Black Holes, Gravity to the Max. By Dr. Harold Williams of Montgomery College Planetarium http://montgomerycollege.edu/Departments/planet/ Given in the planetarium Saturday 20 November 2010. Black Hole in front of the Milky Way, out galaxy with 10 Solar Masses and viewed from 600km away. 0.
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By Dr. Harold Williams
of Montgomery College Planetarium
Given in the planetarium Saturday 20 November 2010
Just like white dwarfs (Chandrasekhar limit: 1.4 Msun), there is a mass limit for neutron stars:
Neutron stars can not exist with masses > 3 Msun
We know of no mechanism to halt the collapse of a compact object with > 3 Msun.
It will collapse into a single point – a singularity:
But only at the end of time relative to an outside observer.
=> A black hole!
Velocity needed to escape Earth’s gravity from the surface: vesc≈ 11.6 km/s.
Now, gravitational force decreases with distance (~ 1/d2) => Starting out high above the surface => lower escape velocity.
If you could compress Earth to a smaller radius => higher escape velocity from the surface.
An astronaut descending down towards the event horizon of the black hole will be stretched vertically (tidal effects) and squeezed laterally unless the black hole is very large like thousands of solar masses, so the multi-million solar mass black hole in the center of the galaxy is safe.
Compact object with > 3 Msun must be a black hole!
Washington Post, Tuesday, November 16, 2010.
Optical spectrum shows spectral lines from material in the jet.
Two sets of lines: one blue-shifted, one red-shifted
Line systems shift back and forth across each other due to jet precession
Accretion disks around black holes
Strong X-ray sources
Rapidly, erratically variable (with flickering on time scales of less than a second)
Sometimes: Quasi-periodic oscillations (QPOs)
Sometimes: Radio-emitting jets
Short (~ a few s), bright bursts of gamma-rays
GRB of May 10, 1999: 1 day after the GRB
2 days after the GRB
Later discovered with X-ray and optical afterglows lasting several hours – a few days
Many have now been associated with host galaxies at large (cosmological) distances.
Probably related to the deaths of very massive (> 25 Msun) stars.
Black Holes: Accreted matter disappears beyond the event horizon without a trace.
Neutron Stars: Accreted matter produces an X-ray flash as it impacts on the neutron star surface.
Fi = mi a
Average curvature of spacetime a energy density