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How many stars can you see in the night sky with the naked eye? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: What's the biggest number that you can really hold in
• 0:12
  Speaker 1: your mind. Let's do a little mental exercise. Start with
• 0:15
  Speaker 1: a number like five. Surely you can imagine like five,
• 0:18
  Speaker 1: I don't know, bananas floating in your mind, right, each
• 0:22
  Speaker 1: one crisp and the unique and different. It's a simple,
• 0:25
  Speaker 1: small number that's easy to visualize, all right, But can
• 0:28
  Speaker 1: you do ten? Can you keep ten individual bananas in
• 0:31
  Speaker 1: your head? Can you do a hundred? Now it starts
• 0:34
  Speaker 1: to get challenging, right, So imagine a thousand, a hundred thousand.
• 0:38
  Speaker 1: Can you imagine a billion individual bananas, all unique, floating
• 0:42
  Speaker 1: in your mind? Unless you're some sort of crazy genius,
• 0:45
  Speaker 1: probably not. But the universe throws these numbers at us
• 0:49
  Speaker 1: all the time, right, you hear millions, billions, trillions, numbers
• 0:53
  Speaker 1: that frankly sound made up. What do these numbers mean?
• 0:56
  Speaker 1: Can we really grasp them? Hi? I'm Daniel. I'm a
• 1:15
  Speaker 1: particle physicist and the co author of the book We
• 1:18
  Speaker 1: Have No Idea, A Guide to the Unknown Universe. My
• 1:21
  Speaker 1: co author in that book, Jorge cham, a cartoonist, is
• 1:25
  Speaker 1: not here today on the podcast, so welcome to the podcast.
• 1:29
  Speaker 1: Daniel and Jorge explain the universe, but without Jorge, who
• 1:33
  Speaker 1: has to be away. This week there's a production of
• 1:35
  Speaker 1: I Heart Media, a podcast in which we zoom all
• 1:38
  Speaker 1: around the universe trying to understand all the crazy, mind
• 1:42
  Speaker 1: blowing things that the universe has to offer. And the
• 1:45
  Speaker 1: universe is filled with things that are difficult to understand
• 1:48
  Speaker 1: but fun to struggle with. And one of those things
• 1:51
  Speaker 1: is numbers. Because the universe is vast, there are so
• 1:56
  Speaker 1: many things out there. They're huge numbers of stuff to
• 1:59
  Speaker 1: think about, to look at, to try to understand. I mean,
• 2:03
  Speaker 1: if you talk about just our galaxy, the Milky Way,
• 2:05
  Speaker 1: there are a hundred billion stars in the universe, how
• 2:09
  Speaker 1: do you comprehend that number? How do you know what
• 2:11
  Speaker 1: that number really means? Frankly, anything bigger than a thousand
• 2:16
  Speaker 1: is to meet infinity? Right, you talk about a million
• 2:18
  Speaker 1: dollars versus ten million dollars, What difference does that really
• 2:21
  Speaker 1: make to me? A billion dollars a trillion dollars. I
• 2:24
  Speaker 1: can never be an economist because to me, all these
• 2:26
  Speaker 1: numbers are ungraspable. They're hard to really put my head around,
• 2:31
  Speaker 1: to really wrap my fingers around and manipulating my mind.
• 2:34
  Speaker 1: I mean, sure, I can do the math with them
• 2:35
  Speaker 1: on paper, but I can't visualize them, and as a
• 2:38
  Speaker 1: visual thinker, it's difficult for me to understand what these
• 2:41
  Speaker 1: things mean. But yet the universe throws these numbers at us, right,
• 2:45
  Speaker 1: And you try to do science at this scale particle physics.
• 2:49
  Speaker 1: Every baseball is filled with ten to the twenty three
• 2:52
  Speaker 1: times some number of atoms. Right, It's hard to imagine.
• 2:55
  Speaker 1: Every time you're holding a banana or a baseball in
• 2:57
  Speaker 1: your hand, you're interacting with a ridiculously huge number of particles.
• 3:02
  Speaker 1: And so that's what we want to focus on today.
