Ch 13 : Star Death

1
Ch 13 Star Death

• Occurs when the fusion fuel is exhausted
• Process depends on star mass three cases

2
(1) Very Low Mass (lt 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 gt 1011 years, gt 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 (Jupiters 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 ( gt20Msun )

• nuclear furnace has much higher pressure
  temperature
• e.g. if Mcore gt 4Msun then Tcore gt 600x106
  K
• ? Carbon core ignites ? ash is Oxygen Neon
• C exhausted, O/Ne core contracts ? C burns in
  shell
• when Tcore gt 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
• fuel duration 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 ? so electrons removed
  (neutron-ization)
• ? 56Fe ? 26p 28n, Fe photo-disintegrates ?
  absorbs energy

• Core collapses ? runaway process, occurs in
  1/50 sec

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(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
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(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 ?) confirmed!!
  In NS 1987A

• 1987, Feb 23, 73541 (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

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(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
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(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
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(7b) a-elements
High T fusion
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(7c) Trans-iron elements from s r process