Neutron Stars Have Quarky Cores

by | December 27, 2023, 12:00 PM | Stars & Nebulae

Artist’s impression of the different layers inside a massive neutron star, with the red circle representing a sizable quark-matter core. Credit: Jyrki Hokkanen, CSC

Ok, hold onto your brains; things are about to get dense around here.

Our universe’s most massive stars – those eight times larger than the Sun and bigger – don’t exit this universe quietly. When they run out of material to burn for energy in their cores, they will suddenly collapse, and that collapsing material will then explode as a supernova. The largest of these stars will either leave nothing but a nebula behind or will leave a black hole surrounded by a supernova remnant. I think “nothing” is a pretty straightforward concept, and black holes are one of those things that no one can definitively explain the insides of.

Let’s discuss what happens on the smaller side of this “massive stars exploding” story. These stars explode away their outer atmosphere and leave behind a core with a mass between about 1.2 and 2.1 solar masses, and if they try to be larger or smaller, physics should say no. 

You see, these stars – neutron stars – are so dense that electrons and protons will combine into space-saving neutrons, and the star is supported by the neutrons pressing back like so many compressed springs. If you add mass, they will no longer be able to support themselves and will collapse into a black hole. If you could somehow remove mass, those neutrons would spring into a much larger white dwarf as the neutrons decay back into those electrons and protons (with a side of neutrino and gamma-ray light). These unique, physics-defined, stellar remnants are just plain weird, but they aren’t so weird that students haven’t been given homework to work out the maths that define their sizes. 

But, it turns out, generations of students and professors may have failed to take into account just how weird these objects can be. Lately, researchers have been finding stars that somehow are reaching masses as high as 2.35 solar masses! This reality has folks taking a new look at an old question: can the cores of neutron stars be made of something even weirder than just neutrons?

This “even weirder” material would open the door for more massive neutron stars by providing a new way to support bigger stars against collapsing into a black hole. 

Let’s back up a bit. Neutrons are made of three quarks held together by gluons. According to team researchers, in the largest neutron stars, “Their constituent quarks and gluons are liberated from their typical confinement and are allowed to move almost freely.”

This work appears in Nature and is led by Eemeli Annala.  

According to co-author Joonas Hirvonen, “We had to use millions of CPU hours of supercomputer time to be able to compare our theoretical predictions to observations and to constrain the likelihood of quark-matter cores.”

They find there is an 80-90% probability that the most massive neutron stars have a quark-filled core, and the larger the star, the larger the pocket of quark soup in its gooey center. 

These are super cool results; we now know there is very likely non-normal matter in otherwise normal-looking objects. This is a cool new case of “don’t judge a book by its cover” because that plain-looking star just might have a core of exotic matter. 

Faculty, it’s time to take neutron stars out of your homework sets… unless you’re teaching computer science.