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Stellar Deaths II Neutron Stars and Black Holes 17

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CPS Question ● The pressure that prevents the gravitational collapse of white dwarfs is a result of ______. – A) Conservation of energy – B) Conservation of angular momentum – C) Einstein's principle of equivalence – D) The Pauli exclusion principle

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CPS Question ● A nova is a result of _____. – A) a supergiant star shedding its envelope – B) the explosion of an extremely high-mass star – C) fusion on the surface of an accreting white dwarf – D) pulsation of an isolated brown dwarf

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CPS Question A low mass object that failed to start nuclear fusion is known as a ______ dwarf. –A) brown –B) red –C) white –D) black

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1. White Dwarf If initial star mass < 8 M Sun or so. 2. Neutron Star If initial mass > 8 M Sun and < 25 M Sun. 3. Black Hole If initial mass > 25 M Sun. Final States of a Star

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Supernova remnants What type of remnant does a carbon- detonation supernova leave behind? What about a core-collapse supernova?

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Neutron Stars Type I supernova (Carbon-detonation): no remnant Remnant of core-collapse supernova - a tightly packed ball of neutrons. Diameter: only ~20 km! Mass: 1.4 - 3 M Sun Incredible densities! Conservation of Ang. Mom. => what? A neutron star over the Sandias?

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Neutron Stars Type I supernova (Carbon-detonation): no remnant Remnant of core-collapse supernova - a tightly packed ball of neutrons. Diameter: only ~20 km! Mass: 1.4 - 3 M Sun Incredible densities! Conservation of Ang. Mom. => Rapid rotation rate Magnetic field: 10 12 x Earth's! A neutron star over the Sandias?

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Pulsars Objects that give off periodic pulses of radiation Frequencies: ~one to several hundred pulses/sec What are they?

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Pulsars What produces the beam of radiation received from a pulsar? Do we see all neutron stars as pulsars? Why or why not?

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The Lighthouse Model of a Pulsar Rapid rotation => huge magnetic field Accelerates charges Narrow “beam” of radiation Not all neutron stars are pulsars Can be too old Orientation may be wrong

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Pulsars are incredibly accurate clocks! Accurate to within a few seconds in a million years! Better than best atomic clocks on Earth! Observed fluctuations in period can be used to detect planets orbiting a pulsar.

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1. White Dwarf If initial star mass < 8 M Sun or so. 2. Neutron Star If initial mass > 8 M Sun and < 25 M Sun. 3. Black Hole If initial mass > 25 M Sun. Final States of a Star

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Concept Review What determines whether or not an object is capable of retaining an atmosphere? How might this concept be related to black holes?

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Review of Escape Speed Speed needed to escape the gravitational pull of an object. v esc = 2GM R Escape speed from Earth's surface is 11 km/sec. If Earth were crushed down to 1 cm size, escape speed would be the speed of light. Schwarzschild Radius = if an object is crushed to within the S r, not even light can escape its gravitational pull

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Black Holes If core with > 3 M Sun collapses, not even neutron pressure (Pauli Exclusion Principle) can stop the collapse (initial mass of star > 25 M Sun ). Collapses to a point, a "singularity". Still searching for a theory of quantum gravity. Gravity is so strong that nothing can escape, not even light (so no information) => black hole. So, how might we detect them?

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Event horizon: “Surface” of black hole. Point of no return. Event horizon Schwarzschild Radius

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Black Holes What are some of the strange phenomena we might encounter if we fell into a black hole?

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Effects around Black Holes Near event horizon: 1) Enormous tidal forces. 2) Bending of light: 2) Gravitational redshift. 3) Time dilation.

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Examples: 1. Bending of light. If light travels in straight lines in free space, then gravity causes light to follow curved paths. First observed in 1919 eclipse.

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Gravitational lensing. The gravity of a foreground cluster of galaxies distorts the images of background galaxies into arc shapes.

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Gravitational Red shift Light emitted from a large gravity well object (large star) will appear red shifted on smaller object Photons need energy to leave gravity well so photons lowers in frequency to provide that energy and become red shifted

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Einstein's Principle of Equivalence What phenomenon produces all of the same physical effects as being in gravity? –(Hint: Think about riding in an elevator.)

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Einstein's Principle of Equivalence According to Einstein, the effects of gravity and acceleration are indistinguishable from one another! The laws of physics are identical in either case. –=> acceleration should be able to reproduce effects of a black hole!

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2. Gravitational Redshift Consider accelerating elevator in free space (no gravity). time zero, speed=0 later, speed > 0 light received when elevator receding at some speed. light emitted when elevator at rest. Gravity must produce same result! But now due to reduction of photon energy required to escape gravitational field, not Doppler effect! Photon Energy Frequency!

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Do Black Holes Really Exist? Good Candidate: Cygnus X-1 - Binary system: 30 M Sun star with unseen companion. - Binary orbit => companion ~10 M Sun. - X-rays => million degree gas falling into black hole.

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