Sunday, February 6, 2011

Stellar Objects Part 2: Neutron Stars

Stellar Objects Part 2: Neutron Stars

A neutron star is a supernova remnant of degenerate matter so dense that it represents matter at the limit of physical laws, atoms balanced at the cusp of total annihilation, a black hole.

A neutron star is a stellar remnant from a supernova explosion. The tiny white dot indicated by the arrow in the photograph shown below represents the first neutron star directly observed in visible light. Neutron stars comprise the densest form of matter known to exist. Matter denser than this collapses into a black hole.

About 2000 known neutron stars populate the Milky Way and the Magellanic Clouds. The closest one, shown here, is 424 light years away from us.

There are various classifications of neutron stars, generally according to what kinds of energy they emit. When these stars are very young, they rapidly pulse radio or X-rays. These pulses are believed to be caused by particles accelerating near the star's magnetic poles. The mechanism for these pulses is not well understood but the beams are coherent and synchronized to the rotation of the star, although the magnetic and rotational axes are not aligned, so the beams sweep around as the star rotates like the spotlight of a lighthouse. Neutron stars that emit these pulses are called pulsars. The rotation of pulsars very gradually slows down over time and the pulses eventually die out because magnetic torque acts against the spin. Pulsar radiation is not generally dangerous to life on Earth. Possible exceptions are neutron stars that may be soft gamma ray repeaters. However, magnetars, which are discussed in a different article, are more commonly associated with soft gamma ray emission. Gamma rays are very energetic photons and a nearby gamma ray burst directed at Earth from a supernova, for example, could cause a mass extinction. Soft gamma rays are slightly less energetic but harmful nonetheless. A gamma ray burst could alternatively come from the merging of two neutron stars or a neutron star and a black hole.

Sometimes a neutron star will experience a glitch in which its rotation momentarily speeds up. This may be caused by transitions in the vortices in its neutron superfluid core into a lower energy state. What results is a star quake.

This is an artist's concept of a 2004 neutron star quake that flared so brightly it momentarily blinded all X-ray satellites in orbit. Some neutron stars aren't isolated but instead are part of a binary system. A neutron star's accretion from its companion star or from gases near a black hole may also affect its rotation and its fate.

How Neutron Stars Form

Stars that are about 4 to 10 solar masses have cores that are hot and dense enough to fuse elements up to iron. Fusion (nucleogenesis) stops at iron because fusing atoms larger than iron requires more energy than it releases. Fusion can produce energy only as long as the sum of the masses of the new nuclei is less than that of the original nuclei. When iron nuclei (26 protons and 30 neutrons) begin to fuse with other nuclei the resulting nuclei have more mass, the process consumes more energy than it produces, and the process stops. An iron core then begins to form inside the star.

Eventually this core reaches what is called Chandrasekhar mass, which is about 1.4 solar masses (this explains why all neutron stars are about the same mass). At this point, not even electron degeneracy pressure can hold it up. It collapses, pushing protons and electrons together to form neutrons and neutrinos. Neutrinos do not normally interact with matter but at this density they exert a tremendous outward pressure. Even so, the core continues to collapse under its own gravity and approach neutron degeneracy (discussed in the next paragraph). The outer layers crash inward and rebound, creating a neutrino outburst and a shock wave, and when they do so, they trigger an enormously powerful supernova explosion.

What remains is a stellar remnant composed almost entirely of neutrons. This extremely hot neutron star is supported against further collapse by neutron degeneracy pressure. This pressure results when neutrons become so tightly packed together they occupy all the lowest possible energy states with some neutrons left over that must then occupy higher energy states. These high-energy neutrons create the outward directed degeneracy pressure. This is a quantum mechanical effect and as a result it is insensitive to temperature. That means that the neutrons stay packed together even as the neutron star cools off, as it eventually does, through neutrino radiation.

A neutron stars packs a mass of between 1.4 and 2 solar masses into a sphere about 20 km in radius. A 1 cm cube of neutron star would weigh as much as a mountain. These stars have been observed in supernova remnants and in binary systems. Four of them are thought to have planets. As a general rule, stars that are between 0.5 and 4 solar masses eventually mature into black dwarfs. More massive stars, between 4 and 10 solar masses, mature into neutron stars and very massive stars, more than 10 solar masses mature into black holes. Neutron stars retain most of their angular momentum. This means that because they are only a small fraction of their parent star's radius, they have extremely high rotational speeds, somewhere between ¼ millisecond to 30 seconds per revolution. These stars, due to their density, also have extreme surface gravity, up to 7 x 1012 m/s2, compared to Earth's 9.98 m/s2. If an object fell from a height of 1 meter onto the surface of a neutron star it would take 1 microsecond to land and it would be landing at a velocity of 7 million km/h! The gravity on a neutron star's surface results in an escape velocity of 100,000 km/s, about 1/3 the speed of light. The extreme gravitational field around a neutron star acts as a gravitational lens, bending radiation that passes through it. This means that distant objects behind neutron stars become visible.

