Chapter 13 Neutron Stars

Copyright © 2010 Pearson Education, Inc.Copyright © 2010 Pearson Education, Inc.

Chapter 13Neutron Stars

Copyright © 2010 Pearson Education, Inc.

Chapter 13Neutron Stars and Black Holes


Units of Chapter 13.1 - 13.4Neutron StarsPulsarsNeutron Star BinariesGamma-Ray Bursts


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.

Neutron Stars


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.


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.

Neutron Stars


White Dwarf

• A white dwarf of 1 solar mass is the size of Earth.

10,000 km

Neutron StarWhite Dwarf and Neutron star have the same mass


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.

Neutron Stars


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


What’s Inside?


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


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.


Neutrons

• Neutrons have no electrical charge• Neutrons do not need to overcome any

Coulomb barrier• 1.675×10−27 kg• Lifetime 881.5 seconds

http://en.wikipedia.org/wiki/File:Quark_structure_neutron.svg


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


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.

Pulsars


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


Question 1

Pulsars usually show all of the following EXCEPT

Pulsars are neutron stars no longer undergoing fusion in

their cores.

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.


But why would a neutron star flash on and off?This figure illustrates the lighthouse effect responsible. 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.

Pulsars

Pulsar Crab Nebula


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


Question 3

The lighthouse model explains how

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 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.

Pulsars

http://en.wikipedia.org/wiki/File:Cycle_of_pulsed_gamma_rays_from_the_Vela_pulsar.gif


There is a pulsar at the center of the Crab Nebula; the images to the right show it in the “off” and “on” positions.

Pulsars


The Crab pulsar also pulses in the gamma ray spectrum, as does the nearby Geminga pulsar.

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.

Pulsars


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.

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.



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.



Millisecond pulsars are thought to be “spun-up” by matter falling in from a companion.

This globular cluster hasbeen found to have 108 separate X-ray sources, about half of which are thought to be millisecond pulsars.

Neutron Star BinariesX-ray Binary


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.

Gamma-Ray Bursts


These are some sample luminosity curves for gamma-ray bursts.

Gamma-Ray Bursts



The Guts


HESS TelescopeHigh Energy Spectroscopic System


Light Cone



Cosmic Rays

• Mostly protons colliding with nitrogen• Creates masons, muons, pions, electrons

http://en.wikipedia.org/wiki/File:Atmospheric_Collision.svg


Spark Chamber

+HV

- HV

Cosmic ray


Pierre Auger Observatory


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.

Gamma-Ray Bursts


Two models – merging neutron stars or a hypernova – have been proposed as the source of gamma-ray bursts. Colliding Binaries

Gamma-Ray Bursts


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.

Question 4

One possible explanation for a gamma-ray burster is

Gamma-ray bursts vary in length, and the coalescence of

two neutron stars seems to account for short bursts.


This burst looks very much like an exceptionally strong supernova, lending credence to the hypernova model.

Gamma-Ray Bursts


• 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.

Summary of Chapter 13

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Chapter 13 Neutron Stars