29:62 GENERAL ASTRONOMY
Tenth Homework Set...Due April 28, 1999
(1) Look at Figure 23-19 of your textbook. What is the minimum value for the
mass of the galaxy UGC 12810
(2) M81 is a pretty galaxy. When I was a student, apparent magnitude 19 was
just about the faintest star that could be measured with large telescopes and
long exposure photographs. What kind of stars could have been measured in M81
in those days?
(3) Very roughly speaking, how fast do you expect the galaxies in the
local group to be moving under their mutual gravitational interaction?
Hint: Approximate things by calculating the speed that the Andromeda
galaxy and the Milky Way would move if they were the only objects in the
local group. Clearly describe your steps.
(4) Imagine the following model for a galaxy. The mass distribution is a uniform
sphere of mean density 
kilograms/m 
and radius 8.0 kiloparsecs.
Plot the rotation curve for this galaxy, giving the correct values for the ordinate and
abscissa, and the correct functional form for V(r).
(5) Look at Figure 23-19 of the book. Show why the statement in the Figure caption
is correct.