When I was a kid, a few things kept me up at night.
Namely the Bermuda Triangle, quicksand, and black holes.
Turns out, as an adult these things are way less of a day-to-day problem than I thought
they’d be.
Now I’m lying awake at night all over again, because astronomers have just found the closest
black hole to us yet, and they suggest there could be even more lurking out there in the universe.
Finding black holes is tricky, what with them capturing everything that strays too close,
including light.
They are ‘unseeable’ by their very nature, and only the effects of their intense gravity
give them away.
They are the sarlaccs of space, the insatiable invisible maw of destruction.
So, it’s a bit ironic that the researchers who found this nearby black hole weren’t
even looking for it.
The astronomers were looking at what they thought was a double star system called HR
6819.
Not the most fear-inducing name I’ve ever heard of, but just wait.
This pair of stars is close enough that it can be seen from the Southern Hemisphere without
any optical aids on a dark and clear night.
While observing the stars, the astronomers noticed that what they were actually seeing
was a three body system.
The inner star appeared to orbit something once every 40 days, while the outer star orbited
the pair much farther away, I assume out of a sense of self-preservation.
So, how did the researchers know the object the inner star was orbiting was a black hole?
Well, whatever it was orbiting was invisible, which is a pretty strong clue.
But they could also use the inner star’s orbit to determine the mass of its invisible
partner, which they determined had a mass about four times that of our Sun.
Based on those two pieces of information, the third member of this stellar tango is
most likely a black hole.
This makes HR 6819 the first star system with a black hole that can be seen with the naked eye.
Perhaps the most intriguing thing, or worrying thing depending on how paranoid you are, is
just how mundane this black hole is.
Up until now, we’ve only spotted a few dozen black holes in our galaxy, and usually they
stick out because of how violently they interact with their surroundings.
They’re siphoning off matter from companion stars and blasting x-rays out into space.
By contrast, the black hole hidden in HR 6819 doesn’t seem to be doing much of anything,
so it appears truly black.
It’s just quietly waltzing with its partners, minding its own business.
But it’s very existence hints at much more.
If we found one this close by and this easily, then in all likelihood there are many more
black holes just like it throughout our galaxy.
We’ve long suspected that throughout the history of the Milky Way many stars must have
collapsed into themselves, and discovering this quiet black hole could explain why we
haven’t seen more yet.
The astronomers who led this study estimate that there must be hundreds of millions of
them out there.
So, should you be terrified that there are scores of black holes around every corner
waiting to get you like an army of darkness?
In all seriousness, no.
HR 6819 is about 1,000 light years away.
For perspective, the nearest star to our solar system is a little over four light years away.
If anything, discovering this quiet black hole opens up opportunities to understand
how large stars evolve or how three body systems can trigger black hole mergers and stellar
collisions.
And besides, if a black hole were to get you… you’d never see it coming.
Well, sleep tight everyone!
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I’ll see you next time.