This star could be a bizarre “strange star” made of matter that shouldn’t exist

When certain large stars use up all their nuclear fuel and die, they collapse and explode, creating a supernova. These end-of-life events are among the most energetic in the universe, sending heavy elements like iron and gold into the vast expanses of space. If a star is more massive than our Sun but not too massive to become a black hole, the atoms inside the star can collapse in on themselves — forming a heavy, spinning ball in space that’s a few kilometers wide, but several Sometimes as massive as our Sun, it’s made entirely of neutrons, made from electrons and protons that have been squeezed together.

The remaining core is called a neutron star and is so dense that a spoonful of this stellar material would weigh about 1 billion tons. At these magnitudes, physics starts to get insane: Some neutron stars spin so fast that they spin over 700 times per second, meaning a single point on its surface is moving through space at about a fifth the speed of light. Neutron stars, too, defy the typical laws of particle physics: a neutron on its own might decay within an hour, but when bound in a dense sphere the size of a small asteroid, they no longer have a half-life, to our knowledge.

All of this dense exotic matter can generate some of the most intense electromagnetic energies of any object known to man. In some cases, the magnetism can be 100 million to 1 quadrillion times stronger than the Earth’s magnetic field. From our earthly perspective, this rotation appears like a blink. We call these pulsars, which are very useful for predictions in astronomy.

But even among the craziest stars in the universe, things can get weirder. A type of neutron star called the Central Compact Object (CCO) may sound like some sort of cellphone accessory, but they’re bizarre even by the standards of this interstellar object.

The smallest and lightest known neutron star appears to be the obliquely named HESS J1731-347, discovered around 2007. It is a CCO surrounded by dust clouds and is about 8,000 light-years from Earth. A new analysis of HESS J1731-347 by astronomers at the Institute for Astronomy and Astrophysics in Tübingen, Germany, has revealed even stranger physics about this neutron star.

Using X-rays and gravitational-wave measurements, the astronomers determined that HESS J1731-347 is either “the lightest known neutron star or a ‘strange star’ with a more exotic equation of state,” they report in the journal Nature Astronomy. Under these conditions, the pressure on atoms would be so great that it would break down the atoms’ neutrons into even more basic components, allowing the formation of strange quarks, a bizarre type of quark rarely seen in our universe. (More on strange quarks in a moment.) Such an object has been appropriately dubbed an “alien star.”

“Our mass estimate makes the CCO in HESS J1731-347 the lightest neutron star known to date, and possibly a more exotic object – that is, a candidate for a ‘strange star,'” write Victor Doroshenko, the study’s lead author, and his colleagues. “Such a light neutron star, regardless of the assumed internal composition, seems to be a very intriguing object from an astrophysical point of view.”

Indeed, this analysis could rewrite our understanding of the origin and physics of neutron stars, the authors argue, writing that “models describing the mass loss of the proto-neutron star after the collapse of the supernova core may need to be revised.”

In fact, it seems we’re still learning a lot about how neutron stars form. If HESS J1731-347 measurements are correct, it could stoke conditions for strange quarks.

Strange quarks live up to their name. Quarks are fundamental components of matter, particles so tiny that they cannot be broken down any further. Atoms are made up of protons, neutrons, and electrons, but each of these components are in turn made up of quarks — more specifically, up and down quarks that combine in triads to form normal matter as we know it. (Protons are made up of two up quarks and one down quark, while neutrons are made up of two down quarks and one up quark.)

Everything you’ve ever touched is made up of elements made up of atoms made entirely of up quarks and down quarks — along with electrons and force carrier particles that hold them together. The other four types of quarks — strange, charm, top, and bottom quarks — are rarely observed and hardly ever created except in particle accelerators and random energetic events in the universe. Typically, the matter that these exotic quarks make up is very, very short-lived, rapidly decaying into more familiar parts of the universe.

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Unlike atoms, which can tolerate solitude, quarks don’t like being alone, so physicists rarely find them alone. That’s why scientists build giant particle accelerators like the Large Hadron Collider to smash protons at each other and watch the quarks spin.

Strange quarks are so called because they have a longer than expected half-life, but they’re still not very stable, especially compared to electrons. If weird quarks exist in large quantities in the universe, this is probably “true only at stupidly high pressures,” as the Pasayten Institute, a physics education center, put it. “For example, it is possible that they exist in neutron stars.”

Now it seems we have even stronger evidence that this is possible.

It had not previously been thought that neutron stars could be as small as HESS J1731-347, so even if the strange star theory doesn’t work, this is still a strange neutron star either way. Astrophysicists need to reconsider some of their prevailing theories about how and why neutron stars form. In other words, whether it is a de facto “strange star” of strange quarks, this star is extremely strange – in the colloquial, non-quark sense of the word.