Mini-Movie Monday: Genesis Episode 3, The Galaxy

All the stars in the night sky are bound in the Milky Way galaxy, just one of billions in the Universe. Here’s the story of where they come from.

“The larger the island of knowledge, the longer the shoreline of wonder.”
–Ralph W. Sockman

Our third episode of the Genesis series is live on YouTube, as we relate yet another installment in the story of where this all comes from. In this case, that’s the story of our galaxy, the Milky Way, including what it is, how it came to be this way, and what it will become in the future.

A full transcript of Episode 3: The Galaxy is attached below. Enjoy!

Perhaps the most spectacular sight in the entire night sky is the iconic swath of the Milky Way, brilliant, diffuse and inimitable overhead. Although it’s our home galaxy, viewed from our vantage point inside of it, it hasn’t even been 100 years since we realized that our home is a giant spiral-like structure, with hundreds of billions of unique stars and solar systems inside.

There are many other galaxies with similar structures to ours out there, but not only didn’t they always exist, but they won’t remain this way forever. If we look far enough back in time, we’d come to a place where there weren’t any galaxies at all, because they hadn’t had time to form.

So where did our Milky Way galaxy come from? Let’s go all the way back to the very young Universe, to a pre-galactic time where even the first stars hadn’t yet formed.

In the early Universe, shortly after the Big Bang, the tiny regions that are born with slightly more matter-and-energy than others begin preferentially attracting matter, through the gravitational force, to them. Eventually, enough matter is attracted that these gas clumps collapse, leading to star formation and forming the first star clusters.

These clusters are part of a larger structure — an asymmetric structure — that collapses in the shortest direction first to “pancake”, and form a disk. This disk rotates, and thanks to gravity, accretes more and more matter, collecting all of the clusters in its vicinity, and develops a spiral-like structure due to density waves. And over time, the small-to-midsize clusters that merge with it get funneled into the galactic core, as the overall shape retains a spiral structure.

Our galaxy, like many of the great spirals in the Universe, became a spiral back when the Universe was very young, and although it may have merged with and swallowed a large number of smaller galaxies over time, none of these interactions were enough to disrupt our galaxy’s basic appearance.

But some interactions in this Universe are! After all, not every galaxy is still a spiral galaxy. In fact, of the largest galaxies in the Universe, virtually none of them are spirals at all! There’s an important reason for this, and it teaches us something important about our own future.

When two large, comparably sized spiral galaxies merge together — something that simply happens due to gravitational attraction over time — their spiral shapes first get distorted, and then as the merger progresses, they settle down into a more elliptical structure, where the stars swarm about the center like a colony of angry bees, rather than orbiting in a single plane.

Our own galaxy, having lived as a beloved spiral for more than ten billion years, will see a tremendous burst of star formation in about 4 billion years as we merge with Andromeda, leaving behind a galaxy very different from our own: a true giant elliptical that we’ll call “Milkdromeda.” If we’re still around.

So spiral galaxies like our own are common today, but as we move farther and farther into the future, they’ll become increasingly rare, as the way we exist now is just an adolescent phase in our ever-evolving Universe.

Have a comment? Leave it at the Starts With A Bang forum on Scienceblogs.
And if you missed episodes 1 or 2, check them out below!