Mysterious object may be ‘strange star’ made of quarks, scientists say – ScienceAlert

A relatively small, dense object hidden within a bursting cloud of its own just a few thousand light-years away is challenging our understanding of stellar physics.

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

previouslythe lightest neutron star ever measured has a mass of 1.17 times the mass of the Sun.

This most recent discovery is not only smaller, but significantly less than the minimum neutron star mass predicted by theory. This suggests that there is some gap in our understanding of these ultradense objects… or that what we are looking at 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 between 8 and 30 times the mass of the Sun has reached the end of its life. When the star runs out of material to fuse in its core, it goes supernova, ejecting its outer layers of material into space.

No longer supported by the outward pressure of fusion, the nucleus collapses in on itself to form such a dense object that the atomic nuclei are crushed together and the electrons are forced into intimate contact with the protons long enough for transform into neutrons.

Most of these compact objects are about 1.4 times the mass of the Sun, although theory says they could vary from something as massive as around. 2.3 solar masses, down to only 1.1 solar masses. All packed into a sphere that just packs a sphere only about 20 kilometers (12 miles) across, making each teaspoon of neutron star material weigh between 10 million i several billions tons

Stars with higher and lower masses than neutron stars can also become dense objects. The most massive stars become black holes. The lighter stars become white dwarfs, less dense than neutron stars, with an upper mass limit of 1.4 solar masses, although still quite compact. This is the the final destination of our own Sun.

The neutron star that is the subject of this study is at the center of a so-called supernova remnant HESS J1731-347which had previously been calculated to sit more than 10,000 light years away. One of the difficulties in studying neutron stars, however, lies in the loosely constrained distance measurements. Without an accurate distance, it is difficult to get accurate measurements of a star’s other characteristics.

Recently, a second optically bright star was discovered hiding in HESS J1731-347. From this, using data from the Gaia mapping survey, a team of astronomers led by Victor Doroshenko of the Eberhard Karls University of Tübingen in Germany was able to recalculate the distance to HESS J1731-347 and found that it is much more close to what was thought, around 8,150. light years away.

This means that previous estimates of the neutron star’s other characteristics had to be refined, including its mass. Combined with observations of the X-ray light emitted by the neutron star (incompatible with the X-radiation of a white dwarf), Doroshenko and his colleagues were able to refine its radius to 10.4 kilometers and its mass down to an absolutely unbelievably low 0.77 solar. masses

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

“Our mass estimate makes the compact central object in HESS J1731-347 the lightest neutron star known to date, and potentially a more exotic object, i.e. 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 fundamental subatomic particles that combine to form composite particles such as protons and neutrons. Quarks come in six different types or flavors, called up, down, charm, weird, top, and bottom. Protons and neutrons are made up of up and down quarks.

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

Strange stars would have to form under masses large enough to actually do this, but since the rule book for neutron stars goes out the window when enough quarks get involved, there’s no lower limit either. This means that we cannot rule out the possibility that this neutron star is indeed a strange star.

That would be extremely cool; physicists have been searching for quark matter and strange quark matter for decades. However, while a strange star is certainly possible, the higher probability is that what we are looking at is a neutron star, and that too is extremely fantastic.

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

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

How such a faint neutron star could have formed is difficult to determine with our current models. So whatever it’s made of, the dense object at the heart of HESS J1731-347 will have something to teach us about the mysterious deaths of massive stars.

The team’s research has been published in Astronomy of nature.