The universe may contain hypothetical extremely dense and exotic cosmic objects known as strange quark stars. While astrophysicists continue to debate the existence of quark stars, a team of physicists has discovered that the remnant of a neutron star merger observed in 2019 has the right mass to be one of these strange quark stars.
When the stars die, their cores compress to such incredible degrees that they become entirely new types of objects. E.g, when the sun finally flickerswill leave behind a white dwarf, a planet-sized ball of highly compressed carbon and oxygen atoms. When even bigger stars explode in cataclysmic explosions called Supernova, they leave behind neutron stars. These incredibly dense objects are only a few miles across, but can weigh several times the mass of the sun. As their name suggests, they are made up almost entirely of pure neutrons, making them essentially atomic nuclei miles wide.
Neutron stars they are so exotic that physicists do not yet fully understand them. While we can observe how neutron stars interact with their surroundings and make some good guesses about what happens to all the neutron matter near the surface, the composition of their cores remains elusive.
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The problem is that neutrons are not totally fundamental particles. Even though they combine with protons to form atomic nuclei, neutrons themselves are made up of even smaller particles called quarks.
There are six types or “flavors” of quarks: up, down, up, down, weird, and charm. A neutron is composed of two down quarks and one up quark. If you smash too many atoms together, they revert to a giant ball of neutrons. So if you smash too many neutrons together, they come back to a giant ball of quarks?
The answers range from “maybe” to “it’s complicated.” The problem is that quarks really don’t like being alone. The strong nuclear force, which binds the quarks in a nucleus, actually increases with distance. If you try to pull two quarks together, the force pulling 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 the separated ones.
If you were to model 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 might be a way that uses strange quarks. By themselves, strange quarks are quite heavy, and when left alone they rapidly decay into lighter up and down quarks. When a large number of quarks clump together, however, the physics can change. Physicists discovered that strange quarks can bind with up and down quarks to form triplets, known as “strangelets,” which could be stable, but only under extreme pressures. Just like the pressures one step above a neutron star.
If you compress a neutron star too much, all the neutrons lose their ability to support the star and everything explodes to make a black hole. But there may be a tight step 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 complete control.
Astronomers don’t expect to find a lot of strange stars in the universe; these objects must be heavier than neutron stars but lighter than black holes, and there’s not much wiggle room there. And because we don’t fully understand the physics of the strange, we don’t even know the precise masses at which the strange stars might exist.
But recently, a team of astronomers looked at GW190425, a gravitational wave event triggered by the merger of two neutron stars observed in 2019. Along with huge amounts of gravitational waves, the merging neutron stars produce a kilonova, an explosion that is more powerful than an ordinary star. nova but weaker than a supernova. Although astronomers have not been able to capture one electromagnetic signal from this event, they saw a similar one in 2017 that produced both gravitational waves and radiation.
When two neutron stars merge, there are several options available depending on their masses, their spins, and the angle of collision. According to theoretical calculations, neutron stars could destroy each other, form a black hole, or form a slightly more massive neutron star.
And according to new research, that was recently posted to the arXiv preprint databasethese cosmic collisions can make a strange quark star.
The team calculated that the mass of the object left behind by the 2019 merger was somewhere between 3.11 and 3.54. solar masses. Based on our best understanding of the structure of neutron stars, it is a bit too heavy and should have collapsed into a black hole. But it also falls within the mass range allowed by models of the structures of these strange stars.
It’s still too early to tell if GW190425 is our first sighting of a rare strange-quark star, but future observations (and more theoretical work) could help astronomers identify one of these exotic creatures.
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