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BLACK HOLES
Gregory R. Sivakoff
University of Virginia
BLACK HOLES
•
What is a Black Hole?
•
How Do We Know
Black Holes Exist?
•
Why Do We Study
Black Holes?
Understanding the
GRAVITY of the Situation
The gravitational
force between two
objects depends on
• their Masses
• their Separation
Understanding the
GRAVITY of the Situation
2
Gravity & Orbits
Escape Velocity
crush
D =8000 miles
M = 1025 kg
Vesc = 25,000 miles/hour
D = 800 miles
M = 1025 kg
Vesc = 79,000 miles/hour
General Relativity:
Mass Warps Spacetime
• Space and time
must be considered
together
• Gravity is really a
warping of space
and time
• Denser objects
warp spacetime
more
Warped Space-time
Gravitational Lensing
Gravitational Lensing
Curving Light
Escape Velocity
crush
(not to scale)
D =8000 miles
M = 1025 kg
Vesc = 25,000 miles/hour
D = Vesc = 2/3 inch
M = 1025 kg
Vesc = 186,000 miles/second
The Event Horizon:
Boundary of a Black Hole
• Nothing inside the
radius of the event
horizon, not even
light, can escape
• Mass  Radius 
• 1.8 miles for Sun
• To the Moon for
400,000 Suns
Knowledge About What Is
Inside The Event Horizon
• Charge (most likely neutral)
• Spin
• Non-spinning (unlikely)
• Spinning
• Mass (In units of the Sun)
• Stellar mass (~3-30)
• Supermassive (~100,000-billions)
Black Holes Are Dense
• Minimum density of
a Solar Mass Black
Hole is similar to
that of an Atomic
Nucleus
• But, we believe
black holes have
(nearly?) infinite
density
Formation of
Stellar Mass Black Holes
Formation of
Supermassive Black Holes
• The first massive stars make
large stellar mass black holes
• Massive stellar mass black holes
sink to center of a galaxy
• Black holes mere and grow in
dense stellar center
How Do We Know
Black Holes Exist?
Cygnus X-1
Cygnus X-1:
X-ray Image
Cygnus X-1:
Optical Image
Supergiant Star
25 Solar Masses
Cygnus X-1:
Determining the Orbit
Orbit Indicates
35 Solar Masses
Cygnus X-1:
The Whole System
Black Hole
10 Solar Masses
X-rays from
orbitting material
Supergiant
25 Solar Masses
Most of optical light
The Galactic Center
1 deg = 450 lyr
Sagittarius
(teapot)
Galactic
Center
Next
Slide
5 arcsec = 0.6 lyr
The Galactic Center
3.6 Million Solar Masses in 70 Earth Radii
Why Do We Study
Black Holes
• Exceptionally energetic
• Stellar Mass Black Holes are
remnants of massive stars
• Supermassive black holes may
interact with galaxy and affect how
galaxies evolve
Comparison of Energetics
Gasoline
12 miles
Nuclear Reactions To the Moon
Falling onto a
~60 times farther
Black Hole
than the Moon
~ 50 millionths
Hiroshima
~ 1 Hiroshima
~ 70 Hiroshimas
Bright Shining Beacons
Stellar Mass Black Holes:
Massive Stellar Remnants
We Are Made Of Star Dust
Supermassive Black Holes
Affect Galaxy Evolution
WHAT IS A BLACK HOLE
•
Black Holes are extremely
(infinitely?) dense concentrations
of matter
•
They are so dense they create
regions of space from which
nothing can escape
The strong effects of gravity lead
to interesting behavior, but they
do not suck things in
•
HOW DO WE KNOW
BLACK HOLES EXIST?
•
We can study the space outside
a Black Hole
•
By measuring the mass and
density of an object, we can
prove it is most likely a Black
Hole
WHY DO WE STUDY
BLACK HOLES
•
Black Holes can lead to
extremely energetic
phenomena
•
Black Holes are the remnants of
the massive stars whose dust
we are made of
•
Supermassive Black Holes may
affect how galaxies evolve