Lecture 16 - Compact Objects Flashcards
what are white dwarfs?
the remaining cores of dead stars
why are white dwarfs different than stars?
there is no fusion to maintain heat and pressure to oppose gravity –> pressure comes from a different source
what are the 3 types of pressures?
- classical pressure
- electron degeneracy pressure
- nucleon degeneracy pressure
CLASSICAL PRESSURE
- what stars is this present in?
- what is it proportional to?
- what does it regulate in the sun?
- in main sequence stars
- proportional to heat
- regulates fusion break in sun
what is electron degeneracy pressure, when does it occur?
what principle causes this?
what type of object is it found in?
electrons are pushed against each other to their lowest energy states possible because subatomic particles are packed as closely as the laws of quantum mechanics allow
Pauli exclusion principle –> 2 fermions/electrons CANNOT simultaneously occupy the same quantum state so
in white dwarfs
what is nucleon degeneracy pressure? what type of object is it used in?
compression of neutrons in contracting core of neutron star
are white dwarfs in balance or does one pressure win?
they are in balance –> electron degeneracy pressure balances gravity
describe the temperature of white dwarfs with time
starts off hot because it was recently in a star, but cools down and gets dimmer with time
describe the size and mass of a white dwarf
~ the mass of the sun compressed into the volume of earth
describe the radius of white dwarfs and how this relates to mass
why does this occur?
high mass = smaller radius because GRAVITY can compress the matter MORE
describe the composition of white dwarfs
since it is the core of a dead star, its composition will reflect the products of star’s final fusion stages –> white dwarf left behind by 1 M_sun will be mostly made of carbon
how does the fact that red giants become more luminous with time relate to the fact that more massive white dwarfs are smaller?
adding mass to the degenerate core of the red giant will cause it to contract more, causing the surrounding H-fusion shell to shrink and increase its rate of fusion
as long as the core remains inert and fusion occurs in a shell overlying it, the luminosity of the red giant must steadily increase
SO the white dwarfs have more mass and are smaller due to the contraction
why is there a fundamental limit on the maximum mass of a white dwarf?
electrons move faster when they are squeezed into a small space –> can only reach the speed of light
what is the Chandrasekhar limit?
electrons can only move max. the speed of light, making the maximum mass of a white dwarf 1.4 M_sun
how does the brightness of a white dwarf change? why?
cools down into a cold black dwarf bc no fuel for fusion
how does the size of a white dwarf change? why?
size NEVER changes bc electron degeneracy pressure will always keep it stable against gravity
when can a white dwarf change size? why?
if it is in a close binary system
will gain mass if its companion is a main sequence or giant star due to angular momentum pulling in matter to form a rotating ACCRETION DISK
what does the accretion disk do to the white dwarf?
- its companion feeds matter into the accretion disk
- friction is produced that makes the accretion disk radiate visible, UV, and X-ray light (disk glows)
- temp increases enough for H fusion
- causes a NOVA
i.e. provides energy to a “dead” white dwarf
what is a nova?
what does it produce?
what determines the time btwn 2 novae in a system?
minor H fusion on the surface of a white dwarf in a close binary system
generates heat and pressure, ejecting material that accreted onto the white dwarf
time btwn novae in a system depends on how quickly H accretes onto the white dwarf’s surface and how compressed it gets
when is H the most compressed?
for most massive white dwarfs bc strongest gravity
when does a supernovae occur in white dwarf?
white dwarf’s mass gets too close to white dwarf limit
describe the 2 types of supernovae
how can we tell them apart (2)?
MASSIVE STAR CORE COLLAPSE (Type II)
- Fe core of a massive star reaches white dwarf limit and collapses into a neutron star to cause total explosion
WHITE DWARF BINARY (Type Ia)
- C fusion suddenly begins as a white dwarf in close binary system reaches limit to cause total explosion
- occurs in binary system if companion gives a lot of mass to white dwarf OR if 2 white dwarfs are too close
use light curve to show how luminosity changes with time AND spectra bc white dwarfs don’t have H absorption lines
are novae or supernovae more luminous?
supernovae
what is a neutron star?
what is the pressure that opposes gravity?
ball of neutrons from massive star supernova
electron degeneracy pressure stops bc electrons combine with protons to make neutrons/neutrinos, end up with NEUTRON DEGENERACY PRESSURE
describe the make-up of neutron stars (crust vs inside)
how does density change throughout?
