Why some neutron stars have the same mass but look different

Neutron stars are already the strangest object in the universe – Urtra’s thick star remains are only a few miles away, but larger than our sun.

But what happens when two neutron stars have the same mass but different sizes? A new study provides new insights into these “binary stars” revealing which physical factors allow these mysterious pairs to coexist and how we detect them in the future.

Decode the state equation

The mysterious core is the equation of state (EOS), which connects the pressure and energy density of the core matter in neutron stars. Since direct experiments inside such stars are not possible, scientists use various models and observations to narrow down the appearance of EOS. In this case, the researchers used metamodels (basically a flexible template that could simulate many different EOS scenarios) to explore when and how binary stars formed.

“Two stars – two stable neutron stars with the same mass but with different radii, have long been proposed to emerge due to possible first-order phase transition appearances,” study authors Nai-Bo Zhang and Bao-an Li wrote in the paper.

What makes double stars possible?

The team analyzed how specific EOS parameters affect binary star formation. This includes the properties of nuclear and quark matter, as well as the details of the phase transition between them. Although Symmetric Nuclear Materials (SNM) EOS has little effect, Nuclear symmetric energy– Especially its slope (L) and curvature (K_symbol) – Play a bigger role.

Key findings include:

• Binaries usually coexist only within a narrow mass range of about 0.05 solar energy (ΔM).
• The maximum radius difference (ΔR) between the binary stars is less than 2 km, that is, the current detection threshold.
• Higher transition density and energy discontinuity (δε) between nuclear and quark matter are conducive to twin formation.
• Slope L and curvature K_symbol The possibility of nuclear symmetric energy significantly affecting the bistar.

The study also confirmed that “the EOS of SNM does not significantly affect the formation of binary stars”, but the symmetrical energy components do. Curvature k_symbol The slope l affects the radius and phase transition behavior of the star, which is crucial for observing the distinction between twins.

Why are double stars hard to find

Although the study confirms that binary stars can exist in real-life EOS scenarios, they are still difficult to detect them. The predicted radius difference is less than 2 km, and for the current X-ray or gravity wave instrument, the tolerance distance is very small.

However, the authors are optimistic that future observations, including next-generation X-ray telescopes and gravitational wave detectors, can identify these elusive twins. “This high-precision radius measurement will certainly help identify binary stars,” they wrote.

Impact on physics and observation

Why do double stars exist? Confirmation of their presence will provide strong evidence for phase transitions within neutron stars – from conventional nuclear material to exotic quark material. This, in turn, will open a new window for how matter behaves under certain most extreme conditions in the universe.

The authors also point to the possibility of “hot twin stars”, a variant that could form at temperatures of about 30 million degrees Kelvin, associated with the early stages of supernova residues or neutron star mergers. This further expands the search space, where twins can be found in cosmic events.

Looking to the future

For now, binary stars are still a theoretical but tempting possibility. But thanks to this work, scientists now have a clearer map of where to look for and signs to focus on. With improved models and clearer observations, questions about whether nature produces these universes may soon be answered.

Journals and Learning Information

Posted in: EPJ manuscript (by Arxiv preprint)
title: The influence of national nuclear equations on the formation of binary stars
Author: Nai-Bo Zhang and Bao-An Li
doi/Preprint: ARXIV: 2406.07396V2

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