Stellar corpses. SESAME Astronomy Winter 2011 Week 7. Thursday, February 24, 2011

Transcription

1 Stellar corpses SESAME Astronomy Winter 2011 Week 7 1 1

2 Warning! YOU ARE ABOUT TO SEE TRIPPY, MIND- BLOWING STUFF! PREPARE TO BE BLOWN AWAY! 2 2

3 3 types White Dwarfs (Dwarves?) low-mass stars Neutron Stars Black Holes high-mass stars high-mass stars 3 3

4 White Dwarfs CO (Carbon and Oxygen) Dying embers degenerate Test of cosmological models 4 4

5 structure Carbon and Oxygen core a little Helium and maybe some Hydrogen on the surface typical mass ~ 0.7M typical size - about the size of Earth (not to scale: 260 is 3% of 8500) 5 5

6 White Dwarfs are dying embers not generating new energy cooling off forever most are very dim and can t be seen from Earth 6 6

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8 Dying embers not generating any energy (just losing it) don t even have gravo-thermal energy (when something contracts it heats up - they don t contract (much) when they cool off) that s cause they re degenerate 8 8

9 Degenerate when electrons (or neutrons) are squeezed as tightly as they can go quantum mechanical principles provide the pressure (instead of atoms zipping around) 9 9

10 degeneracy 2 electrons going the same speed can t be in the same place (or even very close, as it turns out) (Pauli exclusion principle) This principle generates pressure (in any gas, but in normal non-degenerate gas, the gas pressure is much larger than the degeneracy pressure) almost all available states get filled up and electrons just say no 10 10

11 degeneracy 2 electrons going the same speed can t be in the same place (or even very close, as it turns out) (Pauli exclusion principle) This principle generates pressure (in any gas, but in normal nondegenerate gas, the gas pressure is much larger than the degeneracy pressure) almost all available states get filled up and electrons just say no 11 11

12 properties of degenerate matter heating it doesn t make it expand it s REALLY dense (a spoonful would weigh as much as a tractor!) the more mass you add, the smaller the object gets 12 12

13 Test of cosmological models for a given initial temperature, can calculate exactly how long it will take for a white dwarf to cool to a different given temperature compare to how old cosmological model says Universe is if white dwarf time is too long, you know something is wrong with model (this actually happened in the 80s/90s, but we fixed it) 13 13

14 limiting mass the more mass white dwarf has, the smaller it gets can you add so much mass its radius is zero? Yes! Chandrasekhar (famous astrophysicist and former department chair here - funny story - derived it when he was, like, 19, on a train limit is about 1.4 M if you go over this limit you get an explosion and a black hole 14 14

15 Nova white dwarfs have VERY strong surface gravity if they have a companion that is losing mass, they can capture this mass (mostly hydrogen) and have a burst of fusion this is called a nova (nova means new star ) 15 15

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17 Type Ia Supernovae Supernova - exploding star we think if 2 white dwarfs somehow collide they will go over the limit and a Supernova Type Ia occurs These explosions fade away in a very specific way so that we can tell exactly how far away they are - very important for cosmology Lots of Iron is produced in these explosions 17 17

18 Neutron stars neutrons also experience degeneracy pressure, but you can squeeze them much tighter than electrons typical size of a neutron star is about 10 km so how does their density compare to the density of white dwarfs? neutron stars by a lot! 18 18

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20 Neutron stars spinning VERY quickly, especially at first Crab pulsar spins about 30 times per second very strong magnetic fields so they send out pulses of radio waves 20 20

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22 these pulses are very regular 22 22

23 LGMs when first discovered in the 50 s/60 s, they were referred to as LGMs (for Little Green Men) Jocelyn Bell* figured out what was going on *We ladies of astronomy are always proud of pioneering mothers who became famous female astronomers, partially 23 because there are so few 23

24 Black Holes a region of space so dense that not even light can escape phrase was coined by John Wheeler at Princeton in the late 60 s Star Trek (TOS) mentioned one but called it a black star because phrase hadn t been coined yet! 24 24

25 Black holes are outta sight! 25 25

26 Black holes - case 1 Schwarzschild imaginary case (non-rotating) 2 sections of a black hole: event horizon and singularity singularity-the point at the very center where all the mass is horizon - the point of no return - not even light can escape from this point inward 26 26

27 Falling into a Schwarzschild black hole time slows down you look redder (related to time slowing down, or to light having to climb out of a deeper potential well) you get stretched (ripped apart, really) 27 27

28 Spinning Black Hole Kerr solution additional region - ergosphere ergosphere - where you have to go in the direction of the black hole s spin, but you can still escape by flying straight outward this is called frame dragging and instead of singularity there is a ring of neutrons 28 28

29 Frame dragging the black hole actually drags space around with it as it spins accretion disk - stuff that falls in toward the black hole orbits before falling in the stuff (gas and dust) gets VERY hot because of frame dragging 29 29

30 magnetic fields of black holes send jets of particles and X-rays out from the poles (not from inside the horizon, though) 30 30

31 Cygnus X-1 first black hole observed (observed the x-rays, not the hole, which by definition we can t see) famous bet between Stephen Hawking and Kip Thorne - Hawking lost and had to by Thorne a year s subscription to Penthouse 31 31

32 Artist s drawing of a black hole accreting mass from a companion star

33 Stay tuned for Supermassive Black Holes in the Centers of Galaxies! 33 33