LIVES AND DEATHS OF STARS
LIVES AND DEATHS OF STARS AST 309N
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This 2 page Class Notes was uploaded by Keegan Goyette on Sunday September 6, 2015. The Class Notes belongs to AST 309N at University of Texas at Austin taught by John Wheeler in Fall. Since its upload, it has received 70 views. For similar materials see /class/181755/ast-309n-university-of-texas-at-austin in Astronomy at University of Texas at Austin.
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Date Created: 09/06/15
Astronomy Bizarre Wheeler 309N Spring 2001 October 24 2001 46490 46495 Review for Test 3 SUPERNOVAE Historical Supemovae in the Milky Way 778 seen with naked eye in last 1000 years SN 1006 brightest SN 1054 now the Crab Nebula contains a rapidly rotating pulsar Tycho 1572 Kepler 1604 Cas A not clearly seen about 1680 shows evidence for jets and a dim compact object in the center SN 1987A rst obvious naked eye supernova since Kepler Our Galaxy contains about 200 known older supernova remnants 10000 to 100000 years old These are particularly notable by their radio emission The average rate of explosion is about once per 100 years in the Galaxy Extragalactic Supemovae many but dimmer more dif cult to study Type I supernovae no evidence for hydrogen in spectrum Type II supernovae de nite evidence for hydrogen in spectrum Type Ia Supemovae brightest no hydrogen avoid spiral arms occur in elliptical galaxies origin in lower mass stars Observe silicon early on iron later Unregulated burning explosion in quantum pressure supported carbonoxygen white dwarf of Chandrasekhar mass Star is completely disrupted no neutron star or black hole Type Ia must generate explosion in old 1 to 10 billion years stellar system Most plausible mechanism mass transfer onto white dwarf Spectra of Type Ia reveal intermediate elements on outside O Mg Si S Ca and ironlike material on inside Consistent with models of Chandrasekhar mass carbonoxygen white dwarfs that begin with a subsonic de agration and then ignite a supersonic detonation Identifying the binary evolution that makes a Type Ia has been dif cult Too much mass transfer will leave Hydrogen in the spectrum Nova explosions will reduce the mass of the white dwarf not grow it There may be too few white dwarf pairs too few recurrent novae and too few supersoft xray sources Type II Supemovae explode in spiral arms never occur in elliptical galaxies normal hydrogen massive stars recently born short lived Observe H early on O Mg Ca later Probably core collapse in ONeMg or iron core Light curve often shows month slong plateau Type Ib Supemovae no hydrogen but spectrum different in detail than Type Ia Observe helium early on O Mg Ca later Occur in spiral arms Probably core collapse Type Ic Supemovae no hydrogen little or no helium early on O Mg Ca later Occur in spiral arms Probably core collapse Type II supernovae are expected in red giants and are expected to leave behind a neutron star Explosions of massive stars in binary systems are expected to occur in a bare thermal pressuresupported core from which the outer layers of hydrogen have been transferred to the companion star The core will continue to evolve to iron even in the absence of the hydrogen envelope This is probably the origin of Types Ib and Ic Hypemovae a few recent supernovae seem to have ten times the energy of norma Type I and Type II Rate of explosion of Type 11 about one per 100 years in a galaxy like ours suggests they come from stars of about 8 to 20 solar masses These stars probably leave neutron stars Types Ib and Ic occur about as often as Type II probably come from roughly the same mass range Type Ic appear to lose their helium as well Types Ib and Ic are also expected to leave neutron stars Supemovae produce some carbon and all elements heavier than carbon mostly from stars with mass greater than 15 solar masses Above some mass perhaps 30 solar masses black holes may swallow all the heavy elements The collapse of the core a gravitational collapse causes essentially all the protons to be converted to neutrons releasing a ood of neutrinos and forming a neutron star The core collapse explosion of the outer layers of the star may occur in one of three ways 1 Prompt mechanism The neutron star rebounds driving a shock wave into the outer parts of the star 2 Delayed mechanism Neutrinos deposit heat behind shock and reinvigorate it 3 Jet mechanism the collapsing rotating neutron star squeezes the magnetic eld and sends a jet up the rotation axis Polarized light from supernovae the light from a supernova will not be polarized if the explosion is spherically symmetric All corecollapse supernovae measured to date Type Ib Ic and 11 show appreciable polarization and hence are not spherical They may be quotbread stickquot shaped or quotbagelquot shaped or some combination of elongation and attening Jetinduced explosions l l l 39 show that f 39 39 powerful jets can blow up a star The jets plow up and down along one axis creating a quotbread stickquot shape and driving bow shocks The bow shocks propagate away from the jets toward the equator where they collide The result of this collision is to blow much of the star out along the equator in a torus or quotbagelquot shape The nal con guration is far from spherical but has jets in one direction and a torus expanding at right angles to the jet This con guration is consistent with the polarization observations and perhaps with the rings images and spectra of SN 1987A Jet mechanism computer calculations show that rotation wraps up magnetic eld quotlines of forcequot causing the magnetic eld and trapped matter to be expelled up and down the rotation axis The generic phrase for this jet mechanism is the quottube of toothpaste effect It is an open question whether or not suf ciently strong jets to explode a star can be produced in this way when a neutron star forms but the Crab pulsar and other young pulsars show evidence of jetlike features Failed explosion if there is no core collapse explosion outer layers fall in crush neutron star maximum mass 23Me to form a black hole In some cases maybe Cas A SN1987A there might be an explosion and still leave behind a black hole Light curves brightness versus time of supernova Type Ia brightest Type Ib Type Ic Type II dimmer Light curves shock energy plus radioactive decay Ejecta must be large before transparent enough for light to leak out If too small originally Ia Ib Ic all shock energy goes into energy of motion light curve must be from radioactive decay Type Ia brighter needs more nickel than Ib Ic hence different mechanism a thermonuclear explosion of carbonoxygen not core collapse Type 11 show shock energy in plateau with evidence for radioactive decay at later time
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