Introduction to Astronomy

1. Introduction to Astronomy • Announcements • No class on Thursday 24 Jul 2008 • Homework #7 due on Monday

2. Review • Stellar Birth • Stellar Life (a.k.a. the main sequence) • Stellar Death (no more Hydrogen) • Low mass stars • White dwarfs, surrounded by planetary nebula • High mass stars • Neutron stars • Black Holes We pick up here

3. Black Holes (Cue sinister music)

4. The ultimate fate of high-mass stars • About 10 solar masses • Higher mass = more gravitational compression when fuel runs out • Core squeezed harder than that which produces white dwarfs and neutron stars • Degenerate neutron pressure cannot stop collapse • Gravity > Strong nuclear force • Ball of neutrons already at nuclear density is squashed into an even smaller volume • “infinite density”

5. Escape Velocity • Earth: • Jupiter: • Sun: • Notice the trend: the more massive the object is, the faster you have to go to escape its gravitational pull

6. What if gravity is so strong that the escape velocity is > speed of light?! • NOTHING would be able to escape… • No apples, astronauts, radio waves, visible light…absolutely nothing

7. History • Proposed late 1700’s by John Michell • What if Vescape = c ? • “Black Stars” Now known as the “Schwartzschild Radius”

8. ANY object can be turned into a black hole! • If Sun collapsed into a black hole (which it won’t), would only be 1.9 miles in diameter • How would Solar System be affected? • NOT AT ALL! • It’s gravity would be exactly the same as the Sun’s • Only when you get near Rs (the “event horizon”), do you feel strong pull of infinite gravity

9. This book has a mass, M = 1 kg RS

10. Cross-section of core time • A star core that collapses to a black hole still has the same mass, but it’s concentrated in a vanishingly-small volume • Infinite density • “Singularity”

11. This all has to do with gravity, so let’s talk a little about that.

12. Gravity • Instead of “spooky action at a distance”, Einstein thought there was something more elegant about gravity • Einstein’s General Theory of Relativity • Matter tells spacetime how to curve • Spacetime tells matter how to move • Black holes tell space to get bent.

14. This effect was confirmed in early 1900s during a solar eclipse The eclipse allowed astronomers to measure stars that appear close to the Sun, and they watched for subtle changes in the stars’ apparent positions that indicated gravity was pulling on their light … GRAVITATIONAL LENSING

17. The predictions of general relativity have been experimentally verified • In fact, every experiment ever devised has produced results that are exactly in agreement with general relativity! • Mercury’s perihelion shift • Viking lander radio transmissions

18. Curved Spacetime Planetary orbits Sun

19. This is the “boundary” of the black hole… Inside this spherical boundary, we know absolutely nothing

20. Curved Spacetime near a Black Hole Event Horizon Goes all the way down (singularity of infinite density)

22. Conservation of Angular Momentum • As star collapses to a black hole, rotation speed increases • Recall the ice-skater effect • So event horizon actually flattens out a little • Not a spherical horizon, oblate spheroid • In fact, some theories predict “ring singularities” in which the rotation speeds up enough to stop the collapse (centripetal force), and the black hole’s mass is concentrated in an infinitely thin ring

23. Static Limit: the material within this limit must be rotating Result: You cannot fall “straight in” to a black hole. You can only circle the drain.

24. How do we know? • If no light can escape, how do we even know they exist, if we can’t see them? • How do you know the wind exists? • E.g. you can’t see with your eyes that the wind is blowing. You can only see what the wind is blowing. • INDIRECT OBSERVATION • We can’t directly see the black hole, but we can observe its effects on nearby stuff

25. Indirect Observation of Black Holes • Accretion • A black hole may pull (accrete) matter into a fast-spinning disk around its equator • Near the event horizon, infalling matter is speeding around at near the speed of light!!! • This causes extreme frictional heating in the gas, making it emit X-rays and gamma-rays • 10’s of millions of degrees Kelvin

26. Center depression and equatorial bulge from rotational dynamics (STATIC LIMIT)

27. Binary Systems • Just like normal binary systems, black holes can exist in binary pairs with other stars • If we see X-ray emitting gas orbiting a region of space in which we cannot directly see an object, probably a black hole with a companion star • As long as M > 5-10 MSun • Using Kepler’s Laws, can determine mass of black hole

30. Supermassive Black Holes • Millions of times the mass of our Sun! • Thought to lurk at the center of every galaxy • If actively consuming matter, called Active Galactic Nucleus • If not, just a gigantic black hole sitting quietly • in fact, just waiting to wake up and feed again (dust, gas, stars, galactic collisions)

31. By watching many stars silently orbit a strangely invisible object at the exact center of our galaxy, Astronomers have deduced the existence of a huge black hole with a mass equal to many millions of Suns!!!

