Ever since 2016,

astronomers have been puzzling over a strange feature in the galaxy cluster Ophiuchus

that no one could quite explain.

It looked kind of like a crater in the gas surrounding one of the galaxies

near the middle of the cluster.

No one knew what to make of that.

But just last week,

astronomers published a paper in The Astrophysical Journal

announcing that it was caused by the largest explosion we've ever detected!

Which kind of seems like something that'd be hard to keep under wraps.

But even though astronomers have been looking at the remnants of this explosion for four

years,

it wasn't always obvious what they were looking at.

Back in 2016,

a collaboration of astronomers based at Stanford first saw something fishy

in the Ophiuchus galaxy cluster

when they analyzed data from the Chandra X-Ray Observatory.

The cluster is about 390 million light-years away,

and it has a whole bunch of galaxies surrounded by dust and gas.

And in particular,

this team focused on this one big galaxy

toward the center of the cluster with a supermassive black hole in its center.

In their images, they saw a sharp, curved line around that galaxy,

kind of like a border.

And inside that line,

it looked like a lot of the gas had just been scooped out.

Now, a feature like this wasn't totally unheard of.

In those cases, scientists figured out

that they were looking at cavities left over by outbursts from black holes.

So this team thought, maybe this was similar—

just a big explosion.

But when they crunched the numbers

to see what it would take to carve out a cavity that size,

they found it would have had to be five times more energetic

than the largest known explosion at the time.

And they didn't think that was too likely.

They reasoned that such a big explosion

probably would have destroyed the cool gas surrounding the core of this galaxy,

which was definitely still there.

So they ruled out that hypothesis.

But they still couldn't figure out what was causing this weird feature.

So a second team of researchers picked up where they left off.

This team, based at the Naval Research Laboratory in Washington, D.C.,

had access to new data.

Similar to the first team,

they used X-ray data from ESA's XMM-Newton satellite

to confirm that the cavity existed.

But unlike the first team,

they also looked at radio data from the Murchison Widefield Array in Australia

and from the Giant Metrewave Radio Telescope in India.

And that offered them a new clue.

That radio data revealed loads of electrons inside the cavity

accelerated to near the speed of light—

fast enough to emit their own radio waves.

And electrons don't just take off at relativistic speeds

without something really strong accelerating them.

So, looking at this big ol' hole punched out of the gas,

filled with relativistic electrons,

this team could be pretty sure what they were looking at:

an outburst from a supermassive black hole.

Near the poles of some black holes,

sometimes matter doesn't fall in but instead gets accelerated to relativistic speeds

and shot back into space in a huge burst of energy.

Astronomers have observed bursts like this before,

but mostly on much smaller scales.

The explosion they were looking at was truly gigantic—

hundreds of thousands of times larger

than most black hole outbursts

and five times larger than the previous record-holder!

With lots of these bursts, any cool gas around the core of the galaxy can be destroyed,

which is why the first team had ruled out the possibility of an explosion.

But in this galaxy,

the temperature and density of the gas drops off especially quickly with radius,

which might have made it more resilient to a large burst—

especially one that didn't hit head-on.

The cool thing about this discovery is not just that it's a huge explosion in space—

though that is also cool - it also illustrates how science builds upon itself.

The second team was only able to make this incredible discovery

because another team had tackled it first.

They used some of the same data and calculations—

just a few years and a few thousand miles apart!

Much closer to home,

last week we also got an update in cute astronomy news:

Earth has a little baby moon!

A mini-moon!

It's about the size of a car,

and based on its trajectory,

scientists think it's been orbiting us for about three years—

we've probably just been missing it because it's so small.

And it's pretty exciting!

It's only the second time we've ever spotted a natural object

other than the Moon in orbit around the Earth.

But scientists have pointed out that this isn't actually a super-rare phenomenon.

Since everything in the solar system is constantly in motion,

planets capture small objects all the time.

Astronomers estimate that at any given point,

there's probably at least one meter-sized rock orbiting the Earth.

But they can be really hard to detect because they're usually very small,

and they don't tend to stick around for long.

We found the first one back in 2006,

and that one stayed in orbit for less than a year after its discovery.

Sadly, our new friend won't be here for long either.

In fact, it looks like we spotted it just in time.

It's projected to leave Earth's orbit in April.

Unfortunately, orbital dynamics make it hard for mini-moons to stay around Earth.

That's mostly because the Moon—

like the main moon, really throws things off:

it's big and near enough to keep new mini-moons in chaotic orbits that never settle.

They either burn up in Earth's atmosphere or get ejected from the system.

And our newest mini-moon is headed out—

in April, it will leave us for more exciting places.

So pretty soon we'll be back down to just one moon—

but considering how many tiny objects are out there,

we may have another visitor sooner than you'd think!

Thanks for watching this episode of SciShow Space!

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We couldn't keep making these videos without your support.

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