How many stars can you see in the night sky with the naked eye?

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Speaker 1: What's the biggest number that you can really hold in your mind. Let's do a little mental exercise. Start with a number like five. Surely you can imagine like five, I don't know, bananas floating in your mind, right, each one crisp and the unique and different. It's a simple, small number that's easy to visualize, all right, But can you do ten? Can you keep ten individual bananas in your head? Can you do a hundred? Now it starts to get challenging, right, So imagine a thousand, a hundred thousand. Can you imagine a billion individual bananas, all unique, floating in your mind? Unless you're some sort of crazy genius, probably not. But the universe throws these numbers at us all the time, right, you hear millions, billions, trillions, numbers that frankly sound made up. What do these numbers mean? Can we really grasp them? Hi? I'm Daniel. I'm a particle physicist and the co author of the book We Have No Idea, A Guide to the Unknown Universe. My co author in that book, Jorge cham, a cartoonist, is not here today on the podcast, so welcome to the podcast. Daniel and Jorge explain the universe, but without Jorge, who has to be away. This week there's a production of I Heart Media, a podcast in which we zoom all around the universe trying to understand all the crazy, mind blowing things that the universe has to offer. And the universe is filled with things that are difficult to understand but fun to struggle with. And one of those things is numbers. Because the universe is vast, there are so many things out there. They're huge numbers of stuff to think about, to look at, to try to understand. I mean, if you talk about just our galaxy, the Milky Way, there are a hundred billion stars in the universe, how do you comprehend that number? How do you know what that number really means? Frankly, anything bigger than a thousand is to meet infinity? Right, you talk about a million dollars versus ten million dollars, What difference does that really make to me? A billion dollars a trillion dollars. I can never be an economist because to me, all these numbers are ungraspable. They're hard to really put my head around, to really wrap my fingers around and manipulating my mind. I mean, sure, I can do the math with them on paper, but I can't visualize them, and as a visual thinker, it's difficult for me to understand what these things mean. But yet the universe throws these numbers at us, right, And you try to do science at this scale particle physics. Every baseball is filled with ten to the twenty three times some number of atoms. Right, It's hard to imagine. Every time you're holding a banana or a baseball in your hand, you're interacting with a ridiculously huge number of particles. And so that's what we want to focus on today. That's the topic of today's podcast, and most specifically today, we're asking the question how many stars are there in this sky? And of course there are a huge number of stars out there in the Milky Way, right. A hundred billion stars in the Milky Way times two trillion galaxies makes a number that's like, really hard to even understand. Two times ten to the twenty three stars in the observable universe. I don't even know the name or that number, right, somebody's have to make up the new prefix, the new scientific phrase for that number, because it's so big that all you can do is write it in scientific notation, right, it's not even a number anybody ever actually sits down and writes out with all those zeros, you know, we need like a new way to write numbers, just to talk about this kind of number, a number in this category. And so that's a huge number, right, two times ten of the twenty three. But there are lots of big numbers in science, not just in physics. I mean, if somebody asked you, what are there more of stars in the Milky Way or trees on Earth? You might think, well, of course there are more stars in the Milky Way. That's a huge, ridiculous number compared to this tiny little planet we find ourselves on, right, But not true. It turns out that there are three trillion trees on Earth, three trillion, which is more than the hundred billion stars in the Milky Way. Right, So there are numbers all over the place, not just in physics, but also in biology. Here, just on this one planet, there are more of those trees than all the stars in our galaxy. And if you look inside our body, there are forty trillion cells that make up a human body. Every single one of you walking around, squishing and breathing and living, has a huge number of these things all cooperating to make you who you are. So there are more cells in the body than trees on Earth, and more trees on Earth than ours in the galaxy. And you can keep going. The number of grains of sand on all the beaches and all the deserts on the Earth is an even bigger number. That's four times ten to the nineteen. So for every tree on Earth, for every cell in the body, there are millions of grains of sand. And this is what I talk about not being able to grasp a number. Like you walk on a beach. You see all that sand, but you can't count it. If somebody drops a handful of objects on the sand, you can say, oh, look, there's your car, keys and your wallet and your phone. There are three things there, right, that makes sense. You can look at three things and count them instantly. You don't actually have to count. You just sort of recognize the numeracy there. You don't have to actually individually count. But if you wanted to count the