A relatively small, dense object hidden in a self-exploded cloud just a few thousand light-years away challenges our understanding of stellar physics.
By all accounts a neutron star, although there is nothing unusual about it. At just 77 percent of the sun’s mass, it is the lowest mass ever measured for an object of its kind.
PreviouslyThe lightest neutron star ever measured was clocked 1.17 times the mass of the Sun.
This newer discovery is not only smaller, but significantly lower than the minimum neutron star mass predicted by theory. This suggests either that there is a gap in our understanding of these extremely dense objects…or what we are looking at is not a neutron star, but a strange, never-before-seen object known as a ‘strange’ star.
Neutron stars are among the densest objects in the entire Universe. It is what remains after a massive star has reached the end of its life, with about 8 to 30 times the mass of the Sun. When the star runs out of material to fuse in its core, it goes supernova and blasts outer layers of material into space.
No longer supported by the outward pressure of fusion, the nucleus collapses into itself to form a very dense object, atomic nuclei are crushed together and electrons are forced to be in close proximity with protons long enough for them to transform into neutrons.
Most of these compact objects are about 1.4 times the mass of the Sun, although theory says it can vary from a massive object to approximately. 2.3 solar masses, up to just 1.1 solar masses. All of this is packaged in a sphere enclosed in a sphere only 20 kilometers (12 miles) in diameter, making it weigh somewhere in between every teaspoonful of neutron star material. 10 million and several billion ton.
Stars with a higher and lower mass than neutron stars can also turn into dense objects. Heavier stars turn into black holes. Lighter stars turn into white dwarfs – less dense than neutron stars, with an upper mass limit of 1.4 solar masses, but still quite compact. this the ultimate fate of our own Sun.
The neutron star that is the subject of this study is located in the center of a supernova remnant called. HESS J1731-347previously calculated to sit 10,000 light years away. However, one of the difficulties in studying neutron stars lies in their insufficiently constrained distance measurements. Without an exact distance, accurate measurements of a star’s other features are difficult to obtain.
Recently, a second optically bright star was discovered lurking in HESS J1731-347. Using data from the Gaia mapping survey, a team of astronomers led by Victor Doroshenko of the Eberhard Karls University of Tübingen in Germany were able to recalculate the distance to HESS J1731-347, and at about 8,150, recalculate the distance to HESS J1731-347. and found that it was much closer than anticipated. Light years away.
This means that previous estimates of the neutron star’s other properties, including its mass, need to be improved. Combined with observations of the X-ray light emitted by the neutron star (inconsistent with X-radiation from a white dwarf), Doroshenko and colleagues were able to tune its radius to 10.4 kilometers and its mass to an absolutely astonishingly low 0.77 solar energy. the masses.
This means that it may not actually be a neutron star as we know it, but a hypothetical object in the wild that has not yet been positively identified.
“Our mass estimate makes the central compact object in HESS J1731-347 the lightest known neutron star to date, and a potentially more exotic object, namely a ‘strange star’ candidate.” researchers write in their paper.
According to the theory, a strange star is very similar to a neutron star, but contains a larger proportion of elementary particles called strange quarks. Quarks are fundamental subatomic particles that combine to form compound particles such as protons and neutrons. Quarks come in six different types, or flavors, called up, down, charm, strange, top, and bottom. Protons and neutrons are made up of up and down quarks.
The theory proposes that in the extremely compressed environment inside a neutron star, subatomic particles break up 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 must form under masses large enough to actually compress, but there is essentially no lower bound either, as the rulebook for neutron stars goes out the window when enough quarks are involved. So we cannot rule out the possibility that this neutron star is actually a strange star.
That would be extremely cool; Physicists have been investigating quark matter and strange quark matter for decades. However, while a strange star is certainly possible, what we’re looking at is more likely to be a neutron star – and that’s pretty cool too.
“The resulting constraints on mass and radius are still fully consistent with a standard neutron star interpretation and can be used to refine the astrophysical constraints in the equation of state of cold condensed matter under this assumption.” researchers write.
“Such a faint neutron star appears to be a very intriguing object from an astrophysical perspective, regardless of the putative internal composition.”
It is difficult to determine how such a faint neutron star could have formed under our current models. So whatever is done, the dense object at the heart of HESS J1731-347 will teach us something about the mysterious afterlife of massive stars.
The team’s research has been published Nature Astronomy.
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