A small, relatively dense object hidden in an exploded cloud still a few thousand light-years away is challenging our understanding of stellar physics.
By all accounts, this appears to be a neutron star profile, although unusual in that. At just 77% of 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, its mass was 1.17 times the mass of the Sun.
This latest discovery is not only smaller, but it is much smaller than 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 to the “alien” star by name.
Neutron stars are among the densest objects in the entire universe. It’s what’s left after a massive star about 8 to 30 times the mass of the Sun has reached 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 supported by the external pressure of fusion, the nucleus collapses in on itself to form a body so dense that atomic nuclei collide together and force electrons 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 something as massive as around 2.3 solar masses, to just 1.1 solar masses. All of this is packed inside a ball packed into a ball about 20 kilometers (12 miles) in diameter, so each teaspoon of neutron star matter weighs somewhere between 10 million and several billion tons.
Stars with higher and lower masses than neutron stars can also transform 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 quite compact. it’s 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 sit 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 star’s other features.
Recently, a second optically bright star was discovered hiding in HESS J1731-347. From there, 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 it to be much closer than expected, about 8150 light-years away.
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 radiation from a white dwarf), Doroshenko and colleagues were able to improve its radius to 10.4 km and its mass to a completely lower solar level of 0.77. masses.
This means that it may not actually be a neutron star as we know it, but a hypothetical object that has not yet been positively identified in nature.
“Our comprehensive estimate makes the central compact body of HESS J1731-347 the lightest neutron star known to date, and possibly a more exotic object, i.e. a candidate ‘alien star’.” , the researchers write in their article.
According to the theory, a strange star is very similar to a neutron star, but contains a greater proportion of fundamental particles called strange 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 high, low, attractive, odd, high and low. 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. According to this model, exotic stars consist of matter made up of equal proportions of up, down, and strange quarks.
Exotic stars should form under masses big enough to put real stress on them, but since the rulebook for neutron stars disappears when enough quarks are included, there’s no bottom line either. This means that we cannot rule out the possibility that this neutron star is in fact an alien star.
That 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 most likely 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 neutron stars and can be used to improve astrophysical constraints on the cold-density state equation under this assumption,” the researchers write.
“Such a faint neutron star, regardless of its supposed internal composition, appears to be a very interesting object from an astrophysical point of view.”
It is difficult to determine how such a light neutron star formed with our current models. So, regardless, the dense object at the core of HESS J1731-347 will have something to teach us about the mysterious afterlife of massive stars.
The team’s research was published in natural astronomy.