12.3.2007 - Astronomy notes

12.3.2007 - Astronomy notes - RR Lyraes: all have same...

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12.3.2007 – astronomy Globular clusters Disk – we live here Cosmic distance Ladder Brightness is proportional to L/D^2 Distance is proportional to (L/B)^ ½ Pulsating variable stars – can oscillate indefinitely RR Lyraes (can be on main sequence): periods between half a day and one day Pulsating because their envelope is expanding and contracting Cepheids (off main sequence): periods between about a day to 100 days More luminous than RR Lyraes Mechanism Outer layers are shrinking, density increasing, opacity increases, outer layers trap light Light being trapped outer layers are heating up, expand, density decreases, opacity decreases, light escapes Outer layers cool down shrink, density increases, etc Instability strip: region in the H-R diagram where the conditions are appropriate for this oscillation Bigger – brighter, smaller – dimmer
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Unformatted text preview: RR Lyraes: all have same luminosity (about 100x sun) Range good to about 100 kpc Visible portion of our galactic disc is 30 kpc across, light takes 100,000 years to get across Cepheid: luminosity is neatly correlated to pulsation period Much greater range: 25 mega pcs Andromeda (nearest galaxy) is about 0.8 mpc away Globs sun not at center of our galaxy Galaxtic mass: Velocity of a circular objects = (GM <r / r) M <r = rV c 2 / G Mass within 8 kpc =~ 10 11 solar masses Mass within 15 kpc = ~ 2 x 10 11 solar masses Due to information about rotation speed, keplerian motion, distance from galactic center, there must be extra mass than what we see dark matter Mass within 15 kpc must be 6 x 10 11 solar masses Dark halo of dark matter About 15% of the matter in the universe is made of visible matter 85% dark matter...
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