NOTE week day for Exam 3 Exam 3 on Tuesday April 16 - HW Ch. 13 & 14 due Mon. April 8

1. NOTE week day for Exam 3Exam 3 on Tuesday April 16- HW Ch. 13 & 14 due Mon. April 8

2. Outline of Chapter 13 Death of Stars Death of Stars White Dwarfs Neutron Stars Black Holes Cycle of Birth and Death of Stars (borrowed in part from Ch. 14)

3. I.Death of Stars Low mass M.S. stars (M < 0.4 solar Mo) produce White Dwarfs Intermediate mass M.S. stars ( 0.4Mo < M < 4 solar Mo) produce White Dwarfs High mass stars M.S. (M > 4 solar Mo) can produce Neutron Stars and Black holes

4. I.Death of Stars DEAD STARS (i.e., Stellar Copses) White Dwarfs: very dense, about mass of Sun in size of Earth. Atoms stop further collapse. M less than 1.4 solar masses Neutron Stars: even denser, about mass of Sun in size of Orlando. Neutrons stop further collapse. M between 1.4 and 3 solar masses. Some neutron stars can be detected as pulsars Black Holes: M more than 3solar masses. Nothing stops the collapse andproduces an object so compact that escape velocity is higher than speed of light; hence,not evenlight can escape.

5. I.Death of Stars White Dwarfs: very dense, about mass of Sun in size of Earth. Atoms stop further collapse. M less than 1.4 solar masses Neutron Stars: even denser, about mass of Sun in size of Orlando. Neutrons stop further collapse. M between 1.4 and 3 solar masses. Some neutron stars can be detected as pulsars Black Holes: M more than 3solar masses. Nothing stops the collapse andproduces an object so compact that escape velocity is higher than speed of light; hence,not evenlight can escape. NOTE: these are the masses of the dead stars NOT the masses they had when they were on the main sequence

6. Very massive stars are rare Low-mass stars are common Luminosity There are few high-mass stars: Supernovas are rare, white dwarfs are more common Temperature

7. Sirius: a binary star system with a M.S. star and a white dwarf

8. A white dwarf is about the same size as Earth

9. White dwarfs cool off and grow dimmer with time

10. Neutron Star About the size of NYC or Orlando

11. During a supernova explosion in the core of the star electrons can combine with protons, making neutrons and neutrinos forming a neutron star

12. Pulsars are neutron stars that give off very regular pulses of radiation

13. A pulsar’s rotation is not aligned with magnetic poles

14. Pulsar

15. Pulsars are rotating neutron stars that act like lighthouses Beams of radiation coming from poles look like pulses as they sweep by Earth

16. Pulsar at center of Crab Nebula pulses 30 times per second

17. X-rays Visible light

18. Pulsar (in Crab Nebula) This is a confirmation of theories that predicted that neutron stars can be produced by a supernova explosion,because the Crab Nebula was produced by a SN that exploded in the year 1054

19. Pulsar (in Crab Nebula) How do we know that there was a Supernova there in 1054?

20. Question Could there be neutron stars that appear as pulsars to civilizations around other stars but not to us? A. Yes B. No

21. Question Could there be neutron stars that appear as pulsars to civilizations around other stars but not to us? A. Yes B. No

22. I.Death of Stars How do we detect Neutron Stars and Black Holes? Neutron Stars: As pulsars As compact objects in binary stars Black Holes: As compact objects in binary stars

23. I.Death of Stars How do we detect Neutron Stars and Black Holes? Neutron Stars: As pulsars As compact objects in binary stars Black Holes: As compact objects in binary stars When we see compact objects in binary stars how do we distinguish Neutron Stars from Black holes?

24. II.Death of Stars How do we detect Neutron Stars and Black Holes? Neutron Stars: As pulsars As compact objects in binary stars Black Holes: As compact objects in binary stars How do we distinguish Neutron Stars from Black holes? The mass of the object

25. I.Death of Stars How do we detect Neutron Stars and Black Holes? Neutron Stars: As pulsars As compact objects in binary stars Black Holes: As compact objects in binary stars How do we distinguish Neutron Stars from Black holes? The mass of the object How do we measure the masses of Stars?

26. II.Death of Stars How do we detect Neutron Stars and Black Holes? Neutron Stars: As pulsars As compact objects in binary stars Black Holes: As compact objects in binary stars How do we distinguish Neutron Stars from Black holes? The mass of the object How do we measure the masses of Stars? Binary Stars

27. Black Hole in a Binary System If the mass of the compact object is more than 3 solar masses, it is a black hole

28. Black Hole in a Binary System If the mass of the compact object is LESS than 3 solar masses what can it be?

29. Black Hole in a Binary System If the mass of the compact object is LESS than 3 solar masses what can it be? If its invisible and less than 3 solar masses (but more than 1.4): Neutron Star

30. What is a black hole? • A black hole is an object whose gravity is so powerful that not even light can escape it. • A place where gravity has crushed matter into oblivion, creating a true hole in the universe from which nothing can ever escape, not even light.

31. REMEMBER: Escape Velocity  2G M (radius) Escape velocity = When the escape velocity from an object is equal or greater than the speed of light, that object is a black hole. Not even light (photons) can escape from the surface of a black hole

32. Escape Velocity What would happen to Earth’s orbit if the Sun became a black hole now?

33. Escape Velocity What would happen to Earth’s orbit if the Sun became a black hole now? Hint: Remember the force due to gravity:

34. Escape Velocity What would happen to Earth’s orbit if the Sun became a black hole now? Hint: Remember the force due to gravity: F= GM1M2/D2

35. If the Sun shrank into a black hole, its gravity would be different only near the event horizon. At the orbits of the planets the gravity would stay the same! Black holes don’t suck! Unless you are VERY close

36. Time passes more slowly near the event horizon

37. Thought Question Is it easy or hard to fall into a black hole? A. Easy B. Hard

38. Thought Question Is it easy or hard to fall into a black hole? A. Easy B. Hard Hint: A black hole with the same mass as the Sun wouldn’t be much bigger than a college campus

39. Thought Question Is it easy or hard to fall into a black hole? B. Hard Hint: A black hole with the same mass as the Sun wouldn’t be much bigger than a college campus: The orbits of the planets are much further than that

40. Tidal forces near the event horizon of a 3 MSun black hole would be lethal to humans Tidal forces would be gentler near a supermassive black hole because its radius is much bigger

41. Do black holes really exist?

42. Black Hole Verification • Need to measure mass • Use orbital properties of companion • Measure velocity and distance of orbiting gas • It’s a black hole if it’s not a star and its mass exceeds the neutron star limit (~3 MSun)

43. Some X-ray binaries contain compact objects of mass exceeding 3 MSun which are likely to be black holes

44. At the center of the Milky Way stars appear to be orbiting something massive but invisible … a black hole! Orbits of stars indicate a mass of about 4 million MSun

45. II. Cycle of Birth and Death of Stars:Interstellar Medium A.Interstellar Matter: Gas (mostly hydrogen) and dust •Nebulae •Extinction and reddening •Interstellar absorption lines •Radio observations B. Nebulae • Emission • Reflection • Dark C. Cycle of Birth and Death of Stars

46. Interstellar Medium IIA. Interstellar Matter: Gas (mostly hydrogen) and dust How do we know that Interstellar Matter is there: •Nebulae •Extinction and reddening •Interstellar absorption lines •Radio observations

47. Extinction and Reddening: interstellar dust will make stars look fainter and redder

48. Absorption Spectrum

49. Interstellar Absorption Lines

50. Radio Observations: some molecules can be detected with radiotelescopes