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Review of Stars’ Lifetime • Main sequence – all stars Chapter 20 • Converting hydrogen to helium • Low mass (<0.4 MSun) • Ends as inert ball of helium • Gradually releases all its energy Stellar Evolution: The Death of Stars 1 2 Review, cont. Medium Mass Stars • Medium mass (0.4 to 4 MSun) • Helium fusion begins in the core • Helium flash if < 2MSun • Consume only the hydrogen in their cores • Then leave the main sequence • Hydrogen shell burning • Luminosity decreases a bit • Star expands and cools • As rate of burning decreases, the star’s outer layers contract • This causes an increase in temperature • Horizontal (red giant) branch • New energy causes star to enlarge • Becomes a red giant 3 4 Asymptotic Giant Branch (AGB) HR Diagram • The helium in the core is converted into carbon and oxygen • Once the helium is exhausted, the core contracts until stopped by degenerate electron pressure 5 6 1 AGB, cont Helium Shell Burning • A thin shell of helium around the core will begin to undergo fusion • The star enters a second red giant stage • The expansion of the star’s outer layers eventually cause the hydrogen shell burning to stop • Carbon ash is accumulating in the core • Helium is “burning” in a shell surrounding the carbon • Hydrogen is still burning in a shell surrounding the helium 7 8 Stages Shown by a Cluster AGB Structure • The central part (right) is about the size of the Earth 9 10 Dredge-Ups Dredge-Ups, cont • During the final stages of a star’s lifetime, convection cells can reach deeper into the star • First red giant stage • Helium flash • A second dredge-up brings more carbon, nitrogen and oxygen • Third (if > 2 MSun) during AGB • The material produced by the NCO cycle can be brought to the surface • Increases the amount of carbon, nitrogen and oxygen 11 • Large amounts of carbon • Star’s spectrum is heavy in carbon elements • Called a carbon star • Ejects large amounts of material 12 2 Carbon Star, Example Planetary Nebula • Strong solar winds • Relatively low surface temperatures • A solar mass star does not have enough mass to start fusing other materials • Intense burning of the hydrogen and helium layers cause pressure that causes its outer layers to escape • During the AGB stage, the star divests itself of its outer layers • About 3000 K • Ejected material often resembles soot 13 14 Examples of Planetary Nebula Planetary Nebula, cont • The star ends up with two distinct parts • The cool envelop of dust and gas that has escaped • The hot core • Still very luminous • The core can illuminate the cloud of gas and dust • This forms a planetary nebula 15 16 Matter White Dwarf • The planetary nebula will continuing expanding into space carrying many different molecules with it • This is the origin of the “heavier” elements • As the envelop of the red giant disperses, the core becomes visible • It is small, but luminous • It is a white dwarf • “Naked” white dwarfs have lost their envelops (or at least they are so dispersed they are invisible) 17 18 3 Characteristics of White Dwarfs Mass-Luminosity Relationship • Cool, but do not shrink • Electron degeneracy creates pressure not dependent on temperature • Mass-radius relationship • The more massive the white dwarf, the smaller it is • The more degenerate matter you have, the smaller it becomes 19 20 Sample Evolutionary Tracks Chandrasekhar Limit • There is a limit to the amount of pressure the degenerate electrons can produce • Therefore, there is an upper limit to the mass that a white dwarf can have • This limit is the Chandrasekhar limit • 1.4 Msun 21 White Dwarf Example – Sirius B 22 More White Dwarfs • Characteristics: • Mass 1.1 MSun • Radius 0.008 rSun • Luminosity 0.04 LSun • Surface Temp 24,000 K • Average Density 3 x 10 kg/m3 23 24 4 Another White Dwarf Binary White Dwarfs 25 Black Dwarf 26 Sun’s Evolution • The white dwarf continues to cool • No nuclear reactions, only stored heat • The heat dissipates • Eventually it becomes a cold, burnt out ember • A black dwarf • Not much compression, just cooling 27 28 Other Types of White Dwarfs Another Type of White Dwarf • White dwarfs produced by stars of ~ 1 solar mass contain mainly carbon and oxygen • Helium White Dwarfs • Neon-oxygen white dwarf • Very low mass stars may never reach helium fusion • The star’s envelop can still be ejected, leaving behind a white dwarf composed mainly of helium • None have been detected that have formed this way • Some have been found in binary systems 29 • A more massive star could reach temperatures high enough for the oxygen to combine with the helium and form neon • When fusion stops, the neon-oxygen white dwarf would be left • The star would need