astronomy Flashcards
Iron Core Collapse
Stars burn until they reach their iron core. After this there are no fusion fuels left for the star to tap. Iron core collapse follows. There is an explosion and only raw material for planets/next generation of stars is left
The Race to Explosion for BH
The baby neutron star is supported by neutron degeneracy pressure. Neutron star mass is growing because of inflating matter. If M> 2.5 Msun, neutron degeneracy pressure fails and BH is formed. Neutron stars are left behind. They shine by residual heat. Pulsars also remain.
Neutron Stars
proposed to explain extreme energy of supernovae
Pulsars
pulsating radio sources. Rapidly spinning, magnetized neutron stars
Black Holes
gravity wins, core collapses, so all mass goes into infinitely small point, the singularity of a new BH
No Hair Theorem
BH can only have mass, charge, and spin. Any other differences will be radiated away as gravitational waves or electromagnetic radiation.
Electric Charge
BH should be neutral. A charged BH will accrete particles of the opposite charge due to electrostatic force.
Spin
the rate at which an object revolves around an axis
Equatorial Bulge
The earth has an equatorial bulge due to rotation. This is because the centrifugal force from rotation slightly counteracts the gravitational force. Spinning BHs also develop equatorial bulge.
Frame Dragging
The Static Limit is as close as a fixed observer can get with a rocket. Any closer and the rocket must route with BH. Anyone at the event horizon will appear to rotate exactly with the black hole.
Static Limit
edge of the ergosphere. Closest you can get and remain stationary
Finding BHs
gravitational lensing. Binary systems. Accretion.
Binary systems
star-BH will orbit around center of mass. Causes wobbling stars. Doppler effect
Accretion
the gravity of the BH will capture some of this gas. The infalling gas particles will collide and create an accretion disk. Most particles don’t fall directly in but get caught in its orbit and slowly spiral in. The gas gets hot as it falls in and emits light.
Light emitted by accretion disks
x-ray light
Stellar mass BH
are formed as a result of stellar evolution. They are much smaller than supermassive BHs.
Neutron stars emit:
x-ray light. Measurement of the unseen mass is one way to separate BHs from neutron stars
Infall Energy
Most objects don’t fall directly into a BH. They either experience spaghettification or begin to orbit
Orbiting material has angular momentum
MVR (r= radius of orbit) Angular momentum does not change but is conserved. Particles not on perfectly circular orbits will collide, exchanging angular momentum.
ISCO
Innermost stable orbit. Inside the ISCO matter likely plunges directly into the BH without radiating energy.
Temperature Structure of the Disk
more energy comes out closer to the center. Hotter and smaller near center
Max Luminosity/Eddington Luminosity
If great enough, pressure can equal gravitational pull of BH and shut off accretion. More massive BH have higher max luminosities.
Spinning BH are:
hotter
Jets
a small fraction of matter in accretion disks can be ejected perpendicular to the disk at a high velocity
Evidence for BHs
X-ray emissions from normal stars. Velocity measurements indicate unseen massive objects. X-ray study of iron accretion disks show large velocities and gravitational redshift. Low minimum luminosity indicate accretion into event horizon
BHs and brightness
They do not always stay very bright
Conservation on accretion disks
if some particles in an accretion disk moves inward, conservation of angular momentum requires: other particles to move outward.
Spiral Galaxies
blue colors of young stars. Lots of dust and gas
Elliptical Galaxies
Light distribution is smooth. lower mass, red, older stars: little gas.
Irregular galaxies
often blue young stars. Usually much smaller
Expansion
most nebula appear to be receding away from us rapidly
redshift
light from other galaxies is shifted to longer wavelengths
rate universe is expanding
the universe is expanding at a nearly constant rate
observers in all galaxies see:
the same expansion, there’s no center
Quasars:
lass than one light week in size. Very red shifted. Powered by BH accretion
Active Galactic Nuclei
Quasars: most luminous, outshine their host galaxies. Seyfert: galaxies, low luminosity quasars. Radio galaxies: AGN that are unusually bright at radio wavelengths
How do SMBHs get so big?
Start with the collapse of massive stars.
Possible solutions as to why SMBHs are so big
Accretion faster than the eddington luminosity rate? Initial stars and their black holes are unusually massive. SMBHs may have formed directly in early universe?
Disk and Bulge Structure
disk: plate-like structure orbiting stars and gas. younger stars and active star formation. Bulge: inner spherical distribution of stars; older stars, little gas.
The mass of central SMBH:
is correlated to the total stellar velocity dispersion and total stellar luminosity. Also affects availability of material falling into BH
Star Formation
Start with cloud of gas and dust. Gravity makes it collapse. It needs to cool to continue collapsing. Cloud will eventually ignite nuclear fusion and become a star.