number of grains of sand on the beach under your feet, you'd have to actually count them. I mean, the information is there, it's coming into your eyes. Even just the number of grains that you see the number of grains that are visible to your eyes when you're looking at the beach. That imprinciple, you have that information coming into your eyeballs. But of course nobody can count. That it's too big a number to grasp. And so that's what fascinates me, that that there are these numbers out there, that they're everywhere, that they're all around us, and so you might think, well, that's certainly a big number. Two times ten of the nineteen is a huge number of grains of sand. It's hard to imagine. It also means that there are more grains of sand than stars in the galaxy, which means that for every star out there, all those individual balls of plasma, there are more than a million grains of sand on Earth. It's hard to really grasp these ratios. But of course I'm married to a biologist, and the king of big numbers turns out to not be physics. The king of big numbers is biology, and specifically microbes. And of course I'm married to a microbiologist. So she reminded me that there are ten to the thirty one pages in the biome. That means there are ten to the thirty one little viruses out there attacking bacteria. And that's a ridiculous number. That's bigger than all the stars in the observable universe. That was ten to the twenty three. So for every star in every single one of those galaxies, there are like a billion viruses here on Earth. To count all the stars in our galaxy and all the stars and other galaxies is just a ridiculous number. And for each of those there's a billion, which already is hard to grasp. A billion viruses here on Earth. So science is filled with these big numbers. But I was interested in something more specific. You take a drive out to the desert, you look up at the night sky, and you're confronted with enormity. You look up at the night sky, you see all these twinkling lights, You see all these stars, and you wonder, wow, how many are there? Certainly seems like a lot, right. We're used to science and physics having big numbers in it. We know the universe has vast and so we imagine we can see a lot of stars. Turns out we might not be able to see that many. So I was curious what people knew about this, what people thought about the number of stars in the night sky. So I walked around the campus if you see Irvine, and I asked, folks, how many stars can you see in the night sky on a beautiful, crystal clear night, right, no pollution, no clouds, um, no moon, no light from Los Angeles? How many stars can you see in the sky? But first think for yourself, if you've been out on a nice clear night, how many stars do you think you can see in the night sky. Here's what people had to say. Millions. Okay, I know there is many more in the universe. I even know how many there are in the universe. Tend to the five between a billion and a trillion roughly, Let's see. I would say at least Avocata's number, like tend toy three. I don't know I would I would guess less than that, maybe like time stend of the ten power, but I don't know. I'm gonna say tend to the twenty like a million guests somewhere in the billions. But I have no idea. All right, So you hear a lot of big number is people definitely feel like it's got to be a big number because they know there are a lot of stars out there and they're right, there are a lot of stars out there in the sky. That doesn't necessarily mean that you can see all of those, right, or even that you can see very many, but people wanted to say a big number. You hear millions and billions and ten to the ten, and so people want we're prepared to be saying a big number. And maybe it's a bit of a gotcha question because a lot of times you'll be asked a question about science and the answer is supposed to impress you, right, the answer is supposed to be so much bigger than you even imagined. So I don't know, maybe people were rounding up a little bit, trying to make sure they hit an impressive number. We'll break it down and talk about how many stars you can actually see in the night sky, but first let's take a quick break. Alright, So we're talking about how any stars you can see in the night sky. Now, first let's start out with how many stars there are. Right, we talked about this quickly already, But in the observable universe, there are ten to the twenty three stars out there, basically a trillion times a hundred billion. It's a huge number of stars. However, most of the stars are really really far away. So let's talk about the light from the star. The stars this huge burning ball of plasma and it's shooting out a lot of light. Our Sun, for example, is not a particularly bright star, but already it's pretty bright. You know, it's shooting out a huge number of photons. If you look up at the Sun, please don't, then you'll burn your eyeballs. Right, and we're not even that close to it. So the Sun is pretty bright, which means the stars out there must be pretty bright. The problem is, in order to see something, it has to be either pretty close or super duper bright, because the brightness of an object falls really quickly with distance. Imagine all the photons leaving the Sun, right, there's a huge number of them. They're screaming out into space, ready to go on some cosmic journey and land on some alien eyeballs. Right, So all these photons are leaving the Sun. But they're leaving the Sun and there's a fixed number seven gajillion, I don't know, some certain number of photons are