stellar corpses - university of chicagomath.uchicago.edu/~grodnick/corpses.pdf · 2011-02-24 ·...

Stellar corpsesSESAME Astronomy

Winter 2011Week 7

1

1Thursday, February 24, 2011

Warning!

YOU ARE ABOUT TO SEE TRIPPY, MIND-BLOWING STUFF!

PREPARE TO BE BLOWN AWAY!

2


3 types

• White Dwarfs (Dwarves?)

• Neutron Stars

• Black Holes

low-mass stars

high-mass stars

high-mass stars

3


White Dwarfs

• CO (Carbon and Oxygen)

• Dying embers

• degenerate

• Test of cosmological models

4


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


White Dwarfs are dying embers

• not generating new energy

• cooling off forever

• most are very dim and can’t be seen from Earth

6


7


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


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


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


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”

11


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


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


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!

14


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


16


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


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


19


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


21


these pulses are very regular

22


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


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


Black holes are outta sight!

25


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

26


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


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

28


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


magnetic fields of black holes

•VERY STRONG• send jets of particles and X-rays out from

the poles (not from inside the horizon, though)

30


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


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!


stellar corpses - university of chicagomath.uchicago.edu/~grodnick/corpses.pdf · 2011-02-24 ·...

Documents