Chapter 15White DwarfsDegeneracy and the Chandrasekhar LimitWhite Dwarfs in Binary SystemsNovas and Type Ia SupernovasNeutron StarsPulsarsEmission from Neutron Stars“Glitches” and Structure of Neutron StarsX-Ray Binary StarsGravitational Waves from Neutron StarsBlack Holes and Escape VelocityNature of Space Around Black HolesThe Event HorizonFormation of Black HolesIndirect Observation of Black HolesHawking RadiationChapter 15Stellar Remnants: White Dwarfs, Neutron Stars, Black HolesWhite Dwarfs•About the mass of the Sun.•Temp can range from 4,000 to 25,000 K.•Core mainly C and O; can’t undergo fusion anymore.Degeneracy and the Chandrasekhar Limit•Degeneracy: only the density affects the pressure, not the temperature.•At 1.4 M⊙ the white dwarf collapses. This is the Chandrasekhar Limit.•How do we measure the mass? Kepler’s 3rd for binary Gravitational redshift for isolatedWhite Dwarfs in Binary Systems•White dwarf “steals” mass from companion star, increases the mass of the white dwarf.•With enough added mass, fusion can briefly restart.Novas and Type Ia SupernovasNeutron Stars•When the iron core of a massive star reaches 1.4 M⊙ the core collapses —> Type II supernova.•Can create a neutron star (radius 10 km, mass 1-3 M⊙)•VERY hot when formed, 1011 - 1012 K. Cools to 106 K after a few years.Pulsars•Neutron stars that emit radio waves with a rapid and precise pulse rate.•They’re actually spinning, not pulsing.•Magnetic field holds beam in place.Emission from Neutron Stars•Narrow beams of charged particles (in addition to the radio emission)•This is called “non-thermal radiation”•We need to be edge-on to detect the emission.•Magnetic field speeds up the particles, this slows down the rotation.“Glitches” and Structure of Neutron Stars•Should slow down uniformly, but this isn’t what we see.•Mathematical models show a rigid crust around a neutron sea.•Glitches happen when crust readjusts.•Binary neutron stars are rare.X-Ray Binary Stars•X-Ray bursters: irregular period•X-Ray pulsars: regular period, hotspots•Millisecond pulsars: don’t slow down (weird)Gravitational Waves from Neutron Stars•Intense gravity and fast orbit makes ripples in gravity.•Gravitational waves have not been directly observed.•Indirect observation in 1993 led to the Nobel Prize in physics (Einstein’s prediction that gravitational waves would dissipate energy, so the pair were slowing down and moving closer together)Black Holes and Escape Velocity•Stars more than 20 M⊙ will become black holes.•For an object with a large enough mass, the escape velocity is higher than the speed of light.•“Schwarzschild radius” Rs = 2GM/c2Nature of Space Around Black Holes•Objects with gravity distort the “fabric of space”•Black holes distort space-time so strongly they rip a hole in space-time.The Event Horizon•Schwarzschild radius also knows as the “event horizon”•Can’t see anything inside. Nothing from inside (not even light) can escape.•Can find mass of black hole from the strength of gravity.Formation of Black Holes•Same degeneracy process as for neutron stars. If degenerate material is more than 3 M⊙, the Schwarzschild radius is larger than the core’s size.•Some material goes straight into black hole —> less explosive “hypernova”•Any rotation and charge of the material that falls into the black hole is retained by the black hole.Indirect Observation of Black Holes•Accretion disc from companion star or other source. Heated to 10 million Kelvin.•Emits x-rays that we can observe.•If x-rays orbiting something with a mass >3 M⊙, then it can’t be a neutron star.Black holes in our neighborhood?More likely than you think.Hawking Radiation•Wein’s Law still applies to black holes.•Black hole with mass of sun: temperature is 6*10-8 K.•This is known as Hawking radiation (for Stephen Hawking)•Even black holes can
View Full Document
Unlocking...