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 10¹¹x the earths gravity and a powerful magnetic field.

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. Though interesting as an order-of-magnitude comparison, this does not imply that the atoms in the sun are packed in close contact. The neutron stars would generally be formed from stars condiderably more massive than our Sun. The incredible density of neutron stars does come from the fact that from atomic size, the electrons are collapsed into the nucleus to combine with protons to form neutrons so that the entire body approaches nuclear density.

Recent research suggests that the heaviest elements may be formed primarily in neutron star mergers rather than supernovae (Frebel & Beers, Physics Today, Jan 2018).

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.

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.

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 10⁸! 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 cm³, 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!

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.

An exciting close binary was reported in Nature in December 2003 and in Science in early 2004. With the cumbersome designation PSR J0737-3039A, it is composed of pulsars with an eccentric orbit of period just 2.4 hours! The most active of the pulsars spins 44 times per second and its companion just once in 2.8 seconds. Irion in Science described the pair as "two pulsars in a tight orbital embrace, blasting each other with radiation as they spiral toward a mutual doom." General relativity calculations reportedly suggest a convergence of the two pulsars by about 7 millimeters/day with a projected crash in about 85 million years.

At just 2000 light years distance, this binary pulsar is relatively close. Its orbit is almost edge-on from the Earth, optimum for viewing. Part of the promise of this dramatic pair is information about relativistic theories of the gravitational interaction. The discovery of this binary pulsar is credited to the 64-meter Parkes radio telescope in New South Wales, Australia. The measurement of the slower period of the companion is credited to Jodrell Bank Observatory in Macclesfield, U.K.

Planets around pulsar?

Radio emissions from the object labeled PSR B1257+12 some 980 light years away in the Virgo constellation classify it as a pulsar with period 6.33 milliseconds. Observations from Arecibo detected variation in the pulsars period which could be modeled in terms of planets orbiting about the pulsar. Current observations indicate three planets and a possible fourth.

Wiki on PSR B1257+12