Download Neutron Stars and Black Holes

1
Lecture 31
Stellar Remnants
January 14c, 2014
2
Neutron Stars
•
•
•
•
•
Supernova remnant
Tightly packed neutron core.
Size ~ 20 km (small asteroid or medium city)
Mass ~ 1.4 - 3 M
Density very high
– 1 tsp. (5 ml) weighs > 100,000,000 tons on Earth!
• Rotates many times per second due to
conservation of angular momentum; any
rotating body spins faster when it shrinks.
3
• Powerful magnetic field, a trillion times stronger
than the Earth’s
Neutron
Stars
– When star collapses,
magnetic
field is concentrated.
Artist rendering showing a
neutron star is about the size of
lower Manhattan
Figure 22.1, Chaisson and McMillan, 5th ed.
Astronomy Today, © 2005 Pearson Prentice Hall
4
Where would a neutron star be found
on an H-R diagram?
A.
B.
C.
D.
E.
F.
G.
H.
Region A
Region B
Region C
Region D
Region E
Region F
Region G
Region H
5
Where would a neutron star be found
on an H-R diagram?
A.
B.
C.
D.
E.
F.
G.
H.
Region A
Region B
Region C
Region D
Region E
Region F
Region G
Region H
Neutron stars are hot and very tiny so they’d be found
near region F on an H-R diagram.
6
Neutron Star -- HST
7
Pulsars
• 1967 Jocelyn Bell
– Observed object emitting pulses of radio waves.
– Pulses repeated every 1.34 seconds
8
Pulsars
• Hundreds more have been found.
• Some pulse in optical, X-rays, or gamma rays.
• Periods range from 0.03 to 0.30 sec. Periods
gradually increase as pulsar loses energy and
rotates slower
• Some are associated with supernova remnants,
many apparently not… hurled into space at high
speed by the supernova explosion.
9
• Hewitt proposed it
is a rapidly rotating
neutron star
beaming radiation.
– Magnetic pole and
rotational axis not
quite lined up.
– Charged particles at
poles of magnetic
fields emit large
amounts of energy.
Figure 22.3, Chaisson and McMillan, 5th ed.
Astronomy Today, © 2005 Pearson Prentice Hall
Pulsars
“Lighthouse Model”
10
• Not all neutron stars are seen to pulse
– Beam may not be pointed at the Earth
– animation
– Older neutron stars have lost energy and no
longer pulse
Earth never sees
beam of energy
Earth
Earth sees beam of
energy
Earth
11
Crab Nebula
Figure 21.10,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
12
Crab Pulsar
The pulsar must be young because it is seen at visible and
X-ray wavelengths. Old pulsars emit mostly at lower
energy radio wavelengths.
animation
Figure 22.4,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
and
Figure 13-19b,
Comins and Kaufmann,
8th ed. Discovering the Universe,
© 2008 W.H. Freeman & Co.
13
What causes the radio pulses of a pulsar?
A. The star vibrates.
B. We observe pulses when one of the beams of
radio radiation emitted by the spinning star points
toward Earth.
C. The star undergoes periodic nuclear explosions
that generate radio emission.
D. The star’s dark orbiting companion periodically
eclipses the radio waves emitted by the main star.
E. A black hole near the star absorbs energy from it
and re-emits it as radio pulses.
14
What causes the radio pulses of a pulsar?
A. The star vibrates.
B. We observe pulses when one of the beams of
radio radiation emitted by the spinning star
points toward Earth.
C. The star undergoes periodic nuclear explosions
that generate radio emission.
D. The star’s dark orbiting companion periodically
eclipses the radio waves emitted by the main star.
E. A black hole near the star absorbs energy from it
and re-emits it as radio pulses.
15
Einstein’s Special Theory of Relativity
• You cannot determine if a frame of
reference is at rest or moving at constant
velocity
• All observers measure the same speed of
light in vacuum
• The distances and times between events
depend upon your frame of reference
• Length contraction and time dilation
• Time and space are linked together in a
single “fabric” called spacetime
16
Einstein’s General Theory of Relativity
• You cannot determine if a frame of
reference is accelerating or immersed in a
uniform gravitational field
• Space and time are affected by large masses
17
General Relativity
• All matter warps spacetime.
– Like a weight on a rubber sheet.
