Remember, you have a Reading Assignment

1. Remember, you have a Reading Assignment Read Chapter 19 (“Our Galaxy”). Ensure that you: a) know the items listed in the “Summary Of Key Concepts”. b) understand the concepts described in the “Common Misconceptions” boxes. Expect a quiz on Chapter 19 on Monday (Mar. 31)!!

2. Reading Assignment Read Chapter 20 (“Galaxies and the Foundation of Modern Cosmology”). Ensure that you: a) know the items listed in the “Summary Of Key Concepts”. b) understand the concepts described in the “Common Misconceptions” boxes. Expect a quiz on Chapter 20 on Wednesday (Apr. 2)!!

3. Black Holes A supernova may leave behind a neutron star or a black hole Simple definition of a Black Hole: An object that has a strong enough gravitational field that light cannot escape it. Event Horizon – the boundary between the inside of a black hole and the universe outside. The event horizon marks the point of no return for objects “entering” the black hole.

4. Black Hole Rubber Sheet Event Horizon

5. Black Hole Rubber Sheet What is the radius of the event horizon?

6. Schwarzschild Radius Define the radius of the event horizon as the circumference divided by 2p. The radius of the event horizon is known as the Schwarzschild radius. RS Schwarzschild radius G Gravitational constant M Mass of the black hole c Speed of light

7. Schwarzschild Radius Suppose the death of a high-mass star leaves behind a 5 MSun black hole. What will the black hole’s Schwarzschild radius be?

8. Schwarzschild Radius A black hole forms when a collapsing stellar core shrinks to a size that is smaller than its Schwarzschild radius. How does this happen? The iron core of a dying high-mass star becomes more massive until gravity overcomes (electron) degeneracy pressure. Additional neutrons are formed by combining electrons and protons. If the mass is great enough, gravity will overcome neutron degeneracy pressure and the core continues to collapse.

9. Black Holes Once the core is smaller than its Schwarzschild radius, we have no knowledge of it other than the three properties of a black hole: Mass Charge Angular Momentum The core continues to contract until it is a point … … for a rotating black hole it might actually be a loop … for string theory it might be a string but for our purposes it becomes a point. The point is known as a singularity.

10. Singularity General Relativity predicts that spacetime will become infinitely curved as it approaches the singularity. Quantum Mechanics predicts that spacetime will fluctuate randomly near the singularity. Reconciling the two theories is one of the great problems in contemporary physics.

11. Black Holes When the black hole forms it does not start “sucking in” everything around it. Gravitational attraction is determined by mass.The mass has not increased, so the black hole does not attract objects any more than the stellar core it was formed from ... as long as an object doesn’t get any closer than it did to the stellar core.

12. Black Holes Where the boundary of the stellar core was. Black Hole Gravitational attraction out here hasn’t changed.

13. Visiting a Black Hole You and a friend visit a 10 MSunblack hole. What will its Schwarzschild radius be?

14. Visiting a Black Hole You put your spaceship into a circular orbit several thousand km from the event horizon. Do you have to worry about getting “sucked in”? No, you’re in a stable orbit. If you were in orbit close to (but still outside of) the event horizon, would you be in trouble? Yes, because of tidal forces.

15. Visiting a Black Hole What will the view be like?

16. Visiting a Black Hole If you get much closer (about 135 km from the event horizon) the view gets more distorted.

21. Visiting a Black Hole Your friend puts on a rocket pack, grabs a clock, and heads towards the black hole. You watch the clock as it gets closer to the black hole; it appears to slow down. Remember, gravity curves spacetime. What does your friend see? The clock seems fine to him.

22. Visiting a Black Hole As your friend approaches the event horizon he seems to be slowing down. You never see him cross the horizon. Your friend however thinks it just takes a moment to cross the horizon. A few caveats: You wouldn’t be able to see your friend getting close to the event horizon – his image would be extremely redshifted, and dim. Your friend would be dead before reaching the event horizon because of tidal forces.

23. Evidence of Black Holes Do black holes exist? Probably. Observational evidence: X-ray Binaries Some x-ray binaries seem to contain a high-density object too massive to be a neutron star. AGNs Many galaxies seem to contain supermassive black holes in their galactic centers.

24. Active Galaxy Centaurus A Credit: X-ray - NASA, CXC, R.Kraft (CfA), et al.; Radio - NSF, VLA, M.Hardcastle (U Hertfordshire) et al.; Optical - ESO, M.Rejkuba (ESO-Garching) et al.

25. Gamma-Ray Bursts Gamma-Ray Bursts (GRBs) – Occasional bursts of gamma rays typically lasting a few seconds. Gamma rays are a form of electromagnetic radiation. They have a shorter wavelength (and therefore higher energy) than x-rays. Many GRBs have been detected, and there distribution is isotropic. Why does this imply that they are extragalactic?

26. Gamma-Ray Burst: A Milestone Explosion Credit: R. Klebesadel, I. Strong & R. Olson (LANL), Vela Project

27. Gamma-Ray Bursts At least some GRBs are associated with supernova explosions. Gamma-Ray Burst, Supernova Bump Image Credit: S. Kulkarni, J. Bloom, P. Price, Caltech - NRAO GRB Collaboration

28. Gamma-Ray Bursts Since GRBs are extragalactic they must be far away. Given their brightness, these are the most intense explosions in the Universe since the Big Bang. How powerful GRBs are is unknown, because we don’t know whether they are collimated.