Astr 2310 Thurs. April 17, 2009  Today s Topics

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Astr 2310 Thurs. April 17, 2009 Today s Topics

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1. 1 Astr 2310 Thurs. April 17, 2009 Today’s Topics Chapter 16 cont.: The Evolution of Stars Stellar Evolution cont. Evolution off the Main Sequence Massive Star Chemical Composition and Evolution H-R Diagrams of Star Clusters Synthesis of Heavy Elements in Stars Chapter 17: Star Deaths White Dwarfs Physical Properties Observational Evidence White Dwarfs in Binary Stars Neutron Stars Physical Properties Plulsars – Rotating Neutron Stars Supernovae Connection Black Holes Physics of Black Holes Structure of Spacetime Observational Evidence for Blackholes

2. 2 Chapter 17 Homework Chapter 17: #1, 3, 4, 13, 20 (Due Tues. April 28)

3. 3 Post Main-Sequence Evolution – High Mass As Hydrogen is exhausted in the cores the stars evolve off the main sequence. Hydrogen shell burning rapidly begins Star’s envelope rapidly expands Luminosity remain approx. constant and radius increases rapidly. Star becomes a supergiant. Helium burning begins when core temp. ~ 108 K. No core degeneracy so no He flash Helium rapidly exhausted and Carbon burning begins producing Magnesium Elements capture He nuclei to build up even numbered nuclei

4. 4 Onion Skin Model for Massive Stars Star develops a multi-shell, “onion-skin” structure. Heavy elements rapidly built up but little energy released (see curve of binding energy and timescales below) Massive (1 solar mass) core of Iron is eventually formed.

5. 5 Testing Stellar Evolution via Star Clusters Observations of a variety of star clusters allow comparison of their H-R diagrams H-R diagrams of most clusters are devoid of massive, hot stars. Main-sequence lifetime is short for high mass stars Most massive and hottest stars on the main sequence can be used to age-date a star cluster Location of evolved stars in H-R diagram can be fit with model evolutionary tracks.

6. 6 Observations of Youngest Star Clusters Young cluster “NGC 2264” Few million years old High mass stars have reached main sequence Lower mass stars are still approaching main sequence Locus of Deuterium burning Variable stars known as T Tauri stars Earlier stages hidden by dust Infrared observations reveal hot cores (protostars). Accreting material in rotating disk Gaseous outflows along poles

7. 7 Observations of Moderately-Young Clusters Praesepe star cluster Few million years old Higher mass stars on the main sequence A few Red Giants present too. Low mass stars have reached main sequence Entire main sequence populated Evidence of young age Stars rapidly rotating Strong magnetic fields High variability of low-mass stars Note the contamination by stars along the line of sight

8. 8 Observations of Oldest Star Clusters Old cluster “47 Tucanae” About 12 Billion years old Stars more massive than the Sun have evolved off the main sequence Horizontal Branch stars are evident Asymptotic Giant Branch stars evident as well Note that the stars at the Tip of the Red Giant Branch are bright but not extremely bright. Note the accuracy of the stellar models and the age dating.

9. 9 Evolutionary States of Stars in H-R Diagram Evolution state of individual stars can also be evaluated given their location in the H-R diagram Evolutionary tracks “fit” to a star to estimate mass and age. Observational properties of these post-main-sequence stars can then provide context to their evolutionary state. Variability (pulsations) Rotational velocity Stellar winds (mass loss, dust production) Atmospheric compositional differences (dredge-up of enriched material)

10. 10 Synthesis of Heavy Elements - I As fuel is exhausted the core continues to shrink with a rise in temperature until the next nuclear reaction ignites. Recall the curve of binding energy Each new reaction creates heavier elements but the energy produced is more modest (low efficiency) Reactions must go faster and faster to prevent collapse Star takes on an “onion-skin” structure with shells of heavier elements fusing and synethsizing heavier elements

11. 11 The Curve of Binding Energy If you keep adding protons to a nucleus? Coulomb repulsion continues to increase new proton feels repulsion from all other protons Strong force attraction reaches limit new proton can’t feel attraction from protons on far side of a big nucleus Gain energy only up to point where Coulomb repulsion outweighs strong force attraction. Most “stable” nucleus is 56Fe (26 protons, 30 neutrons, 56 total) Release energy by fusion of light nuclei to make heaver ones– up to 56Fe Release energy by fission of heavy nuclei to make lighter ones – down to 56Fe

