29:62 General Astronomy
Ninth Homework Set...Due April 21, 1999
Note: For the following problems it may help to draw a diagram
illustrating the main features of the described observations before undertaking
the problem.
(1) A radio telescope points in the galactic plane at galactic longitude
and takes a spectrum of the 21cm emission of the
neutral hydrogen atom. Three spectral lines, corresponding to three clouds,
are seen in the spectrum.
The spectral lines are shifted to lower frequencies (i.e. longer wavelengths) by 84.3 kHz, 269 kHz, and 592 kHz with respect to the rest frequency of 1420.5 MHz. How far are these clouds from the Sun?
You will need the following galactic model for your calculations. Assume that
the orbital speed of the sun about the galactic center 
is 250
km/sec, that the distance of the Sun from the galactic center 
is 8.0 kiloparsecs, and that the galactic rotation curve V(r)=250 km/sec
for galactocentric distances greater than 2.0 kiloparsecs.
(2) Look at Figure 22-30 of your textbook. Use the data to calculate the
minimum possible mass for the Milky Way galaxy. Show your calculations
and state what assumptions you are making.
(3) A radio telescope points in the galactic plane at a galactic longitude
and takes a spectrum of the 21cm emission. What is the most
highly doppler-shifted frequency that will be observed? Where along the line of
sight will it be observed?
(4) Suppose that someone tells you that there is a hole in the HI distribution
in the inner part of the galaxy, much like the hole in a donut. Using the
ideas we have discussed in class, how would you check this out?
(5) Here's a fun one. On the bulletin board outside my office is a plot of
the intensity of CO emission in the Milky Way galaxy. Concentrate on the portion
of the diagram within 
of the galactic center. From this diagram,
and using the ideas developed in class, what can you say about the distribution
of molecular gas in our galaxy? Don't want to come to my office?. Think
I'll make you do problems on the blackboard and miss your important next class?
Try this: Map of the Galaxy!