What is a neutron star?
|Updated June 01, 2013
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.
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.
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.
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.
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.
Image: 10-14 meter or 10 fermi, it is the distance at which you can see the nucleus of an atom. In the late nineteenth century, it was discovered that the atom is not an indivisible element of matter.
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.
Image: Illustration of the electron.
Disintegration of nuclei
Compression continue our imagination...
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.
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 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.
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).
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.