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We ask Museum educator Janine all your questions about how far away things are, from the Moon to the end of the universe, during this Pulsar podcast brought to you by #MOSatHome. We ask questions submitted by listeners, so if you have a question you'd like us to ask an expert, send it to us at sciencequestions@mos.org.

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**ERIC:** At the Museum of Science, we're often asked how far away things are in space. The simple answer is, really, really far away.

Today on Pulsar, we'll get some more exact answers, starting with the closest things to our home planet and making our way out to the edge of the universe. And along the way, we'll find out: how do we know how far away these things are?

Thanks to Facebook Boston for supporting this episode of Pulsar. I'm your host, Eric, And my guest today is Janine from our forums department. Janine, thanks so much for going on this journey through the universe with me.

**JANINE:** Yeah, absolutely, happy to be here.

**ERIC:** So let's start with the closest natural object to us here on the Earth. How far away is the moon?

**JANINE:** OK, so I'll use a unit of measurement that you're probably pretty familiar with. It's about 238,855 miles on average, and I say on average, because the distance does change.

The moon does not orbit the Earth in a perfect circle, but that's kind of an abstract thing, and it doesn't really mean anything to you, right?

So if the Earth was the size of a basketball, the moon would be about the size of a tennis ball. They would be about 23 feet, 9 inches apart, which is about 30 earths, which is crazy to me.

**ERIC:** It's further away than you would think.

**JANINE:** It really is. I always think everything in space has more space than we expect it to, so even our closest neighbor is 30 times away the size we are.

**ERIC:** And it's the furthest we've ever explored with humans, and we often get asked, how long did it take those humans to get to the moon?

**JANINE:** Apollo 11, so our first astronauts landing on the moon. It took them 102 hours, 45 minutes, and 40 seconds from takeoff to landing to get to the moon.

So that's 4.25 days, but they didn't go in a straight line, and that's because - well, there's a lot of reasons, but mostly it's because it's the most efficient way to get there.

Everything that you put on a mission to go into space costs fuel, so the more fuel you have, so to go faster, would actually make you weigh more, so there's this balance of power and efficiency, and you're always trying to make it as light as possible.

It was kind of more of a circle around the Earth and then a couple of circles around the moon and then a landing rather than a straight shot.

**ERIC:** So we could have got there a little bit quicker than four days, but not too much quicker.

**JANINE:** Yeah, I think they say, on average, over the course of all of the missions is about three days to get from Earth to the moon.

**ERIC:** So we haven't sent any astronauts to the moon in nearly 50 years. Lately, they spend their time on the International Space Station. How far away from Earth's surface is that?

**JANINE:** So that's actually a lot closer. It's only about 254 miles away, and I was trying to figure out what cities on the Earth are at least in the US are close to that distance, and I figured out it's about the distance if you were to fly from LA to Las Vegas.

**ERIC:** And the next object on our list at the center of the solar system, the sun. How far away is that?

**JANINE:** So sun is our closest star, and it's 92 million miles away, which is crazy, and now we're starting to get to these distances in space where talking about them in miles really doesn't mean anything.

So actually, the average distance from the Earth to the sun is a unit that astronomers used called an astronomical unit, so we've just decided that, for math, it's a lot easier to figure out, we'll just say that the distance from the Earth to the sun is 1, and then all of our math can be easier.

If you could travel at the speed of light, which you can't because you're made of mass, but if you could, it would take 8.3 minutes. The thing that blows my mind away about this is, since it takes eight minutes for light to travel, the sun could go out suddenly, and we wouldn't know about it for eight minutes.

**ERIC:** Because it would take eight minutes for light to stop showing up on Earth.

**JANINE:** Yeah, it's crazy.

**ERIC:** So jumping right out to the edge of our neighborhood, we often get asked how big the solar system is. So how far away is the edge of the solar system? Does it even have an edge?

**JANINE:** OK, so it's hard to talk about the solar system and what does it mean to be part of the solar system. We talk about it, where the gravity of the sun is no longer the dominant pole on an object.

