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1. Variable Stars Some Giants and Hypergiants exhibit regular periodic change in luminosity Mira (Fabricius 1595) changes by factor of 100 with period.

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Presentation on theme: "1. Variable Stars Some Giants and Hypergiants exhibit regular periodic change in luminosity Mira (Fabricius 1595) changes by factor of 100 with period."— Presentation transcript:

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2 Variable Stars Some Giants and Hypergiants exhibit regular periodic change in luminosity Mira (Fabricius 1595) changes by factor of 100 with period of 332d LPV like Mira not well modelled 2

3 Instability Strip A nearly vertical region traversed by most massive stars on HB RR Lyrae: PII HB stars with periods of hours. Luminosity varies little (!) Cepheids (PI), W Virginis (PII) periods of days. 3

4 Why They Pulse Cepheids oscillate in size (radial oscillation) Temperature and luminosity peak during rapid expansion Eddington: Compression increases opacity in layer trapping energy and propelling layer up where it expands, releases energy Problem: compression reduces opacity due to heating Solution: compression ionizes Helium so less heating. Expansion reduces ionization – κ- mechanism Instability strip has partially ionized Helium at suitable depth 4

5 Why We Care Leavitt 1908: Period-Luminosity Relation for SMC cepheids Luminous cepheids have longer periods With calibration in globular clusters cepheids become standard candles Later: W Virginis PLR less luminous for same period 5

6 White Dwarfs Bessel 1844: Sirius wobbles: a binary Pup hard to find. Clark 1846 Orbits: Spectrum (Adams 1915): Surface Gravity Spectrum: Very broad Hydrogen absorption lines Estimate: No Hydrogen else fusion 6

7 Degenerate Matter White dwarves are the degenerate cores of stars with Composition is Carbon Oxygen Masses Significant mass loss Chandrasekhar: Relativity: 7

8 Mass-Radius 8

9 Roche Potential In a binary system matter orbits both stars Entire system rotates. If dropped from (rotating) rest, where will a stone fall? Combined gravity and rotation described by Roche potential Inside each star’s Roche lobe orbits stay close to that star 9

10 Algol Eclipsing binary Algol is a puzzle: MS subgiant Massive A should have evolved earlier? B started out as the more massive star In its subgiant phase, atmosphere leaked out of its Roche lobe Gas lost by B forms accretion disk around A 10

11 White Dwarf Nova White dwarves in close binaries can accrete Hydrogen at from partner when it overflows its Roche lobe Infalling gas compressed to degeneracy and heated by immense surface gravity Enriched with CNO by turbulent mixing at base When accumulates, base temperature CNO fusion explosively heats gas to and luminosity Radiation pressure ejects accreted material Total energy released over months Can recur in Ejected matter glows at initial 30/yr in M31 11

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13 Nova Remnants 13

14 Type-Ia Supernova Accretion adds to white dwarf mass. What if it exceeds Chandrasekhar limit? It doesn’t. As increased mass compresses dwarf, pressure and temperature increase A turbulent convection phase leads to ignition of Carbon fusion In degenerate dwarf heating does not lead to expansion so violent explosive process fuses substantial fraction of star in a few seconds Oxygen fusion less complete Internal temperature exceeds Fusion releases blowing star apart completely releasing shock wave ejecting matter at high speeds Luminosity reaches and decays over months Spectrum has absorption lines of Si but little H He Decay of readioactive fusion products near iron mass in shell contributes to luminosity at late times 14

15 What We Know Nature of Mass donor unclear – Single Degenerate: Donor is MS or giant – Double Degenerate: Donor is White dwarf ripped apart by tidal forces in merger Likely both occur Nature of explosion also debated: deflagration or detonation? Degenerate Helium flash trigger or internal CO ignition Fact: Luminosity (corrected by light curve) almost the same for all Ia Supernovae: Standard Candles! 15

16 A Standard Candle 16

17 Post-MS Massive Star Massive stars end Main Sequence life When core Hydrogen fusion ceases core contracts and envelope expands and cools Shell Hydrogen fusion: Red Supergiant Core does not become degenerate 17

