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The Structure of Our Galaxy

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  • 0:03 The Milky Way Galaxy
  • 2:04 The Disk Component
  • 3:53 The Halo, Central…
  • 5:17 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
If you ever looked up at the night sky far away from city lights, you would've seen a band of light across the night sky and lots of stars scattered around the rest of the sky. Why? The structure of our galaxy will explain exactly why.

The Milky Way Galaxy

You live at home. Your home lives in a city. The city lives in a state, which lives in a country, which lives on Earth. The Earth lives in our solar system, which calls our galaxy, the Milky Way Galaxy, home.

You know the structure of your home. Maybe it's made of brick. Maybe it's a split-level home. You even know the outline of your city and the boundaries of your country. You can use a satellite to see our Earth from a distance. But we have no way of seeing our galaxy from far away.

So, what's the structure of something we can't see from afar, something we are immersed within? What's the structure of our galaxy? This lesson will outline the answer astronomers have found from years of research.

When you look up at the night sky, it looks like the stars are pretty evenly distributed all over the cosmos. But the reality is that stars clump together into galaxies. A galaxy is a large but isolated collection of stars, interstellar gas and dust, star clusters, and nebulae that orbit around a common center of mass. When I say they're isolated clumps, I say that because galaxies are separated by vast spaces that are mainly empty.

And so it follows that just about every single thing you see up in the night sky with the unaided eye is part of our galaxy, the Milky Way galaxy, a barred, spiral galaxy that contains our sun and Earth, one that is visible as the Milky Way. The Milky Way is a wispy band of light made by the glow of our galaxy's stars that stretches across the sky. It's best seen when it's very dark outside and no light pollution is present from nearby cities or streetlights.

The Milky Way encircles us, and we see it basically as an edge-on view of our galaxy. Meaning, we are looking at it from within the plane of our galaxy. This is the reason for why it appears as a band around the sky.

The Disk Component

To help better understand how this is so, imagine our galaxy is a wafer disk. The disk component of our galaxy is the disk of our galaxy, the components within the plane of the galaxy. But what is the plane of the galaxy?

Well, let's face it; if you're looking at the wafer face-on, it looks like a flat circle. But if you look at it edge-on, that's the plane of the galaxy, and it looks like a thin band stretching from left to right, just like it would on our own night sky.

This means that when you go outside at night and look up at the night sky, you see one of two things. If you look out at the sky in the plane of the disk, you see the Milky Way. If you look perpendicular to the plane of our galaxy, you see a much sparser collection of stars scattered throughout space.

The disk component of our galaxy contains the majority of the galaxy's stars and almost all of the interstellar gas and dust. The interstellar dust and gas is where stars are born, and thus, almost all star formation occurs in the galaxy's disk. Actually, if you look at the image on your screen, you can see the Milky Way. The dark streaks that stretch across the Milky Way occur thanks to the interstellar dust located in the plane of the Milky Way galaxy.

While most of the stars in the disk are middle-to lower-main-sequence stars, there are exceptions. There are some white dwarfs, red giants, and even very massive class O and B stars. The latter two are rare but are so luminous that they are the ones that provide a lot of light emanating from the disk itself.

The Halo, Central Bulge, and Spherical Component

If we could modify our wafer a bit, we'd be able to describe something known as the central bulge. It is a cloud of billions of stars lying at the center of our galaxy, one that contains little gas or dust. It is called a central bulge for obvious reasons. It bulges out at the center of the galaxy. For some odd reason, I picture it as a cherry in the middle of our wafer, sticking out on either side of it. In reality, the central bulge is more elongated, like a bar, as opposed to being truly spherical. Surrounding the central bulge and disk is something known as the galaxy's halo, a spherical cloud of stars, star clusters, and little gas or dust.

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