• 3:05
  Speaker 1: That's the topic of today's podcast, and most specifically today,
• 3:08
  Speaker 1: we're asking the question how many stars are there in
• 3:16
  Speaker 1: this sky? And of course there are a huge number
• 3:19
  Speaker 1: of stars out there in the Milky Way, right. A
• 3:20
  Speaker 1: hundred billion stars in the Milky Way times two trillion
• 3:24
  Speaker 1: galaxies makes a number that's like, really hard to even understand.
• 3:28
  Speaker 1: Two times ten to the twenty three stars in the
• 3:31
  Speaker 1: observable universe. I don't even know the name or that number, right,
• 3:35
  Speaker 1: somebody's have to make up the new prefix, the new
• 3:38
  Speaker 1: scientific phrase for that number, because it's so big that
• 3:41
  Speaker 1: all you can do is write it in scientific notation, right,
• 3:44
  Speaker 1: it's not even a number anybody ever actually sits down
• 3:46
  Speaker 1: and writes out with all those zeros, you know, we
• 3:49
  Speaker 1: need like a new way to write numbers, just to
• 3:51
  Speaker 1: talk about this kind of number, a number in this category.
• 3:56
  Speaker 1: And so that's a huge number, right, two times ten
• 3:58
  Speaker 1: of the twenty three. But there are lots of big
• 4:00
  Speaker 1: numbers in science, not just in physics. I mean, if
• 4:04
  Speaker 1: somebody asked you, what are there more of stars in
• 4:07
  Speaker 1: the Milky Way or trees on Earth? You might think, well,
• 4:10
  Speaker 1: of course there are more stars in the Milky Way.
• 4:12
  Speaker 1: That's a huge, ridiculous number compared to this tiny little
• 4:16
  Speaker 1: planet we find ourselves on, right, But not true. It
• 4:19
  Speaker 1: turns out that there are three trillion trees on Earth,
• 4:24
  Speaker 1: three trillion, which is more than the hundred billion stars
• 4:28
  Speaker 1: in the Milky Way. Right, So there are numbers all
• 4:31
  Speaker 1: over the place, not just in physics, but also in biology. Here,
• 4:34
  Speaker 1: just on this one planet, there are more of those
• 4:36
  Speaker 1: trees than all the stars in our galaxy. And if
• 4:40
  Speaker 1: you look inside our body, there are forty trillion cells
• 4:44
  Speaker 1: that make up a human body. Every single one of
• 4:46
  Speaker 1: you walking around, squishing and breathing and living, has a
• 4:50
  Speaker 1: huge number of these things all cooperating to make you
• 4:53
  Speaker 1: who you are. So there are more cells in the
• 4:55
  Speaker 1: body than trees on Earth, and more trees on Earth
• 4:59
  Speaker 1: than ours in the galaxy. And you can keep going.
• 5:02
  Speaker 1: The number of grains of sand on all the beaches
• 5:06
  Speaker 1: and all the deserts on the Earth is an even
• 5:07
  Speaker 1: bigger number. That's four times ten to the nineteen. So
• 5:12
  Speaker 1: for every tree on Earth, for every cell in the body,
• 5:16
  Speaker 1: there are millions of grains of sand. And this is
• 5:19
  Speaker 1: what I talk about not being able to grasp a number.
• 5:22
  Speaker 1: Like you walk on a beach. You see all that sand,
• 5:25
  Speaker 1: but you can't count it. If somebody drops a handful
• 5:28
  Speaker 1: of objects on the sand, you can say, oh, look,
• 5:29
  Speaker 1: there's your car, keys and your wallet and your phone.
• 5:32
  Speaker 1: There are three things there, right, that makes sense. You
• 5:34
  Speaker 1: can look at three things and count them instantly. You
• 5:36
  Speaker 1: don't actually have to count. You just sort of recognize
• 5:39
  Speaker 1: the numeracy there. You don't have to actually individually count.
• 5:43
  Speaker 1: But if you wanted to count the number of grains
• 5:45
  Speaker 1: of sand on the beach under your feet, you'd have
• 5:47
  Speaker 1: to actually count them. I mean, the information is there,
• 5:50
  Speaker 1: it's coming into your eyes. Even just the number of
• 5:53
  Speaker 1: grains that you see the number of grains that are
• 5:55
  Speaker 1: visible to your eyes when you're looking at the beach.
• 5:59
  Speaker 1: That imprinciple, you have that information coming into your eyeballs.
