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Variable Stars - PowerPoint PPT Presentation

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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. Instability Strip. A nearly vertical region traversed by most massive stars on HB

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Variable Stars

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variable stars
Variable Stars
  • Some Giants and Hypergiants exhibit regular periodic change in luminosity
  • Mira (Fabricius 1595) changes by factor of 100with period of 332d
  • LPV like Mira not well modelled
instability strip
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.
why they pulse
Why They Pulse
  • Cepheidsoscillate 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
why we care
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
white dwarfs
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
degenerate matter
Degenerate Matter
  • White dwarves are the degenerate cores of stars with
  • Composition is Carbon Oxygen
  • Masses
  • Significant mass loss
  • Chandrasekhar:
  • Relativity:
roche potential
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
  • Eclipsing binary Algolis a puzzle:



  • 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
white dwarf nova
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
type ia supernova
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
what we know
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!
post ms massive star
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
massive star hb
Massive Star HB
  • Helium core ignites
  • Hydrogen fusion in shell
  • Envelope contracts and heats
  • Blue Supergiant
  • Forming CO core
massive star agb
Massive Star AGB
  • CO core collapses until
  • Carbon fusion produces Mg Ne O
  • Helium and Hydrogen fusion in shells
  • Many neutrinos carry energy off
  • Superwindand mass loss
more onion shells
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
end of the si day
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?
the center cannot hold
The Center Cannot Hold
  • As gravitational crush increases, iron core collapses from size of Earth to a few km in
  • In core,emits ϒrays 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?
  • 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 beyondvia r-process
  • As ejecta thin light can escape. Luminosity reaches
  • Energy released type-II supernova – gravitationalin origin
seeing them
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, 1604. Estimated every 300 years but obscured by dust
  • Many visible in other galaxies, currently some 20-30 bright ones
  • SN classified by spectrum:
    • Ia: Strong Si no H He
    • Ib: Weak H Strong He
    • Ic: Weak Si no H He
    • II: Strong H
  • Iaare nuclear explosion of WD
  • II IbIcare gravitational core collapse with degrees of envelope loss

168,000 years ago a B3 I supergiant collapsed in LMC

  • Observed as SN 1987A
  • Progenitor known – changed theory
  • Remnants observed in detail
  • Three hours before the supernova detected, neutrino detectors observed a burst (20) of neutrinos from the right direction.
  • 20 detected implies 1058emitted carrying 1046 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
what is left of core
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
  • 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

what are pulsars
What are Pulsars?
  • Rotating star breaks up
  • Only NS dense enough to survive
  • Emission aligned to magnetic axis - tilted
  • Crab pulsar :

Neutron star SN remnant

how they work
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