The Evolution of the Modern Milky Way Galaxy

**SPEAKER_1** (0:00)
No one goes to Hank's for his spreadsheets. They go for a darn good pizza. Lately though, the shop's been quiet. So Hank decides to bring back the $1 slice. He asks Copilot in Microsoft Excel to look at his sales and costs and help him see if he can afford it. Copilot shows Hank where the money's going and which little extras make the dollar slice work. Now Hank says, line out the door. Hank makes the pizza, Copilot handles the spreadsheets. Learn more at m365copilot.com/work.

**SPEAKER_2** (0:31)
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**Matt O'Dowd** (1:00)
When I was a young graduate student, I got to use one of the giant telescopes at the Las Campanas Observatory in the Atacama Desert in Chile. I was traveling with a much more experienced astronomer from Europe, but it was his first time in the southern hemisphere. He went outside for a weather check and came back looking annoyed to report that there were two gigantic clouds in the sky.
I went to look and it was the most crystal clear night I'd ever seen. Was my colleague suffering altitude sickness? Then I realized there were two clouds in the sky. Two hazy blobs just off the dust street center of the Milky Way band? I laughed out loud. This wasn't water vapor. This was lunch. The Milky Way's lunch. Find a dark night in the southern hemisphere and you'll see it too. The several billion stars of the large and the small Magellanic clouds in their slow death spiral towards the Milky Way.
My colleague needn't have worried. We did some great observing that night. But astronomers watching the resulting collision in around 2 billion years might have more cause for concern.
When we scan the heavens with giant telescopes like those at Las Campanas, we see galactic cannibalism everywhere. We see moments that appear frozen on the human time scale, but are really snapshots of the incredibly violent process of galaxy formation. This is how all galaxies are made. We can piece together a pretty good understanding of this process from countless snapshots. Looking into the distance means looking into the past. So it's possible to stitch together a Frankenstein flip book of galaxy evolution. But what about our home galaxy? It would be nice if we could learn the Milky Way's true history and final fate. And in fact, we can. Since that night on Las Campanas, enormous surveys have tracked the positions and motions of more than a billion Milky Way stars. We are now able to calculate a detailed and dynamical map of the Milky Way.
We can predict its future mergers with the Magellanic Clouds and Andromeda. But perhaps more astonishing, we can reconstruct its past. But before we get to this, let's review what we know about the Milky Way of the present and of galaxy evolution in the general sense. The Milky Way is a pretty typical barred spiral galaxy. Our sun is in the disk on a minor outcropping of one of the spiral arms. It orbits the same direction as all the disk stars once every couple of hundred million years.
In the center we have the bulge. An elongated spheroid of stars that all orbit randomly at different angles. All of this is surrounded by the halo, which is also spheroidal, but twice the diameter of the 100,000 light-year-wide disk. This thing is sprinkled with wayward stars and ancient dense mini-galaxies called globular clusters.
But mostly the halo is made of dark matter, which also suffuses the disk and the bulge, and constitutes 80% of the Milky Way's mass. The entire galaxy is beautiful and intricately structured. Weird to think that it built itself into this through a life of violence. So let's look at what we know about galaxy evolution based on all the other galaxies in the universe. We actually talked about this process recently in our episode on the galactic habitable zone. The first galaxies collapsed from very slight over-dense regions in the hot hydrogen and helium gas that filled the universe after the Big Bang.
It's hard to see the galaxies in the first billion years or so. Galaxies were still growing, and most are too small to see at those great distances. The most distant galaxy known as of the filming of this episode was discovered only in April this year. It shines out from a young universe only 250 million years after the Big Bang. Back then, galaxies were raging storms of star formation due to the...