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Astronomy Bizarre Wheeler309N Fall 2004 October 6, 2004(47855)Review for Test #2SUPERNOVAEHistorical Supernovae in the Milky Way - several seen and recorded with naked eye in last 2000 years.SN 386 earliest on record, SN 1006 brightest, SN 1054, now the Crab Nebula, contains a rapidly rotatingpulsar and suggestions of a jet. Tycho 1572, Kepler 1604. Cas A not clearly seen about 1680 showsevidence for jets, and a dim compact object in the center. The events that show compact objects also seemto show evidence of “elongated” explosions or “jets.”Extragalactic Supernovae - many, but dimmer, more difficult to study.Type I supernovae - no evidence for hydrogen in spectrum.Type II supernovae - definite evidence for hydrogen in spectrum.Type Ia Supernovae - brightest, no hydrogen, avoid spiral arms, occur in elliptical galaxies, origin inlower mass stars. Observe silicon early on, iron later. Unregulated burning, explosion in quantumpressure supported carbon/oxygen white dwarf of Chandrasekhar mass. Star is completely disrupted, noneutron star or black hole. Light curve shows peak lasting about a week.Type II Supernovae - explode in spiral arms, never occur in elliptical galaxies, normal hydrogen, massivestars, recently born, short lived. Observe H early on, O, Mg, Ca later. Probably core collapse in ironcore. Light curve often shows month’s-long “plateau.”Type Ib Supernovae - 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 Supernovae - no hydrogen, little or no helium early on, O, Mg, Ca later. Occur in spiral arms.Probably core collapse.Light curves of Type Ib and Ic are similar to Type Ia, but dimmer at maximum brightness.Type II supernovae are expected in red giants and are expected to leave behind a neutron star. Explosionsof massive stars in binary systems are expected to occur in a bare thermal pressure-supported core fromwhich the outer layers of hydrogen have been transferred to the companion star. The core will continue toevolve to iron, in the absence of the hydrogen envelope. This is probably the origin of Types Ib and Ic.Hypernovae - a few recent supernovae seem to have ten times the expanding motion energy of “normal”Type I and Type II, but they may just explode faster in some directions than others.Rate of explosion of Type II (about one per 100 years in a galaxy like ours) suggests they come from starsof about 8 to 20 solar masses. These stars probably leave neutron stars. Types Ib and Ic occur about asoften as Type II, probably come from roughly the same mass range. Types Ib and Ic are also expected toleave neutron stars.To burn a thermonuclear fuel, the star must get hotter to overcome the charge repulsion. This happensautomatically in massive stars supported by the thermal pressure that regulates their burning. These starsproduce shells of ever-heavier elements.Common elements produced in supernovae, carbon, oxygen, magnesium, silicon, sulfur, calcium, arebuilt up by adding “building block” of helium nuclei consisting of four particles, 2 protons and 2neutrons.Iron (with 26p and 30n) is endothermic, absorbing energy. This will reduce the pressure and cause thecollapse of the iron core to form a neutron star.The collapse of the core, a gravitational collapse, causes essentially all the protons to be converted toneutrons, releasing a flood of neutrinos and forming a neutron star.Repulsive nuclear force between compressed neutrons and neutron quantum pressure halt the collapse andallow the neutron star to form.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 thestar. This is known to occur, but to be insufficient to cause an explosion.2. Delayed mechanism: Neutrinos stirred out by the boiling neutron star deposit heat behind shockand reinvigorate it. Not clear this is sufficient.3. Jet mechanism: the collapsing rotating neutron star squeezes the magnetic field and sends a jetup the rotation axis. Naturally makes asymmetric explosion, but not yet clear sufficientlystrong jets are produced.Polarized light from supernovae - the light from a supernova will not be polarized if the explosion isspherically symmetric. All core-collapse supernovae measured to date, Type Ib, Ic, and II, showappreciable polarization and hence are not spherical. They may be “breadstick” shaped or “bagel” shapedor some combination of elongation and flattening.Jet mechanism - computer calculations show that rotation wraps up magnetic field “lines of force”causing the magnetic field and trapped matter to be expelled up (and down) the rotation axis. The genericphrase for this jet mechanism is the “tube of toothpaste effect.” It is an open question whether or notsufficiently strong jets to explode a star can be produced in this way when a neutron star forms, but theCrab pulsar, other young pulsars, and Cas A show evidence of jet-like features.Jet-induced explosions - Supercomputer computations show that sufficiently powerful jets can blow up astar. The jets plow up and down along one axis creating a “breadstick” shape and driving bow shocks.The bow shocks propagate away from the jets toward the equator where they collide. The result of thiscollision is to blow much of the star out along the equator in a torus or “bagel” shape. The finalconfiguration is far from spherical, but has jets in one direction and a torus expanding at right angles tothe jet. This configuration is consistent with the polarization observations.Jet mechanism – rotation will produce a dynamo amplifying magnetic fields. Rotation also twists themagnetic field with a natural tendency to create a jet-like flow of energy up and down the rotation axis.Failed explosion - if there is no core collapse explosion, outer layers fall in, crush neutron star (maximummass ~2M) to form a black hole.Type Ia - must generate explosion in old (1 to 10 billion years) stellar system. Most plausible mechanismmass transfer onto white dwarf.Spectra of Type Ia reveal intermediate elements on outside (O, Mg, Si, S, Ca) and iron-like material oninside. Consistent with models of Chandrasekhar mass carbon-oxygen white dwarfs that begin with asubsonic deflagration and then ignite a supersonic detonation.Identifying the binary evolution that


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