Transcript Copyright © 2010 Pearson Education, Inc. Chapter 13 Neutron Stars

Chapter 13
Neutron Stars
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Chapter 13
Neutron Stars and Black Holes
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Units of Chapter 13.1 - 13.4
Neutron Stars
Pulsars
Neutron Star Binaries
Gamma-Ray Bursts
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Neutron Stars
After a Type I supernova, little or nothing
remains of the original star.
After a Type II supernova, part of the core
may survive. It is very dense – as dense as
an atomic nucleus – and is called a neutron
star.
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Discovery!
• In 1934 Walter Baade and Fritz Zwicky
proposed the existence of the neutron
star, only a year after the discovery of
the neutron by Sir James Chadwick.
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Neutron Stars
Neutron stars,
although they have
1-3 solar masses,
are so dense that
they are very small.
This image shows
a 1-solar-mass
neutron star, about
10 km in diameter,
compared to
Manhattan.
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White Dwarf
• A white dwarf of 1 solar mass is the size
of Earth.
10,000 km
White Dwarf and
Neutron star have
the same mass
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Neutron Star
Neutron Stars
Other important properties of neutron stars
(beyond mass and size):
• Rotation – as the parent star collapses, the
neutron core spins very rapidly, conserving
angular momentum. Typical periods are
fractions of a second. (Fastest 43,000 rpm)
• Magnetic field – again as a result of the
collapse, the neutron star’s magnetic field
becomes enormously strong.
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How To Gain Weight
• The gravitational field at a neutron star’s
surface is about 2×1011 times stronger
than on Earth
• 200,000,000,000
• one teaspoon (5 milliliters) of its
material would have a mass over
5.5×1012 kg
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What’s Inside?
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Quantum Physics
• Pauli exclusion principle. This principle
states that no two neutrons (or any
other fermionic particles) can occupy
the same place and quantum state
simultaneously.
• Wolfgang Pauli
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What is a Neutron?
• Can be thought of as a proton which
has merged with an electron. The
positive charge of the proton plus the
negative charge of an electron yields
zero net charge.
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Neutrons
• Neutrons have no electrical charge
• Neutrons do not need to overcome any
Coulomb barrier
• 1.675×10−27 kg
• Lifetime 881.5 seconds
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How Many and Where?
• About 2000 neutron stars in
the Milky Way galaxy
• Often detected as radio
pulsars
• Pulsars were discovered by
Jocelyn Bell in 1967
• Predicted by J. Robert
Oppenheimer in 1938
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Pulsars
The first pulsar was discovered in 1967. It
emitted extraordinarily regular pulses; nothing
like it had ever been seen before.
After some initial confusion, it was realized that
this was a neutron star, spinning very rapidly.
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Question 1
Pulsars usually
show all of the
following EXCEPT
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a) extremely rapid rotation.
b) high-temperature fusion reactions.
c) a narrow regular pulse of radiation.
d) high-speed motion through the galaxy.
e) an intense magnetic field.
Question 1
Pulsars usually
show all of the
following
EXCEPT
a) extremely rapid rotation.
b) high-temperature fusion reactions.
c) a narrow regular pulse of radiation.
d) high-speed motion through the galaxy.
e) all of the above.
Pulsars are neutron stars no
longer undergoing fusion in
their cores.
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Pulsars
But why would a neutron star flash on and off?
This figure illustrates the lighthouse effect
responsible.
Pulsar Crab Nebula
Strong jets of matter are emitted at the magnetic
poles, as that is where
they can escape. If the
rotation axis is not the
same as the magnetic
axis, the two beams will
sweep out circular paths.
If Earth lies in one of
those paths, we will see
the star blinking on and off.
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Question 3
The lighthouse
model explains
how
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a) pulsars can be used as interstellar
navigation beacons.
b) the period of pulsation increases as a
neutron star collapses.
c) pulsars have their rotation axis pointing
toward Earth.
d) a rotating neutron star generates an
observable beam of light.
