Neutron Stars: Definition

Neutron stars are the smallest, densest stars in the universe and also represent the densest form in which matter can exist. If a neutron star’s density (about 10^14 g/cm3) were any greater it would collapse into a black hole. Neutron stars also generate the strongest magnetic fields known (1012 gauss) and with a core temperature theorized to exceed 100 million K are the hottest objects known.

A neutron star is the last stage in the life of certain stars. Stars that are between 8 and 20 to 30 times as massive as our sun are the type that eventually ends up as neutron stars. Stars below this limit will end their lives as white dwarf stars, those of greater mass will end up as black holes.

Neutron Stars: Description

Current theory says that the neutron star is spinning it’s fastest just after birth, it then begins to slow down, at a rate of 1013 sec/sec for young neutron stars. Astronomers used this precise slowdown as a way of determining the age of the neutron star. Suddenly a pulsar was discovered that was spinning very, very fast and its spin-down rate showed that it was extremely old, four times the age of the universe in fact! Luckily rather than having to explain how a star could pre-date the universe, a new class of neutron stars (microsecond pulsars) was discovered. In the case of a pulsar in a binary star system matter can be transferred from the other star to the pulsar, increasing its mass and consequently making the star “spin-up” for a while until settling down to a normal pulsar lifestyle.

Because of the great magnetic field near a pulsar the matter can get very close to the surface of the star without falling in, do to the large angular momentum of the particles driven by the star’s magnetic field. Because the matter than is dragged into the neutron star is orbiting so quickly it can quickly cause the neutron star to speed up its spin rate. This is the mechanism that describes the creation of a millisecond neutron star. The matter is moving so fast that there is much friction as the particles jostle together. This causes the gas to heat to temperatures which cause it to radiate x-rays.

This process can also cause x-ray bursts. It is theorized that these hydrogen and helium transferred to the neutron star from the companion builds up in a dense layer. Eventually, the hydrogen and helium reach a density at which thermonuclear fusion starts, which then converts most or all of the gas into iron, releasing a tremendous amount of energy, creating an x-ray burst that can bee seen at great distances.

Neutron Stars: History

Neutron Stars: Life

During a normal star’s lifetime hydrogen is fused into helium in the core and energy is released. As the hydrogen is depleted the solar core becomes dominated by an inert helium core. The core starts to heat, however the temperature is not enough to cause helium fusion so at this point hydrogen fusion continues in a zone around the outside of the core. Eventually a high enough temperature is reached and helium can start to fuse. This happens so quickly it is called the “helium flash”. During this time the core shrinks in size and the remainder of the star expands (an equal and opposite reaction) and the star starts to becomes a giant, or in the stars that will eventually become pulsars, a super-giant. Eventually the star’s atmosphere is cooled by the expansion and the light moves to the red part of the spectrum; a red-giant is created, some greater than 10 A.U. in diameter. The creation of the red-giant occurs in steps that are caused by the underlying nuclear reactions.

The star begins to shrink under gravity and gets hotter. Just like in the previous stage the helium fusion takes place in a zone around a core now becoming carbon based; a mostly pure carbon core begins to form. Again the star bloats getting larger. The carbon core gets hotter and hotter but not hot enough to continue to fuse heavier elements and so begins to rapidly cool. The helium on the surrounding zone is depleted and fusion slows; the stars core shrinks and part of its atmosphere puffs of into space.

This process goes on again and again as the fusion reactions create heavier elements. The only difference is that as heavier elements get created you get a series of rings or zones around the core that are primarily of a single element. It is in these zones that the fusion reactions are taking place. At the point where the core of the star becomes iron dominated the process is forced to end. Iron is endothermic in fusion type reactions; it requires external energy to continue. It gets this energy from the zones around it where the fusion reactions are taking place. The iron core just accumulates until it gets to about 1.4 solar masses (the “Chandrasekhar mass”), at which point the electron degeneracy pressure that had been supporting it against gravity is overcome and the core collapses inward.

Things happen very quickly from there. The core superheats, the remaining heavy natural elements are created in seconds as the temperature rises. Suddenly, there is no more energy for the core to bleed off from the solar atmosphere. Catastrophically, the massive gravity crushing down on the star is no longer balanced by radiation coming from the fusion reactions and the star explodes at the region around the core where the fusion was occurring. The outer layers of the star are blown off at incredible velocity as a supernova explosion. The resulting and opposite force is focused on the core region.

At the very high pressures involved in this collapse, it is energetically favorable to combine protons and electrons to form neutrons plus neutrinos. The neutrinos escape after scattering a bit and helping the supernova happen, and the neutrons settle down to become a neutron star, with neutron degeneracy managing to oppose gravity. The supernova explosion is really a neutrino explosion. While the blast gives off enough light in the visible spectrum to make it as bright as an entire galaxy, the neutrino burst actually outshines the entire universe (in “neutrino light”) for several seconds! To put it in other words, there are more neutrinos released in the first few seconds of a supernova expulsion than are produced by all other processes in the universe.

