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How Big Are Neutron Stars?

[Estimated read time: 3 minutes]
  • Astrophysicists determine new constraints on radii and tidal deformabilities of neutron stars. 
  • They compared 2 billion theoretical models with gravitational waves captured in 2017. 
  • This allowed them to deduce the size of neutron stars within a range of 1.5 kilometers: it varies from 12 to 13.5 km. 

A neutron star is a collapsed core of a massive star with a mass between 1.4 and 2.16 times that of our Sun. Neutron stars (that can be observed) are extremely hot: their surface temperature reaches more than 600,000 Kelvin.

They are compact, and so dense that a single teaspoon (containing sample neutron stars) would weigh a billion tons. Their magnetic and gravitational fields are nearly 1 quadrillion and 200 billion times stronger than the Earth’s.

As far as size is concerned, we don’t exactly know how large or small they really are. According to the previous studies, they vary from 8 to 16 kilometers (a rough estimate).

For over 4 decades, scientists all over the world have been trying to precisely determine the size of neutron stars because it would provide crucial information about the behavior of matter at nuclear densities.

Now astrophysicists at the FIAS and Goethe University Frankfurt have conducted a research, in which they determine new constraints on radii and tidal deformabilities of neutron stars. By comparing billions of theoretical models with gravitational waves, they deduce the size of neutron stars within 1.5 kilometers.

How Did They Do This?

In August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo interferometer network (for detecting gravitational wave) captured a signal from the inspiral of a binary neutron star system, referred as event GW170817.

Just a couple of seconds later, it detected a series of electromagnetic emission, which confirmed that the merging process of neutron star binaries is associated with gamma-ray bursts.

Researchers used this data to solve a long-standing puzzle: what’s the maximum mass a neutron star could have before collapsing into a black hole. Then they worked to put solid constraints on neutron stars’ size.

We don’t know much about the matter inside neutron stars, thus scientists elected to follow another path. They used statistical approaches for measuring neutron stars’ size.

In order to establish narrow limits, scientists processed over 2 billion theoretical models of neutron stars by applying Einstein equations that describe the equilibrium of these relativistic stars. Then, they merged this data with the constraints observed in event GW170817.

Reference: Phys. Rev. Lett. | doi:10.1103/PhysRevLett.120.261103 | Goethe University

How Big Are Neutron Stars - new constraints on radii

This enabled them to deduce the size of an average neutron star within a range of 1.5 kilometers: it varies from 12 to 13.5 km. The accuracy could be further enhanced by analyzing more gravitational waves (originated by binary neutron star systems) in the future.

A Small Twist

Read: Mysterious Fast Radio Bursts Are Coming From Neutron Star Orbiting A Black Hole

The authors also stated that matter at such high densities could alter its attributes and undergo a phase-transition, like water transitions from a solid to liquid phase or vice-verse. It’s speculated that this type of transition can convert usual matter to quark matter, generating new twin stars which will be smaller than their neutron stars but have the same mass.

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