29:50 MODERN ASTRONOMY
Fifth Homework Set...October 27, 1999
Note: Having problems? Don't go around confused and despondent! Ask for help! The purpose
of these problem sets is to help you learn something.
(1) A galaxy is observed to be receding from us at a velocity of 50,000 km/sec. How
far away is it?
Megaparsecs
(2) A spectral line which has a ``rest wavelength'' of 600 nanometers is observed in
a radio galaxy at a wavelength of 660 nanometers. What is the velocity of recession of
the galaxy?
km/sec
(3) For the problem immediately above, what is the distance of the galaxy?
Megaparsecs
(4) The redshift of 3C79 is 0.264. What would be the apparent magnitude of the most
luminous star we see in the night sky? Check Appendix 12 for data. Would we be able to
see it with the Hubble Space Telescope?
Velocity of recession 
. Therefore distance
= 1131 Mpc, so distance modulus = 40 magnitudes. Most luminous stars 
for
absolute magnitude, so their apparent magnitude would be 
.
(5) Quasars with a redshift of 2.0 are very common. When we see the light from such
objects, how long ago was it produced in the quasar galaxy (i.e. what is the lookback
time?)
From Figure 24.4 we see that z=2.0 corresponds to a ``look-back time''
of 80 % of the age of the universe. For an age of the universe of 12 - 14 GYr, this
means the look back time is 9.6 - 11.2 Gyr.