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Lecture Notes Astronomy March 17 & 19

by: Lindsay Notetaker

Lecture Notes Astronomy March 17 & 19 ASTRON 0089

Marketplace > University of Pittsburgh > Astronomy > ASTRON 0089 > Lecture Notes Astronomy March 17 19
Lindsay Notetaker
GPA 3.7
Stars, Galaxies and The Cosmos

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About this Document

These are my detailed lecture notes from class this week on March 17 and 19. I took everything down from her lecture notes, so I hope they're helpful!
Stars, Galaxies and The Cosmos
Class Notes
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This 4 page Class Notes was uploaded by Lindsay Notetaker on Friday March 20, 2015. The Class Notes belongs to ASTRON 0089 at University of Pittsburgh taught by in Spring2015. Since its upload, it has received 46 views. For similar materials see Stars, Galaxies and The Cosmos in Astronomy at University of Pittsburgh.


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Date Created: 03/20/15
317 Chapter 12 Star Formation and Evolution A Nova in a Binary system Hydrogen Ignition on a WD Life after death of a WD Examples semidetached binary semidetached binary with mass transfer overcontact binary Hydrogen can accumulate on the surface of the white dwarf if enough builds up it can reexperience the hydrogen shell burning phase However there s no upper star to hold the fusion in an explosive burning phase engulfs the surface and there s a huge explosion as material is ejected The body of the white dwarf barely cares and settles back down to receiving more hydrogen so novae can reoccur many times Electron degeneracy pressure can only support a mass less than 14 solar masses this is called the Chandrasekhar Limit So things could get out of hand for a white dwarf that s almost 14 solar masses that s being fed more hydrogen Detonating a White Dwarf A Type Ia Supernova Carbon Detonation Supernova l 2 Spectra of these Supernovae don t show hydrogen or helium lines The more massive member of a pair of Sunlike stars exhausts its fuel and turns into a whitedwarf star The white dwarf sucks in gas from its companion eventually reaching a critical mass A ame runaway nuclear reaction ignites in the turbulent core of the dwarf The ame spreads outward converting carbon and oxygen to radioactive nickel Within a few seconds the dwarf has been completely destroyed over the following weeks the radioactive nickel decays powering the bright glow of the debris Type Ia Supernova are much brighter and completely different from nova explosions but both involve a white dwarf Evolution of Stars More Massive than the Sun Stars more massive than the Sun follow very different paths when leaving the Main Sequence Mass Loss from a Supergiant Star Supergiants lose mass at a rapid rate in a strong stellar wind even while on the main sequence As this wind collides with the surrounding interstellar gas and dust it creates a bubble Highmass stars like all stars leave the Main Sequence when there s no more hydrogen fuel in their cores The rst few events are similar to those in lowermass stars 0 First a hydrogen shell then a core burning helium to carbon surrounded by helium and hydrogenburning shells High mass stars burn hydrogen with the CNO cycle not the pp chain and there is no Helium ash The protonproton cycle isn t the only path stars take to fuse hydrogen to helium for stars with masses gt 4 solar masses core temps exceed 20000000 K at these temps the CNO cycle dominates Hydrogen is converted to Helium via a path including carbon the carbon12 nucleus acts as a catalyst C 4 H gives C He neutrinos energy The positrons encounter electrons and are annihilated this produces more energy For stars more massive than 4 solar masses Very high central temps and pressures mean that electron degeneracy pressure won t stop the collapse of the star and elements heavier than C and O are produced Electron degeneracy pressure can only support a mass lt 14 solar masses If the star s CO mass is greater than this then gravity takes over and C begins to burn when the temperature reaches 600 million K Carbon12 Helium4 9 Oxygenl6 this is more likely Or Carbon12 Carbon12 9 Magnesium24 Every successive element is formed by adding on a helium4 nucleus 16 Oxygen 4 Helium 9 20 Neon energy Carbon can fuse either with itself or with a helium nucleus to form more nuclei Evolutionary Stages of a 25 solar mass star Each successive element has smaller returns from fusion it has to burn faster and more vigorously to hold up the star In the stability of nuclei iron is the crossing point when the core has fused to iron no more fusion can take place On the left of iron nuclei gain energy through fusion On the right they gain it through fission Iron56 is the most