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Unformatted text preview: Homework #10: Solutions Astro 10, spring 2010 General Notes to Graders: If numerical answers are roughly correct, do not take marks off. Award part marks if the student has made some progress with the question. Be generous. 1. [5 points] We find ionized gas in HII regions. The gas is in this state because the strong radiation from the O star at the center ionizes the gas. Since HII regions occur around O stars (which live on the main sequence for a short time), HII regions must occur in regions of recent star formation, so there will be young stars nearby. Notes to graders: Students should have a discussion equivalent to this in content to receive full credit. 1 point for ionized gas, 2 points for ionized by the central O star and 2 points for young stars nearby (with a brief explanation why). 2. [10 points] To identify the unseen object, we would first like to determine its mass. Since the masses of the white dwarf and the unseen object may be about the same size, we start with the full form of Keplers 3rd law: P 2 = 4 2 G ( M + m ) a 3 P 2 a 3 M + m We can set this up as a ratio problem, comparing this white dwarf system with the Earth - Sun system: (recalling that since the mass of the Earth is much much smaller than the mass of the Sun, M Sun + M E M Sun ....
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