Lecture 18 : Black holes. Astronomy 111

Transcription

1 Lecture 18 : Black holes Astronomy 111

2 Gravity's final victory A star more massive than about 18 M sun would leave behind a post-supernova core this is larger than 2-3 M sun :Neutron degeneracy pressure would fail and nothing can stop its gravitational collapse. Core would collapse into a singularity, and object with zero radius infinite density

3 Black Hole The ultimate extreme object Gravity is so strong that nothing, not even light, can escape. Infalling matter is shredded by powerful tides and crushed to infinite density. Escape speed exceeds the speed of light.

4 Black Hole Becomes a Black Hole: "Black" because they neither emit nor reflect light. "Hole" because nothing entering can ever escape.

5 Black Hole We know that black holes are formed by the death of massive stars Black holes are also found in the centers of massive galaxies (like the Milky Way)

6 Schwarzschild Radius Light cannot escape from a Black Hole if it comes from a radius closer than the Schwarzschild Radius, R S to the singularity: Where M = Mass of the Black Hole

7 Schwarzschild Radius A black hole with a mass of 1 M sun would have a Schwarzschild Radius of R S =3 km. Compare this with a typical 0.6 M sun White Dwarf, which would have a radius of about 1 R earth (6370km), and a 1.4 M sun neutron star, which would have a radius of about 10km.

8 Schwarzschild Radius

9 Schwarzschild Radius R S is named for German physicist Karl Schwarzschild who in 1916 was one of the first people to explore the implications of Einstein's i then-new General Theory of Relativity, it the modern theory of Gravity.

10 Small and large black holes Black holes are often found in the centers of massive galaxies, and are the end products in the death of massive stars BUT black holes can exist at all masses

11 Small and large black holes A black hole is defined as an object from which light cannot escape Can calculate the escape velocity from a black hole as 2Gm v esc r

12 In-class assignment What if you became a black hole? Take the mass of your own body and compute the radius needed to get an escape velocity equal to the speed of light. This is the radius you would have to squeeze your self in order to transform yourself into a black hole. Compare this radius to the typical size of one atom: m.

13 Curvature of spacetime Matter tells space how to curve and space tells matter how to move.

14 Curvature of spacetime

15 The Event Horizon R S defines the "Event Horizon" surrounding the black hole's singularity: Events occurring inside id R S are invisible i ibl to the outside universe. Anything closer to the singularity than R S can never leave the black hole The Event Horizon hides the singularity from the outside universe. The Event Horizon marks the "Point of No Return" for objects falling into a Black Hole.

16 Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.

17 Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.

18 Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

19 ideo/player.php?videoref=black p p _ holes

20 Gravity around Black Holes Far away from a black hole: Gravity is the same as that of star of the same mass. Close to a black hole: R<3R R S, there are no stable orbits - all matter gets sucked in. At R = 1.5 R S S,, photons would orbit in a circle!

21 Journey to a Black Hole: A Thought Experiment Two observers: Jack & Jill Jack, in a spacesuit, is falling into a black hole. He is carrying a low-power laser beacon that flashes a beam of blue light once a second. Jill is orbiting the black hole in a starship at a safe distance away in a stable circular orbit. She watches Jack fall in by monitoring the incoming flashes from his laser beacon.

22 Journey to a Black Hole

23 He Said, She Said... From Jack's point of view: He sees the ship getting further away. He flashes his blue laser at Jill once a second by his watch. From Jill's point of view: Each laser flash take longer to arrive than the last Each laser flash become redder and fainter than the one before it.

24 Near the Event Horizon... Jack Sees: His blue laser flash every second by his watch The outside world looks oddly distorted (positions of stars have changed since he started). Jill Sees: Jack's laser flashing about once every hour. The laser flashes are now shifted to radio wavelengths, and the flashes are getting fainter with each flash.

25 Down the hole Jill Sees: One last flash from Jack's laser after a long delay (months?) The last flash is very faint and at very long radio wavelengths. She never sees another flash from Jack... Jack Sees: The universe appear to vanish as he crosses the event horizon He gets shredded by strong tides near the singularity and crushed to infinite density.

26 Moral: The powerful gravity of a black hole warps space and time around it: Time appears to stand still at the event horizon as seen by a distant observer. Time flows as it always does as seen by an infalling astronaut. Light emerging from near the black hole is gravitationally redshifted to longer (red) wavelengths.

27 -4uWfiY

28 Seeing what cannot be seen... Question: If black holes are black, how can we hope to see them?

29 Seeing what cannot be seen... Question: If black holes are black, how can we hope to see them? Answer: Look for the effects of their gravity on their surroundings. Look for stars orbiting around an unseen massive object Look for X-rays emitted by gas that is superheated as it falls into a black hole Look for jets of material ejected by the black hole

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32 Galactic jets Jet originating from Active Galactic Nucleus (AGN) in M87 Extends 5000 ly!

33 Milky Way s black hole We can measure the mass of the black hole at the center of our Galaxy Watch stars orbit at Galactic center Calculate orbits using Kepler s laws Derive mass of object stars are orbiting Very massive!

34 X-Ray Binaries Bright, variable X-ray sources identified by X-ray observatory satellites: Spectroscopic binary with only one set of spectral lines - the second object is invisible. Gas from the visible star is dumped on the companion, heats up, and emits X-rays.

35 X-Ray Binaries Estimate the mass of the unseen companion from the parameters of its orbit. A black hole candidate, conservatively, would be a system in which the mass of the unseen companion was larger than 3 M sun, the more massive the better.

36 Binary system with an accretion disk around a compact object being fed by material from the companion star.

37 Black Hole Candidates A number of X-ray binaries have been found with unseen companions with Masses > 3 M sun, too big for a Neutron Star. Currently 20 confirmed black hole candidates in our Galaxy: First was Cygnus X-1: M = 7-13 M sun Largest is GRS : M = M sun Most are in the range 4-10 M sun Estimated to be as many as 1 Billion stellar-mass black holes in our Galaxy, which points out how very hard it is to find something that does not emit any radiation of its own.

38 Black Holes are not totally Black! "Classical" General Relativity says: Black Holes are totally black Can only grow in mass and size Last forever (nothing gets out once inside) But, General Relativity does not include the effects of Quantum Mechanics.

39 Evaporating Black Holes Black Holes evaporate slowly by emitting subatomic particles and photons via "Hawking Radiation": Very cold thermal radiation (Temperatures of ~10 nanokelvin) Bigger Black holes are colder The smaller the mass, the hotter the black hole, and so the faster the evaporation.

40 Evaporating Black Holes For black holes in the real universe, the evaporation rate is VERY slow: A 3 M sun black hole would require about years to completely evaporate. This is about times the present age of the Universe. Probably unimportant today, but it could be an important process in the distant future of the Universe.

41 A Final Word Physicist John Archibald Wheeler (b. 1911), coined the term "black hole" [The black hole] "teaches us that space can be crumpled like a piece of paper p into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that t we regard as 'sacred', as immutable, are anything but.

42 Summary Black Holes are totally collapsed objects gravity so strong not even light can escape predicted by General Relativity Schwarzschild Radius & Event Horizon Unobservable objects are observable through their effect on surroundings X-ray binarys Jets Orbital mechanics Black Hole Evaporation Emit "Hawking Radiation"