12. 12 Synthesis of Heavy Elements - II Nuclear synthesis creates enhancements in the CNO elements a-capture onto C12 during triple-a produces O16, Ne20 Carbon burning produces Mg24 and Si28 Silicon burning produces Fe56 Iron (Fe56) is also enhanced since it is the most tightly bound nucleus Higher atomic numbered nuclei need more neutrons to mitigate Coulomb repulsion Elements heavier than Fe are less tightly bound Fusion cannot synthesize elements beyond Fe56 Odd-even effect due to only a-particles (He nuclei) being present in these hot environments (sort of): AGB stars and SN: a-capture onto C12 and higher also produces: C12, O16, Ne20, Mg24, Si28, S32, Ar36, Ca40 ... Picture is oversimplified as it ignores neutron capture Nuclei experience neutron flux in neutron-rich environments s and r process (slow and rapid neutron capture) Essential for the “trans-Fe” elements Burbidge, Burbidge, Fowler and Hoyle (B2FH) described processes Models can be tested via abundance pattern of very heavy elements

13. 13 Current Elemental Abundances Recall that the abundance of the elements can be modeled with an accurate stellar atmosphere. Once the temperature and density profile of the atmosphere is specified the Boltzman and Saha equations can be solved strengths of individual spectral lines modeled to infer abundances Resulting abundances Li, Be, and Boron (light elements) very rare Triple-a process skips them to form Carbon a-process creates CNO peak Odd-even pattern evident Iron peak evident as well

14. 14 Chapter 17: Star Deaths Moderate-low mass stars produce White Dwarfs Instabilities during double-shell burning within AGB stars produces tremendous amounts of mass-loss UV photons ionize surrounding envelope to form planetary nebula Example of a proto-planetary nebula: Red Rectangle Envelope eventually fully expelled to reveal hot degenerate core and surrounding ionized nebula

15. 15 Proto-planetary Nebula Red Rectangle Prototype of proto-planetary nebula Shells evidence for pulsations expelling stellar envelope Thick, dusty torus surrounds post-AGB star Spectroscopy indicates dust and PAH formation “Rays” probably result of shadowing effects

16. 16 Planetary Nebula Once envelope is expelled the central hot core can ionize surrounding gas (nebula) Temp. ~ 105 K Rapid cooling of central star means lifetime of only ~ 104 yrs. Result will be a White Dwarf

17. 17 Physical Properties of White Dwarfs Chandrasekhar Developed Models for White Dwarfs Degenerate Equation of State: P = kr5/3 (non-relativistic gas) P = kr4/3 (relativistic gas) Combining with the definition of density (r ~ M/R3) we have: P ~ r5/3 ~ M5/3/R5 Hydrostatic Equilibrium Requires P ~ M2/R4 Equating: M2/R4 ~ M5/3/R5 which yields: R ~ 1/M1/3 Note that as Mass increases R decreases (Mass-Radius Relation) Detailed modeling indicates a maximum mass for White Dwarfs: Chandrasekahar Mass ~ 1.44 Msun

18. 18 Observational Evidence of White Dwarfs White Dwarfs in Nearby Binary System Sirius and Procyon Astrometric Orbits yield Masses MSiriusB = 2.1 x 1030 kg (typically 0.7 Msun) Temperatures and Luminosities yield Radii RSiriusB = 5.5 x 105 km (typically 0.01 Rsun ~ Rearth) Redshifts Inconsistant with Velocity of Primary Strong Gravitational Redshift! Consistent with Predictions of General Relativity

19. 19 White Dwarfs in H-R Diagram

20. 20 White Dwarfs in Binary Systems White dwarfs in close binary systems have interesting properties and implications. The White Dwarf was originally the more massive since it evolved first. If the companion begins to expand it can fill its Roche lobe and begin to transfer mass onto the White dwarf. Hot accretion disk can surround White Dwarf Cataclysmic variable stars Strong x-ray emission from accretion disk Accreting White Dwarf can reach Chandrasekhar mass and collapse as a Type-Ia supernova (no Hydrogen lines in the spectrua)

21. 21 Massive stars produce Neutron Stars Silicon burning goes very fast as very little energy is produced Temperature of Fe core rises but no further fusion is possible Core photo-dissociates Fe56 + g 14 He4 + n Process absorbs energy instead of producing it. Core produces burst of neutrinos (n) Core collapses to density of atomic nucleus! Energy absorbed results in huge drop in central pressure Core undergoes free-fall collapse to enormous density He nuclei and electrons squeezed together to form neutron core: p + e n Neutrinos blow off outer layers of star creating type II supernova If Mcore < 3 Msun the neutron degeneracy can support the core Result is a neutron star Inner, dense layers of star’s envelope undergo rapid nuclear fusion due to expanding shock wave Trans-Fe elements synthesized (r-process)

22. 22 Type II Supernovae - I Enormous explosions in our galaxy and distant galaxies. Chinese record of supernovae include (clockwise from upper left): Vela supernova (~ 4000 BC) and the Crab (1054 AD), more recent examples include those named for Tyco (1572) and Kepler (1604).

