Neutron Stars

What is a neutron star?

Here's an interesting excerpt from the book "The fate of the stars" by George Greenstein, allowing us to understand the neutron star. This 1983 article does not reflect the current knowledge of neutron star, which fascinates more and more of astrophysicists. The average density of a neutron star is roughly that of the atomic nucleus. But the star does not point all this density. Starting from its surface toward its center, we encounter densities increasing. To more easily represent the state of matter inside neutron stars, it is instructive to imagine an experiment. Let's start with a block of ordinary matter, rock, and compress it to bring it at densities of larger and larger. It will then undergo a series of transitions to states of increasingly strange, recurring at every stage, the state of matter in regions of deeper and deeper inside the star. Apply to a cube of 1 km of rock from the side, a battery of giant presses, and compress it until it has more than 100 meters high.
Now is denser than any terrestrial material. We would be hard pressed to tear off a small piece of 3 cm square, because it would weigh about 200 kg. Let us recall that neutron stars, unlike ordinary planets and stars, have super-powerful magnetic fields. To reproduce the conditions prevailing inside the star, this cube to apply a magnetic field as intense.

This field is so powerful that it deforms to the atoms that make up matter. In the absence of magnetic fields, atoms have a spherical shape, while subjected to magnetic fields super powerful, they take a tapered shape and align themselves along magnetic field lines, like so many small needles placed end to end.
Atoms exert forces on each chemical the other, joining into thin and long molecular chains. The material is then tapered structure, in lock of hair.
This is the first critical phase of compression, it is the surface material of the star.

Image: Jets of matter and antimatter that stray from the neutron star at the center of the Crab Nebula. This image in the X-ray was taken in 2002 by the Chandra satellite. The central ring has a diameter of about one light year.
Credit: NASA/CXC/ASU/J. Hester et al.

When the atoms no longer exist

The cube, a mile high in the beginning, has been compressed to a height of 100 meters. Compress it further until it has more than 5 meters high. Now, every cubic centimeter of this material super dense reached a weight of 100 tons, and finds himself in a state quite unusual. At this pressure, the atoms that make up ordinary matter no longer exist. They are forced to encroach on each other. Spherical atoms or "needles" are formed from electrons orbiting the nucleus. But once crushed upon each other, the ordered structure is destroyed. That is exactly what would happen if you pressed two brick houses against one another. This corresponds to the second critical phase of compression.
During this phase, material is found dissolved in a mixture homogeneous, uniform, atomic components: electrons and nuclei. It is no longer subject to the laws of chemistry. For example, material can not burn, it is neither acid nor alkaline it has no flavor. These are purely chemical properties of matter, chemistry and results from interactions between atoms, but atoms have disappeared. This material forms a solid, because that forces the nuclei exert on each other.

These forces are quite simple. have a positive electric charge, and charges repel. The nuclei then try to avoid each of them. The most favorable situation is one in which each core is the farthest possible of its neighbors.
This set, in which each particle repels and is repelled by all the others, behaves exactly like a crowd jammed into the subway: to avoid contact, people remain motionless. The material is frozen: it is not cold, but because it is dense.
The neutron star, like Earth, has an outer crust. This crust begins a few yards away under the surface of the star, and extends a few miles inland.

The nuclei absorb electrons

The cube of a mile side has now reached a height of 5 meters. Continuing compression. Nuclei begin to absorb electrons. An atomic nucleus contains roughly as many neutrons as protons, due to compression, the protons react with the electrons now they absorb to form more neutrons. Slowly, steadily, ordinary matter is compressed into neutron matter. Compress the cube until it reaches 50 centimeters. Each cubic centimeter weighs 100,000 tons. It's still a solid, and it is now almost entirely made up of neutron-rich nuclei, with some residual electrons. But at this density, we meet the critical third phase of compression, the neutrons begin to be boiling around the nuclei. The nuclei are so neutron rich that they find themselves unable to contain them all. One by one first, then ever-increasing with increasing density, the neutrons escaping from their nuclei as the bees in the hive. They fill the spaces between the nuclei. They move freely. They flow in all directions. They form a superfluid (see note). Beyond the critical third phase of compression, the material consists of a solid coexists with a superfluid.

The superfluid neutron penetrates the solid and diffuse in all directions. We describe the inner crust neutron star. Located just below the outer crust, it is bathed by the neutron superfluid, a real underground ocean.

NB: There is only one superfluid on Earth, and also it is extremely rare, it is ordinary helium. At 4 degrees above absolute zero (-269 ° C), it liquefies. A 2 degree above absolute zero, it changes from a fluid than ordinary superfluid. The most surprising properties of this superfluid is its complete lack of viscosity. This property makes the movement of vortices in fluids are forced to disappear. Water has a viscosity average, if we move the water in a bathtub, its movement will remain a few minutes. Honey has a high viscosity and vortex motions to cease immediately. The superfluid helium, It has no viscosity. If you waved a bath of superfluid helium, it would maintain the deformations for months.

