The rules for a stellar death seem pretty simple. If the star isn't that massive, it burns out into a carbon-rich remnant called a white dwarf. If it's big enough, the star ends in a bang, exploding in a supernova that can leave behind a neutron star or a black hole. But a number of simulations have suggested that there's another option: big stars that go out not with a bang but a whimper.

The idea is that, rather than exploding, much of the mass of the star falls inward to the core, forming a larger black hole. While some of the outer layers of the star are shed and it brightens briefly, there's no catastrophic explosion. Now, researchers about to publish in Monthly Notices of the Royal Astronomical Society have identified one of these collapses in the form of a star that seems to have vanished.

No bang

While supernovae are often called explosions, they're actually a bit more complicated than that. Deep within the soon-to-be supernova, all the lighter elements have been fused, leaving the core to undergo reactions that absorb energy. Without any excess energy to push back against gravity, the interior of the star collapses, forming a black hole or neutron star. It's only the outer layers that are jettisoned, creating an outburst of light and material.

A number of things have hinted that this isn't the entire story, though. When we can identify the star that gave rise to a supernova, we've found that most of them are below 18 times the mass of the Sun, even though we know that the relevant type of star (a red supergiant, in this case) can get far more massive. Plus, there's evidence that massive stars are formed more frequently than the rate of supernovae would suggest. Both of those suggest that there are fewer visible supernovae than we'd expect.

Then we have the black holes observed merging by LIGO, which were larger than we'd expect to be generated by a supernova. One way to get that is to have more of the star's mass transferred to the black hole, which would leave less to participate in any explosion. Models of high mass stars suggested this was possible. For a red supergiant, it would involve swallowing all but the hydrogen-rich layers near the surface, which would result in a relatively short-lived flare before the star was replaced by a black hole.

How do you look for something like that? Supernova are relatively easy to spot, given that they're characterized by the sudden appearance of an intense burst of light; in fact, amateurs tend to spot many of them. But in this case, we'd need to spot a star vanishing into nothing more than a brief outburst that lasted less than a year.

The search is on

The challenge involved didn't stop people from trying, and they arranged time on the Large Binocular Telescope to survey for stars that seemingly vanished. And they came up with some candidates, one of which was given the name N6946-BH1. A luminous star to begin with, it appeared to dim in visible wavelengths while brightening in the infrared from about 2005 to 2008. In 2009, N6946-BH1 underwent a transient brightening event for several months. And then, it vanished. Observations in 2015 showed nothing at the site.

The researchers considered a variety of processes that might make it look like the star had vanished. Most of these involved the star ejecting some gas and dust that obscured it from view. But most of the processes that would involve that sort of injection would make the gas and dust quite energetic and easy to spot. But careful observations using the Hubble and Spitzer space telescopes confirmed that there was really nothing at the site. "Thus, the progenitor did not survive behind a thick, dusty wind," the authors conclude.

So what happened to it? Obviously, the team favors the direct collapse of most of the star directly into a black hole, one with a mass that's much closer to the ones observed through gravitational waves at LIGO (over 20 times that of the Sun). The outburst would have been the shedding of the star's outermost layers and possibly the energy released as matter falls in toward the black hole.

Further observations need to be done to confirm a black hole has been left behind; they're often active at energetic wavelengths while feeding, so we have a chance of catching some X-ray emissions. But the important challenge will be to identify other events of this type. After all, the hope is that they can account for some of the differences between the number of massive stars and the number of supernovae we observe.

The arXiv. Abstract number: 1609.01283 (About the arXiv). To be published in Monthly Notices of the Royal Astronomical Society.