Astronomy 100: Tuesday & Thursday 2:30-3:45 PM Class Details

1. Astronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom Burbinetburbine@mtholyoke.eduwww.xanga.com/astronomy100

2. OWL assignment (Due Today) • There is be an OWL assignment due on Tuesday April 5 at 11:59 pm. • There are 15 questions and a perfect score will give you 2 homework points.

3. Exam • 40 Questions • Chapters 15, 16, 17, and 18 • Some of the questions are taken straight from OWL questions

4. Other Room • April 7th Goessmann 0020 2:30-3:45 PM Last Names beginning with H, I, J, and K

5. Things you should know • Layers of the Sun • Hydrogen Fusion • Hertzsprung-Russell Diagram • Stellar Classifications • Life Cycle of the Sun • Helium Fusion • CNO cycle • What happens to stars as they “die”

6. Review Session • Wednesday-Review Session • Hasbrouck 134 from 7-8 pm • I will be there at 6 pm if you want to talk to me in a much smaller group

7. PRS Questions

8. PRS question #1 • What is approximately the temperature of the plasma in a sunspot? • A) 2,000 K • B) 4,000 K • C) 6,000 K • D) 8,000 K • E) 10,000 K

9. PRS question #1 • What is approximately the temperature of the plasma in a sunspot? • A) 2,000 K • B) 4,000 K • C) 6,000 K • D) 8,000 K • E) 10,000 K

10. PRS Question #2 • Which of these spectral types have the strongest hydrogen emission lines in their spectra? • A) O • B) B • C) A • D) F • E) G

11. PRS Question #2 • Which of these spectral types have the strongest hydrogen emission lines in their spectra? • A) O • B) B • C) A • D) F • E) G

12. “Deaths” of Stars • White Dwarfs • Neutron Stars • Black Holes

15. White Dwarfs • White Dwarfs is the core left over when a star can no longer undergo fusion • Very dense • Some have densities of 3 million grams per cubic centimeter • A teaspoon of a white dwarf would weigh as much as an elephant

16. White Dwarfs • Some white dwarfs have the same mass as the Sun but slightly bigger than the Earth • 200,000 times as dense as the earth

17. White Dwarfs • Collapsing due to gravity • The collapse is stopped by electron degeneracy pressure

18. Electron Degeneracy Pressure • No two electrons can occupy the same quantum state

19. Electron Degeneracy Pressure • As electrons are moved closer together • Their momentum (velocity) increases • Due to Heisenberg Uncertainty Principle

20. So What Does This Mean • Electron Degeneracy Pressure balances the gravitational force due to gravity in white dwarfs

21. One Interesting Thing • More massive white dwarfs are smaller

22. White Dwarf Limit • The mass of a White Dwarf can not exceed approximately 1.4 Solar Masses • Called the Chandrasekhar Limit • Electrons would have velocities greater than the speed of light

23. The Sun • Will end up as a White Dwarf

24. Neutron Star • Neutron stars are usually 10 kilometers acroos • But more massive than the Sun • Made almost entirely of neutrons • Electrons and protons have fused together

25. How do you make a neutron star? • Remnant of a Supernova

26. How do you get a Supernova? • A high-mass star keeps on fusing elements into ones with larger atomic masses • Is now a Red Supergiant • Energy keeps on being released since the mass of the new nucleus is less than the original ones

28. This stops with Iron • Fusion of Iron with another element does not release energy • Fission of Iron with another element does not release energy • So you keep on making Iron

30. Initially • Gravity keeps on pulling the core together • The core keeps on shrinking • Electron degeneracy keeps the core together for awhile

31. Then • The iron core becomes too massive and collapses • The iron core becomes neutrons when protons and electrons fuse together

33. Density • You could take everybody on Earth and cram them into a volume the size of sugar cube

34. Explosion • The collapse of the core releases a huge amount of energy since the rest of the star collapses and then bounces off the neutron core • 1044-46 Joules • Annual energy generation of Sun is 1034 Joules

35. How do we know there are neutron stars? • The identification of Pulsars • Pulsars give out pulses of radio waves at precise intervals

36. Pulsars • Pulsars were found at the center of supernovae remnants

37. Pulsars • Pulsars were interpreted as rotating neutron stars • Only neutron stars could rotate that fast • Strong magnetic fields can beam radiation out

39. Black Holes • If a collapsing stellar core has a mass greater than 3 solar masses, • It becomes a black hole

40. Black Hole • After a supernova if all the outer mass of the star is not blown off • The mass falls back on the neutron star • The gravity causes the neutron star to keep contracting

41. Black Hole • A black hole is a region where nothing can escape, even light.

42. Event Horizon • Event Horizon is the boundary between the inside and outside of the Black Hole • Within the Event Horizon, the escape velocity is greater than the speed of light • Nothing can escape once it enters the Event Horizon

43. How do calculate the radius of the Event Horizon? • It is called the Schwarzschild Radius • Radius = 2GM/c2 • This is a variation of the escape velocity formula • Escape velocity = square root (2GMplanet/Rplanet)

44. Black Hole Sizes • A Black Hole with the mass of the Earth would have a radius of 0.009 meters • A Black Hole with the mass of the Sun would have a radius of 3 kilometers

46. Can you see a Black Hole?

47. No • Black Holes do not emit any light • So you must see them indirectly • You need to see the effects of their gravity

48. The white area is the core of a Galaxy Inside the core there is a brown spiral-shaped disk. It weighs a hundred thousand times as much as our Sun. Evidence

49. Evidence • Because it is rotating we can measure its radii and speed, and hence determine its mass. • This object is about as large as our solar system, but weighs 1,200,000,000 times as much as our sun. • Gravity is about one million times as strong as on the sun. • Almost certainly this object is a black hole.

50. Questions