- crust of electrons and positively charged atomic nuclei
- deep inside, electrons have combined with protons so the interior is almost ENTIRELY NEUTRONS
density increases inward
what is a PULSAR?
pulses of radio emission due to neutron stars spinning rapidly as it collapses (conservation of angular momentum)
describe the formation of a pulsar and its magnetic field
iron core of massive star collapses into neutron star and its rotation rate increases as it shrinks
as it collapses, magnetic fields get bunched up tightly so the field is stronger and directs BEAMS OF RADIATION along the magnetic poles which we detect
do pulsars actually pulse?
no, they have a steady beam of light but it looks like it pulses bc it spins
what are magnetars? what is their fate (3)
how do we observe the explosions?
the MOST HIGHLY MAGNETIZED neutron stars
- high magnetic fields make the star unstable leading to:
1. cracking crusts
2. magnetic storms
3. huge explosions
observe explosions thru X-ray and Gamma-ray bursts
what happens to a neutron star in a close binary system?
can be REVIVED in a binary system, where gas from a companion star creates an accretion disk and it rotates very rapidly
what are X-ray binaries?
close binaries with accreting neutron stars get very hot so the disk radiates X-rays
why can we detect X-ray/neutron star binaries with X-ray?
the matter accreting on a neutron star eventually gets hot enough for He fusion to produce a burst of X-rays
what is a black hole?
object with such strong gravity that not even light can escape it
what does a black hole’s mass do?
WARPS SPACE AND TIME
can you come back if you enter a black hole?
NO
What does Einstein say about gravity and how does this relate to black holes?
Einstein says that gravity is 2D flat spacetime, then if you add something heavy it deforms spacetime
black hole –> extreme curvature
- but Einstein said there was infinity gravity which we know is wrong bc we know quantum mechanics have to take over
if the sun became a black hole, what would happen to its gravity?
its gravity would be different only near the event horizon
what is the event horizon?
POINT OF NO RETURN where escape velocity equals the speed of light and is not fast enough to exit
how does spacetime change near the black hole?
VERY WARPED –> straight lines not straight
time passes more slowly
why is there a gravitational redshift when light waves try to climb out of a deep hole in spacetime?
time passes more slowly, so light waves take more time to climb out of a deep hole in spacetime, causing waves to stretch and shift more red
what is ESCAPE VELOCITY?
minimum speed required for a free, non-propelled object to escape the gravity of an object, reaching an infinite distance from it
escape velocity = sqrt[(2GM)/d] = sqrt[2gd]
i.e. initial KE = final PE
what are 2 ways that black holes arise? and what does this lead to?
- former core of a star completely collapses
- neutron star acquires a lot of matter
all the mass is crushed into a tiny point of infinite density so gravitational force gets very large
what is the surface of a black hole?
radius at which escape velocity = speed of light
the surface is the EVENT HORIZON, the radius of the event horizon is the SCHWARZSCHILD RADIUS
what does “no escape” mean?
no more contact with something that falls in –> loses its identity
what is the limit called where you can resist the crush of gravity? what happens beyond this?
neutron star limit –> beyond this, no known force can resist the crush of gravity
what is SINGULARITY? what theory describes this?
gravity crushes all the matter onto a single point called SINGULARITY
described by general theory of relativity
why is Einstein’s theory of black holes wrong?
he said the center of a black hole is 0 divided by infinity but this is not possible! quantum mechanics must take over
describe how we perceive Einstein’s theory now
INCOMPLETE, FORMALLY WRONG, DISPROVEN
when could a supernovae produce a black hole?
if enough mass falls into the core
what is the neutron star limit?
neutron degeneracy pressure cannot support a neutron star against gravity if its mass is > 3M_sun
what happens to wavelength of light and time as it gets closer to event horizon? TO AN OBSERVER
shifts to red to infrared to radio, then eventually undetectable
time slows down and eventually freezes
how does wavelength of light and time as it gets closer to event horizon? FROM PERSPECTIVE OF THINGS FALLING IN BLACK HOLE
wavelength and time are normal
from an observer’s perspective, what happens as something reaches the event horizon? vs perspective of something falling in
observer will never see it cross the event horizon and it will seem like it takes infinity to cross
something falling in will see itself falling into oblivion (if it can even perceive that –> object will be stretched and squeezed)
what happens if you plunge into a SUPERMASSIVE black hole?
it has weaker tidal forces so it is less lethal
mass of black hole vs earth
black hole way heavier than earth
3 ways we know black holes exist?
- collide with LIGO telescope
- have effects on Hulse-Taylor binary
- can imagine their surface with Event Horizon Telescope
what happens if X ray binary has compact object with mass > 3 M_sun
likely black holes!
SUPERMASSIVE BLACK HOLES
- location
- how are they born?
- how do we observe them?
- mass
- at the centre of every large galaxy
- born via evolution of galaxies in early universe
- observe thru motions of stars, jets
- few million to several billion solar masses
MINI/MICROSCOPIC/PRIMORDIAL BLACK HOLES
- mass
- possible time of formation?
- how do we observe?
- Hawking Radiation
- < 1 solar mass
- possibly formed in early universe
- not yet observed
- energy may leave via Hawking Radiation and cause black hole to evaporate
what are gamma-ray bursts?
when do they occur?
what are they produced by?
how do they compare to X-ray bursts from massive star supernovae?
most luminous explosions in the cosmos
occur when the core of a massive star runs out of nuclear fuel and collapses under its own weight
produced by supernova explosions or collisions btwn neutron stars
collisions btwn neutron stars may release even more energy than massive star supernova
