A strange, light neutron star could expand our understanding of the universe

The star is way lighter than theoretically expected with a mass of only 77 percent that of the Sun, challenging previous models.
Deena Theresa
Neutron stars are collapsed stars emitting light and x-rays.
Neutron stars are collapsed stars emitting light and x-rays.

Artur Plawgo/iStock 

A discovery could refine our understanding of the cosmos.

In a paper published in Nature Astronomy, scientists detail a "small and extremely light" neutron star, with a radius of approximately 6.2 miles (10 kilometers) and a mass of only 77 percent that of the Sun.

The observation is unusual as neutron stars usually have a mass 1.4 times that of the Sun, with a radius of just tens of kilometers. This makes them ultra-dense objects.

So, the newly founded star, which is way lighter than theoretically expected, could expand "our knowledge of the state in which cold, dense matter exists in the Universe," according to a statement.

"Such a light neutron star, regardless of the assumed internal composition, appears to be a very intriguing object from an astrophysical perspective. Indeed, forming neutron stars with masses lower than ~1.17M is problematic from an evolutionary perspective, and the least massive neutron star known to date (PSR J0453+1559, with an estimated mass of 1.174 ± 0.004M) has been compatible with this constraint," the researchers write in their paper.

Neutron stars are the most dense objects known

Now, what are neutron stars?

When the very central region of a star collapses, protons and electrons are crushed into a neutron. NASA states that if the core of this star is between one and three solar masses, the newly created neutrons could halt the collapse, forming a neutron star, the dense object known in the universe.

Neutron stars can be found scattered in the galaxy and have been studied in the past. They have been found in the centers of supernova remnants, emitting X-rays.

This study focused on a Central compact object ("isolated, radio-quiet, non-accreting, thermally emitting neutron stars") in a supernova remnant called HESS J1731-347. As per previous calculations, the remnant sat more than 10,000 light-years away. But, it was difficult to obtain other accurate measurements of the star due to "poorly constrained distance measurements", according to ScienceAlert.

The star enabled the team to calculate the distance, mass and density of matter

An optically bright star was recently found at the exact location. Using data from the Gaia mapping survey, Victor Doroshenko and a team at the Eberhard Karls University of Tübingen in Germany could recalculate the distance to HESS J1731-347. They found that the supernova remnant was, in fact, some 8,150 light-years away.

The bright star permitted Doroshenko and co-authors to not only determine the distance but also calculate the neutron star's mass and the density of matter within it.

"Our mass estimate makes the CCO in HESS J1731-347 the lightest neutron star known to date, and potentially a more exotic object—that is, a ‘strange star’ candidate. We emphasize that while the first part of the statement above is a robust result, the second is an intriguing possibility consistent with our analysis," the researchers write in their paper.

The researchers stress that the object could either be a ‘strange star’ with a more exotic equation of state or the lightest neutron star is known, which is the greater likelihood. Nevertheless, it is bound to change our perception of the massive stars in the universe.

Study Abstract:

To constrain the equation of state of cold dense matter, astrophysical measurements are essential. These are mostly based on observations of neutron stars in the X-ray band, and, more recently, also on gravitational wave observations. Of particular interest are observations of unusually heavy or light neutron stars which extend the range of central densities probed by observations and thus permit the testing of nuclear-physics predictions over a wider parameter space. Here we report on the analysis of such a star, a central compact object within the supernova remnant HESS J1731-347. We estimate the mass and radius of the neutron star to be M = 0.77+0.20 −0.17 M and R = 10.4+0.86 −0.78 km, respectively, based on modelling of the X-ray spectrum and a robust distance estimate from Gaia observations. Our estimate implies that this object is either the lightest neutron star known, or a ‘strange star’ with a more exotic equation of state. Adopting a standard neutron star matter hypothesis allows the corresponding equations of state to be constrained.

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