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Lecture 25 – Black Holes

Lecture 25 – Black Holes. Comet Siding Spring, Mars, 51 Ophiuchi: Rolando Ligustri ( CARA Project, CAST). Read before class: Galaxies Hubble Types, properties Chapt 24 Milky Way Structure Chapt 23. SNe Type II -- Core Collapse SNe:. Gamma Ray Burst == GRB. Gamma-rays. Optical.

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Lecture 25 – Black Holes

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  1. Lecture 25 – Black Holes Comet Siding Spring, Mars, 51 Ophiuchi: Rolando Ligustri (CARA Project, CAST) • Read before class: • Galaxies • Hubble Types, properties Chapt 24 • Milky Way Structure Chapt 23

  2. SNe Type II -- Core Collapse SNe:

  3. Gamma Ray Burst == GRB Gamma-rays Optical Swift satellite capturing a gamma-ray burst Light curve shows GRB == Super Duper Nova Core collapse  Black Hole ! Progenitor star M ~ 80 to 130 Msun

  4. Black holes (BH): • Discuss with your neighbour what you think a black hole is: • How do they form? (many ways?) • What is their structure? • How do we detect them? • Can anything escape from them? • What is gravity like near them? Are they a cosmic vacuum cleaner or not? • What are their sizes? • Puzzles?

  5. GR • spacetime (spt) • 3 dimensions of space + 1 dimension of time • light follows curved paths in spt • a mass (object) bends spt causing curved paths for particles with or without mass (e.g. photons)

  6. Black Holes: Definition Note that Newton’s Laws hold outside the black hole. 1) A region of space-time where gravitation becomes overwhelming & the curvature of spt is so great that space “folds” over on itself.

  7. Black Holes: Definition: Photon just outside BH also lose energy. 2) Escape velocity > = c. • escape velocity is the energy of motion required to overcome gravity & go into orbit.

  8. Black Holes: Definition: Photon just outside BH also lose energy. Since escape velocity > = c. • Matter & light cannot escape. • photon loses energy rather than change v. E = h * frequency

  9. Black Hole: components • Singularity: A point in the universe where the density of matter & gravitational field are infinite. • Event Horizon: An imaginary spherical surface with a radius = Schwarzschild radius (Rsch). • Rsch == distance from centre of an object such that, if all the M were compressed within that region, escape speed = c.

  10. Black Hole (BH) : Components Standard components • Singularity • Event Horizon (Rsch) • Magnetic fields (note B lines) • rotating • Accretion disk • Jets due to formation only if other objects come near

  11. BH: Accretion Disk • Tidal forces stretch stars or its outer layer towards BH  stellar material orbits in a disk. • Crashing into itself, disk gas loses energy  closer to event horizon. • Some material falls through Rsch. • some particles travel along B lines. bi-polar jets.

  12. Model the equations: • John Hawley at the University of Virginia • general relativistic visualization of a supercomputed magneto-hydrodynamic simulation of a disk and jet around a black hole.

  13. Black Holes: The size of the Event Horizon Only depends on mass! Calculate for Jupiter:

  14. Black Holes: The size of the Event Horizon

  15. Black Holes: The size of the Event Horizon Only depends on mass! • Calculate Rsch for Jupiter. •  roughly the height of a room Need to be close to the event horizon to fall in. However tides. Toothpaste effect.

  16. Black Holes: The size of the Event Horizon • Calculate radius of the event horizon for the Sun. • Out to the orbit of Mercury. • Out to the current radius of the Sun. • 10000 m • 3000 m • 3 m

  17. Black Holes: If our Sun became a black hole right now, nothing would happen to the orbit of the Earth. The force of gravity doesn’t change until one is close to the event horizon & mass of sun hasn’t changed. Only its density has become infinite. Earth would not get sucked into the black hole because black holes do NOT act like Cosmic Vacuum Cleaners.

  18. Near the Event Horizon: Distortions • Orion constellation with & without intervening BH. • stars appear twice & whole sky repeated around event horizon. • Robert Nemiroff at Michigan Tech U.

  19. Alain Riazuelo, IAP/UPMC/CNRS

  20. Near the Event Horizon: Distortions • Unrealistic since tidal destruction would occur • blue- & red-shift • Andrew Hamilton, U Colorado

  21. Near Event Horizon: Distorted Accretion Disk • 1979 professional paper by J-P Luminet in Astronomy & Astrophysics using Kip Thorne & collaborator’s equations. “Gravitation” by Misner, Thorne and Wheeler

  22. Near the Event Horizon: Distortions • Interstellar movie • Equations by Kip Thorne

  23. Into the Black Hole • http://vimeo.com/8723702

  24. Inside the Event Horizon • According to General Relativity, time isn’t a particularly special dimension. •  can be swapped with another dimension. • Outside Rsch move in any direction in space but only 1 direction in time (towards the future). • Inside Rsch only move forward in space towards the singularity but move backwards & forwards in time!

  25. Can anything escape a BH? • E.g. Roger Blandford: Mechanical energy can escape. • B lines threaded through the gas, in the accretion disk & falling into the black hole • B lines twist around rotating black hole, slowing it down. • The energy of rotation travels out along B lines & deposited in disk  explains X-ray hot spots.

  26. Can anything escape a BH? 2. Hawking Radiation: • A Quantum Mechanical effect due to Heisenberg Uncertainty Principle. • Even a vacuum has fluctuations: Pairs of virtual particles appear together at some position in st, move apart, come back together & annihilate. - If close to BH, 1 of the pair falls in & the other escapes to infinity, becoming a real particle.

  27. Can anything escape a BH? 2. Hawking Radiation: • Radiation with a black body T inversely proportional to BH M • Small BH - higher temperatures. • As BH radiates, r decreases, T increases & it radiates faster. • BH evaporate! More detail next lecture.

  28. What can escape from a black hole? Hawking radiation and mechanical energy. Using “rate of evaporation”, can figure out BH lifetime.

  29. On class website Types of Black Holes

  30. Types of Black Holes: M 13 Danny Lee Russell • Intermediate BH. • Form by mergers of stars. • Globular cluster ~ 1 million stars within several pc. • Star density high in globular clusters so more opportunity for the merger of stars.

  31. Types of Black Holes: • Supermassive Black Holes • Centres of Galaxies (including Milky Way) • In both spirals and ellipticals. • BUT If the bulge is very small & featureless then there may not be a BH. • Do galaxies form around black holes? Or do galaxies form & black holes accumulate in centre?

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