Neutron Stars
Neutron stars are thought to be the collapsed cores of massive stars. In many ways they are cousins to black holes. They pack an incredible amount of mass into a relatively small size. In a neutron star, a mass the size of our sun would fit into the area of a city.
Crammed into something this size, the matter of the original star is also compressed dramatically into a much lower energy state. Neutron stars have the strongest magnetic fields in the universe. The theoretically strongest magnetic fields of a neutron star can be some 100 trillion times stronger than the magnetic field of Earth, which itself is 100 time greater than any laboratory generated field. Neutron stars are also high temperature superconductors. While labs can produce superconductivity at about 100 degrees Kelvin, the centers of neutron stars are believed to become superconducting at about 100 million degrees K.
Neutron stars were discovered in 1932 and, in 1934, two physicists named Baade and Zwicky suggested that they were formed in supernovae. However, there was little interest in them as more than a hypothetical phenomena since there was little ability to measure or know more about them. Not much effort was spent on theories as to their origins.
In 1967, an English graduate student named Jocelyn Bell was looking at radio observations of quasars. During her observations and calculations, she began to pick up repeated and regular pulses. More observation concluded with the notion that these came from outside our solar system, and the first radio pulsars were discovered. This generated a greater amount of interest and activity.
A number of pulsars were discovered, including one in the Crab Nebula. This was the site of a known supernova, which occurred in the year 1054. It had been recorded by Chinese, Arabic, and North American astronomers. Within a year of that initial discovery, it became clear that pulsars are fast, with a quite regular period, but that, over time, the period of a pulsar always increased slightly.
Armed with these data, it was determined that pulsars had to be rotating neutron stars. This was due to the mathematics associated with other, potentially competing objects such as white dwarfs, black holes, and “plain” neutron stars. There have been over 1000 radio pulsars discovered to date.
The stars that eventually become neutron stars are thought to begin as stars with a mass of 15 to 30 times the mass of our sun (called solar mass). Stars with a smaller mass become white dwarf stars. Stars with a mass greater than 30 times the mass of our Sun become black holes. The general idea is that when the central part of the star fuses it’s way into iron, it can’t go any further because at low pressures iron 56 has the highest binding energy per nucleon, so fusion or fission of iron 56 requires an energy input.
The iron ore thus just accumulates until it gets to about 1.4 solar masses, at which point the electron degeneracy that had been supporting the star against gravity is not strong enough. The star collapses in on itself. Protons and electrons are combined into neutrons and neutrinos. The neutrinos escape after scattering and helping the supernova to occur. The neutrons settle down to become a neutron star, with neutron degeneracy now managing to oppose gravity.
Neutron stars appear to cool fairly quickly. At some point, it is theorized that the core will probably form a superfluid. The protons will likely also form a superconductor crust. Theoretically, it could be possible to measure the surface temperatures of neutron stars and, coupled with estimates of their core temperatures and estimated age, be able to to learn more about their thermal evolution. The problem is that neutron stars are so small that they can barely be detected. Thus, there really is not enough data to be able to come up with neutron star composition except theoretically.
Some neutrons are isolated neutrons, meaning that they exist by themselves. We do know that isolated neutrons have a tendency to slow down over time in terms of their rotation. However, they can also go through periods of spinup which are called glitches. These decay after a few days. It is thought that the superconductor crust and the superfluid core couple impulsively. Since the crust would be spun down by the magnetic fleid and the superfluid core would keep rotating at its original rate, the coupling would speed up the crust, causing the glitch.
Other neutron stars are called accreting or social neutron stars. Some neutron stars are born in binary systems that survive the supernova explosion that created the neutron star. In dense stellar regions, some neutron stars may also be able to capture companions. In either case, mass may be transferred from the companion to the neutron star, which is, in effect, a black hole.
If the companion is less than one solar mass, the neutron’s gravitational attraction on the gas envelope of the companion is stronger, and the gas falls on the neutron star. However, since the neutron star is quite small, the gas winds up orbiting around the star in an accretion disk. Forces operate within the disk that allow the gas to drift in slowly as it orbits, eventually reaching the surface of the neutron. If the magnetic field at the surface is strong enough, the field couples strongly with the matter and forces it down to the magnetic poles. The friction of the gas heats it to millions of degrees and causes it to emit X-rays.
If the companion has a mass between 1 and 10 solar masses, the mass transfer is unstable and doesn’t last very long. There are thus not many in this category. If the mass of the companion is greater than 10 solar masses, the companion produces a stellar wind. Some of the wind falls on the neutron and will flow to the poles. The X-rays primarily come from hot spots on the poles. If the magnetic and rotational axis is not aligned, the radiation sweeps past once per rotation and results in X-ray pulses. These are different than those discovered by Bell and are called accretion powered pulsars.
Gamma ray bursts have been detected for more than 25 years. They are bursts of gamma rays emanating from outside the earth and are significant in the amount of energy they exhibit. Other than observing them, not much else was known until the late 1990’s when observations made by an Italian satellite brought a lot more information. We know that the bursts can vary in both size, time, and intensity. There is no detailed model yet to describe them. There are theories as to origins. One holds that the bursts are caused by the inspiral and merger of two neutron stars, or a neutron star and a black hole. Another is that the bursts are caused by the collapse of a massive star, greater in size than 20 solar masses, into a black hole.
There are a variety of other observed phenomena believed associated with neutron stars. There are soft gamma ray bursts which last for short times and have lower spectral peaks. There are also incidents of huge spikes indicating the presence of tremendous magnetic fields. As with much of the exciting field of physics, we are only on the bare threshold of understanding what we can observe and even further back in terms of simply being able to observe what is out there.
By Karla Soule |