Star formation is a difficult topic to understand for students and for researchers. The reasons are that the theory is built on the detailed physics of stellar structure, and that the changes which occur during star formation are even more difficult to treat mathematically than the steady state structure of of a main sequence star. Most of the theoretical work is done with complicated computer programs.

The Virial Theorem-Applied Version

A body of gas (a cloud, a star, or a star’s core) being compressed by its own “self” gravity and “collapsing” in “quasi-equilibrium” under it must use half of the potential energy released to heat itself, and must radiate the other half away into space

For a cloud or star the energy is radiated to space as gravitational luminosity. The virial theorem is a consequence of the law of gravity, the laws governing gasses, and the conservation of energy.

The virial theorem was presented in class, and you should know it for the quizzes!

Coverage of Topics and Study Notes:

Misleading statement on page 490: In the second column, it says "But whatever their size, however--large or small--emission nebulae are the birthplaces of all the stars in our night sky." This sentence is an incredible error or maybe an over-simplification. Molecular clouds are the birthplaces of all stars. Emission nebulae identify the birthplaces of only O and B stars, and only after they were born. Cooler, lower mass stars (A, F, G, K, and M) can form in clusters without any O and B stars. There are many cases where lower mass stars are forming without O and B stars. It is probably the case that most star forming clouds don't make any O and B stars.

The stages presented in section 19.2 are impossible to remember as Stage1, Stage 2, etc., and I don't expect anybody to remember what happens by stage number. You should try to remember the general scenario, and it's helpful to try to memorize the evolutionary track for the sun as a way to do so. Although I dislike pure memorization, it's worth memorizing the shape of the evolutionary track because there isn't much in our everyday experience to relate star formation to.  Because the array of details is hard to remember, I am giving a simplified coverage in the lectures. Use the lecture notes as the guide to reading the text, and focus on the notes for the exam!

Overall, the chapter is very well written and illustrated, despite my taking issue with a few aspects of it.

You don't need to study Discovery 19-2 on Eta Carinae

Review Questions:

Review and Discussion: Skip number 7 and 16.  The chapter doesn't provide the answers.  They lie in the models for the processes.

Conceptual Self-Test: True or False/Multiple Choice: Skip numbers 11, 13, 15, 17, 19, and 20.   Neither the text nor the lectures will be able to provide enough information to answer these, except perhaps for 19, for which we'll define "main-sequence turnoff" in class.

Problems: We can skip them, even though many are instructive applications of material we've covered.