Cosmology: partial knowledge about neutron stars (Introduction)

by David Turell , Wednesday, May 26, 2021, 19:41 (1278 days ago) @ David Turell

Measurements of size hints at composition:

https://www.quantamagazine.org/squishy-neutron-star-setback-dampens-hopes-of-exotic-mat...

"In the known universe, there is nothing quite like a neutron star. Born from supernovas, these objects contain a star-size mass in a city-size space. This unique trait has led scientists to believe that some extreme physics might take place in their innards — perhaps even the dissolution of neutrons themselves into a softer goo known as quark matter.

"Yet we cannot peer inside neutron stars, so we must rely on the characteristics we can measure, namely their mass and size. Quark matter should be more compressed by the star’s gravity than intact neutrons, so if neutron stars are full of nuclear goo, they should not only be small, but grow smaller as their masses increase.

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"But then in 2019, NASA’s Neutron star Interior Composition Explorer (NICER), an X-ray telescope installed on the International Space Station two years prior, measured the size of a 1.4-solar-mass neutron star called J0030, which is 1,000 light-years from Earth, to be about 26 kilometers across. Now, using NICER data, two independent teams have performed the same analysis for another neutron star, J0740, located 3,000 light-years from Earth.

"The results are surprising. With 2.1 solar masses, J0740 is the most massive known neutron star — about 50% more massive than J0030. Yet the two are essentially the same size — the two teams arrive at 24.8 or 27.4 kilometers across for the former, with uncertainties of several kilometers. The results, which are not yet peer reviewed, were each posted to the online preprint site arxiv.org earlier this month.

"The finding implies that neutron stars may be bizarre, but not so bizarre that they obliterate neutrons themselves. “It might suggest these very exotic states of matter may not be realized in the core of a neutron star,” said Jorge Piekarewicz, a theoretical physicist at Florida State University.

"Neutron stars are formed when a giant star between about eight and 20 times the mass of our sun exhausts its fuel at the end of its life. With no outward pressure to push against the star’s gravity, it collapses. The outer shells explode outward as a supernova, leaving only the dense core behind — the neutron star — packed into a volume the size of Manhattan.

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"The results suggest that neutron stars form quark matter at some point beyond 2.1 solar masses, or perhaps never. Instead, protons and neutrons may simply persist even at the most extreme scales. “It certainly looks like the most squishy models are ruled out,” said Watts.

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"The NICER results are still in an early stage. They will need to be checked and the uncertainties refined. The radius of a third neutron star is in the process of being measured, which could play a large role in confirming or refuting the findings. “We expect we’ll be able to announce [its size] later this year,” said Zaven Arzoumanian, the science lead on NICER at NASA’s Goddard Space Flight Center. “And then maybe a few more beyond that.”

"But so far, the results are pointing toward something intriguing. Even neutron stars, the densest collections of matter in the universe, might not be dense enough to produce some forms of exotic matter. “This is the first strong evidence against a dramatic phase transition in the core of neutron stars,” said Piekarewicz. And if it doesn’t happen in neutron stars, can it happen anywhere else? “I’m afraid not.'”

Comment: What is most amazing is measuring a neutron star's diameter at such distances. We still do not understand why the universe must have such weird objects, one of dhw's worries.