A star has gone missing.
Not in our own Milky Way, but in a galaxy about 75 million light-years away. The star in question is so hot that it glows crystal blue, and it shines a couple million times brighter than the star we know best, our sun. Even as stars go, it’s massive. Astronomers have studied it for nearly two decades, so it was pretty disconcerting when, one day last year, they looked at the latest observations and realized they couldn’t find it anymore.
Andrew Allan, a doctoral student in astrophysics at Trinity College Dublin, checked and rechecked the data. The star is too far away for telescopes to spot it directly, so astronomers look for the distinct signatures that these luminous blue stars imprint on the light coming from their home galaxy. But those traces had vanished.
Maybe, Allan thought, this was Earth’s fault. The weather hadn’t been great on the day that the telescope, perched in the mountains of northern Chile, had examined the distant galaxy. So he and his colleagues decided to conduct another round of observations on a different instrument at the observatory a few months later. Once again, no star.
“It was a pleasant surprise for us,” Allan told me.
Pleasant? When I asked Allan about this discovery, which was announced yesterday, I had expected some alarm in his response, maybe even a hint of panic, over an apparent cosmic disappearance. Outer space certainly can be a mysterious and surprising realm, but stars like this one aren’t supposed to blink out without a trace. They usually finish their life in a powerful, radiant explosion—a supernova—leaving behind a newly formed black hole.
The observations revealed no evidence of a supernova—and that, to Allan, is the best part. He and his colleagues say that the star might have simply skipped over the supernova and collapsed into a black hole without fanfare.
“If indeed the star turned into a black hole with no supernova at all, then it’s a case of ‘gone without a bang’ that astronomers have been searching for for a while now,” Iair Arcavi, an astronomer at Tel Aviv University who was not involved in this research, told me.
Astronomers have theorized that a star could go out this way, and they have come close to capturing it. In 2009, a star appeared to brighten, a sign that it might soon explode, in a galaxy so well known for stellar explosions that it is nicknamed the “fireworks galaxy.” For several months, the star glowed 1 million times more luminous than our sun. But by 2015, it had vanished instead. When even the most powerful space-based telescopes couldn’t find it, astronomers concluded that it had fizzled out into a black hole without experiencing that final spark.
Allan hopes that’s what happened here, but the researchers don’t yet know for sure. “It’s important to keep in mind that they cannot—and do not try to—show definitively that the star has truly disappeared,” says Daniel Perley, an astronomer at Liverpool John Moores University, in the United Kingdom, who was not involved in the research.
The researchers haven’t ruled out slightly less dramatic possibilities. One explanation rests with this kind of star’s nature. Luminous blue stars are quite moody, prone to dramatic shifts in brightness. After Allan and his colleagues realized that the star had seemingly disappeared, they went through archival observations to search for clues. The data suggest that the star might have dimmed after an especially luminous period. In this scenario, the disappearing act was helped along by another astrophysical phenomenon; in a quirk of timing, a cloud of cosmic dust might have moved right in front of the star at about the same time, blocking its already-subdued shine so that the star became completely hidden from our view.
Still another explanation, put forth by a different team of astronomers in their own study of this distant galaxy this year, suggests that the star, missing or not, isn’t a star at all. According to this group, telescopes have been observing, all this time, the light of a long-lived supernova interacting with cosmic material around it. From Earth, these distant interactions could be mistaken for the dimming and brightening of a star. The first flare of this supernova would have been visible through telescopes in the mid-1990s, but as it happens, none were peering into this galaxy at the time.
To investigate this mystery further, astronomers will need to collect more observations of the distant galaxy. Allan said that astronomers will soon deploy the Hubble space telescope, one of humankind’s finest eyes on the sky, to take a look.
For now, the case of the disappearing star will remain in the annals of unsolved stellar mysteries, alongside a star that seemed to have the opposite problem. A few years ago, a team of researchers led by Arcavi came upon a star that had exploded over and over again, instead of fading into the darkness after a single radiant blast. He’s still not sure what happened: Massive stars aren’t supposed to survive their supernova. But then again, they’re not supposed to quietly vanish into the darkness, either.