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Black Holes The Science Behind The Science Fiction 

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• What is a black 

hole? 

• Do BHs exist inNature?

 –YES!

• How do we find them?• What do they look

like?

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Stars

Pressure Balances Gravity

The Sun

 F  r  o m  w w w . a s  t   r  o n o m y  n o t   e s  . c  o m 

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Eluding Gravity’s Grasp 

Earth: Vesc = 27,000 miles/hour (11 km/s)

Sun: Vesc = 1.4 million miles/hour (600 km/s)

V esc

2GM 

 R

Mass M

Radius R

Escape Velocity

Escape Velocity

Speed Needed To

Escape An Object’s 

Gravitational Pull 

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“Dark Stars”Speed of light 1 billion miles/hour (3x105 km/s)

What if a star were so small, escape

speed > speed of light?

A star we couldn’t see! 

Earth mass: R 1 inch

Solar mass:

R

2 miles

Vesc = speed

of light 

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1915: General Relativity, Einstein’s Theory of Gravity

1916: Schwarzschild’s Discovery of BHs in GR 

BHs only understood & accepted in the 1960s

(Term “Black Hole” coined by John Wheeler in 1967) 

Karl SchwarzschildAlbert Einstein

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If an object is small enough, gravity overwhelms

pressure and the object collapses. Gravity is so

strong that nothing, not even light, can escape.

“Radius” of a BH 

2 miles for a solar mass

1 inch for an Earth mass

NOT a solid surface

All Mass at the Center 

(GR not valid there)

Black Holes in GR

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Gravity bends the path of light

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Massive bodies and escape speed

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Gravity bends the path of light

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.Mass

.Total electric charge

.Spin = angular momentum

Three parameters completely

describe the structure of a black hole 

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Seeing black holes

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Gravitational lensing 

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Basic structure of black hole..

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Event horizon

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Rotating black holes

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A nonrotating black hole has only a “center”

and a “surface” 

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Intro: Of fourth dimension

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mass bends space-time

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Falling into a black holes

• Signals time dilated and red

shifted . (external)

• No communication

• Incoming signals can enter

• Fall for a finite time in event

horizon

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Falling into a black hole

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Gravitational redshift

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Falling into a black hole

• Spaghettification 

Ripped apart

•  Atoms 

• Electrons

nuclei

• Protons• Neutrons

Quarks

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How do we find BHs in Nature?

“It’s black, and it looks like a hole.

I’d say it’s a black hole.” 

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Where are BHs Found?

Centers of Galaxies Binary Stars

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The Milky Way Galaxy: ~ 100,000 light-years across 

4 106 Msun

Black Hole 

Scale: Size of Solar System: 0.01 light-years

Typical Distance btw. Stars: 1 light-year

Central Black Hole Mass: 4 million Msun

Also ~ millions of 10 Msun BHs

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Stars in the Central

Light-Year of the Galaxy

Keep Zooming In … 

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Milky Way

Black Holes and Galaxies:

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• A striking

correlation wasfound between

black holes

and their host

galaxies.

• 0.1% 

•Directly

proportional

Black Holes and Galaxies:

Chicken or Egg? 

Techniques for finding black holes

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Techniques for finding black holes

•Accretion disks

•

Gas jets•Xray images

•Infrared images

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Seeing black holes

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Light From Gas Falling

Into the Black Hole

Infrared Image

BH

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How do black holes forms ?

•Gravitational collapse

•Emerge due toattraction

•Big bang

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These are some of supernova

explosion

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36 Dr. H. Fearn CSUF Physics

How do big Black Holes form

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  Dispelling the Myths … 

• BHs are not cosmic vacuumcleaners:  only inside the horizon

is matter pulled extremely inward

• Far away from a BH, gravity

is no different than for any

other object with the same mass

M87

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Look closer

still:

Disk of gas

orbiting at500 km/sec,

around

one billion

solar massescrammed into

1 parsec.

A black hole.

M87

C A

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Jets from atiny region in

the center

power

tremendousluminosity in

radio and X

rays.

A black hole.

Cen A

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The Moral of the Story … 

• Physicists said that Black Holes could exist

• Astronomers have found that BHs do exist 

• BHs are responsible for the most dramatic and

energetic phenomena in the universe