• 6:02
  Speaker 1: But of course nobody can count. That it's too big
• 6:05
  Speaker 1: a number to grasp. And so that's what fascinates me,
• 6:08
  Speaker 1: that that there are these numbers out there, that they're everywhere,
• 6:11
  Speaker 1: that they're all around us, and so you might think, well,
• 6:13
  Speaker 1: that's certainly a big number. Two times ten of the
• 6:15
  Speaker 1: nineteen is a huge number of grains of sand. It's
• 6:18
  Speaker 1: hard to imagine. It also means that there are more
• 6:20
  Speaker 1: grains of sand than stars in the galaxy, which means
• 6:24
  Speaker 1: that for every star out there, all those individual balls
• 6:27
  Speaker 1: of plasma, there are more than a million grains of
• 6:30
  Speaker 1: sand on Earth. It's hard to really grasp these ratios.
• 6:33
  Speaker 1: But of course I'm married to a biologist, and the
• 6:36
  Speaker 1: king of big numbers turns out to not be physics.
• 6:39
  Speaker 1: The king of big numbers is biology, and specifically microbes.
• 6:44
  Speaker 1: And of course I'm married to a microbiologist. So she
• 6:46
  Speaker 1: reminded me that there are ten to the thirty one
• 6:49
  Speaker 1: pages in the biome. That means there are ten to
• 6:53
  Speaker 1: the thirty one little viruses out there attacking bacteria. And
• 6:56
  Speaker 1: that's a ridiculous number. That's bigger than all the stars
• 7:00
  Speaker 1: in the observable universe. That was ten to the twenty three.
• 7:03
  Speaker 1: So for every star in every single one of those galaxies,
• 7:07
  Speaker 1: there are like a billion viruses here on Earth. To
• 7:11
  Speaker 1: count all the stars in our galaxy and all the
• 7:13
  Speaker 1: stars and other galaxies is just a ridiculous number. And
• 7:17
  Speaker 1: for each of those there's a billion, which already is
• 7:20
  Speaker 1: hard to grasp. A billion viruses here on Earth. So
• 7:24
  Speaker 1: science is filled with these big numbers. But I was
• 7:27
  Speaker 1: interested in something more specific. You take a drive out
• 7:30
  Speaker 1: to the desert, you look up at the night sky,
• 7:32
  Speaker 1: and you're confronted with enormity. You look up at the
• 7:35
  Speaker 1: night sky, you see all these twinkling lights, You see
• 7:38
  Speaker 1: all these stars, and you wonder, wow, how many are there?
• 7:42
  Speaker 1: Certainly seems like a lot, right. We're used to science
• 7:45
  Speaker 1: and physics having big numbers in it. We know the
• 7:48
  Speaker 1: universe has vast and so we imagine we can see
• 7:51
  Speaker 1: a lot of stars. Turns out we might not be
• 7:54
  Speaker 1: able to see that many. So I was curious what
• 7:56
  Speaker 1: people knew about this, what people thought about the number
• 7:59
  Speaker 1: of stars in the night sky. So I walked around
• 8:02
  Speaker 1: the campus if you see Irvine, and I asked, folks,
• 8:04
  Speaker 1: how many stars can you see in the night sky
• 8:07
  Speaker 1: on a beautiful, crystal clear night, right, no pollution, no clouds, um,
• 8:12
  Speaker 1: no moon, no light from Los Angeles? How many stars
• 8:15
  Speaker 1: can you see in the sky? But first think for yourself,
• 8:19
  Speaker 1: if you've been out on a nice clear night, how
• 8:21
  Speaker 1: many stars do you think you can see in the
• 8:23
  Speaker 1: night sky. Here's what people had to say. Millions. Okay,
• 8:28
  Speaker 1: I know there is many more in the universe. I
• 8:30
  Speaker 1: even know how many there are in the universe. Tend
• 8:34
  Speaker 1: to the five between a billion and a trillion roughly,
• 8:39
  Speaker 1: Let's see. I would say at least Avocata's number, like
• 8:43
  Speaker 1: tend toy three. I don't know I would I would
• 8:46
  Speaker 1: guess less than that, maybe like time stend of the
• 8:50
  Speaker 1: ten power, but I don't know. I'm gonna say tend
• 8:53
  Speaker 1: to the twenty like a million guests somewhere in the billions.
