General Astronomy -
Spring 2013
Homework #4 - due February 17
1. (1 pt) How does the spectrum from an emission nebula differ from
that of a reflection nebula?
2. (1 pt) When does a protostar become a star?
3. (3 pt) A gas cloud with a mass of 5 solar masses collapses
from a radius of 1000 solar radii to 10 solar radii (phase A) and
then from 10 solar radii to 1 solar radius (phase B). What is
the ratio of the energy released in phase A to the energy released
in phase B?
4. (2 pt) A gas cloud has a temperature of 10 K and a density
of 1000 cm-3. How large are the pieces that the
cloud fragments into?
5. (3 pt) Our Milky Way galaxy contains about 200 billion
solar masses of material. If the Milky Way formed from a cloud
of intergalactic gas with a density of 10-3 cm-3,
what was the temperature of the cloud?