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Lecture 25 – Black Holes Comet Siding Spring, Mars, 51 Ophiuchi: Rolando Ligustri (CARA Project, CAST) Recall column • Read before class: – Galaxies – Hubble Types, properties Chapt 24 – Milky Way Structure Chapt 23 summary SNe Type II -- Core Collapse SNe: Recall column summary summary Gamma Ray Burst == GRB Recall column Gamma-rays Optical Swift satellite capturing a gamma-ray burst Light curve shows GRB == Super Duper Nova Progenitor star M ~ 80 to 130 Msun Core collapse Black Hole ! Black holes (BH): Recall column summary • 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? GR summary Recall column • 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) Black Holes: Definition summary Recall column 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. Black Holes: Definition: summary Recall column 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. Black Holes: Definition: summary Recall column 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 Black Hole: components summary Recall column • 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. Black Hole (BH) : Components Recall column Standard components due to formation only if other objects come near – Singularity – Event Horizon (Rsch) – Magnetic fields (note B lines) – rotating – Accretion disk – Jets summary BH: Accretion Disk summary Recall column • 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. Model the equations: summary Recall column • John Hawley at the University of Virginia • general relativistic visualization of a supercomputed magnetohydrodynamic simulation of a disk and jet around a black hole. Black Holes: The size of the Event Horizon Recall column Only depends on mass! Calculate for Jupiter: summary Black Holes: The size of the Event Horizon Recall column summary Black Holes: The size of the Event Horizon Recall column 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. summary Black Holes: The size of the Event Horizon Recall column • Calculate radius of the event horizon for the Sun. a) Out to the orbit of Mercury. b) Out to the current radius of the Sun. c) 10000 m d) 3000 m e) 3 m summary Black Holes: summary Recall column 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. Near the Event Horizon: Distortions summary Recall column • Orion constellation with & without intervening BH. • stars appear twice & whole sky repeated around event horizon. • Robert Nemiroff at Michigan Tech U. Alain Riazuelo, IAP/UPMC/CNRS Recall column summary Near the Event Horizon: Distortions Recall column • • • Unrealistic since tidal destruction would occur blue- & red-shift Andrew Hamilton, U Colorado summary Near Event Horizon: Distorted Accretion Disk Recall column “Gravitation” by Misner, Thorne and Wheeler • 1979 professional paper by J-P Luminet in Astronomy & Astrophysics using Kip Thorne & collaborator’s equations. summary Near the Event Horizon: Distortions Recall column • Interstellar movie • Equations by Kip Thorne summary summary Into the Black Hole Recall column • http://vimeo.com/8723702 Inside the Event Horizon Recall column summary • 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! Can anything escape a BH? summary Recall column 1. 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. Can anything escape a BH? summary Recall column 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. summary Can anything escape a BH? Recall column 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. summary Recall column • What can escape from a black hole? Hawking radiation and mechanical energy. Using “rate of evaporation”, can figure out BH lifetime. On class website Types of Black Holes Recall column Type Supermassive Mass Rsch Location Detection Method summary Lifetime Solar Masses yrs >3*10**6 10**97 to 10**106 1/2 light-min Centre of Galaxies Doppler shift of orbiting gas or stars. Period of orbit of stars in images. Mid-mass 500-5000 (Intermediate) Stellar Mass 1 to 10 smaller than Globular Clusters Doppler shift the solar radius X-rays from accretion. 3 to 30 km Throughout disk of galaxies around Doppler Shift of companion. 10**67 X-rays from accretion. Gravitational lenses. Primordial less than 1 1 cm Throughout universe Gamma-rays if evaporating but this is the only type of black hole that has not been observed even indirectly. around 10**10 Types of Black Holes: summary M 13 Danny Lee Russell Recall column • 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. summary Types of Black Holes: Recall column • • • • 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?