When the stars die, their cores compress to such incredible degrees that they become entirely new types of objects. For instance, when the sun finally goes downhe will leave behind him white dwarf, a planet-sized ball of highly compressed carbon and oxygen atoms. When even larger stars explode in cataclysmic explosions called supernovae, they leave behind neutron stars. These incredibly dense objects are only a few kilometers across but can weigh many times the mass of the sun. As their name suggests, they are made almost entirely of pure neutrons, which essentially makes them atomic nuclei several kilometers wide.
Neutron stars are so exotic that physicists still don’t fully understand them. Although we can observe how neutron stars interact with their surroundings and make good guesses about what happens to all that neutron matter near the surface, the composition of their cores remains elusive.
Related: Are quark stars possible?
The problem is that neutrons are not totally fundamental particles. Although they group together with protons to form atomic nuclei, neutrons themselves are made up of even smaller particles called quarks.
There are six kinds, or “flavors,” of quarks: up, down, up, down, strange, and charm. A neutron is made up of two down quarks and one up quark. If you smash too many atoms together, they revert back to a giant ball of neutrons. So if you smash too many neutrons together, do they revert back to a giant ball of quarks?
The answers vary from “maybe” to “it’s complicated”. The problem is that quarks really don’t like to be alone. The strong nuclear force, which binds quarks in a nucleus, actually grows with distance. If you try to bring two quarks closer together, the force that brings them back increases. Eventually, the attractive energy between them becomes so great that new particles appear in the vacuum, including new quarks that are more than happy to bond with those that are apart.
If you were to fashion a macroscopic object from the up or down quarks that make up a neutron, that object would explode very quickly and very violently.
But there could be a way using strange quarks. On their own, strange quarks are quite heavy, and when left alone they quickly decay into lighter up and down quarks. However, when large numbers of quarks come together, the physics can change. Physicists have discovered that strange quarks can bond with up and down quarks to form triplets, called “strangelets”, which could be stable, but only under extreme pressures. Like the pressures a notch above a neutron star.
If you compress a neutron star too much, all the neutrons lose their ability to support the star, and the whole thing implodes to make a black hole. But there may be a stage stuck in between where the pressures are high enough to dissolve neutrons and form a strange quark star but not intense enough to gravity to take full control.
Astronomers don’t expect to find many strange stars in the universe; these objects must be heavier than neutron stars but lighter than black holes, and there’s not a lot of wiggle room there. And because we don’t fully understand the physics of strangelets, we don’t even know the precise masses where the strange stars might exist.
But recently, a team of astronomers looked into GW190425, a gravitational wave event triggered by the merger of two neutron stars observed in 2019. Along with huge amounts of gravitational waves, the star merger neutron produces a kilonova, an explosion more powerful than a normal one. nova but weaker than a supernova. Although astronomers were unable to capture a electromagnetic signal of this event, they saw a similar one in 2017 that produced both gravitational waves and radiation.
When two neutron stars merge, they have several options depending on their masses, spins, and collision angle. According to theoretical calculations, neutron stars could fade out, form a black hole, or form a slightly more massive neutron star.
And according to the new research, which has been recently published in the arXiv prepublication databasethese cosmic collisions could create a strange quark star.
The team calculated the mass of the object left behind by the 2019 meltdown to be between 3.11 and 3.54 solar masses. From our best understanding of the structure of neutron stars, it’s just a bit too heavy, and it should have collapsed into a black hole. But it is also within the mass range allowed by the structural models of these strange stars.
It’s still too early to tell if GW190425 is our first sighting of a strange and rare quark star, but future observations (and more theoretical work) could help astronomers identify one of these exotic creatures.
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