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UCCS PES 1060: Astronomy II
Chapter 18
Problems Worksheet name: __________________________
1. A beam of light shining through a dense molecular cloud is diminished in intensity by a factor of: 2 for
every: 5 pc it travels. By how many magnitudes is the light from a background star dimmed if the
total thickness of the cloud is: 60 pc?
60
5
It gets reduced by: 2 a total of:
=
times as it passes through the cloud.
Thus, the light is dimmed by a factor of:
it entered the cloud.
212  4096 .
times fainter than it was before
This corresponds to: 9 magnitudes, since  2.512   4000 .
9
The number of magnitudes can be directly calculated using the formula:
2.5 Log (
4096
) =
2. A star of apparent magnitude: 10 lies at: 500 pc from Earth.
Interstellar absorption results in an average extinction of: 2 magnitudes per kilo parsec.
The apparent magnitude of the star is being made dimmer by the presence of dust.
Over half a kiloparsec (500 pc), this extinction will be only: 1 magnitude.
Without the effects of dust, the apparent magnitude would be: +9.
a. Find the absolute magnitude (M):
( apparent magnitude without dust )
-
{ 5 log10 [ ( distance from Earth ) / ( apparent magnitude with dust ) ] }
/
[ log10 (
(5)(
minus
)] =
) =
=
Abs Mag
b. Find the luminosity L, (in solar units) of the star with an absolute magnitude M = 0.51.
L (solar units)
= 10 – ( M
L (solar units)
= 10 – ( 0.51
L (solar units)
= 10 – (
) / 2.5
L (solar units)
= 10 – (
)
L (solar units)
= 10
(
)
L (solar units)
=
- 4.83 ) / 2.5
- 4.83 ) / 2.5