A massive white-dwarf merger product before final collapse.

Gravitational-wave emission can lead to the coalescence of close pairs of compact objects orbiting each other. In the case of neutron stars, such mergers may yield masses above the Tolman-Oppenheimer-Volkoff limit (2 to 2.7 solar masses), leading to the formation of black holes.

Interacting supernovae from photoionization-confined shells around red supergiant stars.

Betelgeuse, a nearby red supergiant, is a fast-moving star with a powerful stellar wind that drives a bow shock into its surroundings. This picture has been challenged by the discovery of a dense and almost static shell that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium.