 
Summary: ASTR 3730: Astrophysics 1 Problem Set #3
Due in class Thursday October 6th
1) An optically thick source of blackbody radiation at temperature Tsource is
surrounded by an optically thin cloud of gas at temperature Tcloud:
A
B
If an atomic transition in the optically thin gas corresponds to a frequency 0,
sketch the spectrum you would expect to see in the vicinity of this spectral line
along the two lines of sight A and B:
(a) In the case when Tcloud < Tsource
(b) In the case when Tcloud > Tsource
Explain briefly the rationale for your answers.
2) The Planck function describing the intensity of blackbody radiation is:
B =
2h3
c2
1
eh kT
1
