Transcript lecture24
The death of stars and stellar remnants •He White dwarfs •He White dwarf and planetary nebulae •Carbon White Dwarfs •Neon-Silicon-Oxigen White Dwarfs •Two types of Supernovae •Type Ia, the exploding stars disintegrates •Type II (core collapse), the star leaves remnants: •Neutron stars (basically, a neutron white dwarf, I.e. degenerate gas of neutrons) •Black holes Assigned Reading Ch. 13 and Ch. 14, excluding 14.3 (eliminated from syllabus) Stellar evolution can be deeply altered in a binary system: mass transfer An originally massive star can loose mass and become less massive (longer life) A nearly dead star can be rejuvenated by accretion of fresh fuel Planetary Nebula At the center of the nebula there is the dying star. This is a white dwarf, where small and hot: it photo-ionizes the nebula The nebula formed out of the mass loss during the red super-giant phase. Destiny of stars with roughly M < 8Mo M <0.4 Mo He WD M < 4 Mo, C WD M < 8 Mo, C + O + Si WD Novae are nuclear explosions on the surface of white dwarf and neutron stars Brightness changes by a factor of 4000! Two basic types of supernovae Type Ia – from the thermonuclear detonation of a white dwarf with M ~ 1.4 Msun after accreting matter from its companion. (1.4 Msun is called Chandrasekhar limit) Type II – from core collapse of a massive star neutron star or black hole. Type Ia: White Dwarf Supernova If a White Dwarf accretes enough matter from a companion star, it will eventually nova. If, after the nova, it does not shed all the mass it gained, it will continue to accrete mass until it novas again. If this process continues (accretion, nova, accretion, nova, etc.) such that the WD continues to gain mass, once it has a mass of 1.4Msun, the core will collapse, carbon fusion will occur simultaneously throughout the core, and the WD will supernova. How might it be possible for a White Dwarf to flare back to life? Remnant from a Type Ia supernova: A lot of irons! The Sun will never supernova because It will become a white dwarf before it has the chance. Its surface temperature is not high enough. It is not large enough. It is not massive enough. Remnant from a Type II supernova Crab Nebula The supernova explosion that created the Crab was seen on about July 4, 1054 AD. Neutron stars A neutron star --- a giant nucleus --- is formed from the collapse of a massive star. Supported by neutron degeneracy pressure. Only about 10 km in radius. A teaspoon full would contain 108 tons! Very hot and with very strong magnetic field Jocelyn Bell Neutron stars discovered as pulsar SNR N157B in the LMC pulsar 16ms period The fastest young pulsar known Pulsar, as a light house A fast rotating, magnetized neutron star. Emits both strong radiation (radio) and jets pf highenergy particles. Jets not very well understood; their existence is due to the rotation and to the presence of magnetic fields Pulsar Evolution Pulsar slow down their rotation Pulsar emits radiation (0.1%) and high energy plasma (99.9%): it looses energy This energy is replenished at the expense of rotational energy Eventually, pulsar slows down, radio beams become weaker. Many pulsars not observable, because beams do not sweep Earth; or they have been kicked off their nebula (asymmetric collapse) or slowed down too quickly (because they had ultra strong magnetic field) The Limit of Neutron Degeneracy The upper limit on the mass of stars supported by neutron degeneracy pressure is about 3.0 MSun (predicted by Lev Landau) If the remaining core contains more mass, neutron degeneracy pressure is insufficient to stop the collapse. In fact, nothing can stop the collapse, and the star becomes a black hole. Review Questions 1. 2. 3. 4. What are type-Ia supernovae? What do a type-II supernova leave behind? Why does a neutron star spin fast? What is a pulsar? Black holes When the ball of neutrons collapses, it forms a singularity – a small region in space with small volume and the mass of the parent material. a singularity in space, from which nothing can escape, even light! The most interesting aspects of a black hole are not what it’s made of, but what effect is has on the space and time around it. If we apply General Relativity to a collapsed star, we find that it can be sufficiently dense to trap light in its gravity. The Size of a Black Hole The extent of a black hole is called its event horizon. Nothing escapes the event horizon! The radius of the event