leaving the Sun. So if you're really close to the Sun, then your eyeball is going to get hit with a lot of those photons. But now take a step away, take go a thousand kilometers further, you have the same number of photons. Right, it's not adding any more photons. I mean there's another wave of photons coming along behind it, but in that one blast of light that we're talking about, you don't get any fresh photons. These same photons continue out, but now they cover a larger area. That cover a sphere that has a larger radius. And the area of a sphere grows with the radius squared, and so as the radius of that sphere that the photons are trying to illuminate grows, the number of photons per area drops like one over distance squared one over radius squared. So if you're twice as far from the star, the brightness is one fourth. If you're four times the distance from the star, then the brightness is one And that doesn't seem like a big effect for small numbers, but it adds to pretty quickly. If you're a thousand times further away, then the brightness goes down by a million, And those are pretty big numbers, and so they can impact even really bright stuff like stars. So what that means is that for us to see a star. Right, we have to be able to see a certain number of photons. When you look up at the night sky, you turn your eyes to the sky. You have to get photons from that star. And already that's a fascinating journey. You know, those photons were created in the heat of that cosmic fusion billions of miles away and have flown through space for thousands or millions or billions of years before they hit your eyeball. They've lived all that time, and then they're just sucked up by your eyeball and are no more. Right, you just take that little bit of energy. Anyway, for you to see a star in the sky means that a photon is left that star and arrived at your eyeball. And your eyeballs are pretty good, right, they're pretty good detectors. But you need to see a few photons in order to register a star. So there's a threshold. All the stars are out there, but in order for you to see them with your eye, in order for you to say, hey, I see a star, you have to get a certain number of photons, and so that cuts down on the number of stars that are visible to the naked eye by a huge fraction. If they're really far away, then that have to be super duper bright for you to still get enough photons, right, because remember the density of photons that are hitting your eye from that star falls with a distance squared, So if it's super far away, then has to be ridiculously bright, which is why it's so amazing sometimes when we see things like quasars, which we know are really far away and yet still seem bright, which means at their source they're riduculously bright, or if they're closer they don't have to be quite as bright. But being closer is a really small fraction of the universe. So that's cut to the chase. In the end, you can see the only between five to ten thousand stars in the night sky with the human eye, and of course you can't see the entire night sky from earth right, you can at best see half of it, So cut that number in half. We're talking just a few thousand stars in the night sky that you can see with the naked eye. And you might imagine, no, I'm sure I could see many, many more, right, Well, it might be that you've just seen these pictures either from Hubble, which we'll talk about in a minute, or from long exposure cameras which accumulate a lot of photons over time, so you can see things that are dimmer um. People have the impression, I think, from those photographs or from astronomical pictures from space telescopes, that there's a huge number of stars you can see in the sky, but those are not, of course, using the naked eye. The naked eye, you have to get a certain number of photons per second in order to register the star, and so very few stars a tiny fraction of stars in the universe satisfy the criterion of either being close enough or being bright enough that you get enough individual photons and your personal eyeball to see those stars, and people often use as a sort of metric. There's one particularly bright star in the sky. It's called Vega, and the threshold for being able to see a star using the naked eye for most humans is brightness of about zero point to five per cent the brightness of Vega. So anything brighter than that you can see. Anything dimmer than that you just can't see with the naked eye. You just don't get enough photons to even know it's there. I mean, it's there, and it's sending photons into space, but those photons are just not dense enough by the time they get here for you to spot one. Right, maybe you're standing between is like a photon over there and a photon over there, and none of them hit your eye, right, or maybe one of them hits your eye. But you need a certain number of photons. You need several photons for to really register something. Okay, but then you can soup it up. Right, you can say, well, I don't necessarily just need to use the naked eye. Certainly you can see more. We have tell lescopes, and that's exactly what telescopes are for. And even binoculars. Binoculars and telescopes, they don't actually do much zooming. Mostly the power of a telescope or binoculars is in gathering light. In the front of a telescope, you have a large lens, and that lens gathers more light than your eye, right, it's bigger than your eye. And this is why bigger telescopes are more powerful, because they gather more light and they focus it so you have a larger area to receive those photons, which