What is the structure of a neutron star? Like a chocolate bar

What's the tastiest-looking celestial object? If you like bonbons, it's a neutron star!

Fascinating new research reveals that some neutron stars have hard outer shells and soft cores, while others have soft outer shells and hard inner cores

An artist's depiction of a neutron star containing the entire mass of our sun compressed into a ball the size of the city of Manhattan.

(Image credit: NASA's Goddard Space Flight Center)

New research shows that neutron stars are like various interstellar candies: light neutron stars have a soft outer shell and a hard core, like a chocolate praline filled with nuts; while heavy neutron stars are hard on the outside and soft on the inside, more like chocolate fudge.

Physicists listed a large number of internal state equations of neutron stars and took into account nuclear physics theory and actual astronomical observations of neutron stars to reach this conclusion.

"The result is interesting because it shows us in a visual way how compressed the core of a neutron star can be," said Luciano Rezzolla, an astrophysicist at the Institute for Theoretical Physics at Goethe University in Germany and one of the authors of the study. "It is clear that neutron stars are a bit like chocolates with a filling: light neutron stars are like a soft chocolate with a hazelnut in the center, while heavy neutron stars are more like a hard outer layer with a soft chocolate filling."

Goethe University Frankfurt. Image source: german-u15

When a star's core reaches a mass equal to or greater than that of our Sun, it collapses to form a neutron star, and the remnant is compressed to the size of a city, leading to an extreme state of density and the birth of material so dense that a teaspoon would weigh 4 billion tons.

These extreme conditions make it impossible to effectively simulate neutron stars on Earth, meaning that the interiors of these neutron stars have remained shrouded in mystery since their discovery more than 60 years ago.

Rizzla and his colleagues studied neutron stars by measuring the speed of sound inside them. This technique uses the compression of the medium in which sound propagates to determine the hardness or softness of an object, and is widely used in everyday life. For example, this sound velocity method has achieved remarkable success in exploring the interior of the Earth and oil and other sediments.

The heavy neutron star is on the left and the light neutron star is on the right. They are drawn to look like interstellar candy. (Image copyright: Peter Kiefer, Luciano Rizzella)

Substituting the speed of sound into their state equations, the researchers found that neutron stars with a mass less than 1.7 times that of the sun are soft inside and hard outside, while neutron stars with a mass greater than 1.7 times that of the sun are hard inside and soft outside.

The team's study also revealed some previously unidentified features of neutron stars. In particular, the team calculated that, regardless of their mass, neutron stars appear to have a radius of only about 7.5 miles (12 kilometers), about the diameter of Frankfurt, where Goethe University is located. This uniform size may seem unlikely, but all neutron stars begin their lives with a core of between 1.18 and 1.97 solar masses, and any small differences in the size of these cores are weakened after they are compressed to a radius of only about ten kilometers.

Aerial view of Frankfurt. Source: goodfreeephotos

The research may also help reveal how neutron stars in binary systems emit gravitational waves, tiny ripples in space-time that can be measured using extremely sensitive laser interferometers on Earth.

"Our extensive data analysis not only allows us to predict the radius and maximum mass of neutron stars, but also to establish new deformation limits for binary neutron star systems - that is, the maximum amount by which two neutron stars can bend each other in the gravitational field," said Christian Ecker, a scientist at Utrecht University in the Netherlands and co-author of the study. "These insights will be crucial for accurately interpreting equations of state that are unknown in future astronomical observations and for detecting gravitational waves caused by stellar mergers."

Artist's impression of two neutron stars spiraling toward each other before merging. Image credit: esa

Related knowledge

Artist's depiction of an explosion on a supermagnetic neutron star, also known as a magnetar. Source: NASA

Neutron stars are the collapsed cores of massive supergiants with total masses between 10 and 25 times the mass of our Sun, or more if the star is rich in iron. Aside from black holes and other hypothetical objects (such as white holes, quark stars, and strange stars), neutron stars are the smallest and densest known objects. New content will be added to the most recent focus area in the selected section.

Neutron stars are the cores of collapsed supergiants with total masses between 10 and 25 times the mass of our Sun, or more if the star is rich in iron. Aside from black holes and other hypothetical objects (such as white holes, quark stars, and strange stars), neutron stars are the smallest and densest known stellar objects. They have a radius of about 10 kilometers (6 miles) and a mass of about 1.4 times that of our Sun. They form from supernova explosions and shrink due to gravitational collapse, which makes neutron stars denser than white dwarfs and closer to the density of atomic nuclei.

BY:Robert Lea

FY: A high school veteran

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