23. 23 Type II Supernovae - II Association of historical supernova events with remnants and pulsars No recent supernovae in our Galaxy No modern photometric or spectroscopic data available. Supernovae in distant galaxies provide the only modern data for context and interpretation. Extensive surveys of extragalactic supernovae have resulted in a wealth of data including: Luminosities: 109 Lsun Expansion velocities: V ~ 10,000 km/sec Heavy element abundances: trans-Fe elements, direct evidence for the r-process

24. 24 Possible Supernovae Precursors Several Galactic stars have been suggested as possible precursors for type-II supernovae. These include r Cass, Eta Caraina (right). At present there is no reliable way to predict the event. Such an event could be visible in the daytime sky. Some have suggested that any corresponding Gamma-ray bust would present a hazard to life on Earth.

25. 25 Physical Properties of Neutron Stars Mass: M ~ 2 Msun Radius: R ~ 12 km Density: r ~ 5 x 1017 kg/m3 ~ 4 x 1011 Sun’s Conservation of angular momentum means enormous rotational velocities (near speed of light) Magnetic field: B ~ 2 x 1011 Gauss Surface gravity: g ~ 1011 gearth Escape velocity: Vesc ~ 30% c Initial Temp: T ~ 1011 K Internal Structure: Core: superfluid of neutrons and pions Mantle: superconducting fluid of neutrons and protons Crust: Iron crust few meters thick

26. 26 Observational Evidence of Neutron Stars: Pulsars Pulsars: Pulsating Stars Rapid pulses at radio wavelengths, extremely regular Periods from 1 to few 1000 msec Lighthouse beaming of synchrotron emission (spiraling e- in strong mag. Field) Field strengths of ~ 108 Teslas Deformation into oblate spheroid Steady period changes indicate spindown Abrupt period changes suggest crustal shrinkage (quakes!)

27. 27 Crab Pulsar at X-ray Wavelengths Crab pulsar is the nearest, best observed pulsar P = 0.03 sec. Much of what we know about neutron stars comes from the Crab Optical pulses detected by tuning photon counting detector to the radio period. Pulsar powers the Crab nebula See movie on class website

28. 28 Observational Evidence for Neutron Stars: Binary Systems Mass of neutron stars can be measured if in binary system. Detached systems: No mass transfer or x-ray emission Single line spectroscopic binaries Only limits for mass of neutron star If a pulsar then the time delay provides orbital velocity and hence mass Binary pulsar provides masses for both as well as critical tests of General Relativity Mass Transfer Systems: Strong x-ray source due to hot accretion disk Emission lines from accretion disk (velocity and hence mass)

29. 29 Origin of Black Holes The neutron star equation of state suggests that neutron degeneracy cannot provide sufficient support for M > 4 Msun Nothing can halt collapse and core collapses to a point mass. Models imply that stars with M > 20 Msun will likely produce a Black Hole Amount of mass loss is uncertain and so models are not definitive

30. 30 Physical Properties of Black Holes Cannot be described without General Relativity General Relativistic solutions: Schwarzschild: Static, non-rotating Black Hole Kerr: Rotating Black Hole Hawking: “Black Holes have no hair.” Only three numbers describe a Black Hole: Law of Cosmic Censorship: Mass, angular momentum, and charge Event horizon masks singularity Event Horizon Singularity is surrounded by surface at which the escape velocity reaches speed of lightWithin this volume no light can escape! R = 2GM/c2 (Schwarzschild radius) Angular momentum should be huge but it is limited: Lmax ~ GM2/c (otherwise singularity is exposed)

31. 31 Black Holes in Binary Systems Presence in mass transfer systems can results in strong x-ray emission. Infalling material acquires enormous kinetic energy (v ~ c) Candidate Black Hole binary systems now number ~ 50 systems

32. 32 Chapter 17 Homework Chapter 17: #1, 3, 4, 13, 20 (Due Tues. April 28)

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