Ultracold liquid helium acts very similar to the hot interior of a neutron star.
Yesterday I talked about millisecond pulsars, and the way in which they might gain such rapid rotation. Another property of millisecond pulsars is that they demonstrate very clearly that pulsars are neutron stars. It all has to do with their rapid rotation and the physics of centripetal (or centrifugal) force.
In the 1990s the ROSAT x-ray observatory made an all-sky survey. By 2001, seven soft x-ray sources were found from the survey data, and shown to be neutron stars. They came to be known as the magnificent seven.
A neutron star is the remnant of a large supernova. When a large star explodes, a remnant of its core is compressed so tightly that the electrons are squeezed into protons, resulting in a mass of neutrons. A neutron star typically has a mass of about 2 solar masses, but it is only about 12 kilometers in diameter. Imagine taking two suns and squeeze it into the size of a small city, and you get the idea of how incredibly dense these objects are.
Yesterday I talked about quark stars. These are hypothetical stars similar to neutron stars, but smaller and a bit more massive. Their gravity and density would be enough that instead of being largely made of neutrons (which are made of up and down quarks), they would be made of free up, down and strange quarks. If such quark stars exist, they would lie between neutron stars and solar mass black holes.
Imagine a star twice as massive as the Sun, compressed to the size of a city. All that matter squeezed into a sphere about 15 miles wide. Such an object is known as a neutron star. The matter of a neutron star is so dense and its gravity is so strong that atoms cannot support themselves. Instead they collapse, with the electrons being squeezed into the nuclei until what remains is a mass of neutrons. Hence the name.
Neutron stars typically form when a large star dies in a supernova explosion. The outer layers of the star are cast outward to form a supernova remnant, while the core of the star collapses into a dense neutron star. What keeps a neutron star from collapsing under its own weight is the pressure of the neutrons pushing against each other, similar to the way electron pressure works in a white dwarf star. But there is a limit to how much weight the neutrons can counter, known as the Tolman-Oppenheimer-Volkoff (TOV) limit. This limit means that a neutron star can’t be more massive than about three solar masses. More than that, and it would collapse into a black hole.
A magnetar is a neutron star with an extremely strong magnetic field, a billion times stronger than the strongest fields we can create on Earth. As a neutron star, magnetars also have very strong gravitational fields, with a surface gravity a hundred billion times that of Earth. Such a high gravity would seem to ensure that a magnetar is spherical, but a magnetar’s strong gravitational field could distort the star, making it more of an oblate spheroid. We’ve suspected that such a magnetic distortion could occur with magnetars, but now a research team seems to have found an example of this phenomenon.
You may have heard that CERN announced the discovery (confirmation, actually. See addendum below.) of a strange particle known as Z(4430). A paper summarizing the results has been published on the physics arxiv, which is a repository for preprint (not yet peer reviewed) physics papers. The new particle is about 4 times more massive than a proton, has a negative charge, and appears to be a theoretical particle known as a tetraquark. The results are still young, but if this discovery holds up it could have implications for our understanding of neutron stars.
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