ASTR 1020 1st Edition Lecture 14Q: If the white dwarf collapses, what happens next?A: Carbon fusion will start White Dwarf Supernova (SECOND TIME)- If enough mass is accreted, electron degeneracy pressure is overcomeo Known as the Chandrasekhar Limit White Dwarf Mass < 1.4 Msun- Star then collapses, carbon fusion begins explosively throughout the entire WDNEW KIND OF SUPERNOVAType 1 Supernova (WD SN) –1.4 mass stars + all look exactly the sameType 11 Supernova (massive star SN) -high mass stars3 TYPES OF STELLAR EXPLOSIONS White Dwarf Nova (fusion on surface)- Binary systems only- Occurs in older star populations- White dwarf survives (can repeat again and again)- Luminosity of ~10^5 SunsWhite Dwarf Supernova (fusion of core)- Binary systems only- Occurs in older star populations- Nothing left (can only happen once)- Luminosity of ~10^10 SunsMassive Star Supernova (core collapse)- Found in young star formation regions (young enough for massive stars)- Makes a neutron star or black hole- Luminosity of a few 10^9 SunsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.NEUTRON STAR Structure- Ball of 20 km in diameter- Mass is 1.5 times the sun- Solid crust -1 km thick (crystalline iron)- Heavy liquid interior –mostly neutrons, with other particles (neutronium)- Structure determined by gravity vs. neutron degeneracy pressure- Crushing gravity at its surface- Neutrons spinning very rapidlyWhy Neutron Stars Spin So Fast: Vast shrinking conserves angular momentum = m v r- Collapse to a neutron star increases both rotation and magnetic fields- Gradually slows down as angular momentum is lostSigns of rapidly spinning stars….Jocelyn Bell: Cambridge (UK) graduate student in 1967 discovered it by accident!Named it LGM-1 (Little Green Men #1)Hewish later received the Nobel prize for his discovery with no mention of BellPulse shape from LGM-1 –Joydivision Unknown PleasuresMystery resolved when pulsar discovered in Crab Nebula (known supernova remnant) Pulsar –rotating neutron star (lighthouses in our Galaxy) (all neutron stars are not pulsars)Pulsar beams as pointed to our Galactic Location Neutron Stars in Binary SystemsIf white dwarfs can do it, so can neutron stars!- Mass transfer builds very hot accretion disk around neutron star:intense X-ray emission (continuously) from diskexplosive helium fusion (in bursts) on neutron star = X-ray Burster GRAVITY!F = (GM1M2) / r^2R = distance between the two masses G = gravitational constant “Big G”G = 6.67 x 10^-11 m^3 / (kg s^2)Black Holes- Gravity is balanced by pressure: more gravity higher pressure- But, maximum possible pressure for different materialso White dwarf star: maximum pressure possible for atomso Neutron star: maximum pressure for neutronso Black hole: gravity is too great for anything singularity Singularity – cusp or fold in space time / point of infinite curvatureWhy are black holes called that?- Escape velocity: Vescape = SQUAREROOT OF (2GM) / R- nothing can go faster than the speed of light- so if Vescape > c, nothing can get out black holeSchwartzchild Radius: distance from a black hole where nothing can escapeR-schwartzchild = (2GM) / (c)^2How big is the event horizon?R schwarzchild = (2GM) / c^2 = 3 km per solar massCompare 3 km with the 700,000 km radius of the sunTo make the Sun into a black hole, you need to compress it a LOTCould happen to much larger stars but not the
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