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Ch 14 : Star Corpses

Ch 14 : Star Corpses. Three kinds of skeleton are left behind White Dwarfs Neutron Stars Black Holes These are all examples of compact objects. (1) Endpoints of stellar evolution. review dependence of corpse on original star mass. (2) White Dwarf Stars.

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Ch 14 : Star Corpses

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  1. Ch 14 : Star Corpses • Three kinds of skeleton are left behind • White Dwarfs • Neutron Stars • Black Holes • These are all examples of compact objects

  2. (1) Endpoints of stellar evolution • review dependence of corpse on original star mass

  3. (2) White Dwarf Stars • are the remaining C/O core from low-mass star death • ~ size of earth & ~mass of sun • huge density ~106 gm/cm3 = 1 ton/cm3 • very high surface gravity ~105 gearth high pressure atm. • no nuclear fusion (“dead” star) • Glows by residual heat (was once a hot star core) • Slowly (few x 109 yr) coolsdown (small surface area) • Evolutionary tracks below MS Beach ball size

  4. (2b) White Dwarf Composition • atoms crushed, but nuclei not touching (~1000 x sep) • Nuclei in sea of electrons • Electrons fill global energy levels (shoulder to shoulder) • Very rigid = degeneracy pressure • P independent of Temp (like a solid) • more massive WD are crushed smaller (M ↑ R ↓ ) • R → 0 when M = 1.4 Msun = Chandrasekhar limit

  5. (2c) Reawakening White Dwarfs in binaries • WDs in binary systems can accrete gas from companion • Fresh hydrogen builds on surface, becomes degenerate • Runaway H fusion explodes as a Nova • Repeats as transfer continues. • If WD continues to build in mass …… • Approaches Chandrasekhar limit (1.4Msun) • Collapse begins, T↑ causes C detonation • Huge explosion = Supernova (type Ia) • WD destroyed, elements scattered • Standard “bomb”  good standard candle • Use for distances to remote galaxies after before movie

  6. Recent detection of thermal X-rays from NS in SNR Neutron stars are city sized (3) Neutron Stars • End of massive star’s life: • Fe core > 1.4 Msun collapses (Supernova type II) • e- + p → n + υ  ball of neutrons, R ~ 10 km = neutron star • neutrons “shoulder to shoulder”  neutron degeneracy pressure • Density huge = atomic nucleus density • 1.5 – 2 Msun within ~10 km  ρ ~ 1014 gm/cm3 = 108 tons/cm3 • Surface gravity exceedingly strong: Vesc ~ 0.5c • Hot but tiny  faint X-ray source

  7. (3b) Neutron Stars • Is there another way to detect them ? • Yes – consider two other properties • Collapse  spin very fast (conserve angular momentum) • e.g. 1Msun 1Rsun @ P~25 days  Rns ~ 10 km @ P ~ 0.01 sec ! • Super-fluid & super-conducting •  Very strong magnetic fields ~1010 Tesla • Together these produce a radio lighthouse = pulsar • First detected in 1967, now ~1000 known

  8. (4) Pulsars • Discovered 1967 – unusual radio pulses • First few had P ~ 1 second • Unclear: binary WD; pulsing WD; rotating WD ?? movie • Then Crab pulsar : P ~ 1/30 sec  rotating N.S. • Young are fast  old are slow • Slow-down can be measured • Energy lost via wind & magnetic breaking • c.f. Crab : dErot/dt ~ Lnebula Old: spin-up Vela 104 yr 11 rps 174 rps • Very precise clocks • e.g. binary pulsar timing: •  Orbit contracts due to gravitational waves 642 rps

  9. (4b) Pulsars Emission Mechanism • Basically, like a lighthouse: • Opposite twin beams sweep around as the star rotates. • Not all beams point at us • Many pulsars invisible • Beam axis may be tilted • e.g. earth’s magnetic axis • Beams arise from strong magnetic field • Rotation makes v strong electric field at poles • Electrons accelerated along magnetic axis • Spiral around magnetic fields  EM emission • Faster spin  more energetic  opt + xray

  10. (5) Black Holes YES ! • Is there a limit to neutron degeneracy pressure ? • It behaves like electron degeneracy pressure: • M ↑ R ↓ ; Vn → c ; “Oppenheimer-Volkoff” limit ~ 3Msun • Enter “black hole” regime: light trapped by gravity • Consider “escape velocity” : • a) For sphere of radius R and mass M : Vesc = √ (2GM / R) • e.g. for Earth Vesc = 11 km/s • b) For small R & large M, can have Vesc = c = 300,000 km/s • Condition is Rs = (2GM / c2) = Schwarzchild radius • e.g. Rs (sun) = 3 km ; Rs (earth) = 1 cm …. Very compact ! • c) Note Rs ~ M, so Rs(M/Msun) = 3(M/Msun) km • e.g. for BH of mass 100Msun , Rs = 300 km demo

  11. (5b) Singularities & Event Horizons • • Inside Rs can a new pressure support the mass ? • Situation fundamentally different than before • Pressure makes gravity  helps collapse • Space ↔ time reverse roles  R↓ only allowed • No force can act upwards • Collapse unstoppable • Singularity formed NO ! • Event horizon: • Once inside nothing can leave • Emits no light  Black Hole • Cannot see beyond Rs • Event horizon

  12. 10Ms B A 300 km (5c) Black Hole Properties • Far from BH  normal properties • e.g. if Sun → BH, Earth’s orbit unaffected •  BHs don’t “suck” • Close to BH  bizarre properties A sees B’s time slow down B falls ever more slowly B redder (gravitational redshift) B experiences 1 ms death Feet pulled stronger by 200 tons • BH have only 3 properties: • Mass • Spin : Kerr (1963) BHs; drag space around like whirlpool • (Charge : rarely considered since infalling matter is neutral)

  13. (5d) Why so Weird ? • Because of the nature of Gravity • Newton’s (1680) description fails for strong gravity • Einstein’s (1916) description (General Relativity) is different & better • Start from Equivalence Principle • Gravity & acceleration are indistinguishable •  must mix space & time  space-time • mass affects “geometry” of space-time • BHs have severed space-time region • near BH strong space-time distortion • Observations support Einstein’s view • Many tests done • GR never failed (yet)

  14. (5e) Finding Black Holes • BH themselves are tiny & invisible !! • Must look for their influence on other (visible) things • Gas falling into BHs releases a lot of energy: ~0.5 mc2 (!!) • Gas enters via an Accretion Disk • Becomes very hot & bright, may also flicker as blobs orbit •  Look for smallbrightX-ray sourceswith M>3Msun • Two contexts for black holes: • a) “stellar” mass (10 – 20 Msun) BH in binary system • b) “super-massive” (106 – 109 Msun) BH in galaxy nuclei • Many examples now known • Indisputable evidence was slow to come • Now many (dozens) of convincing stellar mass systems • SMBH in galaxy nuclei now compelling (especially our own galaxy!)

  15. movie Flying over AD & BH • BH accrete gas • From companion wind or overflow • Disks can flicker with ms periods • Jets sometimes produced & seen (5f) BH systems • SMBH in galaxy nuclei • Discovered by rapid star orbits • Contain powerful accretion disks • & stable jets which move at v ~ c Sketch of binary BH Huge stable fast jets Rapid rotation

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