1 / 15

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).

thomas-mclaughlin
Download Presentation

Ch 13 : Star Death

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related