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General Astronomy

General Astronomy. Black Holes. A huge great enormous thing, like — like nothing. A huge big — well, like a — I don’t know — like an enormous big nothing ... Piglet describes the Heffalump, in Winnie the Pooh by A.A. Milne. Black Holes.

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General Astronomy

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  1. General Astronomy Black Holes

  2. A huge great enormous thing, like — like nothing. A huge big — well, like a — I don’t know — like an enormous big nothing ... Piglet describes the Heffalump, in Winnie the Pooh by A.A. Milne

  3. Black Holes • After a massive star supernova, if the core has a mass > 3 M, the force of gravity will be too strong for even neutron degeneracy to stop. • The core will collapse out of existence. GRAVITY FINALLY WINS!! • This is what we call a black hole. • The core becomes infinitely small. • Since 3 M or more are compressed into an infinitely small space, the gravity of the core is HUGE!

  4. SuperNova

  5. Karl Schwarzschild (1876-1916) Black Holes Several months after Einstein presented the General Theory of Relativity in 1915, a young German soldier serving at the Russian front, Karl Schwarzschild (1873-1916), solved the equations and described a black hole. This results in the notion of an Event Horizon with a Schwarzschild radius.

  6. “Size” of a Black Hole • Spacetime is so highly warped around a black hole, even light can not escape. • Schwarzschild Radius – the distance from a black hole where the escape velocity equals the speed of light. Rs = 2GM/c2 (Rs in m; M in kg) • A sphere of radiusRs around theblack hole is called the event horizon.

  7. Structure of a black hole

  8. A nonrotating black hole has only a “center” and a “surface” • The black hole is surrounded by an event horizon which is the sphere from which light cannot escape • The distance between the black hole and its event horizon is the Schwarzschild radius (RSch= 2GM/c2) • The center of the black hole is a point of infinite density and zero volume, called a singularity

  9. Bending of Light Path Around Black Holes At a distance of about 1.5 Rsch of a black hole, spacetime is distorted so much that photons emitted from the back of your head actually go around the black hole and come back to you.

  10. Photon orbits around a black hole

  11. Radius of the Event Horizon For example, suppose the Earth could be compressed enough to become a blackhole

  12. To be a blackhole, Earth would have to be compressed to about the size of a dime – slightly less than 1 centimeter

  13. Rotating black holes • A rotating black hole (one with angular momentum) has an ergosphere around the outside of the event horizon • In the ergosphere, space and time themselves are dragged along with the rotation of the black hole

  14. Schwarzschild Radii

  15. Singularities Singularity Theorem: Every black hole must have a singularity inside itself. A naked singularity is not inside a black hole (not surrounded by an event horizon), and therefore can be seen by someone outside it. Cosmic Censorship Theorem: The laws of physics prevent naked singularities from forming when a star collapses.

  16. Most properties of matter vanish when matter enters a black hole, such as chemical composition, texture, color, shape, size, distinctions between protons and electrons, etc.

  17. Mass As measured by the black hole’s effect on orbiting bodies, such as another star Total Electric Charge As measured by the strength of the electric force Spin (angular momentum) How fast the black hole is spinning Only 3 parameters remain…

  18. Types of black holes • Schwarzschild (1916) • mass • Reissner-Nordström (1916, 1918) • mass, electric charge • Kerr (1963) • mass, angular momentum • Kerr-Newman (1965) • mass, angular momentum, electric charge

  19. Sizes • Black Holes Come in Varying Sizes: • “Stellar Mass” • 5 - 20 times the mass of the sun • Result from supernova explosion of massive star • Massive • Millions times the mass of the sun • Lie in centers of galaxies

  20. Falling into a black hole Falling into a black hole gravitational tidal forces pull spacetime in such a way that time becomes infinitely long (as viewed by distant observer). The falling observer sees ordinary free fall in a finite time.

  21. Falling into a black holes • With a sufficiently large black hole, a freely falling astronaut would pass right through the event horizon in a finite time, would be not feel the event horizon. • A distant observer watching the freely falling observer would never see her fall through the event horizon (takes an infinite time). • Falling into smaller black hole, the freely falling observer would be ripped apart by tidal effects.

  22. Falling into a black hole • Signals sent from the freely falling observer would be time dilated and redshifted. • Once inside the event horizon, no communication with the universe outside the event horizon is possible. • But incoming signals from external world can enter.

  23. Falling into a black hole:time dilation

  24. Falling into a black hole • As matter approaches the event horizon… • the tidal forces are tremendous • the object would be “spaghettified”

  25. Spaghettification!

  26. Falling into a black hole • The tidal force between head and toes is now 1 million g, for a 30 solar mass black hole. • The tides wouldn't be so bad for a very massive black hole. The tide at 1 Schwarzschild radius would be less than 1g if the black hole exceeded 30,000 solar masses. • From the event horizon to the central singularity will take 0.0001 seconds in free fall, for a 30 solar mass black hole. (Although to the observer it would take forever.) • The infall time is proportional to the mass of the black hole.

