The end product of a Supernova event associated with stars greater than about 8-10 solar masses is a Neutron star (see also page 20-2a). Such stars develop from strong internal pressures that create neutrons by intense squeezing together of protons and electrons (remember: p + e ---> n); these neutrons are also degenerate. (Degenerate matter describes a condition in which the pressures exerted by the mass [as in a gaseous state] no longer depend on temperature but only on the [high] density reached at this stage; the matter is said to no longer obey the classical laws of physics). During the formation of a Neutron star, the prior state star (which may have a core as heavy as iron) develops a degeneration pressure that rises until it is capable of instigating a gravity-driven collapse down to a remarkably tiny size.
This class of stars winds up as small objects only a few kilometers wide but containing matter equivalent to 4-5 solar masses. Their densities can exceed 1014 gm/cc (or 107 denser than White Dwarfs). (A feel for this extreme density is gained from this comparison: A volume equivalent to a lump of sugar would contain 100 million metric tons [as measured on Earth] of neutron star matter.) These stars can be detected by telescopes that gather Gamma-ray, X-ray, and Radio radiation. Obviously, being of such small size Neutron stars are very hard to find by optical telescope, even though they can glow with intense radiance, unless they are very near to Earth within the M.W. galaxy. The HST has now provided the first-ever image in Visible light, shown below, coming a Neutron star. It is shining just in front of a nebular dust mass whose distance is just 400 light years away. (The light is produced by processes involving photon escape from a surface whose temperature exceeds 10000°K; the surface area is quite small in keeping with the miniscule size of the star.) The size of this object has been estimated to be only 28 km (16.8 miles) making it the second smallest intrinsically radiating object beyond our Solar System discovered to date by visual means.
The XMM-Newton satellite has produced a strong Gamma ray image of a Neutron star just 500 light years beyond the Solar System, as shown here:
This smallest imaged star, Geminga, is about 20 km (12 miles) in diameter and has a mass 1.5 times that of the Sun. It rotates at a speed of 4 revolutions per second. Its hot surface exudes strong X-rays and Gamma rays, some extending out as filaments (tails), being driven by its huge magnetic field. Electrons and positrons are also a part of the filament, the result of the electric field built by the rotation of the star within its magnetic field. The electrons accelerate outward but some evidence shows that the positrons are coaxed back to the star to settle into hot spots.
Some Neutron stars, called Pulsars, are known to have intense magnetic fields and to emit directional beams of strong pulses, best observed by Radio astronomy but also very evident in the X-ray region, in extremely regular intervals (with periods from about 1/1000th of a second to several seconds) whose cyclical nature is related to their (often rapid) rotation; the Earth must lie within the beam's solid angle in order to detect this Pulsar action (the pulses therefore are bursts of radiation from a constant beam detected intermittently from Earth. That is much like a searchlight's beam which, while sweeping continuously, appears to the viewer only when aligned momentarily as it passes through its cycle). This is illustrated by this diagram:
Pulsars are formed by the Neutron star's immense gravity pulling gas from Supernova debris (most Pulsars seem associated with Type II Supernovae), such that this gas is accelerated to a half or more of the speed of light (thus approaching relativistic speeds [those near light speed]); this gas "detonates" when it strikes the Neutron star surface. The magnetic field tends to funnel the fast-moving gas and particles onto narrow parts of the Neutron star's surface which become "hot spots. This releases great quantities of energy extending in the spectrum from Radio to X-ray regions. There are thousands of bursts of energy that rise from the surface many times each second giving rise to the periodicity detected by Radio telescopes and by X-ray observatories such as Chandra.
One of the Pulsars that has been extensively studied lies in the heart of the Crab Nebula which we have examined earlier. It shows the development of a pair of short jets of very hot gases that radiate strongly in the X-ray region of the spectrum.
There is a very strong Pulsar in the Vega constellation. Here is a Chandra view of this feature, expanded in an inset:
The jet (about 0.5 light years in length) around the Vega Pulsar continually shifts its position and shape, as monitored over many months by Chandra. This bespeaks of a significant variation in the configuration of its driving magnetic field.
Pulsars can be irregular in shape. This asymmetry is mainly the result of unequal distribution of X-ray excited gas around the central Neutron star. This is evident in PSR B1509-38.
Some Pulsars leave a distinctive trail of excited particles behind them, resembling patterns seen in wind tunnel experiments. These are, in fact, dubbed "Wind Pulsars". The best documented example so far is the Mouse Pulsar, moving at a speed of 2 million km/hr (1.25 million mph) through space. This image combines data from Chandra and a Radio telescope:/p>
Most Neutron stars have very strong magnetic fields up to 1012 gauss (a normal star's field has a strength of around 100 gauss). A rare subclass of Neutron stars is called a Magnetar, or more formally, an AXP (Anomalous X-ray Pulsar). Only 15 have been found so far but they are probably much more common (it is estimated that about 1 in 10 pulsars are also magnetars). An AXP has a magnetic field measuring around 1014 Gauss (the current record holder, at 1015 Gauss, is SGR 1806-20, about 1000 times greater than a typical Neutron star and a million billion times that of the Sun's 5 Gauss). A Magnetar is similar to an SGR (Soft Gamma-ray Repeaters), another Neutron star variant that undergoes periodic variations in energy output. Both AXPs and SGRs are detected by their pronounced X-ray signals. The Rossi Explorer satellite is well-suited to study Neutron stars. The Magnetar N 39 has been examined by the HST; while not directly seeable, its presence is evident in the Visible as a collection of filamentous strands formed from shock waves released when a giant star exploded some thousands of years ago leaving a Magnetar behind.
One magnetar, SGR 1900+14, has been found with a ring 7 light years across. This is how it appears in a Spitzer IR image (the ring appears to be heated dust):