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How do we know what the Milky Way looks like if we've never been outside of it?

Hosted by: Caitlin Hofmeister

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Caitlin: Imagine waking up one morning in a room you’ve never seen before. If someone asked, you’d probably have a pretty tough time trying to describe the shape of the building you were in, since you’d never seen it from the outside. But that’s basically what we’re doing when we try to understand what the Milky Way looks like from our position inside it.

These days, we know a lot about our galaxy: it’s a spiral, and we’re about three fifths of the way out from the center, in one of the arms. But it’s taken thousands of years to figure that out. Ancient Greek and Arab astronomers could see that there was this big weird strip running across the sky, and they weren’t all agreed on what it was made of.

Many of them thought that it was an enormous collection of stars, but Aristotle had a different idea. He thought the Milky Way was a huge patch of the atmosphere on fire, and that it looked dim because it was so far away. Almost two thousand years later, Galileo pointed his first telescope at the sky and proved Aristotle wrong for good: the Milky Way was a collection of stars.

Once scientists knew that, the question became: what was that collection shaped like? There are basically two common shapes in the Universe: the blob and the disk. Stars, planets, asteroids, and moons are all of the blob variety, and things like planetary rings and star systems are more-or-less flat disks.

Since we see the Milky Way as a band across the sky, that means it’s more like a disk. And the fact that the band is pretty narrow means that the Solar System must be near the plane of the disk — the middle of the thin side. If we were closer to the top or bottom of the disk, the Milky Way would look much broader, and you might even be able to see some of its structure directly. And if you travel around Earth, you’ll notice that the galaxy completely encircles our planet — so we must not be on its outside edge.

The first person to investigate the situation more deeply than this was British astronomer William Herschel, around 1785. He broke up the night sky into more than 600 zones and used his telescope to count every star he could see in each zone.

The dimmer a star looked, the farther away Herschel assumed it was. Using this information, he worked out what he thought the basic structure of the galaxy must be. Since he saw roughly the same number of stars in every direction, he concluded that we must be somewhere near the center of the Milky Way.

More than a hundred years later, the Dutch astronomer Jacobus Kapteyn tried the same basic technique with photography and came to the same conclusion. Then, in 1917, American astronomer Harlow Shapley tried a different method: he studied globular clusters, bright, dense groups of stars that you can see from very far away. Like Herschel before him, Shapley also assumed that dimmer clusters must be farther away.

But he found something very different from what Herschel did: Shapley realized that if you mapped out these globular clusters, they formed a rough sphere. And, more importantly, that most of these clusters were in one part of the night sky. He figured that the middle of the sphere corresponded to the middle of the galaxy, in which case Earth would be far from the center of the Milky Way.

We now know he was right. Herschel and Kapteyn were wrong because they didn’t know that the galaxy is full of dust, which absorbs starlight and obscures our view of the farthest parts of the Milky Way. A few years after Shapley figured out our position within our galaxy, Edwin Hubble, another American astronomer, discovered that there were lots of other galaxies.

From what we know about those other galaxies, plus what we’ve learned by using the tools of modern astronomy, we’ve been able to piece together an even better picture of the Milky Way. Dust doesn’t block infrared light as much, so astronomers can use infrared telescopes to look right past it. And with radio telescopes, astronomers can ignore the stars themselves and map out the gas that makes up a lot of the galaxy. With these techniques, plus new, better methods of gauging distances, we’ve developed a much more detailed map of the Milky Way.

But that doesn’t mean we have everything figured out. Just in the last few years, astronomers have discovered that our galaxy has what’s called a bar at its center — a rectangular region that joins together the Milky Way’s spiral arms and might provide a nursery where new stars form.

So, it is harder to learn about our galaxy from our position inside it. But with careful observation and some good tools, we’ve been able to discover a lot about our corner of the universe.

Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who make this show possible. If you want to help us keep making episodes like this, you can go to And don’t forget to go to and subscribe!