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Stellar corpsesSESAME Astronomy
Winter 2011Week 7
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1Thursday, February 24, 2011
Warning!
YOU ARE ABOUT TO SEE TRIPPY, MIND-BLOWING STUFF!
PREPARE TO BE BLOWN AWAY!
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2Thursday, February 24, 2011
3 types
• White Dwarfs (Dwarves?)
• Neutron Stars
• Black Holes
low-mass stars
high-mass stars
high-mass stars
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3Thursday, February 24, 2011
White Dwarfs
• CO (Carbon and Oxygen)
• Dying embers
• degenerate
• Test of cosmological models
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4Thursday, February 24, 2011
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)
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5Thursday, February 24, 2011
White Dwarfs are dying embers
• not generating new energy
• cooling off forever
• most are very dim and can’t be seen from Earth
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6Thursday, February 24, 2011
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7Thursday, February 24, 2011
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
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8Thursday, February 24, 2011
Degenerate
• when electrons (or neutrons) are squeezed as tightly as they can go
• quantum mechanical principles provide the pressure (instead of atoms zipping around)
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9Thursday, February 24, 2011
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”
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10Thursday, February 24, 2011
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”
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11Thursday, February 24, 2011
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
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12Thursday, February 24, 2011
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
13Thursday, February 24, 2011
limiting mass
• the more mass white dwarf has, the smaller it gets
• can you add so much mass its radius is zero?
• 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
Yes!
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14Thursday, February 24, 2011
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”)
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15Thursday, February 24, 2011
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16Thursday, February 24, 2011
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
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17Thursday, February 24, 2011
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!
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18Thursday, February 24, 2011
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19Thursday, February 24, 2011
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
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20Thursday, February 24, 2011
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these pulses are very regular
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22Thursday, February 24, 2011
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 because there are so few 23
23Thursday, February 24, 2011
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!
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24Thursday, February 24, 2011
Black holes are outta sight!
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25Thursday, February 24, 2011
Black holes - case 1Schwarzschild
• 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
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26Thursday, February 24, 2011
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)
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27Thursday, February 24, 2011
Spinning Black HoleKerr 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
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28Thursday, February 24, 2011
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
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29Thursday, February 24, 2011
magnetic fields of black holes
•VERY STRONG• send jets of particles and X-rays out from
the poles (not from inside the horizon, though)
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30Thursday, February 24, 2011
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
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31Thursday, February 24, 2011
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Artist’s drawing of a black hole accreting mass from a companion star.
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Stay tuned for
Supermassive Black Holes in the Centers of Galaxies!
33Thursday, February 24, 2011