Neutron stars—extremely dense remnants of stars that have undergone supernova events—have appeared in fiction since the 1960s. Their immense gravitational fields and resulting extreme tidal forces are a recurring point of focus. Some works depict the neutron stars as harbouring exotic alien lifeforms, while others focus on the habitability of the surrounding system of planets. Neutron star mergers, and their potential to cause extinction events at interstellar distances due to the enormous amounts of radiation released, also feature on occasion. Neutronium, the degenerate matter that makes up neutron stars, often turns up as a material existing outside of them in science fiction; in reality, it would likely not be stable.