Earlier this year, a group of astronomers announced that the light from a pair of colliding stars is going to show up in the night sky in 2021 or 2022.

We usually think of astronomical events as taking thousands or even millions of years to happen, so it’s kind of weird to imagine something so sudden and extreme happening in less than a decade. But between things like comets and supernovas, humans were seeing sudden, surprising changes in the sky long before telescopes were invented.

And if you could watch it for long enough — like, a few thousand years — you’d even see the sky itself change before your eyes. Comets are probably the night sky’s most famous intruders. These days, we know comets are icy worlds from the outer solar system that are heated by the Sun until the ice turns directly into gas.

But to ancient people from around the world, the brilliant tail of a comet stretching across the night sky disrupted the perfectly orderly heavens. People thought that comets forecasted a similar sort of disruption down here on Earth, so they sometimes started wars and invaded other countries. Like when what we now call Halley’s Comet streaked across the sky in 1066, and William the Conqueror took it as a sign that it was time to invade England.

But comets aren’t the only thing that have caused sudden changes to the night sky. Sometimes, things that look like new stars appear out of nowhere, stay for a little while, and then disappear back into the darkness. Our first record of this happening comes from Chinese astrologers in 185 CE, who noticed what they called a “guest star” in the winter sky that slowly dimmed over the next few months.

They didn’t know it, but they weren’t seeing a star being born that winter. They were seeing one die. When large stars run out of fuel, they go supernova, giving off as much light in a single explosion as our Sun will over its entire ten billion-year lifetime.

And if a supernova is close enough to Earth, the giant, glowing cloud of gas it gives off can show up in our sky and temporarily look like a new star. Some of the brightest supernovas ever recorded were hundreds of times brighter than the brightest stars and stayed in the sky for years — even showing up during the day! People from all over the world recorded seeing these sorts of bright new stars in the sky, from detailed writings in Europe and Asia to stone carvings in the American Southwest.

And using all these detailed records, modern astronomers have actually been able to find the remains of a lot of the supernovas people have seen throughout history. Like the guest star that people around the world saw in 1054, which turned into what we now call the Crab Nebula. But not all changes to the night sky have been as sudden as a comet or supernova.

It’s also changed more gradually. Earth’s seasons depend on its tilt between its orbit around the Sun and the axis that it spins around every day. And that tilt slowly wobbles, pointing the planet in different directions over time.

Which means that the summer constellations of ancient Greece are now about a month closer to spring. And the patterns of the stars in the sky aren’t even constant. Groups of stars might look close together, but in space they can actually be hundreds of light-years apart.

And since different stars move in different directions at different speeds, the way they’re arranged in the sky has changed over time. So when humans were first leaving Africa fifty to sixty thousand years ago, the stars in the bowl of the Big Dipper were a lot closer to a square, and the handle looked more like a straight line. And for those of you in the Southern Hemisphere, the Southern Cross used to be a lot more stretched out and lopsided.

The stars in the sky will keep moving around, and you might not even recognize the sky 50,000 years from now. But if you don’t want to wait millennia for constellations to change, or centuries for a supernova, or even fifty years for Halley’s Comet, all you have to do is look up at the sky in four or five years. Because the light from that pair of colliding stars is on its way here.

The stars are 1,842 light-years from Earth, which means that it takes the light from them 1,842 years to get here. So we don’t see them as they are now — we'll see them as they were 1,842 years ago. And right now, they look like what’s called a contact binary: they’re orbiting each other so close together that their atmospheres actually touch.

Astronomers have watched these orbits get faster and faster over the last few years, which means that the stars are getting closer and closer together. And based on what happened the last time they saw something like this happen, the researchers are predicting that we should see the light from those stars colliding in about four or five years. Even though the stars are too dim to see with the naked eye right now, that won’t be true for long.

Because the explosion from the merger should be about ten thousand times brighter than the stars themselves. And if the light from the explosion reaches Earth close to when it’s summer in the Northern Hemisphere, it should look like a bright new star in the night sky near the constellation Cygnus. The stars are far enough away that we don’t have to worry about any effects from the explosion — it won’t even be the brightest star in the sky, though it will be one of the brightest.

We’ve seen this sort of collision happen so few times that we don’t even know how long it’ll be in the sky. But it’ll be a cool reminder that the sky is always changing.

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