Mysterious Innards

Neutron stars are very hot when they form, about 1012 K, but they cool rapidly through neutrino radiation, to about 106 K in a few years. As they cool they are thought to form layers and as you explore deeper within these layers, the kinds of matter you might encounter are, well, are just plain bizarre. Remember, neutron stars are made of the densest matter possible. In fact, physicists are beginning to wonder if the name "neutron" is itself a misnomer because they think that in the center of neutron stars the neutrons themselves may be squished down into more exotic types of matter. Some recent computer models have suggested that these stellar corpses may be filled with free quarks, the constituents of neutrons, or even hyperons or kaon condensates.* The October 2010 discovery of a neutron star called J1614-2230 located 4000 light years away not only has broken the record for mass, 2 times solar mass, but it deals a death blow to many proposed models for the kind of matter that makes up a neutron star. Free quarks, kaons and hyperons are out. A neutron star composed of any of these materials would collapse to form a black hole before it could reach 2 times solar mass. What a neutron star is actually made of remains a mystery.

*Under normal conditions all quarks are bound up in atoms as neutrons or protons. Free quarks do not exist except under extraordinary heat and pressure in the form of quark-gluon plasma. Hyperons, like neutrons and protons, are made of three quarks, but the quarks that make them up are of a different kind.  Kaons are particles called mesons that consist of two quarks. Both Kaons and hyperons contain strange quarks. These particles constitute what is called strange matter. When neutrons are compressed beyond a certain limit they dissociate into strange matter quarks. These strange matter quarks in turn transform into a bound state called a strangelet, composed of roughly equal numbers of up, down and strange quarks. Such a state could be as small as the mass of a hydrogen nucleus or as large as meters across and these strangelets are believed to be what make up something very exotic called quark stars or strange stars as they are sometimes called (these stars are discussed in more detail in another article). Normally, strange quarks are unstable and don't exist for long, but in large numbers in neutrons, for example, they may represent the lowest possible energy state. Having three kinds of quarks allows them to be packed in together more efficiently. If this strange matter hypothesis is correct, it could have some potentially dire consequences. For example, if a strangelet came into contact with an ordinary atomic nucleus in a clump of matter on Earth, it would convert that matter into strange matter. By doing so it would release energy, producing a larger more stable strangelet, and this process would continue until all nuclei in all the matter in Earth were converted. Earth would as a result be converted into a hot large clump of strange matter. Not to worry though, about the only way this matter would come into contact with Earth would be if a quark star slammed into it, very unlikely, and we would have bigger worries at that point. If a strangelet hit a neutron star it would theoretically convert it into a quark star or strange star. The strange matter hypothesis is unproven. One nagging question is why aren't all neutron stars strange stars, since strange matter seems to represent a lower energy state. There is an ongoing effort to determine whether the surfaces of known neutron stars consist of strange matter or nuclear matter. The phenomenon of X-ray bursts is well explained in terms of nuclear matter and seismic vibrations of magnetars (see the article on magnetars) also support nuclear matter.

I recommend Dr. Coleman Miller's Neutron Star page as a fun read.  Some of his information is based on his own personal speculations but he backs everything up with scientific argument and some very helpful diagrams.

An Exciting Update

University of Alberta astronomer Craig Heinke and his colleagues have just increased our understanding of the physical nature of the matter inside neutron stars, as described in a February 2011 Edmonton Journal article, specifically how matter behaves inside the core of the neutron star, Cassiopeia A, a remnant from a supernova that exploded 11,000 light years away in the Milky Way.

This is a false colour image of Cassiopeia A, using the Hubble and Spritzer telescopes as well as the Chandra X-ray observatory.

They found direct evidence that the core contains a frictionless superfluid (a fluid that flows with absolutely no friction) that seems to defy gravity, as well as a superconductor (a material through which electrons can flow without losing any energy along the way). They have been observing the neutron star's surface temperature (it's about 2 million °C) for 10 years and have found that only a superfluid core could explain its rapid rate of cooling, of about 4% per year.  The only superfluids observed on Earth are extremely cold, just above absolute zero. And superconductors are observed here only when temepratures drop below - 100 °C. Evidence of superfluid and superconducting hot matter inside neutron stars will hopefully spark a new wave of theoretical research supported by computer modelling as we try to probe further into the workings of matter that is squeezed to its absolute limit, and perhaps, beyond.

Sept 19, 2014 addition: A new 5-minute video just released by Kurzgesagt (which means "in a nutshell" in German) explains neutron stars in a fun and very clear (and accurate!) way. It's an excellent watch:

Next up: Quark Stars.

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