Neutron Star Measures Just 22 Kilometers Across

Neutron Star Measures Just 22 Kilometers Across

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How big is a neutron star? These extreme, ultra-dense collapsed stars are fairly small, as far as stellar objects are concerned. Even though they pack the mass of a full-sized star, their size is often compared to the width of a medium-to-large-sized city. For years, astronomers have pegged neutron stars at somewhere between 19-27 km (12 to 17 miles) across. This is quite actually quite precise, given the distances and characteristics of neutrons stars. But astronomers have been working to narrow that down to an even more precise measurement.

An international team of researchers has now done just that. Using data from several different telescopes and observatories, members of the Max Planck Institute for Gravitational Physics, theAlbert Einstein Institute (AEI) have narrowed the size estimates for neutron stars by a factor of two.

“We find that the typical neutron star, which is about 1.4 times as heavy as our Sun has a radius of about 11 kilometers,” said Badri Krishnan, who led the research team at the AEI Hannover. “Our results limit the radius to likely be somewhere between 10.4 and 11.9 kilometers.”

That translates to between 20.8 – 23.8 km (13-14.8 miles) in diameter.

Afterglow of GW170817 is shown in close-ups captured by the NASA Hubble Space Telescope, showing it dimming in brightness over days and weeks. CREDIT: NASA and ESA: A. Levan (U. Warwick), N. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)

The object of this team’s study is rather famous: the binary neutron star merger GW170817 which created the gravitational waves detected in 2017 by the LIGO (Laser-Interferometer Gravitational Wave Observatory) and Virgo consortium. This object has been studied numerous times by multiple telescopes, including the Fermi satellite, the Hubble Space Telescope and numerous telescopes and observatories around the world. All those observations gave the Max Planck team a boatload of data to work with.

“Binary neutron star mergers are a gold mine of information!” said Collin Capano, researcher at the AEI Hannover and lead author of a paper published in Nature Astronomy. “Neutron stars contain the densest matter in the observable universe. In fact, they are so dense and compact, that you can think of the entire star as a single atomic nucleus, scaled up to the size of a city. By measuring these objects’ properties, we learn about the fundamental physics that governs matter at the sub-atomic level.”

Neutron stars are formed when a massive star runs out of fuel and collapses. The very central region of the star – the core – collapses, crushing together every proton and electron into a neutron. If the core of the collapsing star is between about 1 and 3 solar masses, these newly-created neutrons can stop the collapse, leaving behind a neutron star.

Stars with even higher masses will continue to collapse into stellar-mass black holes.

Artist’s illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet

But the collapse into a neutron star creates the densest object known – again, an object with the mass of a sun crushed down to the size of a city. And you’ve probably heard this other comparison before, but it’s worth a repeat because of how dramatic it is: One sugar cube of neutron star material would weigh about 1 trillion kilograms (or 1 billion tons) on Earth – about as much as Mount Everest.

The research team used a model based on the fundamental understanding of how subatomic particles interact at the high densities found inside neutron stars. Their work shows it is possible to perform theoretical calculations at length scales less than a trillionth of a millimeter, and they can be compared with observations of an astrophysical object more than a hundred million light years away.

The team writes in their paper. “The properties of neutron stars are determined by the nature of the matter that they contain. These properties can be constrained by measurements of the star’s size.” The team says their results and processes can also be applied to the study of astronomical objects, like pulsars, magnetars, and even the way gravitational waves are emitted to provide details of what is creating these waves.

The Bad Astronomer Phil Plait tweeted about this paper today:

(You can read Phil’s article about this same paper here).

“These results are exciting, not just because we have been able to vastly improve neutron star radii measurements, but because it gives us a window into the ultimate fate of neutron stars in merging binaries,” said Stephanie Brown, co-author of the publication and a PhD student at the AEI Hannover.

More:

Paper: Stringent constraints on neutron-star radii from multimessenger observations and nuclear theory
Max Planck Institute press release

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March 12, 2020 at 05:49AM

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