Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.
Stars come in all different sizes and shapes, but none are as unique as the family of neutron stars. In this group, we find an example of an object that is so dense that a tablespoon of material would weigh millions of tons! How could nature have cooked up something so bizarre? Like black holes, neutron stars find their birth begins with a death.
How Neutron Stars Are Made
Massive stars have lots of fuel, initially in the form of hydrogen. Through nuclear fusion, hydrogen is transformed into helium and light. This process happens to helium as well and up and up we go on the periodic table until we get to iron, which cannot be fused together in the interior of the sun. Normally, electron degeneracy pressure, or its tendency to avoid being near other elections, is enough to counter gravity but once we get to iron the pressure is not as great as the electrons are pulled closer to the nucleus of the atom. The pressure decreases and gravity condenses the star’s core to the point where an explosion releases incredible amounts of energy. Depending on the size of the star, anything between 8-20 sun masses will become a neutron star while anything larger becomes a black hole.
So why the name neutron star? The reason is surprisingly simple. As the core collapses, gravity condenses everything so much that the protons and electrons combine to become neutrons, which are charge neutral and thus are happy to be bunched up with one another without care. Thus the neutron star can be quite small (about 10 km in diameter) and yet have as much mass as nearly 2 or 3 Suns! (Seeds 226)
Let the Weirdness Begin
Okay, so gravity. Big deal right? What about a potential new form of matter? It is possible, for the conditions in a neutron star are unlike anywhere else in the Universe. Matter has been condensed to as maximum an extreme as possible. Anymore, and it would have become a black hole upon the supernova. But the form matter takes inside a neutron star has been compared to pasta. Yum?
This was proposed after scientists noticed that no pulsars seem to exist that can have a spin period longer than 12 seconds. Theoretically it could be slower than that but none have been found. Some models showed that the matter inside the pulsar could be responsible for this. When in a pasta formation, the electric resistivity increases which thus causes the electrons to have a difficult time moving around. Electron movement is what causes magnetic fields to form and if the electrons have a hard time moving in the first place then the ability of the pulsar to radiate EM waves is limited. Thus, the ability for the angular momentum to decrease is also limited, for one way to decrease spin is to radiate energy or matter (Moskowitz).
But what if the material inside a neutron star isn't that pasta-property material? Several models have been proposed for what the core of a neutron star really is. One is a quark core, where remaining protons are condensed with the neutrons to break apart and are just a sea of up and down quarks. Another option is a hyperon core, where those nucleons are not broken but instead have a high amount of strange quarks because of the high energy present. Another option is quite catchy - the kaon condensate core, where quark pairs of strange/up or strange/down exist. Figuring out which (if any) are viable is tough because of the conditions needed to generate it. Particle accelerators can make some of them but at temperatures that are billions, even trillions, of degrees warmer than a neutron star. Another standstill (Sokol).
But a possible test to determine what models work best was devised using glitches of a pulsar. Every once and a while, a pulsar should experience a sudden change in speed, a glitch, and change its output. These glitched likely arise from interactions between the crust and a super fluid interior (which moves about with low friction) exchanging momentum, just like 1E 2259+586, or from magnetic field lines breaking. But when scientists watched the Vela pulsar for three years, they had a chance to see the before and after glitch moment, something missing before. Only one glitch was seen through that time. Before the glitch occurred, a "weak and very broad pulse" in polarization was sent, then 90 milliseconds later...no pulse, when one was expected. Then the normal behavior returned. Models are being built with this data to see which theory works best (Timmer "Three").
Neutrons and Neutrinos
Still not sold on this whole odd physics yet? Alright, I think I may have something that may satisfy. It involves that crust we were just mentioning, and it also involves energy release. But you will never belief what is the agent of the energy takeaway. It is one of nature’s most elusive particles that hardly interact with anything at all and yet here plays a big role. That’s right; the tiny neutrino is the culprit.
And a potential problem exists because of that. How? Well, sometimes matter falls into a neutron star. Usually, its gas that gets caught in the magnetic field and sent to the poles but occasionally something can encounter the surface. It will interact with the crust and fall under enormous pressure, enough for it to go thermonuclear and release an X-ray burst. However, for such a burst to occur also requires that the material be hot. So why is that a problem? Most models show the crust to be cold. Very cold. Like nearly absolute zero. This is because a region where double beta-decay (where electrons and neutrinos are released as a particle breaks down) occurs frequently has been potentially found below the crust. Through a process known as Urca, those neutrinos take energy away from the system, effectively cooling it down. Scientists propose a new mechanism to help reconcile this view with the thermonuclear explosion potential that neutron stars have (Francis "Neutrino").
