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Transcript
Black Holes and Neutron Stars Dead Stars Copyright – A. Hobart 13 Goals • • • • What are black holes? How do we see black holes? What happens when black holes are in binaries? Supermassive Black Holes 13 Density • Density = mass per volume • From Red Giant cores to White Dwarfs to Neutron Stars, density has been increasing. • As density increases, the force of gravity on the surface increases. • The greater the force, the higher the escape velocity: – How fast you need to go in order to escape the surface. • How dense can something get? • How strong can the force of gravity be? • What if the escape velocity is faster than light? 13 Singularity • When a high-mass star’s core is greater than ~3 x Msun, then, when it collapses, neutron degeneracy pressure can’t balance gravity. • The star collapses to form a singularity. • No size at all. • Density infinite. • Escape velocity > c 13 Black Hole Diagram Singularity Event Horizon . Schwarzschild Radius 13 Schwarzschild Radius • Distance from object where vesc > c Object Earth Jupiter Sun Mass Radius cm 1M R S 3km M Sun 300 x Earth 3m 6 x 1024 kg 300,000 x Earth 3 km 13 Concept Test • A black hole is best defined as: a. b. c. d. e. a star which sucks all matter into itself. a window to another Universe. any object which is smaller than its event horizon. the final result of all stellar evolution. none of the above 13 Seeing Holes • Can’t see black hole itself, but can see matter falling into a hole. • Gravitational forces stretch and rip matter: heats up. • Very hot objects emit in Xrays (interior of Sun) • Cygnus X-1. http://www.owlnet.rice.edu/~spac250/steve/ident.html 13 Binaries • Gravitational tides pull matter off big low density objects towards small high density objects. Cygnus X-1 13 Holes Don’t Suck • Newton’s Laws of gravity only depend on mass and separation. • Kepler’s Laws of orbits only depend on mass and separation. • At 1 AU, force of gravity from a 1 Msol B.H. is same as from a 1 Msol star. • At surface of each, force of gavity is very different! 13 Tides Mm Fgravity G 2 d M M m m m m m m •While each m is attracted to each other m, the difference in force from M is greater. •The closer you are to the object M, the more extreme this is! 13 Accretion disk Tides Frictional Heating 13 Concept Test • We can see X-rays from black holes because? a. X-rays are more energetic than visible light and so can escape from the event horizon. b. X-rays can pass through ordinary matter showing us things we can’t normally see. c. Light given off by objects as they enter the event horizon are gravitationally redshifted to X-rays. d. Material flowing into a black hole is heated so much that the thermal radiation peaks in X-rays. e. None of the above 13 Cygnus X-1 • 1970s • Intense source X-rays. • “Near” star HDE226868. 13 HDE226868 • Doppler shifts of HDE226868 • Like before, we get mass of star and unseen companion. Separation ( AU ) Period ( yrs ) Total Mass 3 2 13 The Companion • Result: Period = 5.6 days Total Mass ~ 28 x Msun • From spectral type of HDE226868 we estimate its mass ~18 Msun. • Companion M = 10 Msun! • Massive! • But where is its light? • Dark! • Can’t be a normal star, or even neutron star. 13 X-ray Source? • Star brightness fluctuates every 5.6 days. • X-rays drop off every 5.6 days! • Companion must be source of X-rays! • REH = 30 km! R EH M 3km MSun 13 Supermassive Black Holes Distance • Photograph the center of a galaxy. Velocity • Make spectrum of light from center. 13 Heart of Darkness • From Doppler shift get a velocity. • From picture get a separation. • From Kepler’s Laws get a Total Mass. Separation ( AU ) Period ( yrs ) Total Mass 3 2 13 The Dark Truth • Observe: V = 400 km/s within 26 LY of center. • So: Separation ( AU )3 2 Period ( yrs ) – Period = 121,600 yrs – Separation = 26 LY = 1,600,000 AU • Total Mass in central pixel: 300,000,000 x Mass of Sun! • But where’s all the light? • Small, massive, dark black hole? • REH = 6.5 AU! Total Mass 13 Homework #13 • • For 10/22: Read B18.4: Voyage to a Black Hole I am the captain of a starship that gets stuck just outside the event horizon of a black hole. When I am rescued, what am I most likely to find: a. b. c. d. e. I have aged a day, but 1000 years have passed for everyone else. A day has passed for everyone else, but I am now an old man. While I have stayed the same age, I think everyone else has grown old. Strangely, everyone else thinks they have stayed the same age, but I have grown old. While I have grown old, I think everyone else has stayed the same age. Meanwhile, everyone else thinks the reverse. None of the above. 13