Ast 207 F2009 Nov-09Ast 207 F2009Black Holes & Quasars—18 Nov• Black hole– Mass is so concentrated that nothing escapes•Quasar– Black holes in the center of galaxies that is lit by material falling in toward the black hole.BH in center of Milky WayJet in galaxy M87BH Cyg X1Ast 207 F2009Black holes / Quasars —18 Nov• Quasars are black holes lit by material falling into them.• Black holes in center of Milky Way & M87• Our Dusty Universe: Astronomical Horizons Lecture– "Come learn how microscopic particles of dust in the universefrustrate and enlighten astronomers in their quest to understand the cosmos."– Heather Jacobson– Thurs, 7:30 at Planetarium• Hwk 9 is due Fri, 20th.– No late papers. Answers will be handed out and put on angel after class.– Missouri Club today.• Observing today & Thurs, Nov 18 & 19, 6:30-10:00pm– Attend only if stars are visible. See angel after 5:00pm, if weather is ambiguous. Rain predicted.– Quiz. You will be asked to locate a star using the Abrams Planetarium star chart. Quiz counts as one extra-credit clicker assignment.– Go to the south end of the building (toward Wilson Rd.) & take the elevator up to the penthouse.• Test 3 is on Mon, 23rd.– Covers material through “dark matter” (Mon, 16thand Hwk 9).• No class on Wed, 25th.Ast 207 F2009 Nov-09Ast 207 F2009Discovery 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 lightAst 207 F2009Discovery 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 lightAst 207 F2009 Nov-09Ast 207 F2009Energy Source:Accretion disk +Black Hole + Jets• Gas, stars fall into 108Mblack hole.• Gravitational potential energy Î thermal energy Î light[Fig 26.17][Fig 26.18]Ast 207 F2009The Center of our Milky Way Galaxy Visible lightRadio waves200 LYAst 207 F2009 Nov-09Ast 207 F2009Sagittarius A*Radio observations with higher angular resolution.15 LYSmall oval is the point source Sagittarius A* = center of galaxyAst 207 F2009Infrared Images of the Galactic Center1 LYGalacticCenter(Sagittarius A*)Using “adaptive optics” technique on Gemini 8m telescope.Density of stars is 300,000 x greater than in vicinity of SunAst 207 F2009 Nov-09Ast 207 F2009Black Hole at the Center of the Milky Way GalaxyInfrared observations over 6 years.• From 2002.25 to 2002.40 (0.15yr), star moved same as it did from 1995.53 to 1996.43 (0.9yr).1. Why did the star move so fast when it was near Sgr A*?a. Kepler’s law of equal areasb. It got sucked in by the black hole.c. The black hole spit it out.d. There is mass near the black holeAst 207 F2009Black Hole at the Center of the Milky Way GalaxyInfrared observations over 6 years.• Orbit is a ellipse tilted by 46°• Data from 2002 goes within 60AU from black hole. [Orbit of Pluto is 40AU.]• Within 60AU, the mass at SgrA* acts like a point.• Orbit is consistent with Sag A being a black hole.Ast 207 F2009 Nov-09Ast 207 F2009The Black Hole at the the Galactic CenterMeasure orbits of stars in very center.1. To deduce the mass, what quantities do you need from the figure?A. Period & inclinationB. P & eccentricityC. P & semi major axisD. I & EE. I & SInfrared observations over 6 years.Ast 207 F2009Mass of the black hole in the center of our galaxy• A star orbits Sgr A in an elliptical orbit with a period of 14 yr. The semi major axis of the orbit is 1000 AU. Show how to find the mass of Sgr A. Give the numerical answer for the mass. (It is easiest to express the mass in solar mass, rather than in kg.)1. What is the key big idea needed to figuring out the mass from the observations?A. Kepler’s 3rdLawB. Orbit is an ellipse.C. Period is expressed in years.D. Semi major axis is expressed in AU.• Use Newton’s version of Kepler’s Third Law: P2=R3/M, where the period P is in years, the semi major axis R is in AU, and the mass M in solar masses.M = R3/P2= 10003/142= 5×106Msun.• The actual period is 15.2yr, and the actual semi major axis is 950AU. M= 9523 /15.22= 4×106Msun.Ast 207 F2009 Nov-09Ast 207 F2009Black Hole at the Center of the Milky Way GalaxyInfrared observations over 6 years.• Orbit is a ellipse. Data from 2002 goes within 60AU from black hole. [Orbit of Pluto is 40AU.]• Within 60AU, the mass at SgrA* acts like a point.• Orbit is consistent with SgrA* being a black hole.• If SgrA* is made of stars, there would be a million stars packed within the size of the solar system. The stars would collide.• Therefore Sgr A* is a black
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