The universe may contain extremely dense and exotic hypothetical cosmic objects known as strange quark stars. While astrophysicists continue to debate quark stars’ existence, 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 (opens in new tab), their cores compress to such incredible degrees that they become entirely new kinds of objects. For example, when the sun finally flickers out (opens in new tab), it will leave behind a white dwarf (opens in new tab), a planet-size ball of highly compressed carbon and oxygen atoms.

When even larger stars explode in cataclysmic explosions called supernovas (opens in new tab), they leave behind neutron stars. These incredibly dense objects are only a few miles across but can weigh a few times the mass of the sun. As their name suggests, they are made almost entirely of pure neutrons, making them essentially kilometers-wide atomic nuclei.

Neutron stars (opens in new tab) 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 as to what’s happening to all that neutron matter near the surface, the makeup of their cores remains elusive.