The Death of StarsToday’s Lecture:Post main-sequence (Chapter 13, pages 296-323)• How stars explode: supernovae!• White dwarfs• Neutron starsWhite dwarfs• Roughly the size of the Earth with the mass of the Sun!• If you try to pack electrons into the same place they must beat different energy levels (like the energy levels of an atom).Each electron must be at a higher energy than the one before it.• All these energetic electrons in one place give rise to apressure: ELECTRON DEGENERACY PRESSURE• This is weird stuff: one teaspoon of white dwarf weighs 3 tons!If a white dwarf is more massive, it actually has a smallerradius.• No nuclear reactions are taking place, the white dwarf justradiates its heat and continues to cool over time.• White dwarfs are sometimes used as age indicators inglobular clusters.Types of White Dwarfs• The Sun will become a carbon/oxygen white dwarf with amass of 0.6Msun.• Stars up to 8Msun become carbon/oxygen white dwarfswith masses up to ~1.1Msun.• Stars below 0.45Msun aren’t massive enough to burnhelium in their core and become helium white dwarfs.• Stars with masses from 8-10Msun have an extra stage ofburning in their core and make oxygen/neon/magnesiumwhite dwarfs with masses of ~1.2Msun.• White dwarfs have a mass limit 1.4Msun (theChandrasekhar limit), above which electron degeneracypressure can’t hold up the star.Mass exchange in binaries• Single stars evolve in a simple manner. Lifetime on themain sequence mostly depends on mass.• Most stars are in binary systems, allowing exciting thingsto occur: mass exchange!Once materialpasses this point,it flows onto thewhite dwarf.Cataclysmic Variables• If one star is a white dwarf and the other star fills itsRoche lobe (like a growing red giant), material canaccrete onto the white dwarf.• Angular momentumprevents material fromdirectly hitting the whitedwarf, forming an accretiondisk.• Cataclysmic variables arebright source in the blue andultraviolet.White dwarfAccretiondiskRed GiantNovae (this is the plural)• Cataclysmic variables undergo phases of brightening,called novae (Latin for “new star”)• Dwarf Novae: A rush of material flows through the disk,falling onto the white dwarf and releasing gravitationalenergy. Last a few days to weeks, and brightens by afactor of ~100.• Novae (or Classical Novae): Material that has built upon the surface of the white dwarf ignites in athermonuclear explosion. Only happens every 1,000 to100,000 years (need to build up enough material) andbrightens by a factor of ~1,000,000!Supernovae: exploding stars!• Previously “normal” star suddenly (few days to weeks)becomes much more luminous. Up to 10 billions timesbrighter than the Sun!• Rivals the entire galaxy in brightness for a few weeks!Fades over months to years.Two main classes:Type I: no hydrogen linesType II: hydrogen lines visible (in spectra)Also, Type I seen in all kinds of galaxies, while Type IIseen in spiral galaxies in star forming regions. Light curveshape and other differences as well.Spectra are differentLight curves are differentSupernovae and remnants• Supernovae produce remnants: expanding shells of gasrich with heavy elements.• Perhaps the most famous is the “Crab Nebula” from asupernova in 1054 AD. It was so bright, Chinese, Japanese,and Arab astronomers saw it for months during the day,and could be seen for 2 years at night.• The remnant merges with other gas and forms new stars.• Supernovae occur 1 to 3 times per 100 years per galaxy.• The last observed supernova in the Milky Way was in 1604(Kepler’s supernova). Are we overdue? Gas and dust mayhide supernovae on the other side of the Galaxy.Composition of our UniverseType Ia Supernovae• White dwarf in a binary system (white dwarf plus red giant or2 white dwarfs -- we’re not sure!)• White dwarf accretes matter and begins growing• When the mass of white dwarf exceeds 1.4Msun (Chrandralimit), there is a runaway chain of nuclear reactions.Heating happens --> Reactions get faster --> Pressure doesn’tincrease because of electron degeneracy --> more heating -->reactions get fast --> and so on!• Carbon and oxygen burn into heavy elements and areexploded out into space.• ~0.6Msun of 28Ni56 is produced, which is radioactive!28Ni56 --> 27Co56 --> 26Fe56 + lots of energyType Ia SNe are super important!• Most of the iron in our Universe is from Type Iasupernova.• Because all the Type Ia supernovae ignite at a similarmass (1.4Msun), they have similar luminosities: they arestandard candles!• They are really bright 5 billion times brighter than ourSun: so we see them at huge distances.• By comparing the apparent brightness with the intrinsicluminosity we can measure vast distances and measurethe shape of the space in between. This is the mainevidence that our Universe is dominated by Dark Energy.Type II Supernova: massive star• Massive stars (> 10 Msun)continue to burn fuel until iron (Fe)forms in the core. Fe is the mostbound atomic nuclei. No morereactions in the Fe.• The Fe core continues to grow.• When the Fe core reaches1.4Msun the core collapses.p + e- --> n + ν The core is converted into neutrons• The outer layers bounce off the core creating an explosion!(neutrinos also very important for driving the envelope away)• A neutron star is formed.Supernova 1987A (Type II)• Nearby! Only 170,000 light years away in the LargeMagellanic Cloud (a small “satellite” galaxy of the MilkyWay).• It initially was a 20 Msun star (but blue supergiant, NOTred -- this is a mystery, is it because of the low metallicity?Perhaps it was two stars merging?)• Neutrinos detected! (25 of them) Explosion mechanismwas core collapse and rebound. A neutron star wasprobably formed, but we still haven’t seen it -- this isanother mystery.• This supernova confirmed many of our ideas about howstars exploded, but also brought up many new questions.Supernova 1987A - energy output• Total energy (emitted in about 1 second) was comparableto the energy emitted in 1 second by ALL normal stars in theentire observable Universe!> 99% of the energy was in neutrinos< 1% was energy of motion of ejecta< 0.01% was in visible light• Supernovae are truly incredible explosions!• Gamma-rays with certain specific energies were seencoming from SN 1987A. This confirms that radioactive 27Co56(Cobalt) was produced.• Confirms that heavy elements are made in stars andexplosion and then dispersed
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