What is N6946-BH1?

A red supergiant that brightened briefly, then faded from sight — the best-known candidate for a star that collapsed with no explosion at all.

Can you see it?

No small telescope will show you this one; it is here to be understood, not spotted.

Its light took about 25 million years to get here.

Why it stands out: If it is what it looks like, this is one of only two stars anyone has watched vanish.
What it is: A red supergiant, around 25 times the Sun’s mass — exactly the sort of star that is supposed to end in a supernova.
What happened: Astronomers had been photographing the Fireworks Galaxy, NGC 6946, for years, watching a list of big stars to catch one exploding. In 2009 this star glowed more than twice as bright for a few months. Then it faded, and by 2015 it was simply gone from the pictures. There was never an explosion.; How we know: Repeated imaging of the same galaxy over more than a decade: the star is in the early pictures and absent from the later ones.
One explanation: The star may have collapsed straight into a black hole and taken its own light with it — a failed supernova. If that is right, some massive stars end without a bang, which would help explain why we find fewer supernovae than the number of heavy stars suggests we should.
Why it is still an argument: Something faint still glows there in the infrared — warm dust, not the star’s own light. The dust could be warmed by gas falling into a new black hole — or it could be hiding a star that is still alive. Later observations raised the possibility that two stars merged here and wrapped themselves in dust. Nobody has settled it. This is a candidate, not a confirmed case.

The numbers

Distance from us	about 25 million light-years
Discovered	2015

It happened in the Fireworks Galaxy, NGC 6946.