• 8:56
  Speaker 1: But I have no idea. All right, So you hear
• 8:59
  Speaker 1: a lot of big number is people definitely feel like
• 9:02
  Speaker 1: it's got to be a big number because they know
• 9:04
  Speaker 1: there are a lot of stars out there and they're right,
• 9:06
  Speaker 1: there are a lot of stars out there in the sky.
• 9:08
  Speaker 1: That doesn't necessarily mean that you can see all of those, right,
• 9:12
  Speaker 1: or even that you can see very many, but people
• 9:15
  Speaker 1: wanted to say a big number. You hear millions and
• 9:17
  Speaker 1: billions and ten to the ten, and so people want
• 9:20
  Speaker 1: we're prepared to be saying a big number. And maybe
• 9:23
  Speaker 1: it's a bit of a gotcha question because a lot
• 9:25
  Speaker 1: of times you'll be asked a question about science and
• 9:27
  Speaker 1: the answer is supposed to impress you, right, the answer
• 9:30
  Speaker 1: is supposed to be so much bigger than you even imagined.
• 9:33
  Speaker 1: So I don't know, maybe people were rounding up a
• 9:35
  Speaker 1: little bit, trying to make sure they hit an impressive number.
• 9:38
  Speaker 1: We'll break it down and talk about how many stars
• 9:41
  Speaker 1: you can actually see in the night sky, but first
• 9:44
  Speaker 1: let's take a quick break. Alright, So we're talking about
• 9:59
  Speaker 1: how any stars you can see in the night sky. Now,
• 10:03
  Speaker 1: first let's start out with how many stars there are. Right,
• 10:05
  Speaker 1: we talked about this quickly already, But in the observable universe,
• 10:09
  Speaker 1: there are ten to the twenty three stars out there,
• 10:12
  Speaker 1: basically a trillion times a hundred billion. It's a huge
• 10:15
  Speaker 1: number of stars. However, most of the stars are really
• 10:19
  Speaker 1: really far away. So let's talk about the light from
• 10:21
  Speaker 1: the star. The stars this huge burning ball of plasma
• 10:24
  Speaker 1: and it's shooting out a lot of light. Our Sun,
• 10:26
  Speaker 1: for example, is not a particularly bright star, but already
• 10:30
  Speaker 1: it's pretty bright. You know, it's shooting out a huge
• 10:32
  Speaker 1: number of photons. If you look up at the Sun,
• 10:35
  Speaker 1: please don't, then you'll burn your eyeballs. Right, and we're
• 10:38
  Speaker 1: not even that close to it. So the Sun is
• 10:40
  Speaker 1: pretty bright, which means the stars out there must be
• 10:42
  Speaker 1: pretty bright. The problem is, in order to see something,
• 10:46
  Speaker 1: it has to be either pretty close or super duper bright,
• 10:50
  Speaker 1: because the brightness of an object falls really quickly with distance.
• 10:56
  Speaker 1: Imagine all the photons leaving the Sun, right, there's a
• 10:59
  Speaker 1: huge number of them. They're screaming out into space, ready
• 11:03
  Speaker 1: to go on some cosmic journey and land on some
• 11:05
  Speaker 1: alien eyeballs. Right, So all these photons are leaving the Sun.
• 11:09
  Speaker 1: But they're leaving the Sun and there's a fixed number
• 11:11
  Speaker 1: seven gajillion, I don't know, some certain number of photons
• 11:13
  Speaker 1: are leaving the Sun. So if you're really close to
• 11:16
  Speaker 1: the Sun, then your eyeball is going to get hit
• 11:18
  Speaker 1: with a lot of those photons. But now take a
• 11:20
  Speaker 1: step away, take go a thousand kilometers further, you have
• 11:23
  Speaker 1: the same number of photons. Right, it's not adding any
• 11:26
  Speaker 1: more photons. I mean there's another wave of photons coming
• 11:29
  Speaker 1: along behind it, but in that one blast of light
• 11:32
  Speaker 1: that we're talking about, you don't get any fresh photons.
• 11:36
  Speaker 1: These same photons continue out, but now they cover a
• 11:39
  Speaker 1: larger area. That cover a sphere that has a larger radius.
