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Neutron Stars on the Brink of Collapse

Neutron stars are the densest objects in the Universe; however, their exact characteristics remain unknown. Using simulations based on recent observations, a team of scientists including HITS researcher Dr. Andreas Bauswein has managed to narrow down the size of these stars.

hen a very massive star dies, its core contracts. In a supernova explosion, the star’s outer layers are expelled, leaving behind an ultra-compact neutron star. For the first time, the LIGO and Virgo Observatories have recently been able to observe the merger of two neutron stars and measure the mass of the merging stars. Together, the neutron stars had a mass of 2.74 solar masses. Based on these observational data, an international team of scientists from Germany, Greece, and Japan including HITS astrophysicist Dr. Andreas Bauswein has managed to narrow down the size of neutron stars with the aid of computer simulations. The calculations suggest that the neutron star radius must be at least 10.7 km. The international research team’s results have beenpublished in “Astrophysical Journal Letters.”

The Collapse as Evidence
In neutron star collisions, two neutron stars orbit around each other, eventually merging to form a star with approximately twice the mass of the individual stars. In this cosmic event, gravitational waves – oscillations of spacetime – whose signal characteristics are related to the mass of the stars, are emitted. This event resembles what happens when a stone is thrown into water and waves form on the water’s surface. The heavier the stone, the higher the waves.
The scientists simulated different merger scenarios for the recently measured masses to determine the radius of the neutron stars. In so doing, they relied on different models and equations of state describing the exact structure of neutron stars. Then, the team of scientists checked whether the calculated merger scenarios are consistent with the observations. The conclusion: All models that lead to the direct collapse of the merger remnant can be ruled out because a collapse leads to the formation of a black hole, which in turn means that relatively little light is emitted during the collision. However, different telescopes have observed a bright light source at the location of the stars’ collision, which provides clear evidence against the hypothesis of collapse.

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