stable nucleus so it neither fuses nor decays Iron is very stable energy isn t produced in nuclear reactions with iron but is used up For example iron Fe absorbs energy gamma rays and breaks up into helium nuclei and neutrons y 56Fe 9 l3 4He 4n The Beginning of the End Now there is no pressure left to hold up the star gravity takes over As the core collapses energy isn t produced The outer layers of the star lose their pressure support they begin to implode falling inward due to the immense gravity Catastrophic collapse follows in a fraction of a second Formation of a Neutron Core As the core gets denser protons and electrons will combine with one another to become neutrons p e 9 n neutrino Neutrons are more massive than protons so this takes some energy Emc2 cooling the core more reducing pressure and accelerating the collapse With the charged particles gone electrical repulsion is now a nonissue The neutrinos very low mass neutral particles mostly escape just like the ones produced in fusion The neutrons offer rapidly increasing resistance to further compression neutron degeneracy pressure kicks in and stops the collapse By the time the collapse can stop densities are enormous 10Al8 kgmA3 but the outer layers are still falling in The imploding outer layers rebound off the nowrigid core in an enormous explosion helped by the neutrinos pouring out of the core a Type 11 core collapse supernova ensues blowing up most of the star A CoreCollapse Supernova l 2 3 As a massive star nears its end it takes on an onionlayer structure at this point in its evolution the star is hundreds of millions of kilometers in radius Iron doesn t undergo nuclear fusion so the core becomes unable to generate heat the gas pressure drops and overlying material suddenly rushes in Within a second the core collapses to nuclear density inwardfalling material rebounds off the core setting up an outwardgoing pressure wave Neutrinos pouring out of the developing neutron star propel the shock wave outward unevenly The shock wave sweeps through the entire star blowing it apart Elements heaver than iron are created inside supernovae by neutron capture reactions The pathways by which heavier elements are created are quite complicated Luminous Supernovae Maximum luminosity as great as 10A9 solar L rivaling the light output of an entire galaxy for a brief period The Tarantula Nebula in the LMC is 51000 pc from Earth but was the SN bright enough to be seen without a telescope An Unusual Supernova SN 1987A appears to have a set of 3 glowing rings Relics of a hydrogenrich outer atmosphere ejected by gentle stellar winds from the star when it was a red supergiant The gas expanded in an hourglass shape bc it was blocked from expanding around the star s equator by a preexisting ring of gas These rings were ionized by the initial ash of ultraviolet radiation from the supernova Outer ring at edge sweptup gas from earlier mass loss Inner ring sweptup redsupergiant gas Supernova remnant dark invisible outer portion surrounding the brighter inner region lit by radioactive decay A Supernova in a Distant Galaxy Type I vs The progenitor star that later exploded into SN 1993J was a K0 red supergiant SN 1993J resulted from the core collapse and subsequent explosion of a massive star Type II Supernovae Type I Supernova binary star system 9 white dwarf amp planetary nebula 9 growing white dwarf from the red giant 9 detonation Type II Supernova He C and H normal star fusion 9 massive star imploding 9 core rebound 9 explosion Telling the Supernova Apart Type II Supernova massive stars collapsing due to gravitational energy 0 A onetime event releasing metals and emitting a hydrogen spectrum Type Ia Supernovae white dwarfs exploding 100 of their mass into space 0 A onetime event releasing oxygen carbon and silicon with almost no hydrogen spectrum Supernova remnants material blown out by a supernova an expanding cloud of material from the explosion Gum Nebula our supernova neighbor exploded around 9000 BC 0 At max brilliance the exploding star probably was as bright as the Moon at first quarter like the firstquarter moon it would ve been visible in the daytime Seeing Supernova Many supernova remnants are invisible at the visible wavelengths our human eyes can see however when the expanding gases collide with the interstellar medium they emit energy at a wide range of wavelengths from X rays through radio waves This is a radio image of the supernova remnant Cassiopeia A as a rule radio searches for supernova remnants are more fruitful than visiblelight searches Supernova of the Past Historically supernovae were recorded as guest stars giving us clues as to where old cores may be found Some of these cores are neutron stars or pulsars


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