[♪ INTRO] When is a black hole not a black hole?

This isn’t a riddle. It’s an example of scientific camaraderie: astronomers from around the world with different hypotheses, working together to solve an interstellar mystery.

And instead of a dead star, they found one sucking the life out of its companion. Black holes can’t be observed directly. We can only observe the effects they have on other things.

So when astronomers study these celestial objects, they’re usually looking at something we can see, and inferring the presence of a black hole based on the gravitational effect it has on that one or more visible objects. So back in 2020, astronomers used one of the telescopes at the European Southern Observatory in Chile to look at a star system with the very creative name HR 6819. Its light looked a bit wonky, possibly influenced by an object they couldn’t see.

But because there’s only so much time a telescope can dedicate to studying a particular part of space. And when stars are so far away they can look like single pixels, so astronomers can only capture so much data. And that means they use computer models to help fill in the gaps and draw educated conclusions about what is going on, based on equations that describe how we believe the universe works.

In this case, the team felt the model which best fit their data was that of a trinary system: two stars more massive than our Sun, and something with a similar mass, but invisible. A black hole, created after a massive star went supernova. The set-up they describe requires the black hole to tightly co-orbit one of the stars, like once every 40 days, with the other star much further away.

That central star what’s called a B-type star, known for living fast and dying young. It weighs in at a mass about 6 times the mass of our Sun. The more distant one is a Be star, which is a B-type star that is just about to run out of hydrogen fuel.

It also spins way faster than normal and has a disk of debris around its equator. After their results were published in the journal Astronomy and Astrophysics, various news outlets picked it up, because this black hole would be a record setter. At a mere 1000 light years from Earth, it was the closest one ever detected!

But because the data could only provide a partial picture, other teams could run their own models on HR 6819 and reach different conclusions. And they did. Like one team who argued there could actually be four bodies in the system!

And others argued there didn’t need to be a black hole at all. Just two stars with a very special relationship. Instead of the B star closely orbiting a black hole and the Be star further away, the two would be so close to one another that the Be star was acting like a cosmic vampire, using the power of gravity to siphon matter off of its companion.

In fact, it was that process which could explain why it’s spinning so fast. Now, it’s not like the first team wasn’t aware of this phenomenon. But it’s such a blink and you’ll miss it process, at least on the scale of a star’s lifetime, that it would have been extremely unlikely for them to have captured it.

Rather than getting into a big academic fight about which conclusion was right, a group of these astronomers teamed up to collect and analyze new data from more powerful instruments at the same observatory, the ESO. Like, instead of a 2.2-meter-wide telescope to collect data, this time they got to use an 8.2-meter-wide telescope. That allowed them to actually take what was one tiny blob of light, and detect… two tiny blobs of light.

The question was, was that enough to show that one of them was orbiting a black hole? Based on this new data, the team was able to confirm that both stars were orbiting one another more tightly than Earth goes around the Sun, with no sign of a black hole’s influence in sight. While there’s still some amount of uncertainty, like how much mass each star has, and how far away the system actually is from Earth, it’s pretty clear there’s no record-breaking black hole next door.

Which you might think is disappointing. But a vampire star is still pretty cool! It means the B star isn’t as normal as astronomers thought it was.

It’s been stripped of most of its hydrogen, so it’s way less massive, bloated, and mostly made of helium. These observations have managed to capture an event which actually doesn’t last that long, astronomically speaking. like having front row seats to a unique moment in a massive star’s life cycle, without having to leave our cosmic couch. And the team is keeping up with the collab.

They’re going to continue to monitor HR 6819 as the system evolves. And all this extra-close data will help other astronomers figure out all the nuances of other binary systems out there. But as for black holes in the neighborhood, the search continues .

There could be as many as a hundred million of them, peppered throughout the galaxy. And the nearest, as least that we know of, is only about 3000 light years away. It’s studies like these which do a great job at revealing a side of science we don’t always get to see.

That the first answers we arrive at shouldn’t be carved into stone, and you never know what cool update is around the corner. Thanks for watching this episode of SciShow Space, where we bring you the finest in black hole-related content and occasionally other stuff as well. If you’d like to get involved with the amazing community that makes all these black hole videos possible, head over to patreon.com/scishowspace. [♪ OUTRO]