– Warped spacetime affects the behavior of BOTH
objects and light in its vicinity.
Analogy: a rolling pool ball on an
uneven surface is deflected in
much the same way as a planet’s
curved orbit is determined by
warped spacetime near the Sun.
Figure 22.18b, Chaisson and McMillan, 6th ed.
Astronomy Today, © 2008 Pearson Prentice Hall
18
Evidence for General Relativity
• On May 29, 1919 Arthur Eddington carefully
measured star positions around the eclipsed Sun.
Precession of Mercury’s orbit, and that of a neutron star binary
system, offered further confirming evidence in support of General
Relativity
19
Formation of Black Holes
• If core of star has M >3 M the neutron
pressure cannot hold up the core
– Nothing remains to stop collapse.
– Becomes a “singularity” -- object with infinite
density and infinitely small size.
– Rips a hole in the fabric of spacetime
20
Why are Black Holes Black?
• Escape velocity = velocity needed to escape
the gravitational pull of an object.
vescape
2GM

 11 km/s for Earth
R
• As mass increases or size decreases, gravity
on the surface of the star increases, and a
larger velocity is needed to escape surface.
• When the escape velocity at the surface
becomes greater than the speed of light, no
light can escape.
21
Schwarzschild Radius
Distance from the center of a supermassive object at
which the escape velocity would be equal to the speed
of light.
Normal star: Light
can escape surface
(vescape < c)
Black Hole: Light
cannot escape surface
(vescape > c)
Radius > Schwarzschild Radius
Radius  Schwarzschild Radius
22
Event Horizon
• The event horizon of a black hole is one
Schwarzschild Radius away from its center.
– No events or communication inside the event
horizon can be observed.
Event Horizon
Light cannot escape
Black Hole
Light can escape
23
Evidence for Black Holes
Isolated black holes are hard to observe, but we might
be able to detect gravitational lensing.
Figure 23.23,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
24
Evidence for Black Holes
• We can observe how the black hole’s gravity affects
nearby objects.
– Unseen companion
– Accretion disk
– X-ray emission
Figure 14-15,
Comins and Kaufmann,
8th ed. Discovering the Universe,
© 2008 W.H. Freeman & Co.
25
Cygnus X-1
• A flickering X-ray source that must be smaller than the Earth
• The X-ray source seems to force the nearby supergiant star to
wobble
• Conclusion: It’s a 30-solar-mass B0 supergiant and an 11solar-mass black hole that are orbiting each other
Figure 22.23, Chaisson and McMillan, 6th ed. Astronomy Today, ©
2008 Pearson Prentice Hall
26
Black Holes in Galaxies
• BHs may have formed in center when galaxy formed.
• Mass of billions of stars in size of SS (Kepler’s 3rd Law).
• Black hole likely in center of the Milky Way.
Accretion disk
surrounding a
300-million-solarmass black hole
in the galaxy
NGC 7052.
27
The Schwarzschild radius of a body is
A. the distance from its center at which nuclear
fusion ceases.
B. the distance from its surface at which an orbiting
companion will be broken apart.
C. the maximum radius a white dwarf can have
before it collapses.
D. the maximum radius a neutron star can have
before it collapses.
E. the radius of a body at which its escape velocity
equals the speed of light.
28
The Schwarzschild radius of a body is
A. the distance from its center at which nuclear
fusion ceases.
B. the distance from its surface at which an orbiting
companion will be broken apart.
C. the maximum radius a white dwarf can have
before it collapses.
D. the maximum radius a neutron star can have
before it collapses.
E. the radius of a body at which its escape
velocity equals the speed of light.
29
Traveling into a Black Hole -Tidal Forces
• Extremely large tidal forces near a BH.
• Difference in forces of gravity on near and
far side would pull object apart.
30
Time Dilation
• From outside, observer sees clock on board
tick more and more slowly than outside of
craft.
• The closer to black hole, the slower time
appears to run.
• At event horizon, time appears to stop!
– An observer far away never sees the craft fall
into BH
– An observer inside the craft sees time proceed
at its normal rate.
31
Inside of a Black Hole
• Scientists to not know for sure what is
inside of a black hole.
• Theories of physics break down for such
high densities.
• Hard to make and test new model since
black holes cannot be directly observed.