Transcript Document

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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Concept Test
• Which of the following lists, in the correct
order, a possible evolutionary path for a
star?
a. Red Giant, Neutron Star, White Dwarf, Nothing
b. Red Giant, White Dwarf, Black Hole
c. Red Giant, Supernova, Planetary Nebula,
Neutron Star
d. Red Giant, Planetary Nebula, White Dwarf
e. Red Giant, Planetary Nebula, Black Hole
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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. a star which sucks all matter into itself.
b. a window to another Universe.
c. any object which is smaller than its event
horizon.
d. the final result of all stellar evolution.
e. 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
X-rays (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
Fgravity
M
M
Mm
G 2
d
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) 
T otalMass
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  
 M Sun



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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) 
T otalMass
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!
T otalMass
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For next week
•
•
•
For 3/14: Review for Exam #2
For 3/14 after class: re-air Cosmos Chapter 9
For 3/16: Exam #2
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