29:62 GENERAL ASTRONOMY
Sixth Homework Set...Due March 31, 1999

Show calculations and give reasons for your answers. A couple of additional sentences of clarification and explanation may convince the grader you know what you are doing.

1. A star has an absolute magnitude of -2.00. It is at a distance of 2500 parsecs. The interstellar extinction along the line of sight to this star is , i.e. . What is the apparent magnitude of this star?
2. Discuss in about one paragraph how the effect of interstellar extinction (if not properly compensated for) could affect our interpretation of the H-R diagrams of star clusters. Use a simple, hand-drawn sketch to illustrate your argument. Hint: Recall that the term for interstellar absorption is extinction and reddening.
3. Imagine one takes an O star from the middle of an HII region and places it in the middle of a gas of neutral Helium with the same density as that of the hydrogen medium in which it had been immersed. Would the HeII region be smaller or larger than the HII region? Obviously you have to support and justify your answer.
4. Assume that the number of recombining hydrogen atoms in an HII region is recombinations/sec/m . What is the radius of an HII region around an O5 star? What is the radius of one around a B0.5 star? To answer this question, you will have to use some numbers presented in lecture.
5. "Bright Nebulae" is the amateur term for HII regions like we have been discussing in class. A list of them is available at Bunch of Bright Nebulae! , or choose your own source of such objects. Using resources at your disposal, such as the constellation maps in Sky and Telescope magazine, what can you say about the distribution of HII regions in the sky? What is the astronomical significance of this?

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