1	Astro 1050 Fri., Oct. 31, 2003 Today: Ch. 10: The Deaths of Stars Go over exam Friday EC Articles.
2	White Dwarfs
3	Simple Planetary Nebula IC 3568 from the Hubble Space Telescope
4	Complicated P-N in a Binary System M2-9 (from the Hubble Space Telescope)
5	A Gallery of P-N from Hubble
6	Complications in Binary Systems Can move mass between stars 1^st (massive) star becomes red giant Its envelope transferred to other star Hot (white dwarf) core exposed 2^nd 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
7	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
8	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 M_star ® 1.4 M_Sun v_electron ® 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 M_Sun no increase in r can provide enough increase in P – star collapses
9	Implications for Stars Stars less massive than 1.4 M_Sun can end as white dwarfs Stars more massive than 1.4 M_Sun can end as white dwarfs, if they lose enough of their mass (during PN stage) that they end up with less than 1.4 M_Sun Stars whose degenerate cores grow more massive than 1.4 M_Sun will undergo a catastrophic core collapse: Neutron stars Supernova
10	Supernova When the degenerate core of a star exceeds 1.4 M_Sun 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)
11	Supernova in Another Galaxy Supernova 1994D in NGC 4526
12	Tycho’s Supernova of 1572 Now seen by the Chandra X-ray Observatory as an expanding cloud.
13	The Crab Nebula – Supernova from 1050 AD Can see expansion between 1973 and 2001 Kitt Peak National Observatory Images
14	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 M_Sun 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
15	Spinning pulsar powers the Crab nebula Red: Ha Blue: “Synchrotron” emission from high speed electrons trapped in magnetic field