Neutron Star

For a sufficiently massive star, an iron core is formed and still the gravitational collapse has enough energy to heat it up to a high enough temperature to either fuse or fission iron. Either in the aftermath of a supernova or in just a collapsing massive star, the energy gets high enough to break down the iron into alpha particles and other smaller units, and still the pressure continues to build. When it reaches the threshold of energy necessary to force the combining of electrons and protons to form neutrons, the electron degeneracy limit has been passed and the collapse continues until it is stopped by neutron degeneracy. At this point it appears that the collapse will stop for stars with mass less than two or three solar masses, and the resulting collection of neutrons is called a neutron star. The periodic emitters called pulsars are thought to be neutron stars.

If the mass exceeds about three solar masses, then even neutron degeneracy will not stop the collapse, and the core shrinks toward the black hole condition.

This neutron degeneracy radius is about 20 km for a solar mass, compared to about earth size for a solar mass white dwarf. The density is quoted as about a billion tons per teaspoonful compared to 5 tons per teaspoonful for the white dwarf.

Pasachoff suggests that neutron stars may be crystalline with crusts on the order of 100 meters thick and an atmosphere a few centimeters thick. They may have 10^11x the earths gravity and a powerful magnetic field.

A neutron star might have an atmosphere a few centimeters thick and mountain ranges poking up a few centimeters through the atmosphere.

A neutron star is thought to be about 1/100,000 the diameter of the Sun, and a nucleus is on the order of 100,000 times smaller than an atom.

Index

Reference
Pasachoff
Sec 8.4
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Pulsars

Intriguing, precisely repeated radio pulses from the plane of our galaxy were discovered in the late 1960's and half-seriously attributed to "little green men" and called LGMs. By a process of elimination and modeling, these periodic sources, called pulsars, are attributed to rotating neutron stars which emit lighthouse type sweeping beams as they rotate. Variations in the normal periodic rate are interpreted as energy loss mechanisms or, in one case, taken as evidence of planets around the pulsar.

Example of precisionBinary pulsar
Index

Pasachoff
p212
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Neutron Degeneracy

Neutron degeneracy is a stellar application of the Pauli Exclusion Principle, as is electron degeneracy. No two neutrons can occupy identical states, even under the pressure of a collapsing star of several solar masses. For stellar masses less than about 1.44 solar masses (the Chandrasekhar limit), the energy from the gravitational collapse is not sufficient to produce the neutrons of a neutron star, so the collapse is halted by electron degeneracy to form white dwarfs. Above 1.44 solar masses, enough energy is available from the gravitational collapse to force the combination of electrons and protons to form neutrons. As the star contracts further, all the lowest neutron energy levels are filled and the neutrons are forced into higher and higher energy levels, filling the lowest unoccupied energy levels. This creates an effective pressure which prevents further gravitational collapse, forming a neutron star. However, for masses greater than 2 to 3 solar masses, even neutron degeneracy can't prevent further collapse and it continues toward the black hole state.

Index
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Pulsar Examples

In 1967 a repeating RF pulse was discovered in our galaxy with a period of 1.3373011 seconds, reproducible at 1 part in 10^8! At first it generated excitement as a possible beacon from an intelligent civilization. At present it is called a pulsar and viewed as a point source of radiation on a spinning neutron star, a rotating beacon.

A 0.033 sec pulsar was discovered in the Crab Nebula as well as an optical and x-ray counterpart. The discovery of the optical and RF signals from the same source was important in that it gave a probe of the number of free electrons in space between us and the pulsar. The dispersion,or slowing of the RF compared to the visible gave the figure of about 1 electron per 30 cm^3, using the distance to the Crab Nebula obtained by other methods. The Crab pulsar is slowing at the rate of about 10^-8 sec per day, and the corresponding energy loss agrees well with the energy needed to keep the nebula luminous. "Starquakes" on pulsars, glitches which speed up the pulsar for a short time, may represent settling of the pulsar crust by as small an amount as a mm!

Index

Pasachoff
Ch 8
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Binary Pulsar

Hulse and Taylor won the Nobel Prize in 1993 for the discovery of the first binary pulsar in 1974. It has a period of 59 milliseconds but shows an orbital period of 7 hours and 45 minutes. Discovered at Arecibo, it was an important test of general relativity. There have been about 40 binary pulsars discovered to date.

Index

Schwarzschild
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Planets around pulsar?

Radio emissions from the object labeled PSR B1257+12 some 1300 light years away in the Virgo constellation classify it as a millisecond pulsar. Observations from Arecibo detected a variation of +/- 3 milliseconds in the pulsars period. This apparent wobble in the motion implies an orbit and can be modeled in terms of planets orbiting about the pulsar. Over three years of observation of the object with resolutions down to a few microseconds, the models include two planets with distances of 1/3 and 1/2 astronomical units with periods of 66.6 and 98.2 days respectively with some indication of at least one additional planet.

Index

Cowen
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