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Ch 13 : Star Death

Ch 13 : Star Death. Occurs when the fusion fuel is exhausted Process depends on star mass: three cases. (1) Very Low Mass (< 0.4M sun ) : Red Dwarfs. Very slow Hydrogen burning  low Luminosity Fully convective  never form Helium core. Cannot ignite Helium (core too cool).

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Ch 13 : Star Death

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  1. Ch 13 : Star Death • Occurs when the fusion fuel is exhausted • Process depends on star mass: three cases

  2. (1) Very Low Mass (< 0.4Msun) : Red Dwarfs • Very slow Hydrogen burning  low Luminosity • Fully convective  never form Helium core. • Cannot ignite Helium (core too cool) • Very long lives: > 1011 years, > Age of Universe • None yet died

  3. (2) Intermediate Mass : 0.4 – 4 Msun • Burns H & He, but not C/O • recall: C/O core + He & H shell burning •  Red Supergiant (~Jupiter’s orbit size) a) weak gravity at surface b) high luminosity c) unstable He Shell burning high mass-loss rate envelope lost in ~104 yr • Envelope ejection: • first slow wind  exposes interior  then fast wind •  expanding shell, ~ 10 km/s • illuminated/ionized by UV from hot core • glows with emission lines • result: “planetary nebula” & core becomes white dwarf star

  4. (3) Planetary Nebulae Formation: slow/fast winds shell-like appearance

  5. (3b) PN examples • Stellar

  6. (3c) HR Tacks • Very rapid motion across HR diag as hot core exposed • UV from hot core ionized nebula • Cools to become white dwarf star (skeleton; see later)

  7. (4) High Mass Stars : ( >20Msun ) • nuclear furnace has much higher pressure & temperature • e.g. if Mcore > 4Msun then Tcore > 600x106 K •  Carbon core ignites  ash is Oxygen & Neon • C exhausted, O/Ne core contracts  C burns in shell • when Tcore > 109 K then O/Ne core ignites  Na,Mg,Al,Si • repeats  onion shell structure core only • Successive reactions go faster: • e.g. for 25 Msun star: • fuelduration% lifetime • H 7x106 yr 94 • He 0.5x106 yr 6 • C/O 600 yr ~0 • Si ~1 day ~0 •  iron core forms at center

  8. (4b) The Iron Core • 56Fe nucleus is most tightly bound •  it cannot be a nuclear fuel • the Fe core grows and compresses • exceedingly dense: 1 ton/cm3 iron • What supports the iron core ? • not gas pressure, but electron degeneracy pressure • electrons fill global energy levels (like a huge incompressible atom) • (they are “shoulder to shoulder”) • When core is ~1.4 Msun and ~500 km disaster is near…. • Ve ~ c, electrons cannot go faster and provide more pressure • e- + p+ → n + υ so electrons removed (neutron-ization) • γ + 56Fe → 26p + 28n, Fe photo-disintegrates → absorbs energy • Core collapses  runaway process, occurs in ~1/50 sec

  9. (5) Supernova Explosion • Collapsing core stops & briefly bounces at size ~10 km • neutrons shoulder to shoulder (neutron degeneracy pressure) •  will become a neutron star • Lighter material falls down from above, hits core at ~0.1c • strong shock races upwards to the surface; arrives after ~1 hr •  Star envelope ejected at ~10,000 km/s  supernova explosion Computer simulation of core collapse. Blue material moving out. before after movie

  10. (5b) Observing SN • SN are “rare”: ~1/galaxy/century on average • In our galaxy, not all are visible: 1054; 1181; 1572; 1604; 1987 • ~10s seen per year in other galaxies • Light curve : outshines a galaxy for ~week; then fades • Initial fading rapid, then slower • “Afterglow” caused by radioactive heating

  11. (5c) SN Energy Budget • Source of energy is Gravity : ~ GM2 / Rcore ~ 1046 Joules • This energy emerges in 3 forms: • 0.01% Light: as bright as a galaxy • 1% KE: blast wave ~ 10Msun moving at 10,000 km/s • 99% Neutrinos (e- + p → n + υ): confirmed!! In NS 1987A • 1987, Feb 23, 7:35:41 (UT) core collapse of Sk 202-69 • 20Msun B3I star; Age ~ 107 yr; 160,000 ly-yr distant • 10 neutrinos detected in 10 sec • Confirms 1046J total energy • Confirms 10s escape from core 2hr later, 5th mag star seen  Confirms shock delay

  12. (6) Supernova Remnants • Aftermath of the explosion: • Hot bubble expands  X-ray bright • Shocks the ISM  emission lines & radio • Snow-plow effect makes shell • Over time (105yr) dissipates & mixes into ISM Crab 103 yr Cas-A - radio ~104 yrs old Optical - Emission lines Vela SNR (old ~105yr) Cas-A - Xray Cygnus loop - Xray

  13. (7) Nucleosynthesis • Star death recycles matter back into ISM: • This contains newly made atoms from nuclear furnaces • (complex: some locked in corpse, some ejected) • Next generation of stars/planets is “enriched” with these elements • Origin & abundance of 92 elements now well understood: • Why carbon/oxygen/iron are common but gold/silver are rare Abundances of all elements

  14. (7b) α-elements High T fusion 109K rapid/slow neutron capture r s 108K 107K s r s r r s

  15. (7c) Trans-iron elements from s & r process N=Z A~208 Pb 4 s A~192 Pt A~138 Ba A~130 Xe 2 s A ~ 80 Ge, Sr 0.5 s Calculated for T~109 K & ρn~ 1024 cm-3 5 s cycle 0.1 s

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