NGC 6939 & NGC 6946 / C12 Fireworks Galaxy

Cepheus near the border with Cygnus. Bright Moonlight causing some background gradients.

NGC 6939, discovered by William Herschel of course, is quite old for an Open Cluster, between 1 and 1.3 billion years. It also lies about 400 parsecs above the galactic plane, a little unusual for Open Clusters as they are usually within the plane of the galaxy, hence the alternative name of Galactic Clusters.

NGC 6946 (also discovered by William Herschel) is about 25 million light years away and resides in the Virgo Supercluster. It’s known as the Fireworks Galaxy because it seems to be a hive of supernovae; ten have been observed in the 20th and 21st centuries alone. This is about 10 times the rate observed in our own galaxy, even though the Milky Way has twice as many stars. In fact more supernovae have been observed in this galaxy than any other.

During 2009, a bright star within NGC 6946 flared up over several months to become over one million times as bright as the Sun. Shortly thereafter it faded rapidly. Observations with the Hubble Space Telescope suggest that the star did not survive, although there remains some infrared emission from its position. This is thought to come from debris falling onto a black hole that formed when the star died. This potential black hole-forming star is designated N6946-BH1. The progenitor is believed to have been a yellow hypergiant star. Wikipedia