Chapter 13: The Stellar Graveyard 3/31/2009 Habbal Astro110http://chandra.harvard.edu/photo/2001/1227/index.html Chapter 13 Lecture 26 1 Low mass star High mass (>8 Msun) star Ends as a white dwarf. Ends in a supernova, leaving a neutron star or black hole 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 2 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. Optical image X-ray image Sirius A Sirius A high mass star high mass star Sirius B Sirius B white dwarf white dwarf 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 3 White dwarfs are supported against gravitational collapse by electron degeneracy pressure 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 4 A 1 MSun white dwarf is about the same size as the Earth… ⇒ A teaspoon of white dwarf material would weight 10 tons! 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 5 More massive white dwarfs are smaller! Mass ⇒ gravitational compression ⇒ density ⇒ radius Chandrasekhar limit: white dwarfs cannot be more massive than 1.4 MSun 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 6 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 • However, WDs cannot BANG! be more than 1.4 MSun. • If WD accretes too much gas, it is destroyed in a white dwarf supernova 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 7 Nova: a nuclear explosion on the surface of a WD, gas is expelled and system returns to normal 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 8 One way to tell supernova types apart is with a light curve showing how the luminosity changes. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 9 Neutron stars: end state of high-mass stars Aftermath of a massive star supernova. Supported against gravitational collapse by neutron degeneracy pressure. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 10 Neutrons stars pack several MSun into a sphere 10 km in diameter ⇒ A teaspoon of neutron star material would weigh 10 billion tons 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 11 Neutron stars found in 1967 as radio pulsars 1.337 s Discovered in 1967 by graduate student Jocelyn Bell WhatExtraterrestrial astronomical intelligence object can (LGM?!) spin so fast? 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 12 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 13 Pulsars are rotating neutron stars that act like lighthouses. Beams of radiation coming from poles look like pulses as they sweep by Earth. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 14 Optical pulses from the neutron star at the center of the Crab nebula 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 15 The Crab Nebula (supernova remnant) X-rays Visible light 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 16 Is there a limit to the mass of a neutron star? Yes, neutron degeneracy pressure cannot resist gravity for >3 Msun ! (happens for a star with initial mass of >25 Msun) What happens next??? There is no other support against gravity!! Everything collapses to a singularity. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 17 Black holes: Gravityʼs ultimate victory Nothing, not even light, can escape a black hole 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 18 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? 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 19 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 20 The curvature of space-time To understand this, imagine the universe has only two spatial dimensions, instead of three. Empty space 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 21 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) 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 22 The curvature of space-time To understand this, imagine the universe has only two spatial dimensions, instead of three. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 23 The curvature of space-time To understand this, imagine the universe has only two spatial dimensions, instead of three. Space near a black hole 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 24 The curvature of space-time Einsteinʼs General Theory of Relativity 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 25 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/c2 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 26 Schwarzschild radius of a 1MSun black hole ~ 3 km 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 27 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! 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 28 What would it be like to visit a black hole? 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 29 What happens near a black hole? Gravitational redshift: light becomes redder as it leaves an object 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 30 ⇒ 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 3/31/2009that it would be undetectable.Habbal Astro110 Chapter 13 Lecture 26 31 Lethal tidal forces near a black hole… 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 32 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 MSun, object must be a black hole. (*) except for Hawking radiation, which leads to BH evaporation. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 33 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 34 Some X-ray binaries contain compact objects of mass exceeding 3 MSun which are likely to be black holes. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 35 One famous X-ray binary with a likely black hole is in the constellation Cygnus. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 36 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. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 37 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.) 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 38 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. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 39 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. 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 40 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 41 3/31/2009 Habbal Astro110 Chapter 13 Lecture 26 42 .