Stars Within Stars
Possibly one of the strangest concepts a neutron star is involved in is a TZO. This hypothetical object is simply put a neutron star inside a super red giant star and arises from a special binary system where the two merge. But how could we spot one? Turns out, these objects have a shelf life, and after a certain number of years the super red giant layer is cast off, resulting in a neutron star that spins too slow for its age, courtesy of a transfer of angular momentum. Such an object may be like 1F161348-5055, a supernova remnant that is 200 years old but is now an x-ray object and spins at 6.67 hours. This is way too slow, unless it was a part of a TZO in its former life (Cendes).
Symbiotic X-ray Binary
Another type of red star is involved in another weird system. Located in the direction of the Milky Way's center, a red giant star was spotted in the vicinity of an X-ray burst. Upon closer examination, a neutron star was spotted near the giant, and scientists were surprised when they did some number crunching. Turns out, the outer layers of the red giant that are naturally shed off at this stage in its life are being powered by the neutron star and sent out as a burst. Based off the magnetic field readings, the neutron star is young...but the red giant is old. It is possible that the neutron star was initially a white dwarf that gathered enough material to surpass its weight limit and collapse into a neutron star rather than forming from a supernova (Jorgenson).
Evidence for a Quantum Effect
One of the biggest predictions of quantum mechanics is the idea of virtual particles, which rise from differing potentials in vacuum energy and have huge implications for black holes. But as many will tell you, testing out this idea is tough, but fortunately neutron stars offer an easy (?) method of detection of the effects of virtual particles. By looking for vacuum birefringence, an effect arising from virtual particles being affected by an intense magnetic field which causes light to scatter like in a prism, scientists have an indirect method of detecting the mysterious particles. Star RX J1856.5-3754, located 400 light-years away, seems to have this predicted pattern (O'Neill "Quantum").
Magnetars have a lot happening at once. Finding new insights into them can be challenging but it isn't entirely hopeless. One was seen going through a loss of angular momentum, and that proved very insightful. Neutron star 1E 2259+586 (catchy, right?), which is in the direction of the constellation Cassiopeia about 10,000 light-years away, was found to have a rotation rate of 6.978948 seconds based off X-ray pulses. That is, until April of 2012 when it decreased by 2.2 millionths of a second, then sent out a huge burst of X-rays on April 21. Big deal, right? In this magtnetar, however, the magnetic field is several magnitudes greater than a normal neutron star and the crust, which is mostly electrons, encounters great electric resistivity. It thus gains an inability to move as fast as the material underneath it and this causes strain on the crust, which cracks and releases X-rays. As the crust reconstitutes itself, the spin increases. 1E went through such a spin down and a spin up, adding some evidence to this model of neutron stars, according to the May 30, 2013 issue of Nature by Neil Gehrels (from the Goddard Space Flight Center) (NASA, Kruesi "Surprise").
And guess what? If a magnetar slows down enough, the star will lose its structural integrity and it will collapse...into a black hole! We have mentioned above such a mechanism to lose rotational energy, but the powerful magnetic field can also rob energy by speeding along EM waves on their way out of the star. But the neutron star has to be big - as massive as 10 suns minimum - if gravity is to condense the star into a black hole (Redd).
Another surprising magnetar discovery was J1834.9-0846, the first one found with a solar nebula around it. A combination of the spin of the star as well as the magnetic field around it provide the energy required to see the luminosity the nebula projects. But what scientists don't understand is how the nebula has been sustained, for slower spinning objects let their wind nebula go (BEC, Wenz "A never").
But it can get even stranger. Can a neutron star switch between being a magnetar and a pulsar? Yes, yes it can, as PSR J1119-6127 has been seen to do. Observations made by Walid Majid (JPL) show that the star switches between a pulsar and a magnetar, one driven by spin and the other by high magnetic field. Big jumps between emissions and magnetic field readings have been seen to support this view, making this star a unique object. So far (Wenz "This")
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