An international team of astronomers has discovered a new type of nebula around binary stars, a system of two gravitationally bound stars orbiting in closed orbits around a common center, ScienceAlert reports.
The researchers followed the binary star YY Hya, consisting of a K-type dwarf star and a hot white dwarf (WD). They are located in a common gas envelope emitted by the star that eventually became a white dwarf. Prior to this, it went through a red giant phase and dispersed its outer layers of gas into space.
Inside this common envelope, the two stars continue to evolve on their own, and stellar radiation illuminates the dispersed gas. This is the new type of nebula, which has been called the galactic emission nebula.
Normal stars expand into red giants by the end of their lives. Because a considerable proportion of stars are in binary pairs, this affects the evolution at the end of their lives. The inflating outer part of the star merges into a common envelope around both stars.