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Black Holes and
Neutron Stars
Dead Stars
Copyright – A. Hobart
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Goals
•
•
•
•
What are black holes?
How do we see black holes?
What happens when black holes are in binaries?
Supermassive Black Holes
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Density
• Density = mass per volume
• From Red Giant cores to White Dwarfs to Neutron
Stars, density has been increasing.
• As density increases, the force of gravity on the
surface increases.
• The greater the force, the higher the escape
velocity:
– How fast you need to go in order to escape the surface.
• How dense can something get?
• How strong can the force of gravity be?
• What if the escape velocity is faster than light?
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Singularity
• When a high-mass star’s core
is greater than ~3 x Msun,
then, when it collapses,
neutron degeneracy pressure
can’t balance gravity.
• The star collapses to form a
singularity.
• No size at all.
• Density infinite.
• Escape velocity > c
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Black Hole Diagram
Singularity
Event Horizon
.
Schwarzschild Radius
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Schwarzschild Radius
• Distance from object where vesc > c
Object
Earth
Jupiter
Sun
Mass
Radius
cm 
 1M

R S  3km  
M
Sun 

300 x Earth
3m
6 x 1024 kg
300,000 x
Earth
3 km
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Concept Test
•
A black hole is best defined as:
a.
b.
c.
d.
e.
a star which sucks all matter into itself.
a window to another Universe.
any object which is smaller than its event horizon.
the final result of all stellar evolution.
none of the above
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Seeing Holes
• Can’t see black hole itself,
but can see matter falling
into a hole.
• Gravitational forces stretch
and rip matter: heats up.
• Very hot objects emit in Xrays (interior of Sun)
• Cygnus X-1.
http://www.owlnet.rice.edu/~spac250/steve/ident.html
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Binaries
• Gravitational tides pull matter off big low density
objects towards small high density objects.
Cygnus X-1
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Holes Don’t Suck
• Newton’s Laws of gravity only depend on mass
and separation.
• Kepler’s Laws of orbits only depend on mass and
separation.
• At 1 AU, force of gravity from a 1 Msol B.H. is
same as from a 1 Msol star.
• At surface of each, force of gavity is very
different!
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Tides
Mm
Fgravity  G 2
d
M
M
m
m
m
m
m
m
•While each m is attracted to each other m, the difference in force
from M is greater.
•The closer you are to the object M, the more extreme this is!
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Accretion disk
Tides
Frictional
Heating
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Concept Test
•
We can see X-rays from black holes because?
a. X-rays are more energetic than visible light and so can
escape from the event horizon.
b. X-rays can pass through ordinary matter showing us
things we can’t normally see.
c. Light given off by objects as they enter the event
horizon are gravitationally redshifted to X-rays.
d. Material flowing into a black hole is heated so much
that the thermal radiation peaks in X-rays.
e. None of the above
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Cygnus X-1
• 1970s
• Intense source X-rays.
• “Near” star HDE226868.
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HDE226868
• Doppler shifts of
HDE226868
• Like before, we get mass
of star and unseen
companion.
Separation ( AU )
Period ( yrs ) 
Total Mass
3
2
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The Companion
• Result:
Period = 5.6 days
Total Mass ~ 28 x Msun
• From spectral type of HDE226868 we estimate its
mass ~18 Msun.
• Companion M = 10 Msun!
• Massive!
• But where is its light?
• Dark!
• Can’t be a normal star, or even neutron star.
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X-ray Source?
• Star brightness fluctuates every 5.6 days.
• X-rays drop off every 5.6 days!
• Companion must be source of X-rays!
• REH = 30 km!
R EH
 M 

 3km  
 MSun 
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Supermassive Black Holes
Distance
• Photograph the center of a
galaxy.
Velocity
• Make spectrum of light
from center.
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Heart of Darkness
• From Doppler shift get a
velocity.
• From picture get a
separation.
• From Kepler’s Laws get
a Total Mass.
Separation ( AU )
Period ( yrs ) 
Total Mass
3
2
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The Dark Truth
• Observe:
V = 400 km/s within 26 LY of center.
• So:
Separation ( AU )3
2
Period ( yrs ) 
– Period = 121,600 yrs
– Separation = 26 LY = 1,600,000 AU
• Total Mass in central pixel:
300,000,000 x Mass of Sun!
• But where’s all the light?
• Small, massive, dark  black hole?
• REH = 6.5 AU!
Total Mass
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Homework #13
•
•
For 2/22: Read B18.4: Voyage to a Black Hole
I am the captain of a starship that gets stuck just outside the
event horizon of a black hole. When I am rescued, what am I
most likely to find:
a.
b.
c.
d.
e.
I have aged a day, but 1000 years have passed for everyone else.
A day has passed for everyone else, but I am now an old man.
While I have stayed the same age, I think everyone else has grown
old. Strangely, everyone else thinks they have stayed the same age,
but I have grown old.
While I have grown old, I think everyone else has stayed the same
age. Meanwhile, everyone else thinks the reverse.
None of the above.
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