Transcript Wednesday, Nov 5 - Department of Physics and Astronomy

Lecture 25 – Black Holes
Comet Siding Spring, Mars, 51 Ophiuchi: Rolando Ligustri (CARA Project, CAST)
Recall column
• Read before class:
– Galaxies
– Hubble Types, properties Chapt 24
– Milky Way Structure Chapt 23
summary
SNe Type II -- Core Collapse SNe:
Recall column
summary
summary
Gamma Ray Burst == GRB
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Gamma-rays
Optical
Swift
satellite
capturing a
gamma-ray
burst
Light curve shows GRB == Super Duper Nova
Progenitor star
M ~ 80 to 130 Msun
Core collapse  Black Hole !
Black holes (BH):
Recall column
summary
• Discuss with your neighbour what you
think a black hole is:
– How do they form? (many ways?)
– What is their structure?
– How do we detect them?
– Can anything escape from them?
– What is gravity like near them? Are
they a cosmic vacuum cleaner or
not?
– What are their sizes?
– Puzzles?
GR
summary
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• spacetime (spt)
– 3 dimensions of space + 1 dimension
of time
• light follows curved paths in spt
– a mass (object) bends spt causing
curved paths for particles with or
without mass (e.g. photons)
Black Holes: Definition
summary
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Note that Newton’s
Laws hold outside the
black hole.
1) A region of space-time where
gravitation becomes overwhelming
& the curvature of spt is so great
that space “folds” over on itself.
Black Holes: Definition:
summary
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Photon just
outside BH
also lose
energy.
2) Escape velocity > = c.
– escape velocity is the energy of
motion required to overcome
gravity & go into orbit.
Black Holes: Definition:
summary
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Photon just
outside BH
also lose
energy.
Since escape velocity > = c.
Matter & light cannot escape.
• photon loses energy rather than
change v. E = h * frequency
Black Hole: components
summary
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• Singularity: A point in the universe
where the density of matter &
gravitational field are infinite.
• Event Horizon: An imaginary spherical
surface with a radius = Schwarzschild
radius (Rsch).
• Rsch == distance from centre of an
object such that, if all the M were
compressed within that region, escape
speed = c.
Black Hole (BH) : Components
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Standard components
due to formation
only if other objects
come near
– Singularity
– Event Horizon (Rsch)
– Magnetic fields (note B lines)
– rotating
– Accretion disk
– Jets
summary
BH: Accretion Disk
summary
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• Tidal forces stretch stars or its outer layer towards
BH
 stellar material orbits in a disk.
• Crashing into itself, disk gas loses energy
 closer to event horizon.
• Some material falls through Rsch.
• some particles travel along B lines.
bi-polar jets.
Model the equations:
summary
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• John Hawley at the University of
Virginia
• general relativistic visualization of a
supercomputed magnetohydrodynamic simulation of a disk and
jet around a black hole.
Black Holes: The size of the Event Horizon
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Only depends on mass!
Calculate for Jupiter:
summary
Black Holes: The size of the Event Horizon
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summary
Black Holes: The size of the Event Horizon
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Only depends on mass!
• Calculate Rsch for Jupiter.
•  roughly the height of a room
Need to be close to the event horizon to fall in.
However tides.
Toothpaste effect.
summary
Black Holes: The size of the Event Horizon
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• Calculate radius of the event horizon for the Sun.
a) Out to the orbit of Mercury.
b) Out to the current radius of the Sun.
c) 10000 m
d) 3000 m
e) 3 m
summary
Black Holes:
summary
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If our Sun became a black hole right now,
nothing would happen to the orbit of
the Earth. The force of gravity doesn’t
change until one is close to the event
horizon & mass of sun hasn’t changed.
Only its density has become infinite.
Earth would not get sucked into the
black hole because black holes do NOT
act like Cosmic Vacuum Cleaners.
Near the Event Horizon: Distortions
summary
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• Orion constellation with & without
intervening BH.
• stars appear twice & whole sky
repeated around event horizon.
• Robert Nemiroff at Michigan Tech U.
Alain Riazuelo, IAP/UPMC/CNRS
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summary
Near the Event Horizon: Distortions
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•
•
•
Unrealistic since tidal destruction would occur
blue- & red-shift
Andrew Hamilton, U Colorado
summary
Near Event Horizon: Distorted Accretion Disk
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“Gravitation” by
Misner, Thorne
and Wheeler
• 1979 professional paper by J-P
Luminet in Astronomy &
Astrophysics using Kip Thorne &
collaborator’s equations.
summary
Near the Event Horizon: Distortions
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• Interstellar movie
• Equations by Kip Thorne
summary
summary
Into the Black Hole
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• http://vimeo.com/8723702
Inside the Event Horizon
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summary
• According to General Relativity, time
isn’t a particularly special dimension.
•  can be swapped with another
dimension.
• Outside Rsch move in any direction in
space but only 1 direction in time
(towards the future).
• Inside Rsch only move forward in space
towards the singularity but move
backwards & forwards in time!
Can anything escape a BH?
summary
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1. E.g. Roger Blandford: Mechanical
energy can escape.
– B lines threaded through the
gas, in the accretion disk &
falling into the black hole
– B lines twist around rotating
black hole, slowing it down.
– The energy of rotation travels
out along B lines & deposited in
disk  explains X-ray hot spots.
Can anything escape a BH?
summary
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2. Hawking Radiation:
– A Quantum Mechanical effect due to
Heisenberg Uncertainty Principle.
– Even a vacuum has fluctuations:
Pairs of virtual particles appear together at
some position in st, move apart, come back
together & annihilate.
- If close to BH, 1 of the pair falls in & the
other escapes to infinity, becoming a real
particle.
summary
Can anything escape a BH?
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2. Hawking Radiation:
Radiation with a black body T
inversely proportional to BH M
Small BH - higher temperatures.
As BH radiates, r decreases, T
increases & it radiates faster.
BH evaporate!
More detail next lecture.
summary
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• What can escape from a black hole?
Hawking radiation and mechanical
energy.
Using “rate of evaporation”, can figure
out BH lifetime.
On class website
Types of Black Holes
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Type
Supermassive
Mass
Rsch
Location
Detection Method
summary
Lifetime
Solar Masses
yrs
>3*10**6
10**97 to
10**106
1/2 light-min Centre of Galaxies Doppler shift of orbiting
gas or stars.
Period of orbit of stars in
images.
Mid-mass
500-5000
(Intermediate)
Stellar Mass
1 to 10
smaller than Globular Clusters Doppler shift
the solar
radius
X-rays from accretion.
3 to 30 km
Throughout disk
of galaxies
around
Doppler Shift of companion. 10**67
X-rays from accretion.
Gravitational lenses.
Primordial
less than 1
1 cm
Throughout
universe
Gamma-rays if
evaporating but this is
the only type of black hole
that has
not been observed
even indirectly.
around
10**10
Types of Black Holes:
summary
M 13 Danny Lee Russell
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• Intermediate BH.
• Form by mergers of stars.
• Globular cluster ~ 1 million stars within
several pc.
• Star density high in globular clusters so
more opportunity for the merger of stars.
summary
Types of Black Holes:
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•
•
•
•
Supermassive Black Holes
Centres of Galaxies (including Milky Way)
In both spirals and ellipticals.
BUT If the bulge is very small & featureless then
there may not be a BH.
• Do galaxies form around black holes? Or do
galaxies form & black holes accumulate in centre?