ORIGINS 2B03 Lecture 20: Lecture 20

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turquoiseant391

9 Feb 2012

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Anything that crosses the event horizon will never come back. We don"t see anything cross the event horizon, the last photon from the object will reach and we will never see it again. Due to time dilation, you don"t see anything fall into the black hole. Last photon signal are extremely red shifted to very long wavelengths. Black holes are only a few km wide (cant see even horizon due to their distance away) We can only see materials close to the black hole. We cant see them directly, so look for their gravitational effects on the surroundings. Centre of galaxies show the biggest black holes. If the stars are orbiting around nothing then this nothing must be a black hole as it has high mass. In a binary system if we see a star being stripped of it"s material with x-rays this is good evidence for black holes. Measure their speeds as they orbit the center.

Related Questions

a) Theescape velocity of an object is that for which thetotalenergy of an object in a gravitational field iszero. Write anexpression for the escape velocity of an object ofmass m adistance r away from the center of astar/planet of massM. b) The gravity of a black-hole isso strong that, if tooclose, not even light can escape. The pointat which the escapevelocity equals the speed of light is calledthe event horizon. Whatis the radius of the event horizon of a 1.5solar massblack-hole(MSUN=2Ã10³⁰ kg)? c) Expresstheradius of the event horizon of a 2 billion solar massblack-hole(like the one at the center of our galaxy) in terms oftheEarth-Sun distance (rES = 1.5 Ã10¹¹m).

13.40 • In 2005 astronomers announced the discovery of a large black hole in the galaxy Markarian 766 having clumps of matter orbiting around once every 27 hours and moving at 30,000 km/s. (a) How far are these clumps from the center of the black hole? (b) What is the mass of this black hole, assuming circular orbits? Express your answer in kilograms and as a multiple of our sun's mass. (C) What is the radius of its event horizon?

The radius (Rh) and mass (Mh) of a black hole are relatedby Rh = 2GMh/c squared, where (c) is the speed of light. Assumethat the gravitational acceleration (ag) of an object at a distance(ro)= 1.001(Rh) from the center of a black hole is given by (ag) =G M / r squared (it is, for large black holes).
A. What is (ag) at (ro) for a very large black hole whosemass is 1.25 × 1014 times the solar mass of 1.99× 1030 kg?
B.  If an astronaut with a height of 1.73 m is atrowith her feet toward this black hole, what is the difference ingravitational acceleration between her head and her feet?

ORIGINS 2B03 Lecture 20: Lecture 20

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