1Massive Stars AfterThe Main SequenceExplosions, Neutron Stars andBlackHolesIt’s all about gravity……• Quiz #4 next class (wed April 2) covers Lectures 9-16• Exam #2 1 week from today (April 7th), coversLectures 9-16 (everything about stars).• “Sample exam”/review sheet posted soon.• Review sessions (Me on Wednesday 7-8 pm: Ellaon Thursday 7-8 pm)• Next assignment is posted (Lab #1 write-up, dueApril 14th (mon after exam).2Stellar Corpses• A type II supernova leaves behind the collapsed coreof neutrons that started the explosion, a neutron star.• If the neutron star is massive enough, it can collapse,forming a black hole…Interior Structure of a Neutron Star3Black HolesBlack Holes4The Escape Velocity Limit• The velocity necessary toavoid being gravitationallydrawn back from an object (theescape velocity) is:• Note that as R decreases, theescape velocity increases• A white dwarf’s escapevelocity is around 6000 km/s,ten times faster than the Sun’s• (a white dwarf is around 100times smaller than the Sun)• Also recall that nothing can travelfaster than the speed of light, c, or 3×108 m/s• If a stellar core is compressed somuch that its radius is smaller than(the Schwarzschild radius) thennothing can escape from itsgravitational force, including light!• The core would become a black hole! Vesc=2GMR! RS=2 " G " Mc2What would happen to the earth if thesun were instantly turned into a solar-mass black hole?• A) We would fall into the center of the BH.• B) We would instantly be turned into spaghetti.• C) We would immediately be vaporized.• D) Nothing, the Earth would continue to orbit the BH,(although it would be cold.)5Mass Warps Space• Mass warps space in its vicinity• The larger the mass, the bigger“dent” it makes in space• Objects gravitationally attractedto these objects can be seen asrolling “downhill” towards them• If the mass is large enough, spacecan be so warped that objectsentering it can never leave – ablack hole is formed.Black Holes• If a stellar core is massiveenough, it will not stopcollapsing when it becomesa neutron star.• The radius of the corecontinues to shrink, anddensity continues toincrease• Eventually the core iscompressed to a singlepoint, called a singularity.• Again, nothing can escapefrom this black hole. Oncesomething, including aphoton, crosses theSchwarzschild radius (orevent horizon), escape isimpossible.6Viewing a black hole• You may be asking, “If light cannotescape a black hole, how can we seeone?”• If a black hole is in orbit around acompanion star, the black hole canpull material away from it.• This material forms anaccretion disk outside ofthe event horizon andheats to high temperatures• As the gas spirals into theblack hole, it emits X-rays, which we can detect!Blackhole spectrum7Light curves from a black holebinary systemGeneral Relativity• Einstein predicted that notonly space would bewarped, but time would beaffected as well• The presence of mass slowsdown the passage of time,so clocks near a black holewill run noticeably slowerthan clocks more distant• The warping of space hasbeen demonstrated manytimes, including byobservations of the orbit ofMercury• The slowing of clocks hasbeen demonstrated as well!8Gravitational Redshift• Photons traveling away from a massive object willexperience a gravitational redshift.– Their frequency will be shifted toward the red end ofthe