Astronomy 102 Problem Set #7

due April 13, 2000, 5:00 pm

Problem #1:
Problem #1:a) The Sun orbits the center of the Milky Way galaxy in roughly a circle of radius 23000 light years, and with a
speed of 220 km/s. Calculate the time required for the Sun to complete one circuit around the galaxy.

b) How many times will the Sun orbit the galaxy in its 10 billion year lifetime?

Problem #2

The nearest "real" galaxy to us is the Andromeda Galaxy, also known as M 31. This galaxy has an apparent
size of 2 degrees (about four times the size of the full moon!). If its linear size is 25 kpc, how far away from us is it?

Problem #3:
How long does it take for light from the galaxy in problem #2 to reach us?

Problem #4: The visual magnitude of the bulge of M31 is 4.3. The spectral type of andromeda is G5, but for the purpose of this excersize lets assume that it is a G2 like our sun. Using the results of problem #3 find out the number of stars in the bulge of M31. For simplicity assume that ALL STARS are like our sun. Hint: Use box 24.1 in your book FETU to find the absolute magnitude of andromeda (its brightness if it was placed 10 parsecs from us). Compare that with the sun's magnitude, and then use the table to the left of figure 24.2 which tells the equivalence of magnitude difference and brightness difference.