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Transcript
Today’s outline Review Star Remnants White Dwarfs Homework due Thursday morning Test Friday Review: Lives of High Mass Stars Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Stellar remnants I White dwarf stars I Neutron stars I Black holes Question concepts Review Star Remnants Question #1: Heavier nuclei have higher electric charge than lighter nuclei, therefore fusing them together requires higher temperatures in order to overcome the stronger electromagnetic repulsion. Question #2: The creation of a new core burning phase inside a star proceeds through the following events: fuel depletion in the core, formation of a burning shell around th core, contraction of the (now inert) core raising its temperature, eventual ignition of burning in the core. Question #3: When a high mass star dies it will leave behind a neutron star or black hole. A low mass star will leave behind a white dwarf. White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Stellar remnants I White dwarf stars I Neutron stars I Black holes Neutron stars Characteristics Pulsars Black holes Bending light Black Holes White dwarf stars Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes White dwarf I remnant core of low-mass star I cooling from being the hot core of a star I millions of times more dense than water teaspoon of white dwarf core = 100 tons I held up by electron degeneracy pressure I more mass is smaller Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Question concepts Question #4: Higher mass white dwarf stars are smaller in size than lower mass ones. Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Electron degeneracy higher energy Degeneracy – particles like electrons are not allowed to occupy the same energy level high temp low dens low temp low temp low dens high dens Thus Temperature is not necessary to ”hold up” the star more like a ”normal” object - does not contract as it loses energy I At high temperature, particles tend to not be in same levels anyway I At low temperatures particles will ”stack up” in lower energy levels I At high densities the ”top” of the stack can be quite high energy The pressure from electrons on the ”top” of the stack holds up a white dwarf star without thermal pressure (though the interior is still several million K) Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Maximum white dwarf mass Review Electrons can’t move faster than speed of light I Upper limit to pressure I Maximum mass for white dwarf Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars I about 1.4MSun Above this mass, electrons are forced onto protons – form neutrons Take up much less space Collapses to neutron star Characteristics Pulsars Black holes Bending light Black Holes Speed of light and relativity The speed of light is always the same – implies that it is also the highest possible speed I normal objects thrown from moving cars – object thrown from faster car moves faster I light emitted from moving cars – always goes the same speed Thus an object can never go faster than light it emits Has some very counterintuitive results for the nature of space and time – time dilation, distance contraction Hinges on something that we take for granted – Simultaneity - events separated in space can occur at the same time - not as true as you think Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Neutron Stars Without electrons, neutron stars are as dense as nuclei Paperclip of this density has mass of mount everest I ? I about as heavy as sun I We don’t know what material at center is like (quark-gluon plasma!) I has a solid crust as well as oceans (of liquid metal) neutrons crust of heavy nuclei atmosphere of light nuclei about 10km in radius (about 6 miles) Already very much like a black hole - very small has an innermost orbit – orbital radius below which objects can’t stay in orbit Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Review Star Remnants White Dwarfs Question concepts Question #5: Neutrons have no charge, and therefore no electric repulsion. This makes it easy for neutrons to get close to each other and to other nuclei. Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Pulsars Extremely regularly pulsed radio signals Spinning neutron stars I I Caused by radiation beamed from magnetic poles Rotation causes ”flashing” as beam points toward and away from us I Born rapidly rotating as core of star collapses in supernova I spins down over time due to losses due to magnetic field Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Pulsars power supernova remnants Review Star Remnants White Dwarfs Rotation of magnetic neutron star (pulsar) is the energy source that lights up the central part of supernova remnant. Central blue haze from energy and magnetic field shed by neutron star Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Black Holes Review At too high mass, quarks can no longer move at high enough speeds to hold up the neutron star. Happens above about 3Msun Collapse to black hole Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Gravity can bend light around a star Light bends somewhat like particle (planetary) orbits The bending of spacetime influences causality – the ability for events to cause other events – information cannot propagate faster than light I event horizon is the radius inside which events cannot cause events at larger radius If light is subject to very high gravity, very close to a small object, can get trapped. This is a black hole. I Light cannot move outward from this radius Black holes are remarkably small not remarkably heavy Can be just a few times our sun’s mass Far away, no stronger gravity than a similar mass star Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes Gravity warps space and time Clocks run more slowly near gravity source Observed for the earth too, but very weak. Review Star Remnants White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars From outside black hole, you never see an object reach the event horizon. It appears to be in freefall forever, with an ever-slowing rate. The object falling in the hole does not notice passing through the event horizon. But once beyond it can never return Characteristics Pulsars Black holes Bending light Black Holes Review Star Remnants Question concepts Question #6: From a typical planetary orbit, a black hole’s gravitational field is no different than that of a star with the same mass. The difference is that a black hole is very small, so that very close to it gravity is quite strong. Question #7: Events that occur closer to a black hole than the event horizon cannot influence events outside the event horizon. That is, information cannot propagate from a point inside the event horizon to a point outside it. White Dwarfs Degeneracy Maximum mass Speed limit Neutron stars Characteristics Pulsars Black holes Bending light Black Holes
