A bizarre star is either the lightest neutron star ever found, or something even weirder

Why some astronomers think a weird neutron star might actually be made of elusive strange quarks

By Troy Farah

Science & Health Editor

Published October 27, 2022 10:15AM (EDT)

An XMM-Newton Mosaic of the Galactic Supernova Remnant HESS J1731-347 (Image courtesy of Victor Doroshenko, Gerd Pühlhofer and ESA / XMM-Newton, CC BY-SA 3.0 IGO)
An XMM-Newton Mosaic of the Galactic Supernova Remnant HESS J1731-347 (Image courtesy of Victor Doroshenko, Gerd Pühlhofer and ESA / XMM-Newton, CC BY-SA 3.0 IGO)

When certain large stars use up all their nuclear fuel and die, they collapse and detonate, creating a supernova. These end-of-life events are some of the most energetic in the universe, and send heavy elements like iron and gold careening into the reaches of space. If a star is more massive than our sun, but not too heavy to become a black hole, the atoms within the star may collapse in on themselves — creating a heavy, spinning ball in space a few miles wide yet several times as massive as our sun, and made entirely of neutrons born of electrons and protons that have been smushed together.

The remaining core is called a neutron star, and they is so dense that a spoonful of this star material would weigh around 1 billion tons. Physics start to get freaky at these magnitudes: some neutron stars spin so fast that they rotate over 700 times per second, meaning a single point on its surface moves through space at about one-fifth the speed of light. Neutron stars also defy the typical laws of particle physics: a standalone neutron might decay within an hour, but when they are bound up in a dense ball the size of a small asteroid, they no longer have a half-life as far as we know.

All this dense exotic matter can generate some of the most intense electromagnetic energy of any object known to humanity. In some cases, the magnetism can be 100 million times to 1 quadrillion times stronger than Earth's magnetic field. From our Earthly perspective, this spin appears like blinking. We call these pulsars, which are very useful for making predictions in astronomy.

But even among the freakiest stars in the universe, things can get weirder. A type of neutron star called a central compact object (CCO) may sound like some kind of cell phone accessory, but they're bizarre even by the standards of this interstellar object.

The smallest, lightest neutron star on record seems to be the obliquely named HESS J1731-347, discovered around 2007. It's a CCO surrounded by clouds of dust, and situated about 8,000 light-years away from Earth. A new analysis of HESS J1731-347 by astronomers at the Institut für Astronomie und Astrophysik in Tübingen, Germany has revealed some even stranger physics about this neutron star.

This analysis may rewrite our understanding of the origin and physics of neutron stars, the authors argue.

Using X-rays and gravitational wave measurements, the astronomers determined HESS J1731-347 is either "the lightest neutron star known, 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 dissolve the neutrons of atoms into even more basic constituent pieces, and allowing the formation of strange quarks, which are a bizarre breed of quark rarely seen in our universe. (More on strange quarks in a moment.) Such an object has been dubbed, appropriately, a "strange star."

"Our mass estimate makes the CCO in HESS J1731-347 the lightest neutron star known to date, and potentially a more exotic object—that is, a 'strange star' candidate," Victor Doroshenko, the lead study author and his colleagues, write. "Such a light neutron star, regardless of the assumed internal composition, appears to be a very intriguing object from an astrophysical perspective."

In fact, this analysis may 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 supernova core collapse might have to be revisited."

Indeed, it seems like we're still learning a lot about how neutron stars form. If the measurements of HESS J1731-347 are correct, it could foment the conditions for strange quarks.

Strange quarks truly live up to their name. Quarks are fundamental components of matter, particles that are so tiny they can't be broken down any smaller. Atoms are made of protons, neutrons and electrons, but (with the exception of electrons) each of these components are in turn made of quarks — specifically, up and down quarks, which cluster in triads to form normal matter as we know it. (Protons are made of two up quarks and a down, while neutrons are made of two down quarks and one up.)

Everything you've ever touched is made of elements that are made of atoms that are made entirely of up quarks and down quarks — along with electrons and force carrier particles holding them together. The other four types of quark — strange, charm, top and bottom quarks — are rarely observed and scarcely created, except in particle accelerators and random energetic events around the universe. Typically, the matter these exotic quarks form is very, very short-lived, and decay quickly into more familiar pieces of the universe. 

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Unlike atoms, which can tolerate solitude, their constituent quarks don't like being alone, so physicists rarely find them by themselves. That's why scientists build giant particle accelerators like the Large Hadron Collider, to blast protons at each other and watch as the quarks go spinning off.

Strange quarks are so dubbed because they have half-lives longer than expected, but they're still not very stable, especially when compared to electrons. If strange quarks do exist in large quantities in the universe, it's probably "only true at stupidly high pressure," as The Pasayten Institute, a physics education center, put it. "For example, It's possible that they exist inside neutron stars."

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

It wasn't previously assumed that neutron stars could be as small as HESS J1731-347, so even if the strange star theory doesn't pan out, this is still a weird neutron star either way. It will require astrophysicists to rethink some of their dominant theories as to how and why neutron stars form. In other words, whether or not it's a de facto "strange star" made of strange quarks, this star is extremely strange — in the colloquial, non-quark sense of the word. 

By Troy Farah

Troy Farah is Salon's science and health editor specializing in drug policy and pandemics.

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Astronomy Neutron Star Physics Quarks Science Space