Neutron Stars In a type II supernova the shock wave does not start at the very center of the collapsing core. After the explosion, the inner ball of neutrons is all that is left. This core remnant is a neutron star.

Actually, it is not really a “star” because all of its nuclear reactions have ceased.

This leftover core of neutrons is extremely small and very massive This leftover core of neutrons is extremely small and very massive. It is about 20 km across (the size of a small asteroid or a city), but it’s mass is greater than that of the Sun.

It is incredibly dense: 1014 to 1015 g/cm3, nearly a billion times denser than a white dwarf. One thimble full would weigh 100 million tons, equal to a good-sized mountain.

A normal atomic nucleus has a density of 1014 g/cm3 A normal atomic nucleus has a density of 1014 g/cm3. A neutron star can be thought of as a single atomic nucleus with an “atomic mass” of 1057.

A neutron star has a solid surface but the gravity would pull a person thinner than a piece of paper.

Rotation and Magnetism Neutron stars rotate extremely rapidly; the periods can be fractions of a second. These speeds of rotation are due to the fact that the collapse of a star follows the law of conservation of angular momentum.

A neutron star has a strong magnetic field; the collapsing core causes the magnetic field to collapse, squeezing the field lines closer together. The field increases to one trillion times the Earth’s field.

This high speed spin and huge magnetic field provides a means of detection for millions of years.

The first neutron star was observed in 1967, a grad student observed an object emitting radiation in rapid pulses lasting about 0.01 seconds apiece. The gap between the emissions lasted 1.34 seconds.

When the pulse of energy intersects the Earth, these are called pulsars. Pulsars have characteristic pulse periods and pulse durations, and are the most accurate natural clocks in the universe.

A few pulsars are directly associated with supernova remnants, like the Crab Nebula remnant.

The Crab Nebula pulsar (bottom right)

Most of the emissions are radio emissions, but some are of visible light, x-rays, and gamma rays. Most “flash” 3 to 30 times a second.

Anthony Hewish received the Nobel Prize in 1974 for his explanation of pulsars. It is called the “lighthouse model”.

All pulsars are neutron stars, but not all neutron stars are pulsars All pulsars are neutron stars, but not all neutron stars are pulsars. The pulsar beam is narrow, so unless it is oriented correctly the beam won’t sweep across the Earth.

The current belief is that all high mass stars end as supernovas, leaving a neutron star behind.

Some pulsars suddenly decrease their pulse period Some pulsars suddenly decrease their pulse period. The interior of a pulsar is a superfluid that flows without friction. It may also be a superconductor with no resistance to electricity.

The surface is a 1 km thick crust, solid and made up of regular neutrons and ‘normal’ iron nuclei and electrons.

Occasionally the crust cracks and settles, a starquake Occasionally the crust cracks and settles, a starquake. This is far more violent than an “Earthly” quake. This settling reduces the radius of the neutron star, so the spin rate must change.

Neutron Star Binaries form X-Ray Bursters - like white dwarf binary novas. These bursts occur hours apart and are far more violent than novas. (A neutron star has greater gravity.)

There are also Gamma Ray Bursters which are similar to X-ray bursters, but far more violent.

In the 1970’s SS433 was discovered In the 1970’s SS433 was discovered. It is an object that emits radio waves and x-rays, but it has a very unusual optical spectrum.

It has three distinct sets of optical emission lines.

1st set--small Doppler shift (slow moving) that alternates back and forth (moves toward earth, then moves away) every 13 days.

The other two sets show large Doppler shifts (velocities 10% to 20% of light speed). One is red shifted (moves away from earth), one is blue shifted (moves toward earth), and they switch every 164 days.

This object is moving toward us very fast, moving away very fast, and staying still all at the same time!!

The best explanation was given by Bruce Margon of the University of Washington.

It is a binary system composed of a neutron star and an ordinary star orbiting each other every 13 days.

Matter from the companion star falls onto the neutron star forming an accretion disk. The low velocity spectral lines come from that disk.

Some material is ejected in narrow jets perpendicular to the disk at >25% of light speed (believed to be caused by radiation and magnetic fields at the inner edge of the disk).

The pull of the companion star on the neutron star and the accretion disk causes the jets to wobble like a top. One wobble is completed every 164 days.

SS 433

There are also rapidly rotating pulsars called millisecond pulsars There are also rapidly rotating pulsars called millisecond pulsars. They spin 100’s of times a second with a pulse period of a few milliseconds.

This is as fast as a neutron star can spin without flying apart.

Many of these are found in globular clusters, which is confusing.

Globular clusters are very old which means the neutron stars formed should have slowed long ago.

Rotation rates must have been increased by some outside force, probably by collecting matter from a companion star.

The spiral from the accretion disk provides a push to accelerate the star.

Later events may eject or destroy the companion leaving an isolated millisecond pulsar, but some are still binary.

This description is very similar to the description of x-ray and gamma ray bursters.

It is possible that many x-ray bursters will eventually become millisecond pulsars.

How does a neutron star become a member of a binary system?

The blast of the supernova explosion would be expected to destroy the companion.

Perhaps the supernova explosion was smaller because the supernova progenitor lost much matter before the supernova.

Or perhaps the neutron star displaced one member of a binary system after the supernova explosion.