Constellation Cygnus | ||
N6946-BH1 is a disappearing red supergiant star in another galaxy, NGC 6946. The star was 25 times the mass of the sun, and was 20 million light years distant from Earth. In March through to May 2009 the star brightened to several million solar luminosities, but after that by 2015 it had disappeared from optical view. In the mid and near infrared an object is still visible, however, it is fading away with a brightness proportional to t−4/3. The brightening was insufficient to be a supernova, and is called a failed supernova.
The stars coordinates were at RA 20:35:27.56 and Dec +60:08:08.29. The brightness of the star, given by its apparent magnitude in different colour bands on 2 July 2005 is given by R = 21, V = 21, B = 22, U = 23. Prior to the optical outburst the star was about 100,000 times as bright as the sun. After the outburst it was invisible in the visual band, but has declined to 5000 times as bright as the sun in infrared radiation.
One hypothesis is that the core of the star collapsed to form a black hole. The collapsing matter formed a burst of neutrinos that lowered the total mass of the star by a fraction of a percent. This caused a shock wave that blasted out the star's envelope to make it brighter.
Observed type II supernovae do not originate from stars with initial masses greater than about 18 M☉, and the rate of large star formation appears to exceed the rate of supernovae. So the suspicion is that something else is happening to these extra large stars. Failed supernovae and black hole formation is one explanation. If this event indeed reflected the formation of a black hole, it is the first time that black hole formation has been observed.