Thanks to Brilliant.org for supporting SciShow Space. [♪ INTRO].

Space is packed with all kinds of mysteries; that’s part of what makes it cool. Some of those mysteries, though, are closer to home than others.

Like, here’s a surprising one: We actually don’t know where the Sun came from! According to what we know about star formation, the Sun probably formed from a huge cloud of gas, along with a bunch of stellar siblings, stars that formed in the same place out of the same stuff. Except, we can’t find evidence of that cloud, or the Sun’s family, anywhere.

Our little Sun is all on its own. Get your tissues ready, because this is basically waiting to be turned into a Pixar short. Even though we can’t find the Sun’s family, we’re pretty confident it has to exist because of how most stars form.

The current model requires that a bunch of stars get born together out of a big ol’ gas cloud, called a molecular or giant molecular cloud depending on its size. Eventually, because space is super vicious, the clouds get eaten up or dispersed by their star children. After that, the stars, and their planets, if they’ve accumulated any, might separate or might move together for a while in a so-called open cluster.

Then, over time, all the little gravitational pulls and tugs from the member stars build up and finally send the stars on different trajectories. These stars can end up all over a galaxy, but they can still be identified as members of the same family because they typically have similar ages, and often have really similar compositions. There can be plenty of variation within a cluster, but there are still some general trends we can look for.

Most stars don’t stick around to form open clusters, but we have evidence that the Sun was one of the few who stayed with its family a bit longer. For one thing, the orbital motions of the Kuiper Belt objects, most notably the planetoid. Sedna, strongly suggest some gravitational interaction with other stars, probably in a relatively dense group.

Otherwise, they likely couldn’t have been jostled into their current orbits. There’s also evidence, like the excess amount of heavy elements in the Sun, and the presence of uranium in the Earth, that there was a supernova near our star when it formed. It could’ve come from a huge, short-lived member of a larger stellar nursery, like an older sibling who peaked too early.

We have plenty of reason to believe that the Sun was born in an open cluster, but we can’t find anyone else from that stellar nursery, even though we’ve been looking all over. It’s like Finding Dory, but with a lot more math. For a while, astronomers thought the Messier 67 open cluster, or M67 for short, was the most likely candidate.

It’s an open cluster about 2700 light years away, in the Cancer constellation. It’s a pretty dense group, which would match our hypotheses, and it’s also about five billion years old. That’s really old for an open cluster, but it’s about as old as the Sun!

Admittedly, dating stars is really hard, so there are some big error bars on that measurement. But it’s in the ballpark, and astronomers love ballparking. So its age and density make M67 a good candidate, but its orbital path around the galaxy is way different from the Sun’s, and that’s become a big source of debate.

On the one hand, this might not be a problem. Because members of open clusters interact gravitationally, it’s possible that a star could get kicked out and end up with some different orbital parameters. But on the other hand, the kind of kick that the Sun would have needed to get on its current path is super huge, like, too big for it to have kept its baby solar system.

So the fact that we’re here is some evidence against the M67 hypothesis. That is, unless that kick happened really early on. An early, gentle nudge, given a lot of time, could have gotten the Sun way off course.

So, we’re still not exactly sure what’s going on here! What we could really use is more data. Thankfully, there are a few projects, both launched in 2013, that are helping with that.

One is the Gaia mission, run by the European Space Agency, and the other is the GALAH project, run by the Australian Astronomical Observatory. Together, they’re collecting astrometry and astrochemistry data of a truly enormous quantity of stars, or data about their positions, movements, and chemistry. Astrometry is good for finding former and current open clusters, because members of them move together.

And even if the Sun’s siblings have dispersed, we could use current star positions to calculate the cluster’s original location. Astrochemistry may also prove to be a smoking gun. Since the Sun is so metallic from that supernova explosion, its sibling stars may have similarly high metallicities.

All we have to do is find stars that match those descriptions! Gaia is working to get astrometric and astrochemical data on a /billion/ stars from an orbit near. Earth.

GALAH, on the other hand, is just doing astrochemistry on a more modest million stars, and it’s operating from the ground. Both missions will wrap up over the next few years. After that, it’s up to astronomers to make sense of the mountains of data!

Between the two surveys, we’re characterizing tons and tons of stars. We haven’t found the Sun’s siblings yet, but these surveys are currently our best chance at it. Besides being a nice end to a story, understanding where the Sun originated and what it was like there can also help us understand why our solar system turned out like it did.

Still, even if these projects don’t give us the data we need to solve the Sun’s mystery, they’ll give us incredible insights into the formation and evolution of the galaxy, which is a pretty sweet consolation prize. But there are some puzzles we can solve about where stars come from on Brilliant.org. That’s right, they have a whole course on the Life Cycles of Stars which starts with this Star Formation quiz.

What’s cool about Brilliant is that they’ve really embraced how to best use their platform with animations that help you wrap your brain around a complex idea. When I was going to school and regularly taking science quizzes, we didn’t have the internet or computers in the classroom, because I’m old. We had this stuff called paper and it didn’t include moving graphics like this that help you imagine the problem.

But with Brilliant, whether you’re a student or a lifelong learner, you can benefit from these interactive quizzes to help you learn and have fun while you’re doing it. So check it out and see how much you know about star formation. Brilliant is also offering 20% off their annual premium subscription to the first 200 SciShow.

Space viewers who sign up at Brilliant.org/SciShowSpace. So you’ll get a discount, and be helping to support SciShow Space. So thanks!

You are awesome! [♪ OUTRO].