1) Review: main sequence lives of low mass stars Red Giant formation 2) Hotter, more Massive stars CNO cycle nucleosynthesis – creation of the heavier elements 3) Supernova 4) Split class: “the life and times of Albert Einstein” (Planetarium) Today’s Lecture: purpose & goals Supernova 1987a Einstein Memorial, National Academy of Sciences, Washington D.C is the Einstein Centennial! Stellar Midlife/Death – large stars “live fast, die young” AST 1002 Planets, Stars and Galaxies

Review t MS = 1/M 2.5 x years Stellar Lifetimes Most luminous stars live shortest lives also, largest (most massive) stars M  Blue giants  few tens of thousands of years Very few recent young still here. Less luminous stars live longer which are the less massive, smaller stars M  Red dwarf stars  over a trillion years ALL still here!! (age of the universe 15 billion years) Formation of Red Giants remember: Stefan-Boltzmann Law: L = 4  R 2.  T 4 These are the same star at different stages of life!!

Review2: Lives of Less Massive Stars Low mass stars follow a pattern: protostar main sequence star stable when gravity and pressure balance. star dies when hydrogen used up at core. Small stars no  degenerate helium ashe. red giant hydrogen shell and helium core burning  shrinks core which gets hotter; expands outer surface which gets cooler  high luminosity Red giant Planetary nebula Blow off outer surface (stellar wind) leaving behind bare core  white dwarf (no fusion, very hot, very small  low luminosity) Binary stars accretion onto white dwarf  novae (only fuse surface, recurrent),  or type Ia supernovae (collapse and fusion of entire 1.4M  white dwarf)

Hotter Stars More massive stars: Gravity is stronger! so core is denser, with higher pressure These stars are hotter! Hotter stars burn faster! More interesting stuff happens… supernovae (again… but different) neutron stars black holes variable stars

Massive Core Burning Core burns differently in massive stars Core is denser, and hotter over a larger region Burns faster! Convection stirs core mixes elements Converts all of core to helium Not just the very center Core not degenerate Then core starts burning helium Hydrogen burning shell appears Start of multiple-layer burning in star Still basically a main sequence star (process stars while hydrogen still burning at core)

CNO Cycle – More Burning Carbon-Nitrogen-Oxygen (CNO) 12 C H  12 C + 4 He occurs if carbon and hydrogen are together and hot enough (>15,000,000K) needs star to be at least ~1.5 solar masses to be main power source

Nucleosynthesis If temperatures get high enough, additional fusion reactions are available H  He  C  O  Ne  Si  Fe Hotter stars produce heavier elements Each fusion stage produces energy each stage goes faster than previous Iron (Fe) does not burn! needs energy into reaction rather than giving up energy VERY unstable run-away process!! e.g. Betelgeuse, eta Carina

A Giant Onion Degenerate Fe core During its life, a massive star burns each step progressively outward Shells form, w/innermost shells burning heavier elements Fe Si O Ne Elements between He and Fe created in latter stages of massive stars life. Fe cannot fuse and give off energy  the next step will be a catastrophe  Core collapse and Type-II Supernova C He H

Supernovae – Type II Type II At some point the pressure in the core is unable to balance the gravitation- al compression, and the iron core collapses, leaving a void. The outer layers of the star crash onto the core, crushing it further and heating it to extreme temperatures. Core Collapse The collapse of the degenerate iron core fuses every proton with an electron to form a neutron and a neutrino.

Supernovae – Type II,… Neutron Star As the outer layers expand the neutron core continues to be crushed. neutron star Left behind an unimaginably dense object  a neutron star. (>1.4 All elements heavier than Fe are created only in supernovae! Your gold ring is a souvenir of a supernova!! M  ) Black Hole For the most massive stars, the neutron star is unstable black hole and is predicted to collapse to a black hole. (>3.0 M  ) All!!

SN1987A 1987 Supernova In 1987 a fantastic milestone occurred in astronomy. For the first time in history scientists were able to record the neutrino shockwave from a Type II supernova. The supernova occurred in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way, that lies about 160,000 light years from us.

Neutron Stars and Pulsars Neutron Stars: calculated properties Radius: 10 km Radius: about 10 km. Density:10 4 g/cm g/ cm 3 Density: from 10 4 g/cm 3 at the surface to about g/ cm 3 at the center. This is about a billion times denser than a white dwarf! Structure: Structure: the star is a very smooth, rapidly spinning, rigid metallic shell filled with a neutron super-fluid, a fluid that moves without resistance. strong magnetic fields! very strong magnetic fields! pulsars. observable as pulsars. Chandrashekar limit: e.g. Crab nebula

Heavier  becomes a black hole

Nucleosynthesis Some typical times and temperatures and rates of reactions Stage 9 solar masses Temperature (K) H burning 20 million years (3-10)x10 7 He burning 2 million years (1-7.5)x10 8 C burning380 years( )x10 9 Ne burning 1.1 years( )x10 9 O burning8 months( )x10 9 Si burning4 days(2.8-4)x10 9 8

Betelgeuse, eta Carina Betelgeuse eta Catrina iron  In massive stars, like Betelgeuse and eta Catrina, nuclear synthesis of heavier and heavier elements occurs up to the element iron. endothermic  The reaction that creates iron is endothermic (it cools the surroundings), which decreases the core’s pressure.  This process is very unstable!! 9

Supernovae – Type II,… Core Collapse The outgoing neutrinos carry energy outward but do not push against further graviational collapse, accelerating it. Neutrino Shockwave kg The mass of the collapsed core is of order kg kg Each proton has a mass of approximately kg / So the number of protons converted to neutrons is about / = 10 57, equal to the number of neutrinos that rush outwards from the collapsed core at the speed of light. 12