magellanic Cloud (LMC) orbits around the Milky Way, at a current distance of about 50 kpc. Explain what will happen to the LMC over the course of billions of years as it continues orbiting around the Galaxy. Make sure to describe the physical processes involved!

Equations

You will get a page of equations and constants with the test, so you do not need to memorize constants or equations except for these three:
• the definition of magnitudes: m1-m2 = -2.5log10(f1/f2) (m=mag, f=flux)
• the magnitude distance relationship: m-M = 5log10(d) - 5 (m=app mag, M=abs mag, d=distance in parsecs)
• the distance / angular size relationship: d = alpha * D / 206265 (d=physical size, D=distance, alpha=angular size in arcseconds)