Neutron Star Progress

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Neutron Star Progress

Postby PatrickPowers » Thu Apr 29, 2021 7:03 am

It has long been possible to calculate the mass of a neutron star from observations, but not possible to know the radius. If the radius were known then we would have a better idea of the density, which is what we really want to know. Among other things this would tell us whether or not neutron stars with an inner core of quark-gluon plasma exist. The smaller the radius, the more dense the star and the more likely this is.

Perhaps the answer may be found here on Earth. Recently https://physics.aps.org/articles/v14/58 the atomic nuclei of lead have been used as a model to measure a parameter of packed-together neutrons that would help answer the density question. Polarized electrons were scattered off of lead nuclei. Neutrons tend to cluster at the surface, protons in the center. Due to an asymmetry in the weak force, left handed neutrons scatter off the nucleus very slighty less often than right handed ones. Somehow this allows them to measure the thickness of the neutron "skin." The more neutrons tend toward surface of the nucleus, the less dense a neutron star would be.

According to the latest practice, the measured value is deliberately biased to a degree kept secret from the experimenters until the experiment is over. Then a mental preference toward a particular result cannot interfere.

Another way to measure the radius is analyzing the gravity waves from neutron star collisions. This gives clues how the stars deform, which in turn gives clues about the radius. The upshot is there is a "mild" disagreement between these two radically different methods of measurement. So while we still don't know the radius of a neutron star, the day is growing closer.
PatrickPowers
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