Astronomers may have seen a star collapse directly to a black hole

One of the basic truisms in astronomy is that, when a massive star ends its life, it goes out with a bang. A big one. A supernova.

This titanic explosion is triggered when the star runs out of nuclear fuel in its core. The core collapses in a heartbeat, and the energy generated in that collapse is so immense that it blows the outer layers off. This explosion is so colossal it can outshine an entire galaxy! In the meantime, the collapsed core can form an exotic neutron star, or may even squeeze itself down into a black hole.

Now, I’ve skipped some steps there, but that’s the general picture (if you want more, check out my Crash Course Astronomy episode on high mass stars and supernovae). If you want a black hole, you have to blow up a massive star.

Except, maybe not. It turns out there’s a loophole that could allow a star to bypass the supernova part. It collapses directly down to a black hole without the explosion. Some energy is released, but not much compared to a supernova, and in the end what you get is a now-you-see-it-now-you-don’t situation: The star is there, and then suddenly … it isn’t.

The idea of a “failed supernova” is an interesting theoretical astrophysical problem, and one scientists have been working on for a while now. But there’s been a new an exciting development: Astronomers now think they’ve seen one!

NGC 6946
The face-on spiral galaxy NGC 6946, which has hosted 10 supernovae in the past century. N6946-BH1 isn’t annotated, because it didn’t explode. Credit: Damian Peach

The star in question is called N6946-BH1, and it was found in a very cool survey specifically designed to look for failed supernovae. Using the Large Binocular Telescope in Arizona, 27 galaxies all within about 30 million light-years of Earth were observed over and over again. Each image was painstakingly compared to the others to look for “transients”: objects that have changed brightness. Even using rather stringent criteria, thousands were found — stars change brightness for a lot of reasons, but most are not due to them going supernova … or, in this case, failing to supernova.

Eventually, the number of interesting objects was whittled down to just 15. Six of them turned out to be run-of-the-mill exploding stars (if the titanic explosion of a few octillion tons of star screaming outward at a substantial fraction of the speed of light can be called “ho-hum”), but nine of them turned out to be more interesting.

Of these, all but one were likely unusual events, like two stars merging, which can cause a very big (and very pretty) eruption, but again falls short of the outcome of a massive star dying. When all was said and done, after searching 27 galaxies for seven years, only one object was left: N6946-BH1.

In earlier images, the star is there, clearly seen in the galaxy NGC 6946, a lovely face-on spiral galaxy roughly 20 million light-years away (and one that has had no fewer than 10 recorded supernovae in the past century; by coincidence one was seen just this year). Then, in later images, it’s gone. Like, gone: Disappeared. Poof.

Now you see it… The star N6946-BH1 is visible in the earlier 2007 Hubble image (left) but is gone in 2015 (right). Credit: NASA/ESA/C. Kochanek (OSU)

If it had exploded as a supernova it would’ve been seen in the images. Instead, in 2009, it briefly got somewhat brighter, glowing at about a million times brighter than the Sun; then it faded so much it was only about 2% of its previous brightness (that is, pre-collapse) by 2015. And yes, in human terms, a million times the Sun’s luminosity is terrifyingly bright, but in terms of a supernova, it’s barely worth mentioning; a typical one will shine many billions of times brighter than the Sun! So this was, at best, a bit of a pop.

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Source: Bad Astronomy | Astronomers may have seen a star collapse directly to a black hole. | SyfyWire

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