2. Discuss the importance of conservation of angular momentum during the formation of a neutron star and the end result.

3. Discuss the relationship between neutron stars and pulsars, the importance of the magnetic field, and a model that might lead to pulsed radiation.

4. Describe how observations of THE binary pulsar aided in verifying predictions of the General Theory of Relativity.

5. Describe the connections between neutron stars, X-ray binaries, and accretion disks.
 
 

¶ neutron degeneracy

¶ strong magnetic field

¶ conserve angular momentum

¶ pulsar

¶ pulsed radiation

¶ THE binary pulsar

¶ gravitational radiation

¶ LIGO

¶ X-ray binary

¶ SS433

¶ accretion disk

¶ The core is now composed of neutrons, but most of the neutrinos (being particles that pass through most kinds of matter) have easily escaped the star. The energy of the rebounding neutron core and some of the escaping neutrinos eject the outer layers of the star in a powerful supernova explosion.

¶ Since only the neutron core of the star remains, the star is called a neutron star. The condition of the matter in the star is called neutron degeneracy, because the neutrons cannot be packed any closer than they are without the neutrons being pushed together.

¶ Note that if the mass of the neutron core were greater than 2 or 3 solar masses, the force of gravity would be so great that the neutrons would be pushed together.

¶ Main sequence stars are known to have magnetic fields associated with them. For example, sunspots are caused by magnetic fields. Therefore, neutron stars must have incredibly strong magnetic fields, because just as the surface gravity of a star increases as the mass collapses into a small size, so must the magnetic field strength increase.  This is because you have all the material producing the magnetic field now packed into a very small space.

¶ During the collapse of a star's core to a neutron core, the core must rotate faster and faster to conserve angular momentum. The effect is similar to what is seen when the arms of a spinning skater are pulled towards the body. Therefore, in addition to having an incredibly strong magnetic field and gravity, a neutron star must rotate very rapidly.

¶ A neutron star's rotation axis generally will not be aligned with its magnetic axis.  This is not too surprising, because even the Earth's magnetic poles are not aligned with its geographic poles.  (The magnetic North Pole is somewhere in Ontario now, for instance, and it wanders a few miles each year.)

¶ Charged particles from the interstellar matter trapped in the magnetic field of a neutron star would emit electromagnetic radiation as they traveled down towards the surface of a neutron star. The electromagnetic radiation would only be emitted in the direction of the magnetic axis.

¶ Thus, it is easy to envisage how a pulse of radiation could be emitted toward the Earth from a rapidly rotating neutron star once every rotation period.

¶ In 1967, before it was realized that this could happen, objects known as pulsars were discovered using radio telescopes. After their discovery, it was quickly realized that pulsars were rapidly rotating neutron stars.

¶ Today there are several competing theories for how pulsed radiation can be emitted by pulsars, but all of the theories assume that a rapidly rotating neutron star is involved.

¶ The rotation speed of a pulsar can be slowed because angular momentum is lost when the magnetic field of a pulsar passes through the interstellar medium. Thus, pulsars are gradually slowing down as they get older.  Think of this as a drag force on the star's rotation, such as if you were standing in a swimming pool and tried to spin around.  The water would slow you down.

¶ The General Theory of Relativity also predicts that a binary system containing two neutron stars will emit gravitational radiation (sometimes called gravity waves or gravitons). Gravitational radiation is emitted any time masses are accelerated, and in the case of binary systems with neutron stars the radiation must be strong.  Gravitational radiation siphons off some of their orbital energy as they orbit around each other, making them gradually spiral inward.  As they get closer together, they orbit each other faster, making stronger gravitational waves which siphon off even more of their orbital energy, which makes them fall in even closer together, and so on.  Eventually, this will cause them to collide.  The gravitational waves emitted in a neutron star collision are predicted to be very strong.

¶ Gravitational wave detectors on Earth are not yet sensitive enough to detect gravitational radiation. The National Science Foundation is building the Laser Interferometric Gravity Observatory (LIGO) in an attempt to detect gravitational radiation.  They hope to detect it from sources like orbiting neutron stars, and they may even witness a neutron star collision.

¶ However, gravitational radiation emitted by a binary system should also cause the orbits to shrink and this is observed in THE binary pulsar.

 ¶Also note that, unlike isolated pulsars, which gradually slow their rotation rates over time, the neutron star in an x-ray binary can speed up its rotation over time.  The gas swirling around in the accretion disk has angular momentum, and when it falls from the disk onto the neutron star, it adds angular momentum to the star, speeding up its rotation.

¶ Accretion disks are like whirlpool-like disks of gas, with the neutron star at their center.  As the gas orbits the neutron star, its internal friction makes it radiate energy as light.  As it loses energy this way, it slowly falls in towards the center of the disk, towards the star.  Now it orbits faster, and it builds up even more friction as it rubs against the slower-orbiting gas that is farther away from the star.  So it gets even hotter, radiating this heat as higher-energy light, and the gas falls even closer in towards the star, and so on.  The result is that the outer regions of the disk are the coolest and glow in infrared light.  As you look closer to the center of the disk, you notice that the gas is much hotter and glows with higher-energy light, getting all the way up to x-rays in the innermost regions.  The gas here is orbiting at very high speeds and eventually falls into the star.  In the disk's innermost regions, the hot gas also emits high winds from its surface, blowing some gas off into space.