Earlier this year, astronomers proposed instead that LRDs were dense spheres of gas with a black hole at their centre, called black hole stars. “When material falls into the black hole, a lot of gravitational energy is released, and this could make the whole ball of gas around it glow like a star,” says Anna de Graaff at Harvard University. Although the energy doesn’t come from nuclear fusion, as in a regular star, the end effect is a similar glowing ball of dense gas, just on a far bigger scale, billions of times brighter than our sun, says de Graaff.