horizon is the Schwarzschild radius given by: Rs = 2GM/c2 Some Examples of Black Hole Sizes A 3MSun black hole would have a Schwarzschild radius of ~10km. It would fit in Amherst. A 3 billion MSun black hole would have a radius of 60 AU – just twice the radius of our solar system. Some primordial black holes may have been created with a mass equal to that of Mount Everest. They would have a radius of just 1.5x10-15 m – smaller than a hydrogen atom! What would a Black Hole look like? Gravitational lensing Some Odd Properties of Space Around a Black Hole Light emitted near the surface of a black hole is redshifted as it leaves the intense gravitational field. For someone far away, time seems to runs more slowly near the surface of a black hole. An astronaut falling into a black hole would seem to take forever to fall in. Gravitational Redshifts A photon will give up energy while climbing away from a mass. It is trading its own energy for gravitational potential energy. Survey Question If your buddy were falling into a black hole, what kind of telescope would you need in order to see him/her wave goodbye as they crossed the event horizon? 1) A large radio telescope. 2) A large infrared telescope. 3) A large visible light telescope? 4) A large X-ray telescope? Black Holes Don’t Suck! Many people are under the impression that the gravity of black holes is so strong that they suck in everything around them. Imagine what would happen if the Sun were to instantly turn into a black hole. What would happen to the Earth? Black Holes Don’t Suck! Since the mass of the Sun and Earth don’t change, and the Earth is no further from the Sun than it was before, the force on the Earth would remain exactly the same. The Earth would continue to orbit the black hole at a distance of 1 AU! Black Holes Don’t Suck! So why are black holes so infamous? • The reason is that the mass is so compact that you can get within a few kilometers of a full solar mass of material. Today, if you stood on the surface of the Sun, much of the material is hundreds of thousands of kilometers away. With a black hole, the mass is so concentrated that you can get very close to the full mass. Gravity strength is extreme near a B.H. The tidal forces near a moderate sized black hole are lethal! How Do We See A Black Hole? Short answer … we don’t. But we can see radiation from the material falling into one. When matter falls into a B.H. it gets very, very hot. It emits X-ray. Candidate B.H.’s are powerful X-ray emitters, especially if they show very rapid variability (=small size) Evidence for Black Holes If black holes are black, how do we know that they exist? The star HD 226868 is an excellent example. It is a B supergiant. The spectral lines in the star clearly show that it is in a binary system with a period of 5.6 days, however, we see no companion star. It is one of the brightest X-ray sources in the sky and is called Cygnus X-1 HD 226868 Cygnus X-1 The blue supergiant is so large, that its outer atmosphere can be drawn into the black hole. As the material spirals into the black hole, it heats up to millions of degrees and emits X-ray radiation. What did you think? Are black holes just holes in space? No, black holes contain highly compressed matter (with infinitely small volume) at their center. They are not empty. What is at the surface of a black hole? The surface of a black hole, called the event horizon, is empty space-there is no stationary matter there. What power or force enables black holes to draw things in? The only force that pulls things in is the gravitational attraction of the matter in the black hole. Time runs more slowly in the presence of a gravitational field. 1s Strobe light No gravitational field. Time runs more slowly in the presence of a gravitational field. Strobe light (according to the clock) 1s Big gravitational field. Discussion Question Your doomed friend remembers that he has a rocket that he can use to temporarily stop his descent into the black hole. With visions of heroism in your head, you tie a rope to your waist and jump out of your spaceship to go and rescue him. How does time appear (to you) to progress for you and your friend as you approach him? 1) Your own time seems to run normally and your friend’s time seems to run faster and faster as you approach him. 2) Your own time seems to run slower and slower as you fall and your friend’s time seems to continue to run at the same slow rate.