to be near the 8 solar mass maximum that form carbon cores 30 5 Evolution of High Mass Stars (> 4 MSun) Core Contraction • As high mass stars leave the main sequence, they tend to move horizontally along the giants branch instead of vertically • They “burn” successively heavier elements • More massive than Chandrasekhar limit • Electron degeneracy cannot stop the contraction of the core • Enters a new round of fusion • Exactly which elements will depend on their mass 31 HR Diagram Tracks High Mass Star Comparison 32 HR Diagram – Massive Stars • The 4 Solar Mass star will burn carbon then stop • Higher masses will burn more elements 33 Fusion Sequence 34 Red Supergiant • As the star moves back and forth in the red giant area of the HR diagram, • Its luminosity decreases • Its radius increases • It reaches the red supergiant stage 35 36 6 Mass Ejections from Supergiants Shell Burning 37 V838 Close Up 38 Betelgeuse 39 40 Shell Burning in Massive Stars Red Supergiant Group • There is no noticeable different in the appearance of the star when each new element starts to burn • At the supergiant stage • All nuclear fusion stops at iron 41 42 7 Evolution and Mass – Summary Evolution and Mass – Summary, cont • The Russell-Vogt Theorem • 0.4 M < M* < 1.2 M – Life-cycle like the Sun • The most important determinant for the life-cycle of a star is its mass • Hydrogen burning by p-p chain • Helium burn to carbon • M* < 0.01 M - Planet • 0.01 M < M* < 0.085 M - Brown Dwarf • 0.085 M < M* < 0.4 M – long-lived, low main-sequence • M* > 1.2 • Hydrogen burning by CNO cycle • End as helium white dwarfs 43 Evolution and Mass – Summary, Final 44 Core Collapse • M* > 8 M • Will have a larger number of nuclear burning cycles • Core Mass of M ~ 1.4 solar masses • Electron degeneracy • Largest mass for white dwarf • As the iron builds up, fusion ceases • Gravity overcomes the thermal radiation pressure • Core collapses • Some estimates are collapse in less than 1 sec • Photodisintegration occurs • The photons have enough energy to divide the iron atoms • Continues until protons and neutrons are separated • End their lives with a supernova. 45 46 Core Collapse Supernova Core Collapse, final • Core consists of protons, neutrons, electrons and photons • High densities cause the electrons and protons to form neutrons • Releases neutrinos • Escape carrying energy with them • Collapse stops when the neutrons cannot be forced closer together • Rebounds and releases energy in a core-collapse supernova 47 48 8 Supernovae Type II Supernova • Types of Supernovae • Caused by core collapse • Type I • Details were discussed previously • Type Ia – include strong absorption lines of ionized silicon • Type Ib – lack silicon lines, but have strong helium absorption lines • Type Ic – have neither silicon or helium lines • Spectra shows a lot of hydrogen and helium • Out shells were blown off during the supernova • Type II • Core collapse of a massive star • Have strong hydrogen absorption lines 49 Type II Example – SN 1987 A 50 SN 1987 A 51 SN 1987 A 52 SN 1987 A Today 53 54 9 Another Example – SN 1993J SN 1987 A 55 56 Type Ia Supernova Type Ia Supernova, cont • A white dwarf’s mass tends to increase with each nova cycle • If the 1.4 solar mass limit is exceeded, the star starts to collapse • The temperature increases to the point where carbon can fuse • Not all the accumulated mass is ejected • This is occur in all parts of the star ~simultaneously • If a mass > 1.4 solar masses is accumulated, then another process occurs • A carbon-detonation supernova results 57 Type I Supernova, final 58 Type Ia – Summary • Some theories indicate other paths to the same result • Collisions between white dwarfs, for example • Spectra shows virtually no hydrogen • The white dwarf was from a low mass star and so the system already lost all of its hydrogen 59 60 10 Type Ib and Ic Supernovae Supernovae Comparison • Also core collapse • Progenitor stars have already lost outer layers • Accounts for lack of hydrogen spectral lines 61 Supernovae Comparison 62 Comparison, cont • Type Ia and Type II supernovae are unrelated to each other • All high mass stars will undergo a Type II supernova • Some low mass stars will become white dwarfs that will experience Type Ia supernovae • The two types occur at approximately the same rate 63 Crab Nebula (top) Vela Supernova Remnants (bottom) 64 More SN Remnants 65 66 11 Even More SN After the SN? • Examples of • Neutron stars • Black holes • G292.0+1.8 • Crab • SN 1987 A 67 68 The Cycle of Stellar Evolution • Star formation is cyclical: stars form, evolve, and die • In dying, they send heavy elements into the interstellar medium • These elements then become parts of new stars 69 12