AST207 F2010 11/19/20101Black Holes & Quasars—19 Nov• Test 3: Mon• Missouri Club– Mon, 8:00‐9:00am, 1420BPS• No class on Wed, 11/24• No pre‐class questions for 11/29• Black hole– Mass is so concentrated that nothing escapes• Einstein’s gravity• Quasar– Black holes in the center of galaxies that is lit by material falling in toward the black hole.• Evidence for black hole in center of Milky Way.BH in center of Milky WayJet in galaxy M87Objectives• What is a black hole?• How does Einstein’s theory of grav ity modify ideas about space and time?AST207 F2010 11/19/20102Laplace’s argument for black holes (1796)• The rotational speed at distance R from a mass M is/• Important Principle: Speed of light c is maximum speed.– If a signal can go faster than c, then you affect the past. You can kill your mom before your birth so that you were never born.• For , 1.5km.• If the sun were squeezed to be smaller than 1.5km, nothing can escape from itnot even light.MercuryVenusEarthMarsJupiterSaturn05101520253035404550024681012R [AU]v [km/s]Black hole in Einstein’s Gravity• Event horizon: Regions outside the event horizon can send light to distant observers. Inside the event horizon, no signal can be sent.• The Schwarzschild radius is the radius of the event horizon./• For a solar mass, 3km.AST207 F2010 11/19/20103Space is cur ved near a black hole• You learned that the circumference of a circle is 2.• The presence of a black hole modifies this.– There is extra distance along a radial line.• The circumferences of circles at RS and at 2RSare 2RS and at 4 RS.1. If there were no black hole nearby, the radial distance between these two circles isA. 0B. RS.C. 2RS.• The actual radial distance is 2.296RS.Time is distorted near a black hole• Joe is at 1.007RSand Colin is at 2RS. Joe’s mom and Colin’s mom send them birthday emails every year.• The presence of a black hole modifies this.– Less time passes near the black hole.1. If there were no black hole nearby, Joe & Colin receive emails A. every year.B. More often than every year.C. Less often than every year.• Joe gets a birthday email every month. Colin gets one every 8.5mo.AST207 F2010 11/19/20104Time and space are distorted by gravity• Einstein’s Law of Gravity: an object feels the distortion of time and space and changes its direction and speed.• Einstein’s theory using distortion is equivalent to Newton’s theory if GM/(rc2) is small.• In solar system, E’s theory shows orbit of Mercury is not exactly an ellipse. Perihelion of ellipse rotates once every 3.2Myr.• Gravity bends path of light.Light bent by galaxy cluster A221811/19/2010 Ast 207, F2010AST207 F2010 11/19/20105Objectives• How was the first quasar discovered?• What supplies the energy of a quasar?Discovery of quasars (quasi‐stellar objects)• Some sources of radio waves are coincident with stars.• Stars do not emit light at radio wavelengths.• Are they some kind of weird star within the Milky Way Galaxy?Visible lightRadio lightAST207 F2010 11/19/20106Discovery of quasars (quasi‐stellar objects)• Maarten Schmidt gets a spectrum.– “Star” was moving at 40,000 km/s. (Hwk 7)– Fastest stars in Milky Way move at 200km/s. 1. Why is it moving so fast?A. It is in a distant galaxy.B. It was shot out of the MW.Visible lightRadio lightEnergy source of quasars• A quasar is brighter than a whole galaxy with billions of stars radiating.• When hydrogen fuses into helium, 0.7% of the mass turns into energy. In the sun, 10% of the mass is fused, and it takes 10Byr.• When mass falls towards a black hole to radius R,/• If R=1.1RS, v is 95% speed of light.– Temperature is hot. Hot enough to emit x‐rays.• Energy creation is efficient. About 10% of the mass can be converted into energy. Time is very short.
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