A35 Lectures 22-23 - 22 Black Holes and Dark Matter Schwarzschild radius(boundary of black hole = 2GM\/c-things go in black hole but nothing comes

# A35 Lectures 22-23 - 22 Black Holes and Dark Matter...

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22 Black Holes and Dark Matter Schwarzschild radius (boundary of black hole) = 2GM/c² -things go in black hole but nothing comes out -radius can go to zero, therefore force can be infinite, making seem “scary”, but only very close to black hole that time/space get distorted Looking at binary stars: gas is in orbit around the black hole – inner orbits are the fastest (just like the solar system)V²R/G. The fast moving gas rubs on less rapidly rotating gas and friction heats the accretion disk. It gets very hot and emits X-rays, hence x-ray astronomy. Orbits around black holes M = V²R/G Rsch = 2GM/c² Ex. what is the orbital speed at 5 Rsch? M = V² (5 x 2GM / c²) / G c²/10 = V² c/s ~ V/3 The orbital speed is 1/3 of the speed of light. Cygnus X-1: HDE 226868 5.6 day period 30Msun + [7 Msun] We can derive the masses of unseen objects from the motions they produce in visible stars. Looking into the Galactic Center -Dust obscures visible light – but not Infrared and radio emission. -Bright stars at the Galactic Center can be seen in the infrared – and seen better with adaptive optics that remove blurring effects of the Earth’s atmosphere. -Motions of those stars provide information for a mass estimate of the very center of our galaxy --> stars are moving 1400 km/sec. -M ~ v²R, M ~ few x 10^6 Msun, size ~ 45 AU, very dense! Massive black hole? Motions of individuals stars in the center of the Milky Way reveal the presence of a very massive (4 x 10^6 solar mass) object that’s smaller than the solar system – the Black Hole at the Galactic Center. ~ 4 x 10^6 Msun in a region that is ~ 100 AU across (Rs is much smaller – 107 km ~ 0.1 AU at 8 kpc ~ 10^-5 arcsec). Where are we in the galaxy? (Harlow Shapley, 1920s Harvard, figured it out) Dust makes it hard to tell. We need to look at the distribution of distant objects that are distributed around the center of the Galaxy. Use globular clusters to find the center of our stellar

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• Spring '09
• RobertKirshner
• Globular cluster