November 18, 2011 – 10am Class

1. November 18, 2011 – 10am Class No class on Wed. before Thanksgiving No on-line quiz this week (eat turkey!) 21inch reopened, sign up for telescope lab Work on Citizen Astronomy assignments TODAY: Where I was this week Info on Midterm 2, Midterm 3 and the Final Finish General Relativity Stellar black holes The super-massive black hole in the Milky Way Dark Matter

2. MMT Observatoryon Mt. Hopkins, near Amado, AZ Before: Six 72-inch mirrors After: 6.5m single mirror

3. MAESTRO: The MMT Advanced Echelle Spectrometer

4. General Relativity Theory of Gravity for very strong gravity Instead of mass, treat gravity as a curvature of space Really curvature in 3-dimensions

6. GPS: Global Positioning Systems and Relativity • GPS system consists of a network of 24 satellites in high orbits aroundthe Earth. • Each satellite has an orbital period of about 12 hours, and an orbital • speed of about 14,000 km/hour • The satellite orbits are arranged so that at least 4 and sometimes as • many as 12 satellites are visible from any point on Earth. • Each satellite carries an atomic clock which “ticks” with an accuracy of1 nanosecond. (Actually they each carry 2 cesium clocks and 2 • rubidium clocks).

7. A GPS receiver determines your position by comparing time of • arrival signals from a number of different satellites, and thus figuring • out how far you are from each satellite -- “triangulation.” The inexpensive hand-held GPS receivers can determine your position to 5 or 10 meters. To achieve this accuracy you need to know the clock ticks to an accuracy of 20-30 nanoseconds. Military applications and airliners have more accurate GPS receivers.

8. Special relativity: the clocks on board the satellites will run slower thanatomic clocks on the ground by 7 microseconds per day because of timedilation. • General relativity: The clocks on board the satellites run FASTER thanatomic clocks on the ground by 45 microseconds a day because the curvature of space-time from the Earth’s gravitational field is LESSfarther from the surface. • If you don’t take these two effects into account then you would • be as much as 10 km off from the build up of errors each DAY. • To solve this basically the number of cesium atom “ticks” per second isredefined for the clocks on the satellites so that “one second” on the satellite is the same as “one second” on the ground. • (on the ground a CS-133 clock frequency is 9,192,631,770 Hz)

9. Schwarzchild Black Hole Karl Schwarzchild (1916) investigated the structure of a black hole, solving the equations of General Relativity.

10. Schwarzchild Black Hole Singularity: The star's mass has compressed to infinite density at the singularity Event Horizon: Within the event horizon, no light or anything else can escape Photon Sphere: Photons can get trapped here, and just orbit the event horizon -- they don't fall in and they don't escape

11. Kerr Black Hole New Zealand astronomer Roy Kerr described a rotating black hole in 1963 We expect that as material falls into the black hole, it brings angular momentum, causing the black hole to "spin" ever faster The singularity becomes a ring

12. Kerr Black Hole New Zealand astronomer Roy Kerr described a rotating black hole in 1963 Stationary Limit: Objects at the stationary limit, if they are traveling at the speed of light, they will be at rest with respect to the rest of the Universe.

13. Naked Singularity If a Kerr black hole gets spinning fast enough, the speed of rotation at the edge of the singularity approaches the speed of light. When it reaches the speed of light, the ergosphere disappears, and what's left is a "naked singularity".

14. Wormholes In science fiction stories, people talk about wormholes, which are naked singularities which are connected to "parallel universes" and would allow time travel faster than the speed of light.

15. Falling into a black hole Tidal stretching will be very severe next to a black hole. (The force of gravity at your head is much less than the force of gravity at your toes). Time slows down and an external observer thinks you never reach the black hole horizon.

16. Hawking Radiation Stephen Hawking (author of a Brief History of Time, and other popular books)

17. Hawking Radiation Particle-antiparticle pairs are sometimes created outside the event horizon of a black hole – these are called virtual pairs. Mass conservation (or equivalently energy since E=mc2) can be VIOLATED if you do it fast enough (Heisenberg Uncertainty principle).

18. Three things can happen to a pair of particles just outside the event horizon: (1) Both particles are pulled into the black hole. (2) Both particles escape from the black hole. (3) One particle escapes while the other is pulled into the black hole.

19. For the third possibility – one particle absorbed by the black hole, one particle escaping -- the particle that has escaped becomes real and can therefore be observed from Earth. The particle that was pulled into the black hole remains virtual and must restore its conservation of energy by giving itself a negative mass-energy. The black hole absorbs this negative mass-energy and as a result, loses mass and appears to shrink.

