Astronomy > Ask a Scientist About the Milky Way Galaxy

What happens to the stars during the day?

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JACKIE FAHERTY: When you're outside during the daytime, you're playing, you're having fun, the sun is high in the sky, it's bright outside, you can see a blue sky. It's gorgeous. Wait till the end of the day. Go outside and watch a sunset. You'll notice the Sun getting lower and lower on the horizon, and the light gets dimmer and dimmer. 

And then the Sun will go below the horizon. And as it does, and you look up at that same sky, the same one you were looking at when the Sun was up, the brightness of the Sun comes down, and you can start to see all of these dots start to emerge -- bright points of light. Those are the stars. Some of them might be planets in our Solar System as well. But those same stars were there when the Sun was up. 

But they're so much fainter than the Sun that they can't possibly shine through so that you could see them. You have to wait until the Sun sets in order for you to be able to see their light. But just know that even when the Sun is up, those stars are there. They're there in the sky. They're just too faint for you to see them. 

My name is Jackie Faherty, and I'm an astrophysicist at the American Museum of Natural History. An astrophysicist means that, for a living, I get to study things out in space. I unlock the secrets of the universe for my job every single day. 

What is a star, and what is the Sun?

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JACKIE FAHERTY: A star is a really large collection of gas, and the gas has been pushed together really tightly so that it ignites and you get, in the core of it, a really big explosion that gets it radiating out, giving off light and heat. And our Sun is exactly one of these kinds of stars. It's not too big. It's not too small. It's kind of in the middle of stars when it comes to how massive they are or how big they are, how actually large they are in their physical size. 

When you go outside and look up the nighttime sky, you'll notice some of the stars are brighter, some of the stars are fainter. And there's two reasons for that. One, some of them might just be closer to you. Or two, some of them might be much farther than our Sun but way bigger than our Sun — more massive, so brighter. And so their light can reach you from a further distance. 

Is there anything else in our Solar System besides the Sun and planets?

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JACKIE FAHERTY: Our Solar System is made up of the Sun and then all the planets and then other things. So you actually get all these tiny rocks. We call them asteroids. They orbit in an area between Mars and Jupiter. 

Then there's things called comets that have way more ices in them than, say, the rocky asteroids. 

The Oort cloud is composed of a lot of comets and maybe some asteroids, but it's basically the leftover materials from when the Solar System was forming. But it got pushed to the outer edges of the Solar System, and it stays there. And the Oort cloud is made of millions and millions of these objects. 

What is the Milky Way Galaxy?

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JACKIE FAHERTY: So when you go outside at night, go to a really dark sky, and you might see a stream of stars going up above your head and down. Depends on the time of year. It might look a little different. But we call that the Milky Way. It looks like a river of stars, and it's gorgeous in your nighttime sky. 

Now, we didn't know exactly what it was for a very long time. It took until the 1900s for us to figure out what that band of stars might be. But we now know that it's actually this big structure that we're inside of, that the Solar System that the Earth that us, humans on the Earth, are inside of. It's called a galaxy. 

Just like the planets orbit the Sun and make a Solar System, stars orbit a central area. And that whole structure of many, many, many billions of stars is called a galaxy. And the one that we're in we've called the Milky Way. We're inside of a part of the Milky Way that we call the disk of the galaxy. 

So it's flat like a pancake. It's not a sphere. It's flat like a pancake. And we think it's got these gorgeous spiral arms that stretch out from it. And based on our position, we can see towards the center of the galaxy. And when we're looking towards it, you're seeing that stretch of stars that's kind of like seeing out from a tire that's wrapped around you. So you're going to see the band all around you. 

What is a galactic year?

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JACKIE FAHERTY: The Earth goes around the Sun roughly 365 days. Mercury goes around in 88 days. All of the planets have a different number of when they go around the Sun. The Sun takes a certain amount of time to go all the way around the galaxy. 

Just like the planets are in motion around the Sun, and the Earth is spinning on an axis, the Sun is also in motion. It's moving around the galaxy, changing its position, moving through a different neighborhood of the galaxy as it goes. It takes roughly 225 million years for the Sun to make its lap around. So that means the Solar System has made roughly 20 laps around the galaxy. Or in other words, the Solar System is 20 galactic years old. 

What do you mean we’re in a bubble?

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There are these structures in the galaxy that astronomers have lovingly called bubbles, and what they are is some stars are so massive that when they run out of fuel at their core, when they run out of the stuff that gets them to light up, they collapse. And when they collapse, they explode in a mega explosion that we call a supernova. 

And when the supernovas go off, they send a lot of energy out into the galaxy. And that energy can push stuff around, so it can push the gas and the dust that makes up the galaxy far away from where that original star was. And it makes a clearing. And the clearing kind of looks like a bubble. 

Right now, the Solar System, you and I and everybody else that's on this planet, is passing through one of those bubbles. Just happenstance. It's just an accident that we're in this area. Now, that led to us having a gorgeous view of the nighttime sky, as the area that we're currently in the galactic neighborhood is relatively clear of gas and dust because these supernovas swept out the area. We don't know how many more of these bubbles are on our path as we move around the galaxy. 

What does the rest of the Milky Way look like?

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JACKIE FAHERTY: Our Sun is one of maybe 200 to 300 billion stars that make up the Milky Way Galaxy, so we're not completely sure what it looks like on the other side of the galaxy. That is a bit of a mystery still for astronomers. It's an exciting mystery. We also don't know exactly all of the structures that exist in the Milky Way. We don't know how many bubbles exist from when supernova went off. All of that is still a mystery. 

And really, anyone watching this could help solve those mysteries. If you decide you love astronomy and you want to be one of the people that studies the stars, maybe, when you're older, you could help us figure out what's happening on the other side of the galaxy. Where are all the bubbles in the Milky Way Galaxy? Will the Solar System pass through another one anytime soon? It's open for you if you want to be an astronomer one day.