Wednesday, Oct. 29Syllabus, class notes, and homeworks are at:www.as.utexas.edu courses AST 301, LacyReading for this week: chapter 11The Wednesday help session is in GRG 424 at 5:00 (forthe entire semester).Our ScheduleAug 27: Ch 1+App A The Scale of the CosmosSep 3: Ch 2+3 The Sky, Cycles in the SkySep 8: Ch 4 The Origin of Modern AstronomySep 15: Ch 5 TelescopesSep 19: Exam #1, Ch 1-5Sep 22: Ch 6 Starlight and AtomsSep 29: Ch 7 The SunOct 6: Ch 8 The Family of StarsOct 13: Ch 9 The Formation and Structure of StarsOct 20: Ch 10 The Lives and Deaths of StarsOct 24: Exam #2, Ch 6-10Oct 27: Ch 11 Neutron Stars and Black HolesNov 3: Ch 12 The Milky Way GalaxyNov 10: Ch 15 CosmologyNov 17: Ch 16 The Origin of the Solar SystemNov 24: Ch 17 The Terrestrial PlanetsDec 1: Ch 18 The Outer Solar System Dec 5: Exam #3, Ch 10-12,15-19Topics for this weekCompare the two types of supernova: how do they differ inthe cause of the explosion and in what is left behind?Describe neutron stars.Describe pulsars.Why do neutron stars rotate so quickly?Why couldn’t white dwarfs or other stars rotate as quickly?Define ‘escape speed’.Describe black holes.What evidence do we have that there is a very massiveblack hole at the center of the Milky Way?The collapse of a massive starStars more massive than about 8 Msun can go through aseries of fusion reaction leading up to iron.Fusion of iron absorbs energy instead of releasing it.That makes the Fe core unstable, and it collapses.As the core collapses, electrons fuse with protons in the Fenuclei: p+ + e- n + νBut this reaction absorbs even more energy, decreasingthe pressure even more.The collapse stops when the density reaches that of anatomic nucleus, and the core is made of neutrons.Neutron degeneracy pressure (and repulsion betweenneutrons when so tightly packed) stops the collapse.The dense ball of neutrons is a neutron star.Type II SupernovaThe shells fall onto the core and also fuse to makeneutrons.The envelope falls onto the neutron core and bounces.It is thrown off at speed as high as 1/10 the speed of light.The hot exploding gas can emit as much light as 1011 starsfor a few days.This happened in the Large Magellanic Cloud (a group ofabout 108 stars about 60,000 pc from here) in 1987.Testing the theoryWhat kind of a star was it that exploded in the LargeMagellanic Cloud?We can see the star on photographs taken before theexplosion. We know it’s the right star, since it’s not therein photos taken after the explosion.It was a luminous blue star, near the top end of the mainsequence.Is this what the theory predicts?The discovery of pulsarsJocelyn Bell, a student in England, was observing ‘radiostars’ with a radio telescope in 1967.She noticed that one of the stars seemed to flickerregularly.Perhaps jokingly, they at first thought it was a signal froman extraterrestrial civilization, but soon other stars like itwere found, and they concluded that it was a naturalphenomenon.(The professor she was working for got the Nobel Prize forthe discovery.)How can a star flash 30 times a second?Even if the Sun could turn on and off in 1/30 second, itsradius is about 2 light-seconds, so it wouldn’t appear tous to all turn on and off together.White dwarfs are small enough to avoid this problem, butwhat could make them flash?We know of pulsating stars that vary in brightness byvarying in size, but they take minutes to years to vary.They also don’t turn off between flashes like pulsars do.Can a star rotate 30 times per second?A star can’t rotate faster than the time for a satellite wouldtake to orbit near its surface.Otherwise the gas near the surface of the star would gointo orbit.So the Earth can’t rotate in less than 90 minutes.The Sun can’t rotate in less than 3 hours.A white dwarf can’t rotate in less than about 10 seconds.But neutron stars are so compact that they can rotate 1000times a second without flying apart.They also have strong magnetic fields to direct theirbeacons.Density of matter in a neutron starDensity = mass / volumeThe density of the Sun is about equal to the density of water,and the mass of a neutron star is somewhat more than themass of the Sun.The volume of a neutron star is about 1015 times smaller thanthe volume of the Sun.How does the density of a neutron star compare to thedensity of the Sun?Why do neutron stars rotate so fast?What happens when an ice skater goes into a spin andthen pulls his hands in?Or what happens to a planet orbiting the Sun if its orbittakes it from far from the Sun in closer to the Sun?The Sun is rotating, with its surface moving at about 1km/sec.If the Sun suddenly collapsed to the size of a neutron star,about 105 times smaller than it is now, and gas on thesurface of the Sun followed an elliptical path going 105times closer to the center of the Sun, how fast would itgo?How fast could a collapsed star rotate?If the Sun suddenly collapsed to the size of a neutron starits surface would be moving at 105 km/sec.Neutron stars don’t actually rotate this fast because theylost some of their angular momentum when they werered giants.Orbital speed around a neutron starWe can use Newton’s version of Kepler’s 3rd law tocalculate the speed that an object would have whenorbiting a neutron star. The formula is:For a mass of 2 Msun and an orbital radius of 10 km, theorbital speed is about 100,000 km/sec.This is 1/3 the speed of light.aGMvstarorbit=Escape speedTo leave Earth orbit and go to the Moon, the Apolloastronauts had to fire their rockets to increase theirspeed to about 1.4 times the orbital speed.For a neutron star with M = 2Msun, vescape ~ 0.45 cIf a neutron star had a mass of about 4 Msun, its gravitywould make it smaller than 10 km, and its escape speedwould be greater than the speed of light.Neutron stars with masses as big as 4 MSun can’t exist.RGMvstarescape2=RelativityEinstein showed that Newton’s laws aren’t valid whenobjects move at speeds near the speed of light.When an object moving at nearly the speed of light is givenenergy it doesn’t go much faster. Instead it gets moremassive.He also showed that it is better to look at gravity not as aforce, but as a distortion of space around massiveobjects, making objects that come near massive objectsfollow curved paths.That is his explanation for the fact that Galileo’s two ballsfell together. They were both following the natural paththrough curved space.Type I supernovaeIf two stars form together, orbiting each
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