The universe may contain extremely dense and exotic hypothetical cosmic objects known as strange quark stars. As astrophysicists continue to debate the existence of quark stars, a team of physicists has found that the remnant of a neutron star merger observed in 2019 has just the right mass to be one of these strange quark stars.
when stars die, their cores compress so incredibly much that they become completely new objects. For example, when the sun finally comes outit leaves a white dwarf, a planet-sized sphere of highly compressed carbon and oxygen atoms. When even larger stars explode in cataclysmic explosions, that’s called supernovae, they leave behind neutron stars. These incredibly dense objects are only a few kilometers wide but can weigh a few times the Sun. As their name suggests, they consist almost entirely of pure neutrons, making them essentially kilometer-sized atomic nuclei.
neutron stars 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 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 absolutely fundamental particles. Although 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, Top, Bottom, Strange, and Charm. A neutron consists of two down quarks and one up quark. If you squeeze too many atoms together, they turn back into a giant ball of neutrons. So if you squeeze too many neutrons together, do they turn back into a giant ball of quarks?
Strange Matters
The answers vary from “maybe” to “it’s complicated”. The problem is that Quarks really don’t like being alone. That strong nuclear power, which binds quarks in a nucleus, actually increases with distance. If you try to pull two quarks together, the force pulling them back increases. Eventually, the attraction between them becomes so great that new particles appear in the vacuum, including new quarks, which more than happily bind to the separated ones.
If you were to make a macroscopic object out of 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 using weird quarks. Strange quarks are inherently quite heavy, and if left alone they quickly break down into the lighter up and down quarks. However, when a large number of quarks group together, the physics can change. Physicists have found that strange quarks can combine with up and down quarks to form triplets known as “strangelets,” which could be stable – but only under extreme pressure. Like the pressure a level above a neutron star.
Cosmic Collisions
If you compress a neutron star too much, all of the neutrons lose their ability to support the star and the whole thing implodes to make a black hole. But maybe there’s a stage squeezed in between where the pressures are high enough to break up the neutrons and form a weird quark star, but not intense enough for that heaviness to take full control.
Astronomers don’t expect to find many strange stars in the universe; These objects need to be heavier than neutron stars but lighter than black holes, and there’s not much wiggle room. And because we don’t fully understand the physics of strangelets, we don’t even know the exact masses in which strange stars might exist.
But recently, a team of astronomers studied GW190425, a gravitational-wave event triggered by the merger of two neutron stars observed in 2019. Combined with massive amounts of gravitational waves, merging neutron stars produce a kilonova, an explosion more powerful than a normal nova but weaker than a supernova. Although astronomers have not been able to capture one electromagnetic Signal from this event, they saw a similar event in 2017 that produced both gravitational waves and radiation.
When two neutron stars merge, a few options are available depending on their mass, spin, and the angle of collision. According to theoretical calculations, the neutron stars could annihilate 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 preprint database arXivthese cosmic collisions could create 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 neutron star structure, this is just 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 too early to tell if GW190425 is our first observation of a rare odd quark star, but future observations (and further theoretical work) could help astronomers pinpoint one of these exotic creatures.
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