1ASTR 1020: Stars & GalaxiesMarch 5, 2008• Reading: Chapter 18, section 18.3; summary of key concepts.• MasteringAstronomy Homework on Star Death is due March 10th.• Exam 2 on Friday, March 14th(Chapters 15.3-19.2).• Meet Friday at Fiske Planetarium for “Dr. Einstein’s Universe!Astronomy In the NewsVarsha ShirhattiChandra VideoThe Stellar GraveyardLow mass stars Æ white dwarfsgravity vs. electron degeneracy pressureHigh mass stars Æ neutron starsGravity vs. neutron degeneracy pressureEven more massive cores Æ black holesGravity wins…..White Dwarfs• For solar-mass star, a hot core of carbon(can also be oxygen for higher mass stars)Size ~ Earth !! Density – 1 cm3weighs about 5 tonsCool from white-blue through red to blackToday: Neutron Stars• Gravity vs. Neutron degeneracy pressure • Size ~ 10 km !!Crushing gravity at its surfaceNeutron star over NYC• Supernova remnant (386 AD) and pulsar, seen in X-ray light2Pulsars• Collapse to a neutron stars increases both rotation and magnetic field• Newly collapsed neutron stars rotate 100s to 1000s of times per second• Magnetic fields focus energy/radiation along magnetic polesNew form of light =synchrotron radiation• When the “beam”sweeps across the Earth, we see a pulsar• Earth lies at the unique intersection of many pulsar beams– use these as galactic pointers to our locationPioneer 10 spacecraft panel- now past PlutoSynchrotron Radiation• Fast electrons in strong magnetic fields Æ neutron stars, black holes• Different shape from thermal radiation: strongest emission in radioClicker Question• The coolest objects in the galaxy are at about 29 K, and the hottest stars are at 29,000 K. At what wavelengths do synchrotron radiation dominate?Wien’s law: wavelength = 2,900,000 nm / Ta) less than 0.1 mm, more than 1000 nmb) less than 0.1 mm, more than 100 nmc) less than 1 mm, more than 1000 nm1 mm = 0.001 m = 1,000,000 nm3a) Cold Wavelength = 2,900,000/29 =100,000 nm= 0.1 mmThis is in the far IR, near the edge of radio.b) Hot Wavelength = 2,900,000 / 29,000= 100 nmThis is in ultraviolet light.c) So, this is answer – less than 1mm, more than 1000 nmThis is the radio part of the spectrum.• Really stands out in radio and X-ray where there is little thermal radiation• Visible light versus X-rays show stars versus “collapsed objects”Not matched scales!Normal starsNeutron stars, black holesVisible light vs. Radio Thermal versus SynchrotronObserving Pulsars• Jocelyn Bell: Cambridge graduate student in 1967 discovered pulsars by accident from an early radio telescope•LGM’s?Pulsar “Lighthouses” don’t actually pulse• Must be very compact object to spin so fast• Spin slows down gradually (thousands of years)4Neutron Stars in Binary Systems• Mass transfer:• Gravitational potential energy Æ X-ray radiation emission X-Ray Binary system, X-ray burstersMatter falling through the spinning disk can spin UP the pulsar!Visible versus X-ray• Thermal light from stars Æ visible and IR• Synchrotron light from neutron stars Æ X-ray and radioVisible lightX-ray lightWhen the mass is too great….• For even neutron degeneracy to hold up, supernova core collapses to an infinitely small point• Æ Black Hole:Next class at FiskePlanetarium on “Dr. Einstein’s
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