ASTRONOMY 292
Dr. Ryden – Winter 2006
Problem Set 7
due Wednesday, March 8
at class time
Note: In solving these problems,
you’ll probably find my lecture
notes more useful than the textbook.
1) Suppose you are in an infinitely large, infinitely old universe in which the
average number density of stars is
n
?
= 10
9
Mpc

3
and the average stellar
radius is equal to the Sun’s radius:
r
?
= 1 r
fl
.
How far, on average, can
you see in any direction before your line of sight hits a star? (Assume that
standard Euclidean geometry holds true.)
If the stars are clumped into
galaxies with number density
n
gal
= 1 Mpc

3
and average radius
r
gal
=
2 kpc, how far, on average, can you see in any direction before your line of
sight hits a galaxy?
2) Imagine a universe full of regulation baseballs, each of mass
m
bb
=
0
.
145 kg and radius
r
bb
= 0
.
0369 m.
If the baseballs are uniformly dis
tributed throughout the universe, what number density of baseballs is re
quired to make the density equal to the current critical density,
ρ
c,
0
=
(3
H
2
0
)
/
(8
πG
)? Given this density of baseballs, how far, on average, would
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 Winter '06
 RYDEN
 Astronomy, Big Bang, Redshift, Physical cosmology, Cosmic microwave background radiation, number density

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