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Black Holes
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Slide 1
Introduction
What are black holes
Evidence of Black Holes
Star like black holes
Massive black holes
Exotic properties of black holes
Introduction
Escape velocity v = [2GM/R]½
If v(escape) is more than c = 300,000 km/sec
then light cannot escape and we have a black
hole.
Examples of escape velocities:
• Earth = 11 km/sec
• Sun = 610 km/sec.
Slide 2
Introduction
• Event horizon [Schwarzschild radius] R at
v(escape) = c
• c = [2GM/R] ½
• R = 2GM/c².
• Example for M = mass of Sun, R = 3 km.
• R(km) = 3 × (M / M)
• Density = M/V = M/[(4/3)πR³] α M/M³ =1/M²
• So density decreases as mass increases.
Slide 3
What are Black Holes?
• Any object where the mass is inside the event horizon.
• Even though the mass can have any value, we only
know the existence of black holes with masses greater
than the mass of the Sun, all the way up to a several
billion solar masses.
• Since we cannot “see” inside a black hole, we cannot
find out the composition.
• In stellar size black holes the density exceeds that of
neutron stars. Matter is squeezed beyond neutron star
size.
• Black holes have mass (has gravitational force outside
event horizon), angular momentum and charge.
Slide 4
How Black Holes are formed
Two sources
1. Original
2. Formed during supernova
Slide 5
NGC 6240
Two Black Holes
Observed by
Chandra X-ray
observatory
Slide 6
p.280
Slide 7
Fig. 14-2, p.282
Slide 8
Fig. 14-3, p.282
Slide 9
Fig. 14-4, p.282
Slide 10
Fig. 14-8, p.285
Evidence of Stellar Size Black Holes
• Produced during supernova of massive stars.
• Look for double stars with only one visible star, but
non visible star being massive.
Kepler’s Third Law (1618)
M1 + M2 = a3/p2.
where M is in units of M, a distance between stars in
AU and p is the period of revolution in years.
• Cygnus X1 is the first detected black hole and has a
mass of at least 6M.
• X-ray produced by in falling matter from outside of
event horizon.
• So far about 10 stellar size black holes been observed.
Slide 11
Cygnus X-1
Slide 12
Fig. 14-10a, p.286
Slide 13
Fig. 14-11, p.288
A few black hole examples
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Slide 14
Name
Distance Period
Cygnus X1 8K ly
5.6 d
LMC X3
175K
1.7
LMC X1
175K
4.2
V616
3K
0.3
V404
11K
6.5
J0422+32
8K
0.2
J1655-40
10K
2.6
GS2000+25 8K
0.3
Nova Ophiuchi
0.7
Mass Companion
10-15 Type O
4-11
B
4-10
O
3-4
K
8-15
K
4.5
M
4-5
F
5-8
K
4-6
K
“Black Hole in Binary Stars
Slide 15
Star Like Black Holes
• Mass about that of mass of the Sun up to about
20 solar masses.
• Formed during supernovae.
• Evidence from binary (double) stars, where the
mass of the unseen star can be computed.
• About ten such objects have been observed.
Slide 16
Massive Black Holes
• Center of each galaxy, except irregular galaxies.
• Center of globular clusters.
M = 109 M
M = 3×109 M
Slide 17
Massive Black Hole examples
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Slide 18
Milky Way
Andromeda
M32
NGC4261
M87
M104
NNGC 3377
2.6 Million Solar mass
30
3
400
3,000
1,000
100
Exotic Properties of Black Holes
• Black hole is a singularity.
• Black holes with spin collapse into a ring.
• If you can pass through the ring of a black
hole, information on space and time is
discontinuous. I.e. you can end up at a
different time and space. Not too likely.
• Black holes that connect two different parts
of the universe is called worm hole.
Slide 19
Black Holes are not very BLACK
• Hawkins has shown that black holes can
radiate energy, called Hawkins radiation.
• The decay time of the black hole is related to
the mass. The more massive , the longer the
decay time.
• Very low mass black holes (micro black holes)
decay almost instantly.
• Stellar or galactic black hole decay times are
longer than the age of the universe.
Slide 20