General Astronomy - Spring 2011
Homework #4 - due February 14



1. (2 pt) How does the spectrum from an emission nebula differ from that of a reflection nebula?

2. (2 pt) When does a protostar become a star?

3.  (6 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.  (5 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.  (5 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?