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This time, Marc, visitors to the Golden Pond Planetarium in Golden Pond, Kentucky, want to know how the Milky Way got its name.

Rayman: Okay. Maybe first we should talk about what the Milky Way is. It’s the name of our own home galaxy. A galaxy is a big collection of stars, and ours has a few hundred billion of them, including the Sun. There are about 100 times more stars in our galaxy than there are people on Earth.

Narrator: How big is our galaxy?

Rayman: Even traveling at the speed of light, it would take about 100,000 years to cross the Milky Way.

Narrator: So it is unimaginably huge! But even that is much smaller than the universe, right?

Rayman: Absolutely! The universe is far, far larger than the Milky Way. Our galaxy is simply one of hundreds of billions in the known universe. A galaxy like ours—as enormous as it is—is but a tiny island of stars in the vast ocean of the cosmos.

Narrator: So, is there anything in our galaxy besides stars—and us?

Rayman: Well, many of the stars probably have planets orbiting them. The galaxy also includes clouds of gas and dust, some of which are in the process of forming new stars and planets. We’ve talked about that in other Space Place Musings. And, most of all, the galaxy holds a large amount of what astronomers call dark matter.

Narrator: Oh, yes. That spooky stuff. Just what is it?

Rayman: No one knows. Dark matter is a mysterious substance that we can’t see, but we know it exists because of how its gravity affects the stars we can see. Another strange object lurks at the center of the Milky Way. It’s a tremendous black hole, perhaps 4 million times more massive than the Sun.

Narrator: I hope we are nowhere near the center of the Milky Way!

Rayman: You’ll be relieved to know we live at a safe distance. Our solar system is about half way from the center to the outer edge, and it takes light 25,000 years to cross that distance.

Narrator: So what does the Milky Way look like?

Rayman: If you could look at our galaxy from the outside, you would mostly see the spiral arms and the bright ball of stars at the center. It might look almost like a starfish with its arms curved around in one direction. Most of the stars, including our Sun, and most of the gas and dust are in those spiral arms.

Narrator: But since we’re inside it, that’s not how it looks to us, is it?

Rayman: No it’s not. The spiral arms are part of a wide, thin disc. Imagine you could shrink to be a tiny observer inside a disc like a DVD, halfway from the center to the outer edge. Now this disc would have far more atoms than the Milky Way has stars, but suppose you were looking around at the atoms. If you looked toward the center or toward the outer edge, you would see a large number of atoms, but if you looked up or down, you wouldn’t see many. So it would look to you as if you were surrounded by a band of atoms.

That’s how the stars appear to us in the Milky Way. When we look up at the night sky in a direction out of the disc of the spiral arms, we see some stars, but when we look within the disc of the Milky Way, we see so many stars that their light blends together to make a white band.

Narrator: We see it only if we are lucky enough to have a dark sky!

Rayman: True, and that is rare for us city dwellers now. But the ancient Greeks had plenty of dark skies. And they had a myth that this white band was milk left in the sky by the goddess Hera. In fact, the word “galaxy” comes from the Greek word for milk.

Narrator: But the Milky Way isn’t as even and homogenized—so to speak—as milk.

Rayman: No. There are some dark patches in our view of it where clouds of dust block the starlight. I named one of my cats Milky Way because her fur has white and dark areas that remind me of the beautiful pattern of our galaxy’s stars.

Narrator: Good cat name. But, how can our listeners find the same band of light if they don’t have a cat like yours?

Rayman: With a clear dark sky, it’s easy to see the Milky Way. To help locate it, use a star chart.

Narrator: Ah, we have star charts for each month as part of our Star Finder game on The Space Place web site. Our listeners can find them by going to spaceplace.nasa.gov and typing “star finder” in the “Find it @ Space Place” field.

Rayman: Great! On those star charts, the Milky Way is the shaded band cutting across the sky. During the summer and fall months, you may see a grouping of stars in the constellation Sagittarius that resembles a teapot, and whenever you see that, you can look for the Milky Way rising from the spout like steam.

Narrator: Thank you, Marc, for another enlightening discussion. To our listeners, we’ll be back soon with more Space Place Musings.

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