{♫Intro♫}.

Compared to a human lifetime, timescales in space are enormous. So it's easy to imagine that our Galaxy is basically frozen during the handful of decades that we're alive.

After all, generations and generations of our ancestors have looked at the same planets and constellations that we see today. But, that being said, galaxies are really dynamic, and the Milky Way is changing all the time. In fact, in your lifetime — let's call it a hundred years — it will undergo some pretty amazing changes.

For one, it will probably grow — like, a lot. According to a 2018 paper, spiral galaxies like ours are steadily expanding at around 500 meters per second. That's roughly twice the speed of a jet!

And if this rate also applies to the Milky Way, that means it will grow about one point five billion kilometers over the next century. That's not just a statistic, either. This number can also teach scientists about galactic evolution in general.

See, the fact that these galaxies grow wasn't a total surprise. For years, scientists have known that, every once in a while, they can eat up smaller galaxies that get captured by their gravity. But that 2018 paper was important because it confirmed that this isn't the only way these neighborhoods get bigger.

They also expand because new stars are being born — and in a pretty odd place, too. In this study, researchers observed two spiral galaxies like ours. And after calculating how stars on the fringes were moving, they concluded that these galaxies were growing because stars were being born on their edges.

Models had predicted this, but it was hard to prove they were right just by looking at the Milky Way since, well, we're inside it. So by looking at other galaxies, scientists were able to confirm their hypothesis. This finding was mainly strange because most stars form toward the center of their galaxy.

So this paper demonstrated that there can still activity way out in the galactic outskirts. That means that, even if it never interacts with another galaxy, the Milky Way will likely keep growing while you're alive — as long as it has enough gas around the edges to make new stars. Of course, at this point in its life, our Galaxy is making stars pretty slowly, churning out maybe one or two a year.

But that means that in the course of your lifetime, it could gain around a hundred new stars! Now, sure, for a place with at least a hundred billion stars, that's barely a sprinkling. Things have slowed down now that our Galaxy is well into adulthood, at a healthy thirteen and a half billion years old.

It's a long ways from its wild youth, about nine billion years ago, when it was forming around a dozen stars a year. Still, from a human perspective, a hundred new stars is nothing to scoff at. And besides, understanding that number can also teach scientists how the Galaxy has evolved.

Because the thing is, the Milky Way's star formation hasn't just tapered off — it's been more of a rollercoaster. After that peak around nine billion years ago, star formation dropped to a tenth of its previous rate. This shutdown happened around the same time that our Galaxy formed its thick disk.

Scientists aren't exactly sure how the two events are connected, but they think it's possible that the formation of this disk stirred things up and made the gas so hot that it stopped condensing into stars. Fortunately, star formation has picked up again since then although these days, things are pretty quiet. Still, that's relatively normal for older neighborhoods, like ours, that don't have a lot of interaction with other galaxies.

Even so, the Milky Way is popping out the occasional new star, as regions of dust condense and ignite. And over the course of a century, our Galaxy is likely to have dozens of new studs of light. Finally, the Milky Way won't just gain things during your lifetime.

It will also lose some. After all, the Milky Way's new stars are just the recycled remains of old ones — and in the next hundred years, it will likely lose about as many stars as it gets. Two or three might even explode as supernovas.

This will only happen to the really massive stars, but when they die, they'll spew their contents into space, and some of the elements from their cores will be incorporated into new stars. As far as we know, the most recent supernova in our Galaxy blew up around 140 years ago. But a 2006 estimate suggests that, on average, the Milky Way has seen a supernova explode around every 50 years.

So in a sense, it seems like we're kind of overdue for some fireworks. And scientists may have found the next culprit. They have their eyes on a triple-star system nicknamed Apep, which is about eight thousand light-years away and seems to be on the brink of explosion at least, based on what we can see of it.

One of its stars is releasing streams of charged particles at a speed that suggests it's at the point of collapse. Thankfully, because of the way the star is oriented, it shouldn't do any harm to Earth even if we see evidence of that explosion soon. And either way, cosmic “brinks” can be human lifetimes, so it's hard to say exactly when this thing will go.

We might not see evidence of its explosion for another thousand years or more. Whether or not the Galaxy lights up with a new supernova in the near future, the Milky Way is far from the frozen river of stars that you see on a dark night. In your lifetime alone, dozens of stars will blink in and out of existence, and the whole Galaxy will likely push its own boundaries by more than a billion kilometers.

And it's not just cool to find events in space on the scale of our lives. Understanding these short-term events also helps us get a handle on how galaxies evolve and sort out what it's like to be in the galactic middle age. Thanks for watching this episode of SciShow Space!

The universe is super fascinating and can also be surprisingly accessible, and we love getting to explore it with you. If you want to support the show and help us keep making more free content like this, you can go to patreon.com/scishow. {♫Outro♫}.