Exotic stars, unlike typical stars found in the observable universe, are stellar remnants or theoretical models that feature unusual properties and behaviors. Among the best-known types of exotic stars are neutron stars, which are incredibly dense remnants left after a supernova explosion of a massive star. Neutron stars can have masses greater than the Sun but compressed into a sphere with a diameter of about 20 kilometers. This extreme density means that a sugar-cube-sized amount of material from a neutron star would weigh about a billion tons on Earth. Another intriguing type is the quark star, hypothesized to exist when neutron stars are further compressed, causing their constituent neutrons to dissolve into a soup of quarks.
Pulsars, a subset of neutron stars, emit beams of electromagnetic radiation from their magnetic poles. These beams are detectable on Earth as pulses because the rotation of the star sweeps the beam across the sky, much like a lighthouse beam. This phenomenon has allowed astronomers to use pulsars as cosmic clocks, with some pulsars spinning hundreds of times per second. Another exotic star type, the magnetar, represents an extreme version of a neutron star, possessing magnetic fields a thousand times stronger than typical neutron stars. These intense magnetic fields generate powerful starquakes and gamma-ray flares detectable from Earth.
Black dwarfs are another theoretical type of exotic star, which are hypothesized to be the final evolutionary state of white dwarfs. However, since the universe is not old enough for any white dwarfs to have cooled into black dwarfs, none have yet been observed. These stars, if they exist, would no longer emit significant heat or light, essentially making them cosmic cinders drifting through space. The concept of black dwarfs presents intriguing implications for the future of star evolution and the long-term dynamics of the universe.
Finally, boson stars and preon stars are even more speculative and represent exotic possibilities in theoretical astrophysics. Boson stars could be formed out of hypothetical particles such as bosons, rather than baryonic matter, and might exist as invisible entities except through their gravitational effects. Preon stars, built from hypothetical sub-quark particles called preons, would be even more extreme in density and exotic physics. The exploration of these and other exotic stars not only stretches our understanding of the cosmos but also challenges the limits of known physics, potentially leading to new discoveries in both astrophysical phenomena and fundamental particle physics.