Chapter 13: The Stellar Graveyard

Transcription

1 Chapter 13: The Stellar Graveyard Habbal Astro110 Chapter 13 Lecture 26 1

2 Low mass star High mass (>8 M sun ) star Ends as a white dwarf. Ends in a supernova, leaving a neutron star or black hole 2

3 White dwarfs: end state of low-mass stars Inert remaining cores of dead low-mass stars. No internal energy generation: start hot and steadily cool off. Sirius A high mass star Optical image Sirius A high mass star X-ray image Sirius B white dwarf Sirius B white dwarf 3

4 White dwarfs are supported against gravitational collapse by electron degeneracy pressure 4

5 A 1 M Sun white dwarf is about the same size as the Earth A teaspoon of white dwarf material would weight 10 tons! 5

6 More massive white dwarfs are smaller! Mass gravitational compression density radius Chandrasekhar limit: white dwarfs cannot be more massive than 1.4 M Sun 6

7 White dwarfs in binary systems WDʼs gravity can accrete gas from companion star. Accreted gas can erupt in a short modest burst of nuclear fusion: novae BANG! However, WDs cannot be more than 1.4 M Sun. If WD accretes too much gas, it is destroyed in a white dwarf supernova 7

8 Nova: a nuclear explosion on the surface of a WD, gas is expelled and system returns to normal 8

9 One way to tell supernova types apart is with a light curve showing how the luminosity changes. 9

10 Neutron stars: end state of high-mass stars Aftermath of a massive star supernova. Supported against gravitational collapse by neutron degeneracy pressure. 10

11 Neutrons stars pack several M Sun into a sphere 10 km in diameter A teaspoon of neutron star material would weigh 10 billion tons 11

12 Neutron stars found in 1967 as radio pulsars s Discovered in 1967 by graduate student Jocelyn Bell What Extraterrestrial astronomical intelligence object can (LGM?!) spin so fast? 12

13 13

14 Pulsars are rotating neutron stars that act like lighthouses. Beams of radiation coming from poles look like pulses as they sweep by Earth. 14

15 Optical pulses from the neutron star at the center of the Crab nebula 15

16 The Crab Nebula (supernova remnant) X-rays Visible light 16

17 Is there a limit to the mass of a neutron star? Yes, neutron degeneracy pressure cannot resist gravity for >3 M sun! (happens for a star with initial mass of >25 M sun ) What happens next??? There is no other support against gravity!! Everything collapses to a singularity. 17

18 Black holes: Gravityʼs ultimate victory Nothing, not even light, can escape a black hole 18

19 What happens to the escape speed of an object as it becomes smaller and denser? Is there a limit to how fast the escape speed can be? 19

20 20

21 The curvature of space-time To understand this, imagine the universe has only two spatial dimensions, instead of three. Empty space 21

22 The curvature of space-time To understand this, imagine the universe has only two spatial dimensions, instead of three. Space near a large mass (e.g. the Sun) 22

23 The curvature of space-time To understand this, imagine the universe has only two spatial dimensions, instead of three. 23

24 The curvature of space-time To understand this, imagine the universe has only two spatial dimensions, instead of three. Space near a black hole 24

25 The curvature of space-time Einsteinʼs General Theory of Relativity 25

26 What is the size of a black hole? The event horizon the surface of the BH, where escape velocity is the speed of light (c) Escape speed > c Escape speed < c Size of event horizon = Schwarzschild radius = 2GM/c 2 26

27 Schwarzschild radius of a 1M Sun black hole ~ 3 km 27

28 If the Sun shrank into a black hole, its gravity would be different only near the event horizon Black holes donʼt suck things into them! 28

29 What would it be like to visit a black hole? 29

30 What happens near a black hole? Gravitational redshift: light becomes redder as it leaves an object 30

31 Time passes more slowly near the event horizon. An object would appear to never quite reach the event horizon but would disappear from view as its light became so redshifted that it would be undetectable. 31

32 Lethal tidal forces near a black hole 32

33 Do black holes really exist? Black holes donʼt emit light! (*) How do we detect black holes? Look for material falling into a BH: will be moving very fast ( hot X-rays) around a dark compact object. Measuring the velocity and distance of this hot gas can give the mass (Newtonʼs form of Keplerʼs Third Law). If >3 M Sun, object must be a black hole. (*) except for Hawking radiation, which leads to BH evaporation. 33

34 34

35 Some X-ray binaries contain compact objects of mass exceeding 3 M Sun which are likely to be black holes. 35

36 One famous X-ray binary with a likely black hole is in the constellation Cygnus. 36

37 Two Types of Supernova White dwarf supernova Carbon fusion suddenly begins as white dwarf in close binary system reaches white dwarf limit, causing total explosion Massive star supernova Iron core of massive star reaches white dwarf limit (1.4 MSun) and collapses into a neutron star, causing an explosion. 37

38 Summary: What is a black hole? A black hole is a place where gravity is so powerful that nothing can ever escape from it, not even light. (Therefore, out of contact with the rest of the Universe.) 38

39 What would it be like to visit a black hole? You could orbit a black hole just like any other object of the same mass. However, you d see strange effects for an object falling toward the black hole: Time would seem to run slowly for the object. Its light would be increasingly redshifted as it approached the black hole. The object would never quite reach the event horizon, but it would soon disappear from view as its light became so redshifted that no instrument could detect it. 39

40 Do black holes really exist? No known force can stop the collapse of a stellar corpse with a mass above the neutron star limit of 2 to 3 solar masses. Theoretical studies of supernovae suggest that such objects should sometimes form. Observational evidence supports this idea. 40

41 41

42 42