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Wednesday January 30 astro lecture.docx

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Bozidar Mitrovic

Wednesday January 30, 2013 Astro Lecture: Black Holes & Einstein’s General Theory of Relativity: Black Holes - if the core of a high mass star has mass above about 3M the degenerate neutrons cannot prevent its gravitational collapse and it collapses into a single point (so-called singularity) - the escape velocity from a body (Earth, Moon, Sun, Star…) is the velocity that an object has to have so that it can escape the gravitational confine of the body - for earth the escape velocity from it is about 11 km/s - it can be shown that V*2esc (escape velocity) = 2GM/R (gravitational constant) - for given mass (M) Vesc increases with decreasing R - singularity, escape velocity > the speed of light, escape velocity is equal to the speed of light, event horizon, escape velocity < speed of light - Schwarzschild Radius Rs is the radius of the event horizon - Rs (in km)=3M (in solar masses) - Example: o For M=10 M Rs= 30km o For M=20 M Rs=60 km - Nothing, including light, can escape the region within the event horizon hence the name “black hole” How is one to detect a Black Hole? - look for a binary system consisting of a regular star and an invisible object with a mass of at least 3M - [center of mass: is in the middle] - third kepler’s law as formulated by Newton tells us that if you take the size of the orbit cube and the orbital period squared you get the mass o a*3/p*2 = Mpm - from (1) and (2) one can find M and m - also the invisible companion needs to be an intense source of x-rays - stellar wind particles produced by the regular star system are accelerated by the strong gravitational pull near the event horizon and as a result produce x- ray radiation - first discovered in 1971 by
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