means you're gonna get more photons, which means you're more likely to see something. Imagine you had a telescope the size of the Earth, right, you would gather a huge number of photons per second from some distant star compared to only the photons that fall on the tiny little spot on Earth that is your eyeball. So the power of a telescope or binoculars is frankly just to gather more light so you can see dimmer things, right, because it takes the light from a large area and it puts it onto a small area. So it's like multiplying the number of photons you're seeing from every source by some factor, and that makes a really big difference because it means that you can see sort of a larger sphere away from the Earth and that spear right. The radius of that sphere, the number of stars net fear grows very quickly with the radius, and so if you're multiplying the power of your eyeballs using a telescope, you can actually see up to about three hundred thousand stars in the sky. Right. You can go down to compare to Vega again, you can you can go down to a number of about zero point zero one percent of Vega. Vega is a particularly bright star in the sky, and you can see a lot more stars if you just use a telescope or binoculars. It's a huge multiplication factor. Now, another thing you can do, of course, is you can just put your camera on right, and you can turn it on, and you can leave it open for a while. And if you just leave a camera fixed and looking at the night sky, then you'll see these traces because stars move right there with its hours don't move, but the earth moves right the earth rotates relative to the star field. And so you take a picture and you just leave your camera fixed, and you'll notice the stars making this circular trail across the film. And so you see like this photon hit and then that photon hit, and then that photon hit. Each of those is a photon. It's taken its journey across the cosmos from the star to your film, from the star to your eyeballs. And so the power of the photography there is to again accumulate photons over time. And if you'd like to see a really crisp picture of the night sky, what you need to do is get a camera that moves with the stars right, that tracks the stars. Usually finds one guide star. It's got a little motor on it which will turn the camera so that it captures photons from the same star in the same place and sort of adds them up. And that's how you can do this long exposure photography that gives you the sense of seeing something really deep in space, because that's exactly what you're doing. The longer you look at the night sky, the more you accumulate the those photons, the more you're seeing stuff that's far away right where the photons are spread out more by the time they get here, so it takes longer to gather enough photons from them in order to see them in your eye or to see them in the camera. All right, but let's talk about what we can do with crazy telescopes and how deep we can see into the night sky using technology. But first let's take another break. All right, So we're talking about how many stars there are are in the sky and how many of them we can see. Now, of course, we're not limited to looking at the night sky with our eyeballs, and we're not limited to looking at the night sky with binoculars or dinky telescopes that we have in our backyard. As a species. We have invested incredible technology which floats out in space, so it's not even limited by the resolution of the atmosphere. It floats in space and it looks deep out into the universe to give us pictures. And of course I'm talking about the Hubble space telescope and a whole battery of other space telescopes which have given us amazing insights into what's going on in the universe. So how does that work, Well, it's the same story. All you need to do to see deeper into the universe is gather more light, and your options there are make your telescope bigger, and so Hubble is huge, right compared to your back backyard telescope. It's got a really big light gathering lens that gives it a multiplication. But Hubble also can point it stuff and you can just stay pointed at it for a while, and so we can gather enough light to see really distant objects. Imagine some galaxy from very early in the universe. It's super far away, right, it's billions and billions of light years, say ten billion light years away. Then the intensity of the light from that star here ten billion light years away is a tiny factor compared to the intensity. If you were like you know, one a like earth distance from that star, something like one over ten billion squared. In order to see that star, then you need to gather photons from it, and there just aren't very many. If you imagine a sphere centered at that star, and the radius of that sphere is our distance from it, we're sort of sitting out on a sphere that's that distance from the star. Then all the light from that star is spread out across this enormous sphere right that radius is ten billion light years. Then the photons are really dispersed, right, Not very many of them land in any particular area. So what you have to do is you have to wait a long time. You have to point Hubble at it for a while and just gather light. And this is why Hubble time is so valuable, because if you point Hubble at any random speck of space, the Hubble deep field, for example, is a tiny little patch of space that they pointed the camera at for a long time, long enough to gather light from super distant galaxies. And of course the longer they look, the more they see. Because the longer they look, the