ASTR 20 Lecture Notes - Lecture 28: Gravitational Lens, Dark Matter, Cosmic Distance Ladder

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16 Jul 2020
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Cepheid variable stars with longer periods have greater luminosities. Hubble measured the distance to the andromeda galaxy using cepheid variables as standard candles. Kepler"s laws tell us that objects orbiting a single massive object have a lower orbital velocity at larger distances. Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter. The visible portion of a galaxy lies deep in the heart of a large halo of dark matter. We can measure the velocities of galaxies in a cluster from their doppler shifts. The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars. Clusters contain large amounts of x-ray emitting hot gas. The temperature of hot gas tells us cluster mass: Gravitational lensing: the bending of light rays by gravity, can also tell us a cluster"s mass. All three methods of measuring cluster mass indicate similar amounts of dark matter.

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