18 Massive Star HB Helium core ignites Hydrogen fusion in shell Envelope contracts and heats Blue Supergiant Forming CO core 18

19 Massive Star AGB CO core collapses until Carbon fusion produces Mg Ne O Helium and Hydrogen fusion in shells Many neutrinos carry energy off Superwind and mass loss 19

20 More Onion Shells At ignite Neon fusion – Produce O Mg… – Neutrinos carry off – Last a few years Oxygen fusion – Produce Si S P… – Neutrinos carry off – Last about a year Si fusion – Produce Ni Fe – Neutrinos carry off – Last about a day Build up inert Fe core Changes rapid. Envelope never responds s-process nucleosynthesis produces heavier elements 20

21 End of the (Si) Day Inert Fe core High T photons cause photodisintegration destroying heavy nuclei and absorbing energy Fe is the end: no more nuclear energy. What next? 21

22 The Center Cannot Hold As gravitational crush increases, iron core collapses from size of Earth to a few km in In core, emits ϒ rays leading to photodisintegration of heavy nuclei Outer layers fall inward at speeds up to As core collapses electron degeneracy overcome Electrons forced into Left with a small, incredibly dense core that is mostly neutrons Does collapse stop? 22

23 Boom! Within 0.25s core is neutrons with radius 20 km and super-nuclear density Very little light can escape, energy carried off by neutrinos. Power emitted in these exceeds all known stars for 10 s At this density core collapse stops with bounce Colliding with infalling layers this triggers shock wave blowing outer star into space (96% of mass for star) In compressed heated shock wave fusion to Fe and beyond via r- process As ejecta thin light can escape. Luminosity reaches Energy released type-II supernova – gravitational in origin 23

24 Seeing Them Sung dynasty history describes a supernova in 1054 whose remnant – Crab nebula in Taurus – is still visible (M1) Japanese, Arabic, Native American records concur Milky Way supernovae also in 1006, 1572, Estimated every 300 years but obscured by dust Many visible in other galaxies, currently some bright ones 24

25 SN 2011dh 25

26 Classification SN classified by spectrum: – Ia: Strong Si no H He – Ib: Weak H Strong He – Ic: Weak Si no H He – II: Strong H Ia are nuclear explosion of WD II Ib Ic are gravitational core collapse with degrees of envelope loss 26

27 168,000 years ago a B3 I supergiant collapsed in LMC Observed as SN 1987A Progenitor known – changed theory Remnants observed in detail 27

28 The Nebula 28

29 What we are Seeing 29

30 Neutrinos Three hours before the supernova detected, neutrino detectors observed a burst (20) of neutrinos from the right direction. 20 detected implies emitted carrying J in agreement with models Neutrinos get out before shock wave disperses outer layers, so got here before the light Neutrino Astronomy launched, many new experiments planned 30

31 What is Left of Core? Electron degeneracy cannot stop collapse – few electrons Neutron degeneracy pressure at density in Surface gravity Physics is relativistic Chandrasekhar Limit depends on rotation Rapid Rotation expected High magnetic field frozen in 31

32 Discovery Physics Predictions: – Rapid Rotation – Intense magnetic field – High Temperature Bell 1967: Periodic 1.337s Radio pulses: LGM? Quickly found other sources: natural Soon find many pulsars Slow down in 32

33 LGM Data 33

34 What are Pulsars? Rotating star breaks up Only NS dense enough to survive Emission aligned to magnetic axis - tilted 34 Crab pulsar : Neutron star SN remnant

35 How They Work General Idea: Rapidly changing intense magnetic field creates intense electric field Lifts charged particles from polar regions into magnetosphere dragged around by rotation Accelerated to relativistic speeds – emit synchrotron radiation at all wavelengths in direction of magnetic axis Emitted energy slows rotation Luminosity of Crab nebula agrees with observed rate of slowing of pulsar Pulsars observed in all bands 35


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