Neutron stars behave somewhat like chocolate pralines. Light stars resemble chocolates with a hazelnut in the center and soft chocolate. On the other hand, heavy stars resemble chocolates with a hard layer and a soft filling.
Neutron stars are collapsed cores of massive supergiant stars with masses between 10 and 25 solar masses. They are caused by the supernova explosion of a massive star. This process combines with gravitational collapse to compress the core beyond the density of white dwarfs to the density of atomic nuclei. Their formation ceases heat generation, and they cool over time; however, they may evolve through collisions or accretions. According to estimates of the number of stars that have undergone supernova explosions, the Milky Way contains around one billion neutron stars and at least several hundred million. Their interiors, however, are poorly understood.
Now, scientists have revealed that neutron stars, where matter reaches enormous densities, have a core that can either be very stiff or very soft, depending on their mass. This is according to physicists at Goethe University Frankfurt, based on extensive model calculations. Two articles describing the findings were published today simultaneously. A neutron star is an extremely compact object formed after a star dies. Few details about its interior are known so far. The mass of our sun or even more is compressed into a sphere the diameter of a large city in neutron stars. Over 60 years after they were first discovered, scientists have been attempting to understand their structure.
Since neutron stars can hardly be recreated in the laboratory on Earth, simulating their extreme conditions is the biggest challenge. Therefore, so-called equations of state can be used to describe various properties, including density and temperature. Using these equations, experts can describe the structure of neutron stars from their surface to their inner core.
Scientists at Goethe University Frankfurt have added further pieces to the puzzle. More than a million equations of state have been developed by a team led by Professor Luciano Rezzolla at the Institute of Theoretical Physics. These equations satisfy constraints imposed by theoretical nuclear physics and astronomical observations, respectively. Researchers found an unexpected result when evaluating the equations of state. “Light” neutron stars are thought to have soft mantles and stiff cores. In contrast, “heavy” neutron stars are thought to have stiff mantles and soft cores (with masses larger than 1.7 solar masses). According to Prof. Luciano Rezzolla, this result gives us a direct measurement of the compressibility of neutron stars’ centers.
According to Rezzolla, neutron stars behave somewhat like chocolate pralines. Light stars resemble chocolates with a hazelnut in the center and soft chocolate. On the other hand, heavy stars resemble chocolates with a hard layer and a soft filling.
Only 12 km in radius
Sinan Altiparmak’s studies on the speed of sound contributed to this insight. Based on how stiff or soft matter is within an object, this quantity measure describes how fast sound waves propagate. We use the velocity of sound to explore the interior of our planet and discover oil deposits here on Earth. In addition to uncovering previously unknown properties of neutron stars, physicists could also model the equations of state. No matter how large their mass is, their radius is very likely only to be 12 km. This means that neutron stars are just as large as a city on Earth.
As the author of the paper Dr. Christian Ecker, explains, the extensive numerical study not only allows scientists to predict the radius and mass of neutron stars, but it also sets new limits on their deformability in binary systems, namely, how strongly gravitational fields distort each other. These insights will be especially important in pinpointing the unknown equation of state with future astronomical observations and gravitational wave detections from merging stars. Yet, we still do not fully understand the composition and structure of matter inside neutron stars. The papers detailing the discovery have been published in the Astrophysical Journal Letters. You can access them here and here.