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lec30 - Lecture 30 The Milky Way Overview Lecture 29 Purdue...

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Lecture 30 The Milky Way: Overview Purdue University, Astronomy 364 1 Lecture 29
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Reminder HW6 can be picked up from RM 144. HW7 will be due on Wed (Apr. 13). Lecture 29 Purdue University, Astronomy 364 2
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Crude Star Counts All stars have the same absolute magnitude M . The number density of stars, n , is constant within the Galaxy. Interstellar extinction is negligible. Assumptions: A star of apparent magnitude m lies at a distance: d = 10 0.2( m M + 5) pc . All wrong! But …
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Brightness Distribution Lecture 29 Purdue University, Astronomy 364 4 Every star closer than a distance d will be brighter than m . Therefore, the total number of stars brighter than m will be N ( < m ) = 4 π 3 d 3 n = 4 π 3 10 0.6( m M + 5) log 10 N ( < m ) = 0.6 m + const By going 1 magnitude fainter, the number of stars increases by about a factor of 10 0.6 ~ 4 . If there are no stars beyond a distance d max , then there will be no stars fainter than m max = M + 5log 10 d max 5
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Extent of Stellar Distribution Lecture 29 Purdue University, Astronomy 364 5 If we find m max
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