So everything on space is pulling on each other. That's how gravity works. That's how mass works.

We're considering the things in the solar system to be the things that are most pulled on by the sun, and so that's at the edge of the Oort cloud, and to go back to that unit of the astronomical unit, that's about 100,000 astronomical units away.

**ERIC:** So start on Earth, head past the sun, then go 100,000 times further than that before you leave the solar system.

**JANINE:** Yeah, isn't that nuts?

**ERIC:** It is. That's already so far, and speaking of that, when we mentioned the outer part of the solar system, we get asked about the robots that we've sent deep into space. So how far away is the furthest spacecraft that we've launched from the earth?

**JANINE:** OK, so I looked this up yesterday. So it's a little bit further out now, but since we're talking about astronomy, everything in astronomy has a big error range anyway, so that's fine. Voyager 1, which was launched in 1977 is about 150 astronomical units away from the Earth.

**ERIC:** So that's wicked far, but it's not anywhere close to leaving behind the effect of the sun's gravity. OK, so leaving the solar system behind, what's the next closest star to us and how far away is it? And since this question comes up a lot how long would it take a rocket to get there?

**JANINE:** So the closest star to us is actually part of a three star system.

The closest one of those three stars is Proxima Centauri, which is 4.22 light years away, and so if you could travel at the speed of light, it would take you 4.22 years to get there, but we can't travel at the speed of light, so how long would it take Voyager 1 to get there? It would take over 73,000 years.

**ERIC:** So using current rocket technology, we're just not going to get there any time soon.

**JANINE:** No. No, space, as I think we're going to establish in this podcast, is very big.

**ERIC:** Now, before we continue our journey, this would be a good place to bring up a question we got from Sophie. She wanted to know how we measure distance to things in the universe that are really far away.

The planets are pretty easy to measure, we've been to them all, we can see them moving, how can we measure the distance to stars and galaxies?

**JANINE:** Yeah, so astronomers actually use a bunch of different tools, and we call it the distance ladder, although I like to think about it as if you had a bunch of yardsticks and you tried to tape them together and that first yardstick is really strong and by the end it's bending over and not super great, because our error of knowing what is correct and how accurate something is increases as we use different steps on this ladder.

But the first step that you can use is called parallax, and you can actually do an experiment with this right now if you want to.

You can hold a finger in front of your face and close your left eye and then close your right eye and look at what happens behind it. And you'll notice that, with respect to the things behind it, it moves in front, just because there's a little bit of distance between each eye.

And so we can do that with stars, but not with our eyes, because that's too small of a distance with respect to how far away stars are.

**ERIC:** Yeah, stars don't seem to move too much if you just go outside and wink at them back and forth a bunch of times.

**JANINE:** Yeah, so what we can actually do is use the Earth in its orbit as that kind of blinking, and so if we go out and measure in June and then we go out and measure in December, now we've got six months apart so we're halfway around the sun.

So we've got that entire distance, which is 2 AU, going back to that astronomical unit is the longest baseline we can get while we're on Earth. And we can look at stars and see how they change with respect to the things behind them, and that's how we can get a direct distance.

**ERIC:** So parallax seems pretty good for stars that are fairly close, but you mentioned other methods too. So what's next?

**JANINE:** Yeah, so the next step is something called a standard candle, and actually the first standard candle was discovered not too far from the Museum of Science by Henrietta Swann Levitt at the Harvard College Observatory back in the early 1900s.

She was a computer there. If you're interested in this at all, there's a really good book called The Glass Universe that talks about all of these computers who worked at the Harvard College Observatory, including Annie Jump Cannon, who's very famous for figuring out the brightness of stars, a relationship about that.

Henrietta Swan Levitt determined this first standard candle. So she was working at the Harvard College Observatory, examining photographic plates from telescopes. So these telescopes were taking all these images and they needed people to reduce the data, which is something that a lot of physical computers do now, but people did back then.