20. Hawking Radiation

21. Hawking Radiation This process makes black holes EVAPORATE: if you wait long enough an isolated black hole will lose enough mass and no longer exist.

22. Evidence for Black Holes If light can’t escape from a black hole, how can we “see” them? There appear to be an estimated 10 million stellar black holes in the Milky Way galaxy. We see these black holes as X-ray sources, because of the very hot accretion disk which forms, as material "accretes" onto the black hole from a companion star.

23. We see these black holes as X-ray sources, because of the very hot accretion disk which forms, as material "accretes" onto the black hole from a companion star. Material the black hole until FRICTION slows down the angular motion and it falls in. The FRICTION makes the material in the accretion disk VERY hot, and luminous.

24. Example: X-ray binary called Puppis A, accretion disk around a neutron star X-ray picture: Million to billion Degree gas

25. X-ray Binary stars: Cygnus X-1

26. Cygnus X-1

27. Why do we think Cygnus X-1 is a black hole? Cygnus X-1 is a very bright X-ray source, observed to vary on timescales less than about 0.01 second. Thus, the X-ray emitting region is less than 0.01 light seconds across, about the size of the Moon. Cyg X-1 has a stellar companion, and spectra of the star can be analyzed like any spectroscopic binary to yield a mass. The mass of the unseen companion star is at least 10 solar masses. The only way the companion can be that small and that massive is if it is a black hole

28. Amazing Fact About the Milky Way #4: At the center of the Milky Way is a BLACK HOLE which has a mass of about 2 million solar masses. We can’t see the center of the Milky Way in the optical, because there’s too much DUST. But we can look at the center of the Milky Way in the IR, X-rays and Radio

29. Center of the Milky Way Optical, or Visible Light Infrared Light

31. In the bulge of the Milky Way, near its center, • there are lots of stars. • Near the Sun, recall that the nearest star • is Proxima Centauri (about a parsec away). • If we were near the Galactic Center, there’d be • 1 million stars • between us and Proxima Centauri

32. Radio Picture of the Milky Way Center Hot gas streaming along magnetic field lines Center

34. Center of the Milky Way radio IR: lots of very massive young stars, 30 solar masses, about to go supernovae radio

35. Very deep picture of the inner region of the Galactic Center (infrared) Blue Supergiants Gemini-N

36. X-ray Picture of the Galactic Center Million Degree Gas X-ray Binary Star systems

37. IR Movie: Motion of stars at the Galactic Center

39. FLAREat theGalacticCenter:swallowing a star or a bit of gas?

40. Out to the edge of its visible disk, the Milky Way Galaxy contains: 200 billion solar masses, but only 20 billion solar luminosities. Conclusion: There must be dark matter in the outer regions of the Galaxy.

41. Andromeda Galaxy Dark matter= stuff that doesn’t emit, absorb, or otherwise interact with light. Other galaxies are found to be mostly dark matter, too.

42. Dark Matter:Most of the Mass of the Milky Wayis NOT normal atomsMost of the Mass in Galaxies is NOTstars, gas or any normal atomic material

43. What the Milky Way REALLY is like: bright stars dark “halo”: nearly spherical distribution of dark matter

44. Dark matter could also be called “invisible matter”. The properties of invisible objects are rather difficult to determine. We know dark matter exists because of its gravitational pull on luminous matter (stars, Gas clouds); otherwise, we know very little About it.

45. Some of the dark matter in galaxy “halos” consists of Massive Compact Halo Objects (MACHOs, for short). MACHOs can be “failed stars” i.e. brown dwarfs (balls of gas smaller than a star but bigger than Jupiter) MACHOs can be “ex-stars” i.e. white dwarfs, neutron stars, black holes

46. Only 20% of the dark matter is MACHOs: Some of the dark matter in galaxy “halos” consists of exotic matter. Suppose there existed a type of massive elementary particle that didn’t absorb, emit, or scatter photons. We’d detect such a particle only by its gravitational pull on luminous matter. Neutrinos?

47. Cosmic Gall by John Updike Neutrinos, they are very small. They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a pane of glass.

48. Although we don’t know the mass of neutrinos exactly, we know it’s tiny… neutrinos electron Neutrinos provide < 10% of the dark matter.

49. Most of the dark matter must be particles other than neutrinos. ANOTHER exotic particle which is a leading candidate for the Dark Matter THE WIMP WIMP = Weakly Interacting Massive Particle

50. According to particle physics theory, WIMPs should be much like neutrinos only more massive. Neutrinos have already been detected Particle physicists are still trying to detect WIMPs.