Space News Deep Dive: Solar Migration

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Do you ever stop and think about the Sun? Not, like, as just the bright thing in our daytime sky, but as a cosmic nuclear fusion reactor that has existed for a third of the time the universe has even been around? It’s a little bit like realizing your parents were living their lives for years before you showed up, but our Sun was around for a long time before some extra smart apes decided to try giving this “standing upright on two feet” thing a try.

And just as you are not the same person you were when you were first born, the Sun is not the same star it was when it first started shoving hydrogen atoms together 4.6 billion years ago. And, as it turns out, it’s not hanging out in the same neighborhood it first grew up in either. We’ve suspected that for a while, but now we’ve got a new study that not only seems to confirm that fact but also suggests a location for our Sun’s hometown—the place it was born. 

So let’s do a deep dive and see where it is we think our Sun came from, and why! 

Where Stars Come From

First, a quick refresher on where stars come from in general. In a process that makes sense logically and is hard to wrap your head around intuitively, these enormous, energetic, brilliant, bright objects are born from shadowy wisps of cold gas drifting through the ether in darkness. Which is a poetic (sort of) way to say that it all starts with a nebula.

“Nebula” can refer to any sort of cloud of gas and dust in space, but nebulas that will be able to give birth to stars generally start out as enormous, cold conglomerations of hydrogen and helium, with some small amounts of other stuff mixed in. The cold part is actually important, because it’s only when it’s cold that the hydrogen molecules will start to clump together if something gives them a push. This can be the gravity of a passing star, the shockwave of a nearby supernova, a collision with another gas cloud, really anything that breaks the cloud’s equilibrium and starts pushing atoms and molecules into each other. And once they start to clump, well. Then, given some time to play with, gravity can take over.

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You know what has more gravity than a single hydrogen molecule? Two hydrogen molecules clumped together. It’s still a very tiny amount of gravity, but it could be enough to attract a third molecule, and then there’s just a bit more gravity added to it. Let this process run for a while and you’ll have a gassy clump of steadily increasing size and mass.

As this clump grows more massive, it begins to collapse under its own increasing gravity. This long, slow collapse starts to generate heat energy at its core. At first this new energy just gets radiated away, but once this gas clump is dense enough it’s hard for that energy to get out. The heat stays trapped inside the clump, creating a pressure that begins to push back against the continuous inward pull of gravity. Even as it continues to gain mass and grow, it maintains this equilibrium by creating more heat energy in its insides. It’s now a protostar—the seed of a sun.

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There’s still a lot of process to go through before it can become a full-fledged star, but the key points are that eventually the protostar is energetic enough to start splitting its stable hydrogen molecules into individual, far more energetic hydrogen atoms. And then it hits a critical point where it begins forcing those hydrogen atoms together and fusing them into helium, beginning the process of stellar fusion and igniting the protostar.

This doesn’t tend to happen one star at a time. Usually whatever event destabilized the parent nebula in the first place triggers the formation of many stars, even thousands at a time if the nebula is massive enough. That means there can be many thousands of stars all originating from the same cloud around the same time. A whole family of stellar siblings, if you will, all of whom will have the chemical markers of their parent nebula reflected in their makeup.

Barring the Way

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We already knew our Sun didn’t form here. By “here” I mean the spot where it currently sits, about 26,000 or so light years from the center of the Milky Way. Its ratio of metals (which to an astronomer and exactly no one else means anything that’s not hydrogen or helium) doesn’t match our local space. Its parent nebula wasn’t from around here. The inner regions of the Milky Way have higher metal contents, so it was suspected that our Sun probably came from a place about 10,000 light years closer in than it sits now, and migrated farther out after it formed. 

But there’s always been a problem with that theory. Our Milky Way is a barred spiral galaxy, which means its central region is an elongated densely-packed cluster of stars. This is the “bar” of the barred galaxy, and theoretically its presence should be exerting gravitational forces preventing stars from moving away from the central region.

So on the one hand we have stellar chemistry strongly suggesting our star formed much closer to the Milky Way’s center, and on the other we have a giant cosmic structure that should have made it impossible for the Sun to get out if it did form there. What gives?

Twinsies!

The new study that came out this week looked at what might be called solar twins. These are stars that look like the Sun—similar gravity, similar temperature, similar chemistry. We’ve looked at such stars before, but we only had limited datasets to work with which made things tricky.

That was solved by the wonderful, departed Gaia telescope, an unmatched star-mapping master (say that five times fast). Gaia found 6,594 solar twins within 1,000 light years of the Sun, a dataset 30 times larger than any previous study of solar twins had to work with!

*Sigh*, I miss you Gaia. You really were the best at what you did.

Anyway, within this giant dataset astronomers found 1,551 stars that were about 4-6 billion years old, which puts them in the Sun’s age bracket. That’s nearly a quarter of the dataset, which is a statistically significant chunk. It means there is a suspiciously large number of stars of the same mass and age that would have formed in the same general area and all wound up in a different same general area billions of years later. Coincidence?? 

Look, science doesn’t love coincidences, so the answer is probably not.

Tracing the Sun’s Roots

The identification of all of these stars suggests a few major things. It suggests that there was a burst of star formation near the Milky Way’s center about 4-6 billion years ago that made, among many other new stars, our Sun. It suggests that sometime very shortly thereafter something occurred to cause these stars to run outward together. And it suggests that when that scattering occurred the Milky Way’s central bar structure did not exist to prevent them from going.

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And, in a fun turn of events, that bar might be the very answer to the question of what happened to trigger all of this. If it began to form about 4-6 billion years ago, the slow buildup of so many stars in the growing bar would have started gravitationally moving things around, concentrating gas into larger and larger nebulas and giving them the exact kinds of shoves they need to begin making protostars.

Then, as the bar began to really take shape, it could have given those new stars and protostars a kick, sending them flying outwards before it finished settling into the gravity powerhouse it is today. Among those stars sent speeding outward would have been the infant Sun, probably still surrounded by the protoplanetary disk that would eventually collapse down to form the rest of our solar system, including Earth.

A Better Neighborhood

It sounds rough—the forcible ejection of a star while it’s still trying to form its solar system. But this story has a good ending, at least if you’re a human (which, since you’re reading this, I’ll assume you are. If not, please get in touch!).

The Sun eventually settled itself in the region of the Milky Way it resides in today, and if it had not we wouldn’t be here. Even if our Earth had formed with the exact size and distance from the Sun it currently has, if our star hadn’t been forced to settle away from its birthplace Earth would not be able to support life. We think no planet can in those regions.

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The Milky Way’s center is much denser, with many, many stars packed into small amounts of space. That means a hyper-radioactive area with a larger than normal number of supernova popping off and also stars randomly smashing into each other from time to time, creating more massive stars which will then also go supernova. Not a great place for life to try and set itself up.

Out where we are, things are quiet. There’s a good distance between the stars, and no massive star close enough to us to have a big effect if it decides to die violently. It’s a much better place for simple life to take hold on a promising planet, and a sufficiently stable region to let that small beginning to flourish into a long-term planet-wide biosphere. And if the Sun had not been sent migrating in its youth, it never would have happened.

Just goes to show, sometimes gravity taketh away but sometimes gravity giveth in the most awesome way possible.