Question 3
The lighthouse
model explains
how
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a) pulsars can be used as interstellar
navigation beacons.
b) the period of pulsation increases as a
neutron star collapses.
c) pulsars have their rotation axis pointing
toward Earth.
d) a rotating neutron star generates an
observable beam of light.
Pulsars
Pulsars radiate their energy away quite rapidly;
the radiation weakens and stops in a few tens
of millions of years, making the neutron star
virtually undetectable.
Pulsars also will not be visible on Earth if their
jets are not pointing our way.
All Pulsars are neutron stars but not all
neutron stars are pulsars.
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Pulsars
There is a pulsar at the
center of the Crab
Nebula; the images to
the right show it in the
“off” and “on”
positions.
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Pulsars
The Crab pulsar also
pulses in the gamma
ray spectrum, as
does the nearby
Geminga pulsar.
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Pulsars
An isolated
neutron star has
been observed by
the Hubble
telescope; it is
moving rapidly,
has a surface
temperature of
700,000 K, and is
about 1 million
years old.
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Neutron Star Binaries
Bursts of X rays
have been
observed near
the center of
our Galaxy. A
typical one
appears at
right, as imaged
in the X-ray
spectrum.
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Neutron Star Binaries
These X-ray bursts are thought to
originate on neutron stars that have
binary partners.
The process is very similar to a nova,
but much more energy is emitted due to
the extremely strong gravitational field
of the neutron star.
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Neutron Star Binaries
Most pulsars have periods between 0.03 and 0.3
seconds, but a new class of pulsar was
discovered in the early 1980s: the millisecond
pulsar.
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Neutron Star Binaries
X-ray Binary
Millisecond pulsars are
thought to be “spun-up”
by matter falling in from a
companion.
This globular cluster has
been found to
have 108
separate X-ray
sources, about
half of which are
thought to be
millisecond
pulsars.
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Gamma-Ray Bursts
Gamma-ray bursts also occur, and were first
spotted by satellites looking for violations of
nuclear test-ban treaties. This map of where the
bursts have been observed shows no “clumping”
of bursts anywhere, particularly not within the
Milky Way. Therefore, the bursts must originate
from outside our Galaxy.
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Gamma-Ray Bursts
These are some sample luminosity curves for
gamma-ray bursts.
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The Guts
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HESS Telescope
High Energy Spectroscopic System
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Light Cone
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Cosmic Rays
• Mostly protons colliding with nitrogen
• Creates masons, muons, pions, electrons
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Spark Chamber
Cosmic ray
+HV
- HV
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Pierre Auger Observatory
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Gamma-Ray Bursts
Distance measurements of some gamma bursts show
them to be very far away – 2 billion parsecs for the first
one measured.
Occasionally the spectrum of a burst can be
measured, allowing distance determination.
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Gamma-Ray Bursts
Two models – merging neutron stars or a
hypernova – have been proposed as the source
of gamma-ray bursts. Colliding Binaries
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Question 4
One possible
explanation for a
gamma-ray
burster is
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a) matter spiraling into a large black
hole.
b) the collision of neutron stars in a
binary system.
c) variations in the magnetic fields of
a pulsar.
d) repeated nova explosions.
e) All of the above are possible.
Question 4
One possible
explanation for a
gamma-ray
burster is
a) matter spiraling into a large black
hole.
b) the collision of neutron stars in a
binary system.
c) variations in the magnetic fields of
a pulsar.
d) repeated nova explosions.
e) All of the above are possible.
Gamma-ray bursts vary in
length, and the coalescence of
two neutron stars seems to
account for short bursts.
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Gamma-Ray Bursts
This burst looks very much like an exceptionally
strong supernova, lending credence to the
hypernova model.
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Summary of Chapter 13
• Supernova may leave behind neutron star.
• Neutron stars are very dense, spin rapidly,
and have intense magnetic fields.
• Neutron stars may appear as pulsars due
to lighthouse effect.
• Neutron star in close binary may become
X-ray burster or millisecond pulsar.
• Gamma-ray bursts probably are due to two
neutron stars colliding, or to hypernova.
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