Chapter 23 - Physics1902 TheMilkyWay Lecture W9.a Chapter...

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1 Physics 1902 The Milky Way Lecture W9.a Chapter 23
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From Earth, we see few stars when looking out of our galaxy (red arrows) and many stars when looking in (blue arrows). Milky Way is what our galaxy appears as in the night sky. 23.1 Our Parent Galaxy
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Our galaxy is a spiral galaxy. The Andromeda Galaxy, our closest spiral neighbor, probably resembles the Milky Way fairly closely. 23.1 Our Parent Galaxy
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Here are two other spiral galaxies, one viewed from the side and the other from the top : 23.1 Our Parent Galaxy
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One of the first attempts to measure the Milky Way was done by Herschel using visible stars. Unfortunately, he was not aware that most of the galaxy, particularly the center, is blocked from view by vast clouds of gas and dust . 23.2 Measuring the Milky Way
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We have already encountered variable stars —novae, supernovae, and related phenomena. These are called cataclysmic variables . There are other stars whose luminosity varies in a regular way, but much more subtly. These are called intrinsic variables . Two types of intrinsic variables have been found: RR Lyrae stars and Cepheids . 23.2 Measuring the Milky Way
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7 Cepheid Variable Stars (Ch 23) There are regions in the evolution of high mass stars where the star is unstable The driving force in the instability is the temperature dependence of the opacity As usual there is a balance between gravity and pressure and the star may oscillate about the equilibrium point Oscillations would be damped but there is a positive feedback from the opacity
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8 Cepheid Cycle Star contracts past equilibrium Pressure rises, T rises Opacity drops as atoms -> ions Star expands, P falls, T falls, Opacity rises Cycle continues The changing opacity acts as a Dam holding back the flow of energy
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9 Overlapped photos taken at max & Min
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10
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11 RR Lyrae – Period less than 1 day
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What are  these?
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The usefulness of these stars comes from their period–luminosity relation :
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