deeper into space they can see, and there's really no limit, right, Hubble can just point at something basically as long as we want. And since the universe has a finite age, is a maximum distance that anything can be that we could see. And there's really no limit on the brightness of something that we can see with Hubble, right, because we could just keep gathering light and wait until eventually photons hit us. But there is a limit. There is a limit to what Hubble can see. And that's because the universe is expanding. Things that are far away in the universe are moving away from us, and that expansion is accelerating. And all this means that that the light from those objects, because it's moving away from us, the wavelength of the light from those objects is shifted in color, called this red shifting, and things that are further away are moving away more quickly. So you can think about the red shift as a sort of measure of distance. And this is how astronomers talk about the distance to something. They say red shift four, red shift five, or ever. And this talks about how much the light is changed in color from where when it was admitted to by the time it gets here on Earth, and along the way it gets shifted down into the red end of the spectrum because things that are moving away from us have their wavelength stretched. And Hubble of course has a certain frequency of light that it can see. It can't just see photons throughout the entire electromagnetic spectrum. It's got a certain range of photons that can see because of the you know, the material that the lens is made out of, and the camera is sensitive. Right, There's no device that is sensitive across the entire electromagnetic spectrum. So there are some things that are so red shifted that Hubble can't see them. They're basically invisible to Hubble. No matter how long you point Hubble at these things, you just will never see them because the light that's getting here is at the wrong frequency. And that's why we have other space telescopes, right. We have the Spitzer Infrared Telescope, for example. It works in the infrared and specifically to see things that are super duper red shifted, right, because those are the things that are super duper far away. Now, the furthest thing ever seen by Hubble is a galaxy at red shift called seven point seven, which means twenty nine billion light years away. This is something that's so old that when it was made, this galaxy was made, the whole universe was just about six hundred million years old. I mean the universe basically a baby when this thing was born. This thing has been around for a super long time, and now it's really far away. That's the furthest thing the Hubble has ever seen. But remember Hubble can't see things that are super red shifted because it's out of the range of frequency that Hubble is sensitive to. So we use other cameras and we can see things that are out to redshift eleven point nine, which means more than thirty two billion light years away. The universe was a mere four hundred million years old when this thing was formed, So that's the record, a red shift eleven point nine, thirty two billion light years away. That's the furthest thing we've seen. And with our telescopes and with our cameras, and we can look really deep into the universe and we can see a huge number of objects. But even if we see these things that, first of all, we can't see the entire sky this way because looking this far into space seeing things that are this dim takes time, and Hubble just hasn't existed long enough to scan the entire sky at this resolution to spend enough time looking at every little patch of the sky in order to see this, So most of the night sky is unexamined. What we think of as the Hubble deep field, if you look it up, is actually a tiny little patch of the sky. Most of the night sky has photons coming to us from distant, old, ancient objects which might not even exist anymore, and nobody's looking at them right because we don't have enough Hubbles and it would take forever for Hubble to scan the entire sky. So most of the things that are out there, uncountably huge numbers of stars out there, nobody can see them, and nobody's even looking. But maybe one day we'll build enough telescopes, or we'll spend enough time to look all the way through the entire sky, we will be able to see all of those uncountably many objects. But even if you've made a pile of bananas, one banana per per star in the sky, I would still not be able to get my mind around the huge number of objects out there. So next time you're out there camping, look up at the night sky and try to count how many stars can you see? I think you'll be surprised to discover it's not actually that many. It's hard to count up to thousands and thousands of stars, and maybe you get bored before you get there. But remember it's a tiny fraction of the stars that are out there, and we're lucky that as humans we can build devices that let us peer deeper and deeper into the universe and unravel the secrets that those distant objects hold. Thanks for listening and tune in next time for more crazy facts about the universe. If you still have a question after listening to all these explanations, please drop us a line. We'd love to hear from you. You can find us at Facebook, Twitter, and Instagram at Daniel and Jorge That's one word, or email us at Feedback at Daniel and Jorge dot com. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of I Heart Radio. For more podcast from my Heart Radio, visit the I Heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.