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13 Black Holes and Neutron Stars Dead Stars Copyright – A. Hobart.

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Presentation on theme: "13 Black Holes and Neutron Stars Dead Stars Copyright – A. Hobart."— Presentation transcript:

1 13 Black Holes and Neutron Stars Dead Stars Copyright – A. Hobart

2 13 Goals What are black holes? How do we see black holes? What happens when black holes are in binaries? Supermassive Black Holes

3 13 Supernova Remnant Recall: In the death of a high-mass star, the core is converted to neutrons and collapses catastrophically. The collapse and rebound creates a supernova. But what happens to the neutrons already at the very center of the core? The central core is left behind as a small, dense, sphere of neutrons  a neutron star.

4 13 Neutron Stars A giant ball of neutrons. Mass : at least 1.4 x mass of the Sun. Diameter: 20 km! Density: 10 18 kg/m 3 –A thimble weighs as much as a mountain Day: 1 – 0.001 seconds! Magnetic fields as strong as the Sun, but in the space of a city.

5 13 Pulsars Interstellar Lighthouses. See periodic bursts of radiation. Perfect clocks. While every pulsar is a neutron star, the opposite isn’t true.

6 13 Crab Nebula Pulsar

7 13 Pulsar Motion Pulsars born in the center of supernovae explosions. Non-symmetric explosions lead to huge “kick.” Large velocity pulsars. v = 800 – 1000 km/s! Guitar Nebula – copyright J.M. Cordes

8 13 Neutron Degeneracy Neutron stars are held up by neutron degeneracy pressure. –Recall electron degeneracy pressure for white dwarfs. –For white dwarfs, maximum mass of 1.4 M sun For neutron stars, maximum mass ~3M sun What happens if a high-mass star is SO big that its central core is bigger than this? What happens when gravity is stronger than even neutron degeneracy pressure?

9 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?

10 13 Singularity When a high-mass star’s core is greater than ~3 x M sun, 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

11 13 Black Hole Diagram. Singularity Event Horizon Schwarzschild Radius

12 13 Distance from object where v esc > c ObjectMassRadius Earth6 x 10 24 kg1 cm Jupiter300 x Earth3 m Sun300,000 x Earth 3 km

13 13 Concept Test A black hole is best defined as: a.a star which sucks all matter into itself. b.a window to another Universe. c.any object which is smaller than its event horizon. d.the final result of all stellar evolution. e.none of the above

14 13

15 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 X-rays (interior of Sun) Cygnus X-1. http://www.owlnet.rice.edu/~spac250/steve/ident.html

16 13 Binaries Gravitational tides pull matter off big low density objects towards small high density objects. Cygnus X-1

17 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 M sol B.H. is same as from a 1 M sol star. At surface of each, force of gavity is very different!

18 13 m Tides M mm 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! Mmmm

19 13 Tides Frictional Heating Accretion disk

20 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

21 13 Cygnus X-1 1970s Intense source X-rays. “Near” star HDE226868.

22 13 HDE226868 Doppler shifts of HDE226868 Like before, we get mass of star and unseen companion.

23 13 The Companion Result: Period = 5.6 days Total Mass ~ 28 x M sun From spectral type of HDE226868 we estimate its mass ~18 M sun. Companion M = 10 M sun ! Massive! But where is its light? Dark! Can’t be a normal star, or even neutron star.

24 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! R EH = 30 km!

25 13 Supermassive Black Holes Photograph the center of a galaxy. Make spectrum of light from center. Velocity Distance

26 13 Heart of Darkness From Doppler shift get a velocity. From picture get a separation. From Kepler’s Laws get a Total Mass.

27 13 The Dark Truth Observe: V = 400 km/s within 26 LY of center. So: –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? R EH = 6.5 AU!

28 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.I have aged a day, but 1000 years have passed for everyone else. b.A day has passed for everyone else, but I am now an old man. c.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. d.While I have grown old, I think everyone else has stayed the same age. Meanwhile, everyone else thinks the reverse. e.None of the above.


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