Image: Illustration of the electron.
The electron has no precise location. It appears and disappears continuously in a vacuum, in a timeless sort of vague, both a little here and a little there.

Disintegration of nuclei

Compression continue our imagination...
Compress the cube until it is 5 centimeters.
10 billion tons of matter are trapped in this volume.
The nuclei are so close now that they are touching each other. They interpenetrate. They mix and lose their identity. Beyond this fourth critical phase of compression, the nuclei are completely disintegrated into a homogeneous soup, almost entirely composed of superfluid neutrons, with some traces of protons and electrons. The solid was dissolved by the compression. At this stage we have reached an area located roughly halfway between the surface and the center of the star, and this marks the lower limit of the crust of the star.
Below this border, a sea of superfluid neutrons extends into the depths of the star.
Dives into the ocean, to the heart of the star. In fact, the density does not increase much. Compared to our imaginary experiment, the conditions at the center of the star are equivalent to reduce the cube to a quarter of its current size. This makes a relatively modest increase in density. But as a result of this increase, something important happens.

We are reaching the limits of our knowledge. With this increased density, an untold number of elementary particles appears inside the star. The denser, more neutrons it contains move quickly to the center of the star, they are so fast that every time they collide, a sheaf of new particles appear.
On Earth, these strange particles are rarely created in experiments in giant particle accelerators. But the star, this happens constantly.

Image: Simulation of collision of particles showing a multitude of complex particles evanescent.

At frontiers of knowledge

The physics of elementary particles is an area located at the frontiers of current knowledge. There is virtually hundreds of exotic particles, but none are included in detail. The reason is that they do not live long enough to be properly investigated. Once created in an accelerator, they decay into other exotic particles which themselves survive only a short time before decaying in turn. The pi meson, for example, survive on average only 300 millionths of a second, and relatively long-lived compared to other particles of its kind. However, during their brief existence, the particles exert on each other forces of great complexity, and interact in various ways.
The new elementary particles disintegrate in a lab, but not in a neutron star. Under great pressure, they become stable. They are extremely numerous in the great depths of the star.

The center of a neutron star consists of matter which we barely understand the properties. But there is still more bizarre, this material is denser than an elementary particle. It is subjected to a pressure such that its fundamental constituents are compressed on each other. Any object in daily life, even as dense as a lead block, contains a good amount of vacuum. Individual particles, which form the ordinary matter, do not touch. It is also true in the heart of the sun or in the depths of planets. But in a neutron star matter is completely compressed, there is more empty space.
But even at this stage we have not yet reached the center of the star...

Image: LHC CERN Control Centre, credit: CERN

Discovery of first pulsar

Pulsars are dead stars, extremely dense dwarf who turn on itself, much faster than the other stars (from 10 to 1000 times per second).
Their light sweeps through space like a lighthouse.
It was in 1967 with a telescope sensitive to flicker, as Jocelyn Bell, a student from Hewish, detected an anomaly in the twinkling of radio waves: the scruff. Jocelyn Bell sought the scruff for months and discovered a series of regular pulsations. These impulses seemed too regular to be natural. This clock 1.33 seconds was too perfect to be from a natural process. At first the scientists wondered if it was not there, the signs of intelligence. Hewish through the Doppler effect, put an end to this hope of signals from another civilization. Subsequently several pulsars have been discovered. Radio sources from neutron star and a pulsar is a neutron star rotates rapidly, corresponding to the heart of a collapsed massive star that exploded as a supernova at the end of life. In general the supernova explosion leaves a super compact celestial object in his heart called SNR (Super Nova Rest). This is the FGST (Fermi Gamma-ray Space Telescope) of NASA who discovered the first pulsar, whose name comes from the abbreviation of pulsating radio source.

The neutron star rotates rapidly on the image above cons is aged 10 000 years, it flashes about three times per second by expelling its gamma rays in space. Five French teams from IN2P3 / CNRS, CEA / IRFU and INSU / CNRS contributed to the analysis and interpretation of these results, published in the journal Science (Science Express on 16 October 2008). Astronomers have identified nearly 1800 pulsars in the Milky Way. These pulsars have been found with their radio signals or their low pulse in visible light and X-rays

Image: Fermi telescope has discovered this pulsar with its gamma-ray emission. The pulsar is in the remnants of supernova CTA1, located about 4600 light years away in the constellation Cepheus.
Credit Nasa /S. Pineault, DRAO