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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 20: Black Holes

Readings: Ch 24, sections 24-3, 5, 6, 7 & 8

Key Ideas

Black Holes are totally collapsed objects
gravity so strong not even light can escape
predicted by General Relativity

Schwarzschild Radius & Event Horizon

Find them by their Gravity
X-ray Binary Stars

Black Hole Evaporation
Emit "Hawking Radiation"

Gravity's Final Victory

A star more massive than about 18 Msun would leave behind a post-supernova core this is larger than 2-3 Msun:

Neutron degeneracy pressure would fail and nothing can stop its gravitational collapse.

Core would collapse into a singularity, and object with

Black Hole

The Ultimate Extreme Object

Becomes a Black Hole:

Schwarzschild Radius

Light cannot escape from a Black Hole if it comes from a radius closer than the Schwarzschild Radius, RS to the singularity:
Schwarzschild Radius
Where M = Mass of the Black Hole

A black hole with a mass of 1 Msun would have a Schwarzschild Radius of RS=3 km.

Compare this with a typical 0.6 Msun White Dwarf, which would have a radius of about 1 Rearth (6370km), and a 1.4 Msun neutron star, which would have a radius of about 10km.

1.5Msun Black Hole, 1.5Msun Neutron Star,
    and Manhattan to scale.
Comparison of a 1.5 Msun Black Hole and Neutron Star with Island of Manhattan for scale.

RS is named for German physicist Karl Schwarzschild who in 1916 was one of the first people to explore the implications of Einstein's then-new General Theory of Relativity, the modern theory of Gravity.

The Event Horizon

RS defines the "Event Horizon" surrounding the black hole's singularity:

The Event Horizon marks the "Point of No Return" for objects falling into a Black Hole.

Gravity around Black Holes

Far away from a black hole:

Close to a black hole:

Journey to a Black Hole: A Thought Experiment

Two observers: Jack & Jill
Jack, in a spacesuit, is falling into a black hole. He is carrying a low-power laser beacon that flashes a beam of blue light once a second.

Jill is orbiting the black hole in a starship at a safe distance away in a stable circular orbit. She watches Jack fall in by monitoring the incoming flashes from his laser beacon.
Black Hole Thought Experiment

He Said, She Said...

From Jack's point of view:

From Jill's point of view:

Near the Event Horizon...

Jack Sees:

Jill Sees:

Down the hole...

Jill Sees:

Jack Sees:


The powerful gravity of a black hole warps space and time around it:

Take a Virtual Trip to a Black Hole or Neutron Star. Pictures & movies by relativist Robert Nemiroff at the Michigan Technical University.

Seeing what cannot be seen...

If black holes are black, how can we hope to see them?

Look for the effects of their gravity on their surroundings.

X-Ray Binaries

Bright, variable X-ray sources identified by X-ray observatory satellites:

Estimate the mass of the unseen companion from the parameters of its orbit.

Black Hole Candidates

A number of X-ray binaries have been found with unseen companions with Masses > 3 Msun, too big for a Neutron Star.

Currently 20 confirmed black hole candidates in our Galaxy:

First was Cygnus X-1: M = 7-13 Msun
Largest is GRS1915+105: M = 10-18 Msun
Most are in the range 4-10 Msun

Estimated to be as many as 1 Billion stellar-mass black holes in our Galaxy, which points out how very hard it is to find something that does not emit any radiation of its own.

Black Holes are not totally Black!

"Classical" General Relativity says:

But, General Relativity does not include the effects of Quantum Mechanics.

Evaporating Black Holes

Black Holes evaporate slowly by emitting subatomic particles and photons via "Hawking Radiation": The smaller the mass, the hotter the black hole, and so the faster the evaporation.

For black holes in the real universe, the evaporation rate is VERY slow:

Probably unimportant today, but it could be an important process in the distant future of the Universe.

A Final Word

Physicist John Archibald Wheeler (b. 1911), who coined the term "black hole", has been one of the most innovative thinkers on space and time. The singularity at the center of the black hole, which we passed over quickly, is not a simple beast. Its existance, at least in the classical theory of relativity, is a real problem that is telling us about the limits of our ideas. Prof. Wheeler has put it this way:

[The black hole] "teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as 'sacred', as immutable, are anything but."
from "Geons, Black Holes, & Quantum Foams: A Life in Physics" by Wheeler and Ford (1998, AIP Press)
Return to [ Unit 3 Index | Astronomy 162 Main Page ]
Updated: 2007 August 3
Copyright Richard W. Pogge, All Rights Reserved.