And she was looking at a particular type of star called a Cepheid variable, and she realized that there was some sort of a relationship between how fast they dimmed and brightened and what their brightness was.

These Cepheid variables are very consistent, so she had this idea that, because luminosity and period are the same, maybe they could be used to figure out how far away something is.

So the standard candle idea is that a candle has an intrinsic brightness that we know. We can determine it because of some sort of physical relationship or just studying physics in general.

This star, if we know this other thing about it, we know how bright it is if you were standing at a certain distance from it. OK, so if we know how bright it should be and we know how bright we're observing it, we can actually figure out the distance based on that, right?

If you know how bright your flashlight is and you know how bright you're seeing it, you can figure out how far away it is.

**ERIC:** So the further away something is, the dimmer it appears to us, and if we know its true brightness, it's pretty easy math to calculate how far away it must be to appear how we see it.

**JANINE:** Yeah, exactly. So they figured out that these Cepheid variables could be used in this way as a standard candle. Although, my personal favorite standard candle is a type 1A supernova.

And that's entirely because, when I was in college, I worked on a project on SS Cygni, which is a very well known cataclysmic variable.

And what a cataclysmic variable is is it's a red giant star, and it has a partner a star, a binary star companion, called a white dwarf, and actually, most stars in the galaxy are in multiple star systems, so it's pretty normal to find a binary star system.

So in a cataclysmic variable, you have this red giant and you had this white dwarf, and the white dwarf is close enough to the red giant that it steals mass from the red giant.

It doesn't know what that mass belongs to and it takes it on and it turns into this disk that goes around the white dwarf and there is a point at which there's too much mass in the disk, it becomes unstable, it all falls on to the white dwarf and the white dwarf brightness suddenly.

And because we know what that mass is, there's a mathematical physical relationship between how much mass is in that disk.

You then know how bright it is. You've got your E equals mc squared, so you know how much mass is going to turn into an energy, and then you can figure out how far away is.

**ERIC:** And this takes us even further out on the distance ladder, because these things are so bright, we can see them from really far away and we can measure larger distances.

**JANINE:** Yeah. Yeah, and actually, that's how we got our first distance to the Andromeda galaxy was Edwin Hubble, who you may have heard of because of a certain telescope. There was a person that that's named after.

So Edwin Hubble in 1924 used Cepheid variables that, as Henrietta Swan Levitt had posited you could, to figure out how far away the Andromeda nebula was, because at that point they didn't know that galaxies were galaxies.

But he used it to prove that it wasn't inside of the Milky Way, and his number was about 900,000 light years. He used 12 Cepheids to figure that out. We now think it's about 2.537 million light years, but.

**ERIC:** So in the ballpark, not too bad for telescopes from 100 years ago.

**JANINE:** It's astronomy, right? So it's pretty close.

**ERIC:** All right, we can use these methods to estimate distances to other galaxies that make up the universe, and now, we're at the end of our journey. How far away is the edge of the universe?

**JANINE:** This one's harder. There isn't an edge to the universe, at least not one that we know of, and people who are trying to figure out this are actually called cosmologists.

So there are people who study what the shape of the universe is, how big it is, how it formed, all of these kinds of things. But we can talk about the edge of the visible universe or actually how far back in time, we can see.

We talked about that time limit and how long it would take light from the sun to get to the earth and how we wouldn't know for eight minutes. Well, that applies to everything that we see in space, which means looking out into space is basically a time machine, right?

We're looking back in time the further out we go because it takes time for light to travel to us.

So the furthest out we can see is about 46.5 billion light years away, which is crazy, but it also means you can look back into the past and try to figure out how the universe formed, which again, is what cosmologists do.

**ERIC:** Well, Janine, thanks so much for telling us how far away everything in the universe is.

**JANINE:** You are so, so welcome.

**ERIC:** You can find out more about the structure of the universe by tuning in to one of our virtual planetarium shows from the comfort of your own home. Visit mos.org/mosathome to see our schedule.

Until next time, keep asking questions.

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