  27. Orbiting a Black Hole

  28. S. Harris Black Holes: Do They Really Exist? We cannot see black holes directly, so we have to look for indirect evidence… What would you look for to find a stellar-massed black hole, like those formed after the death of high mass stars?

  29. “Seeing” a Blackhole If no light can escape from a black hole, how can we know that they are there? You can’t see one directly, but we can see what is happening to the accretion disk surrounding the event horizon.

  30. Seeing black holes

  31. Optical • Material swirls around central black hole. • Gas near black hole heats up to UV and X-ray temperatures. • This heats surrounding gas, which glows in the optical.

  32. Binary Star Systems • Black holes are often part of a binary star system, two stars revolving around each other. • What we see from Earth is a visible star orbiting around what appears to be nothing. • We can infer the mass of the black hole by the way the visible star is orbiting around it. • The larger the black hole, the greater the gravitational pull, and the greater the effect on the visible star. • Chandra illustration

  33. The Case of Cygnus X-1 Cygnus X-1 is a X-ray binary system with a bright star of 18 M⊙, and an unseen (invisible in the visible) companion of about 10 M⊙. If the mass estimate is of the X-ray source is correct, than it certainly exceeds the upper mass limit of neutron star, making it a prime stellar black hole candidate.

  34. Gas from the companion star is drawn by gravity onto the black hole in a swirling pattern. As the gas nears the event horizon, a strong gravitational redshift makes it appear redder and dimmer. When the gas finally crosses the event horizon, it disappears from view. Because a black hole has no surface, the central region is black. As the gas approaches the neutron star, a similar gravitational redshift makes the gas appear redder and dimmer. However, when the gas strikes the solid surface of the neutron star, it glows brightly. Accretion Disks

  35. The orbit of a particle near a black hole depends on the curvature of space around the black hole, which also depends on how fast the black hole is spinning. A spinning black hole drags space around with it and allows atoms to orbit nearer to the black hole than is possible for a non-spinning black hole. The tighter orbit means stronger gravitational effects, which means that more of the X-rays from iron atoms are shifted to lower energies. Rotating Blackholes

  36. X-rays • Black holes capture nearby stellar material. • As the gas gets closer to the black hole, it heats up. • Gas heats to temperatures in the range of millions of degrees. • Gas heated to these temperatures releases tremendous amounts of energy in the form of X-rays. XMM-Newton Illustration

  37. Seeing black holes

  38. X-ray: Frame Dragging Detection of a period in GRO J1655-40 due to precession of the disk. This precession period matches that expected for frame dragging of space- time around the black hole. Credit: J. Bergeron, Sky & Telescope Magazine

  39. But it is also a strong X-ray emitter, and has an X-ray jet. Chandra image of Cen A X-ray: Jets Cen A is known to be a peculiar galaxy with strong radio emission. Optical image of Cen A

  40. SS 433 The observation is of a familiar source named SS 433 -- a binary star system within our Galaxy in the constellation Aquila, the Eagle, about 16,000 light years away. The black hole and its companion are about two-thirds closer to each other than the planet Mercury is to the Sun. The jets shoot off at 175 million miles per hour, 26 percent of light speed.

  41. X-rays from Black Holes In close binary systems, material flows from normal star to black hole. X-rays are emitted from disk of hot gas swirling around the black hole.

  42. What’s Happenin’ Magnetic field from surrounding disk funnels material into the jet

  43. RX J1242-11 The first strong evidence of a supermassive black hole ripping apart a star and consuming a portion of it. Chandra detected a powerful X-ray outburst from the center of the galaxy RX J1242-11. This outburst, one of the most extreme ever detected in a galaxy, was caused by gas from the destroyed star that was heated to millions of degrees Celsius before being swallowed by the black hole.

  44. Radio Jets from Black Holes • Many black holes emit jets. • Material in jet moving at 0.9c. • Jet likely composed of electrons and positrons. • Magnetic fields surrounding black hole expel material and form the jet. • Interaction of jet material with magnetic field gives rise to Radio emission.

  45. With a curious feature M87 - An Elliptical Galaxy

  46. Radio shows the origin of the Jet

  47. Sagittarius A* If the black hole at the center of the Milky Galaxy is about 3 million solar masses, then its size must be smaller than 3 million km, or 10 light seconds. This is only a tiny spot near Sagittarius A* in the picture on the right. The star approached the central Black Hole to within 17 light-hours - only three times the distance between the Sun and planet Pluto - while traveling at no less than 5000 km/sec

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