Astr 222 Practice Midterm #1

Short Answer

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.
Your answer should be ~ 2-3 blue book pages long

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)

Helpful Advice

Study using your notes, the online course notes, and the HW assignments and solutions. Arguments and explanations you make in your answers must be based on material covered in class or on HW sets.
When answering questions, try to address both "what" and "why" in your answer. That is, both describe/define whatever is being asked about (that's the "what") and also give an explanation for how or why something works (the "why").
Sketches are useful, but if you are sketching a plot you should always have axes labeled (i.e., "Time", "Apparent Magnitude", etc). It's also a good idea to indicate which direction the values increase (i.e., "the Y-axis is magnitude, with brighter objects to the top").
When calculating something, if your answer doesn't make sense, say so! ("I got a size for the Sun of 10 meters, which is crazy small -- I must have done something wrong!"). Otherwise I would worry that you don't really understand how big the Sun really is...
Also on calculations, SHOW YOUR WORK and EXPLAIN YOUR STEPS. Without that information, if you make a mistake, it is very difficult to see where you went wrong and award proper partial credit. Wrong answers with no explanations will get zero points.
If you are absolutely stuck on a calculation and don't know how to finish (or even start), if you can at least make a qualitative estimate for what you expect the answer to be, and why, that can help with partial points.
Make sure to always have units attached to your numbers.