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Ask a NASA astronomer! How many galaxies are out there?

If we ever discover the true size of the universe, we'll likely have galaxies to thank. The trillions of massive star clusters we've observed are sending light from the early universe back to us.

MICHELLE THALLER: Evie, you ask a wonderful question: how many galaxies are there?

And this is something that we actually don’t know the answer to, but I can tell you a wonderful story about what we do know.

So let me first talk about what a galaxy is. And a galaxy is a family of stars, but usually in the hundreds of billions of stars. We live in a galaxy called the Milky Way and there are about 500 billion stars, we think, in the Milky Way Galaxy. Galaxies are absolutely huge.

The Milky Way Galaxy is about 100,000 light-years across, and that’s not really a number I can get my mind around, seeing as one light-year is about six trillion miles, so our single galaxy is 100,000-times-six-trillion miles across. It’s absolutely huge.

The best analogy I know is that if you think about the sun—the sun is a giant thing, the sun is so big you can fit a million Earths inside it. It’s really, really big. And if we made the sun the size of a dot of an “i”, so pretend that the sun is only the size of—like take a regular page of a book, look at the dot of an “i”, if the sun were that big, how big would our one Milky Way Galaxy be? It would be about the size of the earth. So that’s how big a single galaxy is.

If the sun were the dot of an “i”, the Milky Way galaxy would be roughly the size of our planet.

Now how many galaxies do we know of?

And this is a wonderful result from the Hubble Space Telescope. The Hubble Space Telescope decided to try to answer that question, and what it did is it looked at an area of the sky that, as far as we knew, was blank, it was just black; we couldn’t see many stars there, we didn’t see any galaxies there, and it decided to take a very, very deep distant look at the universe.

Now, the way the Hubble Space Telescope (and any camera) works is it works kind of like a “light bucket.” You can actually open up the eyes of the telescope and tell it to just keep staring, and the longer it stares the fainter and more distant objects you can see.

For those of you that like photography, it’s called doing a time exposure. You leave your camera open for a certain amount of time and you can see fainter and fainter things.

Well, incredibly, the Hubble Space Telescope kept its eyes open on this one little part in the sky for more than a month, and it just let any light come and build up this beautiful image, and what we discovered is that in this empty part of the sky—empty we say!—we counted over 5000 galaxies. Five thousand galaxies we didn’t even know were there. They were just so faint we’d never seen them before.

When we finally had a sensitive enough telescope up in space and we were able to keep it staring at a tiny little part of the sky for a month 5000 galaxies turned out to be hiding there that we’d never seen.

So, how much of the sky was this tiny little part that the Hubble Space Telescope looked at?

So let’s go back to the dot of an “i”. So think about the dot of an “i” in a book, and now hold of that book at arm’s length. It’s a tiny little point, you can almost barely see the dot of an “i” held at arm’s length. That’s how much of the sky the Hubble Space Telescope counted 5000 galaxies in.

And if you do the statistics, if you take that little dot in the sky, and by the way we’ve done this, we’ve taken other deep images in different regions of the sky, and we get about the same count of galaxies anywhere we look.

If you do the math, tiny little dot all over the sky, 5000 galaxies in each dot, there are actually several trillion galaxies that we can see with the Hubble Space Telescope if we had the time to observe the entire sky.

So we know that there are several trillion galaxies that the Hubble Space Telescope can see, but is that really the number? Is that how many galaxies there really are?

The universe we think is far larger than we’re able to see right now.

So we’re actually building telescopes that are so powerful—the James Webb Space Telescope we hope will launch in the year 2020, and that will be so powerful it will be able to see every single galaxy all the way back to the beginning of time.

So it will be able to see so far away that the light has taken 13 billion years to get to us, and we should be able to see in any part of the sky every galaxy that’s ever formed in the history of the universe. Is that the answer? Is that how many galaxies there are?

Well, I ask you to consider there’s something that astronomers call the observable universe. And that is that if we take a telescope and we look in every direction around us, there is in fact a limit to how far we can see.

And that’s because the universe is not infinitely old. We think the universe began about 13.8 billion years ago with the Big Bang. So we can only see as far out as there’s been time for light to get to us.

So we see in every direction in the sky out to a distance where we’re looking back, say, about 13 billion years, and that’s wonderful, but does that mean that’s how big the universe is? No.

The universe must be much larger than that, it’s just that there hasn’t been time for light to get to us from the rest of the universe.

If the universe is around for another 20 billion years, we will see farther and farther and farther out into the true universe. We have no idea how big the universe really is.

We can only see the part of it where there’s been time for light to travel to us.

The universe is not a sphere centered on us—that would be very, very strange. That’s just how far we can see in every direction since the beginning of time.

So what’s the real answer? How many galaxies are there? And this gets to one of the most amazing things that we don’t know and that is what is the size of the universe. For all we know the universe could be infinite.

It may be that the real answer is that there are infinite number of galaxies, space never ends and there are galaxies throughout an infinite universe.

Or maybe the universe isn’t infinite. Maybe there really is some vast shape to it that we’re not aware of yet. Just like how we’re standing on a spherical earth right now, but it seems flat to us because we can only actually walk around a little part of it at once.

It could be that the universe has a larger shape and maybe it even has boundaries, maybe there really is an answer to how big the universe is.

But in the time the universe has existed there hasn’t been time for light to travel to us from the farthest reaches of the universe.

So right now we honestly have no idea how many galaxies there are.

If we ever discover the true size of the universe—is it infinite or just too big to measure?—we'll likely have galaxies to thank. The trillions of massive star clusters we've observed are sending light from the early universe back to us. But our measuring instruments—the strongest of which is NASA's Hubble Space Telescope—aren't powerful enough to detect light from furthest points of the universe. But in 2020, the James Webb Telescope should be able to, revealing a truer number of galaxies and perhaps the boundaries of the universe itself.

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