32. “Black Holes ain’t so black.” • Stephen Hawking • They can actually be assigned a “temperature” • Strictly speaking, assigned Entropy (a thermodynamic quantity related to the “disorder” in a system) • Entropy depends on surface area of event horizon • An entropy can be loosely interpreted in terms of a temperature • For Mbh = MSun, T ~ 10-8 K

33. Temperature is small, but not zero • Therefore it radiates some energy • Because it radiates, it’s losing energy (equivalent to mass, recall E = mc2) • So black holes actually evaporate away! • Very slowly though, lifetime ~ 1067 years 10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 times the age of the Universe!

34. Other types of radiation • Quantum mechanical • Uncertainty principle: Mass-energy equivalence Despite rumors to the contrary, you can get something for nothing! You can “borrow” energy from the quantum nature of spacetime to create pairs of particles

35. Constantly changing & dynamic… …like the bubbles in Champagne, new ones are constantly emerging and old ones are constantly being destroyed

36. Introduction to Astronomy

37. Here, an electron and anti-electron are created from the quantum “foam” within reach of the gravitational field of the black hole This just so happens to occur near the event horizon. The positron crosses the event horizon and is lost forever. The black hole then appears to radiate the electron!

38. The Casimir Effect • Two small neutral conducting plates placed ~ 1 micrometer apart in a vacuum exert an attractive force between them • “virtual photons” • ~10-7 N (1 kg = 10 N) • Small, but measureable • One of the rare situations where quantum effects have large-scale observable consequences

39. What happens if you get too close to a black hole?

40. Out here, very small curvature, so normal gravity Oh no, not again. But here, gravity is very strong

41. Weaker gravity here Turns you into spaghetti! Stronger gravity here Same phenomena responsible for Saturn’s rings…

42. Other strange properties • Time actually slows down near the event horizon (time dilation) • Time slows down near any massive body • So your buddy watching you fall into a black hole would actually see you frozen in time the instant you crossed the event horizon • In fact, time actually stops at the event horizon…

43. Watching a beam of light from inside the falling elevator Watching a beam of light from outside

44. What you would see if you looked up out of the black hole as you fell in… • As you cross the event horizon, your time slows down infinitely according to an outside observer • On the other hand, this means that for every nanosecond that passes for you, many billions/trillions of years pass outside • Result: the light from many trillions of years of the Universe’s evolution pours in behind you!

45. Extreme Black Hole Theories Einstein-Rosen Bridge Perhaps a pathway to spatially-distant parts of the Universe?

46. White Holes • Counterpart to black holes • Just as nothing can leave a black hole, nothing can enter a white hole • Incredibly theoretical • But think about it…the water going down your drain has to go somewhere, right?

47. Observations of Black Holes • Cygnus X-1 ( ~ 7000 ly distant) • HDE 226868 Blue supergiant Source of mysterious X-rays… Stellar wobble indicates a mass Of 20 5 solar masses They orbit each other every 5½ days

48. Merging Black Holes ( in galaxy cluster Abell 400 ) Composite X-Ray & Radio observations Jets of Radio synchrotron radiation

49. Gravitational Waves • Relatively new science (last 2 decades) • But predicted by Einstein in 1920s • Like dropping a pebble in a puddle creates ripples that spread out, so too do massive bodies create “gravitational ripples” under the right circumstances • Particularly, high masses and high accelerations

50. General Relativity: Spacetime is curved, the result of which we observe as a “Gravitational Force” The curvature around a fast-moving, massive body changes rapidly, giving rise to “Waves” In the fabric of spacetime