A relatively small, dense object disguised within an exploded cloud still a few thousand light-years away challenges our understanding of stellar physics.
By all accounts, it appears to be a neutron star, though unusual in 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 was 1.17 times the mass of the Sun.
This latest discovery is not only smaller, it’s well below the minimum mass of the neutron star predicted by theory. This either indicates that there is a gap in our understanding of these super-dense objects…or that what we are looking at is not a neutron star at all, but 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 material from the star runs out to coalesce into 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 core collapses in on itself to form an extremely dense object, atomic nuclei smash together and electrons forced to become intimate with protons long enough to turn into neutrons.
Most of these compact objects have a mass of about 1.4 times the mass of the Sun, although theory says they could range from a mass as massive as about 2.3 solar masses, all the way to just 1.1 solar masses. All of this is packed inside a ball packed into a ball 20 kilometers (12 miles) or so wide, making each teaspoon of neutron star matter weigh between 10 million and several billion tons.
Stars with higher and lower masses than neutron stars can also turn into dense bodies. Heavier stars turn into black holes. Lighter 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 The final fate of our sun.
The neutron star the subject of this study is located at the center of a supernova remnant called HESS J1731-347, which was previously calculated to be more than 10,000 light-years away. However, one difficulty in studying neutron stars lies in the weakness of the distance measurements. Without an accurate distance, it is difficult to obtain accurate measurements of the other properties of the star.
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 in Germany was able to recalculate the distance to HESS J1731-347, and found it to be much closer than thought, at about 8,150 light-years.
This means that previous estimates of other properties of the neutron star 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 the wild.
“Our comprehensive estimate makes the central compact body of HESS J1731-347 the lightest neutron star known to date, and possibly an even more exotic object—that is, a candidate ‘alien star’,” the researchers wrote in their paper.
According to the theory, an exotic star is very similar to a neutron star, but contains a larger proportion of fundamental particles called alien quarks. Quarks are fundamental subatomic particles that combine to form complex particles such as protons and neutrons. Quarks come in six different types, or flavours, called up, down, charm, strange, up, and down. Protons and neutrons are made up of up and down quarks.
The 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 under clumps big enough to squeeze in, but since the rulebook for neutron stars goes out the window when enough quarks get involved, there’s no bottom line either. Meaning we can’t rule out the possibility that this neutron star is actually an exotic star.
This would be very cool; Physicists have been searching for quark matter and strange quark matter for decades. However, while a strange star is certainly possible, the greater possibility is that what we’re looking at is a neutron star – that’s also pretty cool.
“The obtained constraints on mass and radius are still fully compatible with the standard interpretation of a neutron star and can be used to improve astrophysical constraints on the cold dense matter state equation under this assumption,” the researchers wrote.
“Such a light neutron star, regardless of its supposed internal composition, appears to be a very interesting object from an astrophysical perspective.”
It is difficult to ascertain how such a light neutron star would have formed under our current models. So, whatever it is made of, the dense object at the core of HESS J1731-347 will have something to teach us about the mysterious life of massive stars.
The team’s research was published in natural astronomy.
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