Astro 1050     Mon., Nov. 3, 2003
   Today: Ch. 10: The Deaths of Stars

White Dwarfs

Simple Planetary Nebula
IC 3568   from the Hubble Space Telescope

Complicated P-N in a Binary System
M2-9 (from the Hubble Space Telescope)

A Gallery of P-N from Hubble

Complications in Binary Systems
Can move mass between stars
1st (massive) star becomes red giant
Its envelope transferred to other star
Hot (white dwarf) core exposed
2nd star becomes red giant
Its envelope transferred to white dwarf
Accretion disk around white dwarf
Angular momentum doesn’t let material fall directly to white dwarf surface
Recurrent nova explosions
White dwarf hot enough for fusion, but no Hydrogen fuel
New fuel comes in from companion
Occasionally ignites explosively,
 blowing away remaining fuel

Is a star stable against catastrophic collapse?
Imagine compressing a star slightly (without removing energy)
Pressure goes up (trying to make star expand)
Gravity also goes up (trying to make star collapse)
Does pressure go up faster than gravity?
If Yes:  star is stable – it bounces back to original size
If No:   star is unstable – gravity makes it collapses
Ordinary gas: P does go up fast –  stable
Non-relativistic degenerate gas:   P does go up fast –  stable
Relativistic degenerate gas: P does not go up fast –  unstable
Relativistic:   Mean are the electrons moving at close to the speed of light
Non-relativistic degenerate gas:   increasing r means not only more electrons, but faster electrons, which raises pressure a lot.
Relativistic degenerate gas:   increasing r can’t increase electron velocity (they are already going close to speed of light) so pressure doesn’t go up as much

Chandrasekhar Limit for White Dwarfs
Add mass to an existing white dwarf
Pressure (P) must increase to balance stronger gravity
For degenerate matter, P depends only on density (r), not temperature, so must have higher density
P vs. r rule such that higher mass star must actually have smaller radius to provide enough P
As Mstar ® 1.4 MSun      velectron ® c
Requires much higher r to provide high enough P, so star must be much smaller.
Strong gravity which goes with higher r makes this a losing game.
For M ³ 1.4 MSun no increase in r can provide enough increase in P – star collapses

Implications for Stars
Stars less massive than 1.4 MSun can end as white dwarfs
Stars more massive than 1.4 MSun can end as white dwarfs, if they lose enough of their mass (during PN stage) that they end up with less than 1.4 MSun
Stars whose degenerate cores grow more massive than 1.4 MSun will undergo a catastrophic core collapse:
Neutron stars
Supernova

Supernova
When the degenerate core of a star exceeds 1.4 MSun it collapses
Type II:  Massive star where it runs out of fuel after converting core to Fe
Type  I:  White dwarf in binary, which receives mass from its companion.
Events:
Star’s core begins to collapse
Huge amounts of gravitational energy liberated
Extreme densities allows weak force to convert matter to neutrons
p+ + e-
®  n + n
Neutrinos (n) escape, carrying away much of energy, aiding collapse
Collapsing outer part is heated, “bounces” off core, is ejected into space
Light from very hot ejected matter makes supernova very bright
Ejected matter contains heavy elements from fusion and neutron capture
Core collapses into either:
Neutron stars or Black Holes (Chapter 11)

Supernova in Another Galaxy
Supernova 1994D in NGC 4526

Tycho’s Supernova of 1572
Now seen by the Chandra X-ray Observatory as an expanding cloud.

The Crab Nebula – Supernova from 1050 AD
Can see expansion between 1973 and 2001
Kitt Peak National Observatory Images

What happens to the collapsing core?
Neutron star (more in next chapter)
Quantum rules also resist neutron packing
Densities much higher than white dwarfs allowed
R ~ 5 km      r ~ 1014 gm/cm3   (similar to nucleus)
M limit uncertain,  ~2 or ~3 MSun before it collapses
Spins very fast (by conservation of angular momentum)
Trapped spinning magnetic field makes it:
Act like a “lighthouse” beaming out E-M radiation (radio, light)
pulsars
Accelerates nearby charged particles

Spinning pulsar powers the
 Crab nebula
Red:  Ha
Blue:  “Synchrotron” emission from high speed electrons trapped in magnetic field