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Astronomy Notes
Death of Big Mass Stars - Black Holes
BRING: yellow wishing well, Black Holes video, ppt
DISCLAIMER: These notes do NOT cover everything you need to know. You may need to look up some
item or concept online or in a text. Test questions are not exact copies of the OBJECTIVES but if you
know the OBJECTIVES thoroughly, you should do well on the exams.
HANDOUTS: None
OBJECTIVES:
Give a one sentence description of a black hole.
According to Newton, describe how gravitational force depends on mass and distance.
State the fundamental assumption of the Special Theory of Relativity? Has it been tested? Has it been
disproved?
How does the universe make a black hole?
According to Einstein, describe why one mass exerts a force on a second mass. (Why does gravity occur?)
Describe the basic structure of a black hole. Describe event horizon. Describe singularity.
What is an accretion disk?
List at least three ways astronomers detect black holes.
BLACK HOLE: A black hole is a mass condensed so tightly that nothing, not even light, can escape from
its gravitational effects.
NEWTONIAN DESCRIPTION OF GRAVITY
Gravity is a force
mm
F  G 1 22
D
More mass means more force
Smaller separation means more force
Predicts orbits of satellites, moon, planets very nicely (weak gravitational fields)
ESCAPE SPEED
D
Earth
ME
Vesc
2𝐺𝑀𝐸
𝑉𝑒𝑠𝑐 = √
𝐷
From Earth
From Sun
From 3 M condensed into size of Moon
From 3M condensed into 2-3 miles
~10 km/s
~100 km/s
~10 000 km/s
~300 000 km/s
But NOTHING CAN TRAVEL IN SPACE FASTER THAN THE SPEED OF LIGHT





Fundamental assumption of Special Theory of Relativity
Tested many times in many ways. Ex- Muons reaching Earth.
Many tests have verified. None have disproved.
http://www.exphy.uni-duesseldorf.de/ResearchInst/FundPhys.html
http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html
So if you condense ~3M into 2-3 miles, nothing (not even light) can escape.
HOW DO YOU DO THAT?
Big supernovas (hypernovas)
Perhaps just a big gas cloud collapse (see http://antwrp.gsfc.nasa.gov/apod/ap050402.html )
WHY DOESN’T LIGHT ESCAPE (OR HOW CAN GRAVITY PULL ON LIGHT?)
Einstein’s description of gravity (General Theory of Relativity)
Gravity is the curvature of space-time
Analogy:
Stretch a sheet. Place bowling ball on sheet. Sheet curves.
Marble placed on sheet, “falls” toward bowling ball.
Resident of bowling ball “sees” an attractive force between marble and ball
Calls that force gravity.
But if space-time is curved then light should follow that curve too!
Tested during an eclipse.
Starlight bent by the sun’s gravitational well.
Stars (whose light passed near the sun) appeared in slightly different positions as predicted.
Star appears here
Light bent by suns’s gravity
Sun
Earth
Also tested in microwaves (quasars)
Also tested in radio waves
a. Signals from various Mars missions
b. Signals Cassini probe at Saturn
Star is here
See http://saturn.jpl.nasa.gov/news/press-releases-03/20031002-pr-a.cfm
So if you cram enough mass into a small enough volume, than the gravitational well becomes so steep
that not even light (moving at the speed of light) can climb out.
That is a black hole.
WHAT IS THE STRUCTURE OF A BLACK HOLE?
If you cram 3 M into 2-3 miles,
then gravity forces are so strong that no known force can stop collapse.
All the mass collapses to a point
0 size,
Infinite density
Infinite pressure
ANATOMY OF A BLACK HOLE
Singularity at center
Surrounded by event horizon
EVENT HORIZON
Distance from the singularity at which escape speed = c
Ex: Escape speed from a 3M crammed into ~3 miles is 300 000 km/s
So event horizon for a 3M black hole is ~3 miles
Inside the event horizon, nothing, not even light can escape
Don’t get any closer to a 3M black hole then ~3 miles
Size of this black hole is ~3 miles across
Ex: 100 million solar mass black hole event horizon ~Earth’s orbit size
Compared to MW, that is a grand of sand to US
Side note, not for exam
Inside the event horizon, space-time mixed together
No concept of space and time
Laws of physics depend on knowing what space and time are
Outside event horizon: 3 dim. freedom but no time freedom
Inside event horizon: little 3 dim. freedom but some time control
HOW TO FIND BLACK HOLES
1. Binary stars: Star dancing with an unseen companion of mass > 3 M
Use Doppler Effect to measure speed of star
Use Kepler’s Laws to calculate mass of companion
Binary star simulator: http://www.unm.edu/~astro1/101lab/lab9/lab9_C1.html
Examples of stellar mass black holes
Cygnus X-1 (3 M )
V404 Cygnii (12 M )
SS433 (16M) http://chandra.harvard.edu/press/04_releases/press_010504.html
Side note: This is also one way astronomers detect planets around other stars, by the wobble in the
spectrum of the star
2. Accretion disk: Gas swirling around a black hole
a. See disk itself
b. See x-rays or U.V.from colliding matter in disk or heading toward disk
Example: NGC 4261 http://www.seds.org/hst/ngc4261.html
Example: Whirlpool galaxy (M51) Edge on view of accretion disk 100 L.Y. across
3. Jets of material and radiation exiting perpendicular to accretion disk
Example: See SS433 above
Example: M87 (Giant elliptical galaxy in Virgo Cluster)
4. Gas, dust swirling really rapidly around the central black hole of a galaxy
Use Doppler Effect to measure speed of revolution
Use Kepler’s Laws to calculate mass of black hole
Examples
Sombrero (M104) Inner 2000 LY contains 1 Billion M
Andromeda
Inner 10 L.Y. contains 20 million M
Milky Way
Swirling gasses imply ~3 million M at center
5. Star revolving around central black hole
Example in our own galaxy, Milky Way
http://www.eso.org/outreach/press-rel/pr-2002/pr-17-02.html
Star S2 orbiting center of MW in 15 years! (5000 km/s)
Implies 3.7 million M crammed into area of solar system
Space-time dragging
http://heasarc.gsfc.nasa.gov/docs/xte/learning_center/discover_1197.html
Star ripped apart by a black hole
http://xrtpub.harvard.edu/photo/2004/rxj1242/
Gas falling onto event horizon vs onto neutron star
Example: http://chandra.harvard.edu/photo/cycle1/blackholes/index.html
Mid size mass black holes
Example: http://chandra.harvard.edu/press/00_releases/press_091200.html
OTHER BLACK HOLE WEB RESOURCES
HOMEWORK
Work the tutorial on star lifetime and mass.
Make flash cards for the objectives.
Revised 2 November 2015