# Homework 4 : Galaxies

```Homework 4 : Galaxies
(due at the start of class December 9, 2003)
1. Properties of galaxies.
a. Using the web or some other suitable resource, find a picture of a spiral
galaxy and write down its catalog number (e.g. NGC 224 or M31) and/or
common name (e.g. Andromeda).
b. Sketch your galaxy and label the following parts: spiral arms (if visible),
bulge, nucleus, and disk. Feel free to label anything else of interest, such
as dust lanes, etc.
c. What is the galaxy’s classification on the Hubble sequence (e.g., Sa, SBc,
etc.)? If you can’t find the classification in the literature, make your best
guess from the picture and explain your reasoning.
d. Can you tell if star formation is actively going on in this galaxy? Explain.
e. Finally, from the literature, report the size (diameter) of the galaxy in kpc
or lightyears, and its distance from us, if known.
2. Hubble’s law.
a. State Hubble’s law.
b. Suppose we observe a galaxy to be receding from us at a speed of 15,000
km/s. If Hubble’s constant H0 has a value of 65 km/s/Mpc, how far away
is this galaxy?
c. An astronomer observes one end of an edge-on spiral galaxy to have a
recession speed of 1,200 km/s and the other end to have a recession speed
of 800 km/s. How would you interpret this result? If H0 = 65 km/s/Mpc,
how far away is this galaxy?
3. Dark matter.
a. Explain in your own words why we think galaxies like our own are made
up mostly of “dark matter”.
b. Describe one candidate for dark matter.
```