9A Black Holes and Neutron Stars Dead Stars Copyright – A. Hobart
9A Goals What are neutron stars and pulsars? What are black holes? What happens near a black hole? How do we see black holes? What happens when neutron stars and black holes are in binaries?
9A Supernova Remnant Recall: In the death of a high-mass star, the core is converted to neutrons and collapses catastrophically. The collapse and rebound creates a supernova. But what happens to the neutrons already at the very center of the core? The central core is left behind as a small, dense, sphere of neutrons a neutron star.
9A Neutron Stars A giant ball of neutrons. Mass : at least 1.4 x mass of the Sun. Diameter: 20 km! Density: 10 18 kg/m 3 –A thimble weighs as much as a mountain Day: 1 – 0.001 seconds! Magnetic fields as strong as the Sun, but in the space of a city.
9A Pulsars Interstellar Lighthouses. See periodic bursts of radiation. Perfect clocks. While every pulsar is a neutron star, the opposite isn’t true.
9A Pulsar Motion Pulsars born in the center of supernovae explosions. Non-symmetric explosions lead to huge “kick.” Large velocity pulsars. v = 800 – 1000 km/s! Guitar Nebula – copyright J.M. Cordes
9A Neutron Degeneracy Neutron stars are held up by neutron degeneracy pressure. –Recall electron degeneracy pressure for white dwarfs. –For white dwarfs, maximum mass of 1.4 M sun For neutron stars, maximum mass ~3M sun What happens if a high-mass star is SO big that its central core is bigger than this? What happens when gravity is stronger than even neutron degeneracy pressure?
9A Density Density = mass per volume From Red Giant cores to White Dwarfs to Neutron Stars, density has been increasing. As density increases, the force of gravity on the surface increases. The greater the force, the higher the escape velocity: –How fast you need to go in order to escape the surface. How dense can something get? How strong can the force of gravity be? What if the escape velocity is faster than light?
9A Black Hole When a high-mass star’s core is greater than ~3 x M sun, then, when it collapses, the density becomes so high not even light can escape! The star collapses to form a Black Hole.
9A Black Holes Light is bent by the gravity of a black hole. The event horizon is the boundary inside which light is bent into the black hole. Approaching the event horizon time slows down relative to distant observers. Time stops at the event horizon.
9A Seeing Holes Can’t see black hole itself, but can see matter falling into a hole. Gravitational forces stretch and rip matter: heats up. Very hot objects emit in X-rays (interior of Sun) Cygnus X-1. http://www.owlnet.rice.edu/~spac250/steve/ident.html
9A What would be a good source of material for a hungry black hole?
9A Binaries Gravitational tides pull matter off big low density objects towards small high density objects. Cygnus X-1
9A Nova and Supernova Similar situation with white dwarf-gas giant pairs. White dwarf accretes matter from giant. If enough material falls fast enough it will ignite and fuse on the w.d surface: Nova! If enough mass falls onto white dwarf that M wd > 1.4 x M sun : White dwarf collapses to a neutron star. Supernova! (now there are two types)
9A Binary Pulsars Neutron stars can also be in binaries. General Relativity says they will eventually spiral into one another. Result: -ray bursts: Most violent explosions in the universe.