A bright star in a nearby galaxy has essentially vanished. Astronomers believe that it died and collapsed in on itself, transforming into the eerie cosmic phenomenon known as a black hole.
"It used to be one of the brightest stars in the Andromeda galaxy," says Kishalay De, an astronomer with Columbia University and the Flatiron Institute. "Today, it is nowhere to be seen, even with the most sensitive telescopes."
In the journal Science, he and his colleagues report that they noticed this disappearing star as they went looking through archival data collected over about 15 years by NASA's NEOWISE spacecraft. De says their plan was to make a map of how stars change in brightness in infrared light, so they set out to track the changes in millions of stars over time.
One star, they noticed, was a real outlier. Around 2015, it suddenly brightened for about a year. After that, it abruptly started fading away, in both infrared light and the optical light that human eyes can see. Just a few years later, in optical light, it was completely gone.
Scientists had known about this star for decades, and people used to be able to see it from their backyards, using small telescopes, notes De. But now, "we can't even detect this source today with the Hubble Space Telescope."
And he says in infrared light, it's so faint that it's only barely detectable with the powerful James Webb Space Telescope.
These strange events are consistent with the star's internal nuclear reactor running out of fuel, causing it to collapse into itself and form a black hole, he says. If that's what happened, the faint infrared glow that's left is powered by the remains of the star continuing to fall into the black hole.
"We would predict that this continues to fade away into darkness," says De, though it could take decades to watch that happen.
When massive stars die, they generally explode. These cataclysmic events, called supernovas, happen often and they're easy to spot, because an exploding star becomes intensely bright and can briefly outshine its galaxy, says De.
But even though theoretical astrophysicists believe that a star could die by collapsing into itself and forming a black hole, that kind of event is quiet and much less noticeable.
Suvi Gezari, an astronomer with the University of Maryland who wasn't part of the research team, says this study used infrared light observations over a long time period "to open up this process that is otherwise obscured by dust and very faint and difficult to observe."
Astronomers know of one other case of an apparently disappearing star, but it was farther away and fainter, so observations aren't as detailed. "It's not quite a twin object, but it's pretty similar," says Christopher Kochanek, an astronomer with Ohio State University, who has studied that peculiar event.
"This approach is the only game in town for seeing the formation of a black hole," he notes.
Of course, not everyone is convinced. Some astronomers take the position that these apparently vanishing stars could actually be merging stars that then get their combined light obscured by a disc of dust, says Kochanek. Who's right should be revealed by additional telescope checks in the years ahead, so see how things evolve.
"Fundamentally, the only way to clearly answer this either way is that one thing distinguishes the black hole case from any other scenario," says Kochanek, "and that is that death is forever. Ultimately, it needs to fade to black."
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