For a star to form a black hole, the thought was that they had to go supernova first. A new discovery, however, may force a rethink, hinting at a previously unseen event.
This discovery is a star called N6946-BH1, found 22 million light-years away in the NGC 6946 galaxy. It’s nicknamed the “Fireworks Galaxy” because we have seen so many supernovae there.
But scientists from The Ohio State University found something peculiar. Using the Large Binocular Telescope (LBT) in Arizona, they noticed that N6946-BH1, which was 25 times more massive than the Sun, dimmed out of existence between 2009 and 2015. The findings were published in the Monthly Notices of the Royal Astronomical Society.
Using the Hubble Space Telescope and the Spitzer Space Telescope, they attempted to find the star in its last known location after it had dimmed from view. But they found nothing there, not even evidence that it was being obscured by a dust cloud. The star had simply vanished without a bang.
This may be the first evidence for what’s called a failed supernova. While most massive stars are thought to explode before becoming black holes, 30 percent may actually just collapse directly into being a black hole.
“The typical view is that a star can form a black hole only after it goes supernova,” Christopher Kochanek, professor of astronomy at The Ohio State University and a co-author on the study, said in a statement. “If a star can fall short of a supernova and still make a black hole, that would help to explain why we don’t see supernovae from the most massive stars.”
This particular star had become 1 million times more luminous in 2009 as it neared the end of its life, which suggested it was about to go supernova. But it then became six times fainter in a matter of months, before later disappearing from view.
The scientists think that the mass might be key. It could be that the star was so massive, it simply didn’t have time to go supernova before its core collapsed. This may link into how very massive black holes form – it may be that it's easier to form more massive black holes without an accompanying explosion.
If this turns out to be the case for N6946-BH1, this would be the first failed supernova we’ve ever found. We’ve seen stars exploding all over the place, but it seems some may also simply vanish from view in a massive gravitational collapse.