Astrologers finally find general neutron star structure
Frankfurt, Germany: The mass of our sun or even more is compressed into a sphere with the diameter of a large city in stars">neutron stars, which can form following the death of a star. However, nothing is now known about the core of these highly compact objects. Scientists have been attempting to understand their structure ever since they were discovered more than 60 years ago.
Since they can barely be duplicated on Earth in a laboratory, the biggest obstacle is simulating the severe circumstances inside stars">neutron stars. As a result, there are numerous models in which different attributes, such as temperature and density, are defined using so-called equations of state. From the stellar surface to the inner core, these equations make an attempt to characterise the structure of stars">neutron stars.
Now, physicists from Goethe University Frankfurt have been successful in completing the jigsaw by adding more significant pieces. More than a million different equations of state were created by the working group at the Institute of Theoretical Physics under the direction of Prof. Luciano Rezzolla that satisfy the constraints imposed by data from theoretical nuclear physics on the one hand and astronomical observations on the other.
The working group unexpectedly found that "heavy" stars">neutron stars (with masses greater than 1.7 solar masses) have a stiff mantle and a soft core, in contrast to "light" stars">neutron stars (with masses smaller than 1.7 solar masses), which appear to have a soft mantle and a stiff core.
"This result is very interesting because it gives us a direct measure of how compressible the centre of stars">neutron stars can be," says Prof Luciano Rezzolla, "Neutron stars apparently behave a bit like chocolate pralines: light stars resemble those chocolates that have a hazelnut in their centre surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling."
The speed of sound, which Sinan Altiparmak, a bachelor's student, focused on studying, was essential to this realisation. This quantitative metric, which relies on how stiff or flexible the matter is, describes the speed at which sound waves move about inside an item. On Earth, oil deposits are found and the planet's interior is explored using the speed of sound.
By modelling the equations of state, the physicists were also able to uncover other previously unexplained properties of stars">neutron stars. For example, regardless of their mass, they very probably have a radius of only 12 km. Thus, they are just as large in diameter as Goethe University's hometown Frankfurt.
Author Dr Christian Ecker explained: "Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of stars">neutron stars but also to set new limits on their deformability in binary systems, that is, how strongly they distort each other through their gravitational fields. These insights will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars."
