A relatively small, dense object shrouded in a cloud of its own debris that exploded just a few thousand light-years away is challenging our understanding of stellar physics.
By all accounts, it appears to be a neutron stars, although it is unusual in that. 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 clocked in at 1.17 times the mass of the Sun.
This most recent discovery is not just smaller, it is significantly lower than the minimum neutron star mass predicted by theory. This suggests that there is some gap in our understanding of these ultra-dense objects…or what we are seeing 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 at its core, it goes 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 an object so dense that the atomic nuclei are crushed and the electrons are forced to become intimate with the protons long enough for them to transform into neutrons.
Most of these compact objects are around 1.4 times the mass of the Sun, although in theory they could range from something as massive as around 2.3 solar masses, down to just 1.1 solar masses. All of this packed inside a sphere simply packed into a sphere just 20 kilometers (12 miles) wide, making each teaspoon of neutron star material weigh somewhere between 10 million Y several billion lots.
Stars with masses higher and lower than neutron stars can also become dense objects. The heaviest stars become black holes. The lightest stars become white dwarfs, less dense than neutron stars, with an upper mass limit of 1.4 solar masses, although still fairly compact. This is the final destination of our own sun.
The neutron star being studied is at the center of a supernova remnant called HESS J1731-347which had previously been calculated to sit more than 10,000 light years away. However, one of the difficulties in studying neutron stars lies in poorly constrained distance measurements. Without a precise distance, it is difficult to obtain precise measurements of the other features of a star.
Recently, a second optically bright star was discovered lurking 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 were able to recalculate the distance to HESS J1731-347 and found that it is much closer than expected. it was thought, about 8,150 light-years away.
This means that previous estimates of the neutron star’s other features, including its mass, needed to be refined. Combined with observations of X-ray light emitted by the neutron star (inconsistent with X-radiation from a white dwarf), Doroshenko and colleagues were able to refine its radius to 10.4 kilometers, and its mass to 0. Absolutely amazingly low solar 77. masses.
This means that it might not actually be a neutron star as we know it, but a hypothetical object not yet positively identified in nature.
“Our mass estimate makes the central compact 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 article.
According to theory, a strange star looks a lot like a neutron star, but contains a higher proportion of fundamental 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.
Theory suggests that, in the extremely compressed environment inside a neutron star, subatomic particles break down into their constituent quarks. Under this model, strange stars are made of matter consisting of equal proportions of up, down, and strange quarks.
Strange stars should form under large enough masses to really push them down, but since the rulebook for neutron stars disappears when enough quarks are involved, there’s essentially no lower limit either. Which means that we cannot rule out the possibility that this neutron star is indeed a strange star.
This 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’re seeing is a neutron star, and that’s also extremely cool.
“The obtained constraints on mass and radius remain fully consistent with a standard interpretation of a neutron star and can be used to improve the astrophysical constraints on the equation of state for cold, dense matter under this assumption.” the researchers write.
“Such a light neutron star, regardless of the assumed internal composition, appears to be a very intriguing object from an astrophysical perspective.”
It is a challenge to determine how such a light neutron star could have formed with our current models. So whatever it is made of, the dense object at the heart of HESS J1731-347 will have something to teach us about the mysterious future lives of massive stars.
The team’s research has been published in nature astronomy.
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