A relatively small, dense object disguised within a cloud that exploded a few thousand light-years away challenges our understanding of stellar physics.

By all accounts, it looks like a neutron star file, although that’s unusual. At only 77 percent of the mass of the Sun, it is the lowest mass ever measured for such an object.

previously the lightest neutron star ever measured had 1.17 times the mass of the Sun.

This latest discovery is not only smaller, but well below the minimum neutron star mass predicted by theory. This indicates that there is either a gap in our understanding of these super-dense objects… or that what we are seeing is not a neutron star at all, but rather a strange, never-before-seen object known as an “alien.” ” star.

Neutron stars are among the densest objects in the entire universe. It is what remains after a massive star with a mass between 8 and 30 times the mass of the Sun reaches the end of its life. When the star’s material runs out to coalesce at its core, it travels into a supernova, expelling its outer layers of material into space.

No longer fueled by the external pressure of fusion, the nucleus collapses in on itself to form an extremely dense object, atomic nuclei collide with each other, and electrons are forced to become intimate with protons long enough to become neutrons.

Most of these compact objects have a mass of about 1.4 times the mass of the Sun, although theory says that they could be as massive as their surroundings. 2.3 solar masses, down to only 1.1 solar masses. All of this is packed into a ball about 20 kilometers (12 miles) wide, making each teaspoon of neutron star matter a medium weight. 10 million dollars And the several billion lots.

Stars with masses higher and lower than neutron stars can also become dense bodies. The heaviest stars become black holes. The lightest stars turn out to be white dwarfs, less dense than neutron stars, with a maximum mass of 1.4 solar masses, although they are still somewhat compact. This is the final destination of our sun.

The neutron star in this study is located at the center of a supernova remnant called HessJ1731-347, which was previously calculated to sit more than 10,000 light-years away. However, one difficulty in studying neutron stars lies in the weakness of distance measurements. Without a precise distance, it is difficult to obtain precise measurements of the star’s other properties.

Recently, a second optically bright star was discovered lurking in HESS J1731-347. From this, using data from the Gaia Map Survey, a team of astronomers led by Viktor Doroshenko of Eberhard Karls University in Tübingen, Germany, were able to recalculate the distance to HESS J1731-347 and found that it was much closer than previously thought. I thought, about 8,150 light years.

This means that previous estimates of the neutron star’s other properties need to be refined, including its mass. Combined with observations of X-ray light emitted by a neutron star (inconsistent with X-ray light emitted by a white dwarf), Doroshenko and colleagues were able to improve its radius to 10.4 km and its mass to a very low 0.77 solar. masses.

This means that it may not actually be a neutron star as we know it, but a hypothetical object that has not been positively recognized in nature.

“Our mass estimate makes the central compact body of HESS J1731-347 the lightest known neutron star to date, and possibly an even more exotic object, i.e. an ‘alien star’ candidate.” The researchers write in their article.

According to theory, an exotic star is very similar to a neutron star, but contains a higher proportion of fundamental particles called alien quarks. Quarks are fundamental subatomic particles that combine to form complex particles like protons and neutrons. Quarks come in six different types or flavors, called up, down, charm, strange, up, and down. Protons and neutrons are made up of up and down quarks.

Theory suggests that in the highly compressed environment inside a neutron star, subatomic particles disintegrate into their constituent quarks. Under this model, exotic stars consist of matter made up of equal proportions of up, down, and strange quarks.

Exotic stars should form beneath clusters large enough to fit in, but since the rulebook for neutron stars disappears when enough quarks are involved, there’s no bottom line either. Which means that we cannot rule out the possibility that this neutron star is actually an exotic star.

That would be great; Physicists have been searching for quark matter and strange quark matter for decades. However, while a strange star is certainly possible, the biggest possibility is that what we’re looking at is a neutron star, that’s cool too.

“The obtained constraints on mass and radius remain fully consistent with the standard neutron star interpretation and can be used to improve astrophysical constraints on the cold dense matter equation of state under this assumption.” The researchers write.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-022-01800-1