The matter that remains in the core is squashed to enormous density. Normal matter keeps electron orbitals from encroaching on those of other pieces of matter. In fact when the orbitals do just begin to overlap we have solid matter. When they overlap a little more we have the densest types of solid matter: iron, lead, etc. When the orbitals have a great deal of overlap the electrons are forced to significantly increase velocity; velocity being inversely proportional to the radius of the orbit. When velocity becomes large enough the electrons can move freely among all the orbitals. Electrons no longer belong to any individual atomic nucleus. This is known as the degenerate state of matter. A white dwarf star is made up of this material.

In the supernova explosion so much force is focused on the stellar core that the electrons run out of space all together and are forced to spiral into the nucleus of an atom. The electron combines with a proton to create a neutron (and a neutrino). Eventually the material that is left is made up of neutrons packed together surrounded by a crust of iron and lighter elements. The neutron star is born.

Because of the significant differential gravity from surface to core even in the tiny (~6 miles in diameter) neutron star; it is not homogenous throughout. The star consists of an atmosphere and crust made up of nuclei of iron and lighter elements. Becusae of the extreme conditions these elements are not the same as on Earth but rather form elemental polymers! The iron polymer is 10,000 times as dense as terrestrial iron, one million times as strong, has virtually perfect electrical conduction parallel to the magnetic field, but is a is a good insulator in the direction orthogonal to the magnetic field of the star. At deeper levels the energy becomes great enough that nuclei can have half their protons converted to neutrons. At about 4x106 g/cm3) the material becomes degenerate and the electrons can move freely between the nuclei, making the material superconductive in its electrical and thermal properties.

When density exceeds 4 x 1011g/cm3) a zone that is called the “neutron drip” layer is reached. At this density the energy profile is such that neutrons can “drip” out of the nuclei. At higher densities a layer that consists of neutrons with at most 10% protons and electrons.

At even higher densities (more than1012 g/cm3) you reach the “pasta-antipasta” sequence layer. First the nuclei spread significantly far away from each other, resembling little meatballs. At higher densities the nuclei will coagulate into spaghetti-like strings. Increasing density causes the strings to coalesce into “lazagna sheets”. As density increases you get the “anti-pasta” layers, from “anti-lasagna” to “anti-meatball”. The anti-pasta having holes where the “pasat” layers have matter, so that the “anti-meatball” phase looks like Swiss-cheese with the holes corresponding to the meatballs of the corresponding layer. At densities greater than 2.8 x 10^14 g/cm3) exotic particles are hypothesized: quark-gluon plasma, heavy mass hyperons and isobars particles. The physics at these extreme conditions are still being worked out and there is much to learn.

Death of a Neutron Star

It is interesting that while the star that eventually became a neutron star had one of the shortest lifetimes of any class of stars the neutron star itself can last a long, long time. The actual end of the neutron star depends, ironically, on protons. If protons decay then all matter will remain in galaxies and eventually be swallowed up by black holes; the black holes eventually evaporate. The timescale for this is on the order of 10100 years. Compared to the current age of the universe, at about 1.37x10^10years. Put another way, by then the universe would be over 100 trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion times as old as it is today.

If protons do not decay the timescale greatly increases, in that scenario the black holes don’t grow to engulf any and all mass in the universe. The neutron stars will have to wait until it undergoes quantum tunnel into black holes themselves, which then “quickly” evaporate. The time scale here is not very meaningful because of its size. An attempt to write out the number of years till the neutrons stars all die would require every baryon in the universe to write a zero upon, or 1x101076 years.

Neutron Stars: Morphology

Because of the significant differential gravity from surface to core even in the tiny (~6 miles in diameter) neutron star; it is not homogenous throughout. The star consists of an atmosphere and crust made up of nuclei of iron and lighter elements. Becusae of the extreme conditions these elements are not the same as on Earth but rather form elemental polymers! The iron polymer is 10,000 times as dense as terrestrial iron, one million times as strong, has virtually perfect electrical conduction parallel to the magnetic field, but is a is a good insulator in the direction orthogonal to the magnetic field of the star. At deeper levels the energy becomes great enough that nuclei can have half their protons converted to neutrons. At about 4x106 g/cm3) the material becomes degenerate and the electrons can move freely between the nuclei, making the material superconductive in its electrical and thermal properties.

When density exceeds 4 x 1011 g/cm3) a zone that is called the “neutron drip” layer is reached. At this density the energy profile is such that neutrons can “drip” out of the nuclei. At higher densities a layer that consists of neutrons with at most 10% protons and electrons.

At even higher densities (more than1012 g/cm3) you reach the “pasta-antipasta” sequence layer. First the nuclei spread significantly far away from each other, resembling little meatballs. At higher densities the nuclei will coagulate into spaghetti-like strings. Increasing density causes the strings to coalesce into “lazagna sheets”. As density increases you get the “anti-pasta” layers, from “anti-lasagna” to “anti-meatball”. The anti-pasta having holes where the “pasat” layers have matter, so that the “anti-meatball” phase looks like Swiss-cheese with the holes corresponding to the meatballs of the corresponding layer. At densities greater than 2.8 x 1014 g/cc exotic particles are hypothesized: quark-gluon plasma, heavy mass hyperons and isobars particles. The physics at these extreme conditions are still being worked out and there is much to learn.

The Neutron Star