lecture_tan29 - Key Concepts: Lecture 29: The Milky Way and...

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The Milky Way and Other Galaxies Our first encounter with Dark Matter: too much mass in our Galaxy The Center of Our Galaxy: A Supermassive Black Hole Historical debate on the existence of galaxies beyond the Milky Way Weighing the Milky Way orbits – This keeps gravity from collapsing the galaxy – This allows us to measure the mass of the galaxy using Newton’s adaptation of Kepler’s Third Law M galaxy + M sun = a 3 /P 2 – a = 26,000 LY = 1.6 x 10 9 A.U. – P = 2.25 x 10 8 years M galaxy + M sun = 2x10 11 M sun • This estimate tells us how much mass is inside the Sun’s orbit. Question? • If we were to measure the velocities of more distant parts of the Milky Way and we find that they are moving faster than the Sun, so that their orbital periods are all about the same as the Sun, what does this tell us about the total mass of the Galaxy? Hint in the solar system the more distant planets orbit much more slowly. M galaxy + M sun = a 3 /P 2 Can we write this in terms of the speed? M galaxy + M sun = a 3 /P 2 P = distance / speed P = 2 ! a / v " a / v M galaxy + M sun " a v 2 We see that v is approximately constant in galaxies (does not depend on a). Therefore the Mass inside radius a increases linearly with a. Newton’s version of
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lecture_tan29 - Key Concepts: Lecture 29: The Milky Way and...

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