Mystery object could be a ‘strange star’ made of quarks, scientists say

A relatively small, dense object surrounded by a cloud of its own exploded remains just a few thousand light-years away defies our understanding of stellar physics.

By all accounts, it appears to be a neutron star, although an unusual one at that. At just 77 percent the mass of the Sun, it is the lowest mass ever measured for an object of this type.

Previously, the lightest neutron star ever measured had reached 1.17 times the mass of the Sun.

This recent discovery is not only smaller, but also significantly smaller than the theoretically predicted minimum neutron star mass. This either suggests there is a gap in our understanding of these ultradense objects… or what we are looking at here is not a neutron star at all, but a peculiar, never-before-seen object known as a “strange” star.

Neutron stars are among the densest objects in the entire universe. They are what remains after a massive star of about 8 to 30 times the mass of our Sun has reached the end of its life. When the star runs out of material to fuse at its core, it goes supernova and ejects its outer layers of material into space.

Unsupported by the outward pressures of fusion, the nucleus collapses on itself to form an object so dense that nuclei are crushed together and electrons are forced to become intimate with protons long enough for them to turn into neutrons be able.

Most of these compact objects are about 1.4 times the mass of the Sun, although theory suggests they could range from about 2.3 solar masses to as little as 1.1 solar masses. All of this is packed into a sphere just packed into a sphere only about 20 kilometers (12 miles) across, making each teaspoon of neutron star material weigh anywhere from 10 million to several billion tons.

Stars with higher and lower masses than neutron stars can also become dense objects. Heavier stars turn into black holes. Lighter stars become white dwarfs – less dense than neutron stars, with an upper mass limit of 1.4 solar masses, but still quite compact. this is that ultimate destiny of our own sun.

The neutron star that is the subject of this study is at the center of a supernova remnant called HESS J1731-347, previously calculated to be more than 10,000 light-years away. However, one of the difficulties in studying neutron stars lies in poorly constrained distance measurements. Without an accurate distance, it is difficult to get accurate measurements of a star’s other properties.

A second optically bright star was recently discovered in HESS J1731-347. From this, a team of astronomers led by Victor Doroshenko of the Eberhard-Karls-Universität Tübingen in Germany, using data from the Gaia mapping survey, was able to recalculate the distance to HESS J1731-347 and found that it was about 8,150 is much closer than thought light years away.

This means that previous estimates of the neutron star’s other properties, including its mass, had to be refined. Combined with observations of the X-ray light emitted by the neutron star (which does not match the X-ray emission of a white dwarf), Doroshenko and his colleagues were able to refine its radius to 10.4 kilometers and its mass to an absolutely amazingly low 0.77 solar masses.

This means it may not be a neutron star as we know it, but a hypothetical object that has not yet been positively identified in the wild.

“Our mass estimate makes the central compact object in HESS J1731-347 the lightest neutron star known to date, and possibly a more exotic object – that is, a ‘strange star’ candidate,” the researchers write in their paper.

According to the theory, a strange star looks a lot like a neutron star, but contains a larger proportion of fundamental particles called strange quarks. Quarks are elementary subatomic particles that combine to form composite particles such as protons and neutrons. Quarks come in six different types or flavors referred to as Up, Down, Charm, Strange, Top and Bottom. Protons and neutrons are made up of up and down quarks.

The theory suggests that subatomic particles decay into their quarks in the extremely compressed environment inside a neutron star. According to this model, strange stars are made of matter made up of equal parts up, down, and strange quarks.

Strange stars should form under masses large enough to really exert pressure, but since the rule book for neutron stars goes out the window when enough quarks are involved, there’s essentially no lower limit either. That said, we cannot rule out the possibility that this neutron star is actually a strange star.

That would be extremely cool; Physicists have been searching for quark matter and strange quark matter for decades. While a strange star is certainly possible, what we’re looking at is more likely to be a neutron star — and that’s extremely cool, too.

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

“Such a light neutron star, regardless of the assumed internal composition, seems to be a very intriguing object from an astrophysical point of view.”

It is difficult to determine how such a light neutron star could have formed under our current models. Whatever it is made of, the dense object at the heart of HESS J1731-347 will teach us about the mysterious afterlife of massive stars.

The team’s research was published in natural astronomy.