• 11:43
  Speaker 1: And the area of a sphere grows with the radius squared,
• 11:47
  Speaker 1: and so as the radius of that sphere that the
• 11:49
  Speaker 1: photons are trying to illuminate grows, the number of photons
• 11:54
  Speaker 1: per area drops like one over distance squared one over
• 11:58
  Speaker 1: radius squared. So if you're twice as far from the star,
• 12:02
  Speaker 1: the brightness is one fourth. If you're four times the
• 12:05
  Speaker 1: distance from the star, then the brightness is one And
• 12:09
  Speaker 1: that doesn't seem like a big effect for small numbers,
• 12:11
  Speaker 1: but it adds to pretty quickly. If you're a thousand
• 12:14
  Speaker 1: times further away, then the brightness goes down by a million,
• 12:17
  Speaker 1: And those are pretty big numbers, and so they can
• 12:20
  Speaker 1: impact even really bright stuff like stars. So what that
• 12:23
  Speaker 1: means is that for us to see a star. Right,
• 12:26
  Speaker 1: we have to be able to see a certain number
• 12:28
  Speaker 1: of photons. When you look up at the night sky,
• 12:30
  Speaker 1: you turn your eyes to the sky. You have to
• 12:33
  Speaker 1: get photons from that star. And already that's a fascinating journey.
• 12:37
  Speaker 1: You know, those photons were created in the heat of
• 12:40
  Speaker 1: that cosmic fusion billions of miles away and have flown
• 12:44
  Speaker 1: through space for thousands or millions or billions of years
• 12:47
  Speaker 1: before they hit your eyeball. They've lived all that time,
• 12:50
  Speaker 1: and then they're just sucked up by your eyeball and
• 12:53
  Speaker 1: are no more. Right, you just take that little bit
• 12:55
  Speaker 1: of energy. Anyway, for you to see a star in
• 12:58
  Speaker 1: the sky means that a photon is left that star
• 13:01
  Speaker 1: and arrived at your eyeball. And your eyeballs are pretty good, right,
• 13:04
  Speaker 1: they're pretty good detectors. But you need to see a
• 13:06
  Speaker 1: few photons in order to register a star. So there's
• 13:10
  Speaker 1: a threshold. All the stars are out there, but in
• 13:13
  Speaker 1: order for you to see them with your eye, in
• 13:14
  Speaker 1: order for you to say, hey, I see a star,
• 13:16
  Speaker 1: you have to get a certain number of photons, and
• 13:19
  Speaker 1: so that cuts down on the number of stars that
• 13:22
  Speaker 1: are visible to the naked eye by a huge fraction.
• 13:25
  Speaker 1: If they're really far away, then that have to be
• 13:27
  Speaker 1: super duper bright for you to still get enough photons, right,
• 13:31
  Speaker 1: because remember the density of photons that are hitting your
• 13:34
  Speaker 1: eye from that star falls with a distance squared, So
• 13:37
  Speaker 1: if it's super far away, then has to be ridiculously bright,
• 13:40
  Speaker 1: which is why it's so amazing sometimes when we see
• 13:43
  Speaker 1: things like quasars, which we know are really far away
• 13:46
  Speaker 1: and yet still seem bright, which means at their source
• 13:49
  Speaker 1: they're riduculously bright, or if they're closer they don't have
• 13:52
  Speaker 1: to be quite as bright. But being closer is a
• 13:54
  Speaker 1: really small fraction of the universe. So that's cut to
• 13:58
  Speaker 1: the chase. In the end, you can see the only
• 14:00
  Speaker 1: between five to ten thousand stars in the night sky
• 14:04
  Speaker 1: with the human eye, and of course you can't see
• 14:07
  Speaker 1: the entire night sky from earth right, you can at
• 14:10
  Speaker 1: best see half of it, So cut that number in half.
• 14:13
  Speaker 1: We're talking just a few thousand stars in the night
• 14:17
  Speaker 1: sky that you can see with the naked eye. And
• 14:20
  Speaker 1: you might imagine, no, I'm sure I could see many,
• 14:22
  Speaker 1: many more, right, Well, it might be that you've just
• 14:24
  Speaker 1: seen these pictures either from Hubble, which we'll talk about
• 14:27
  Speaker 1: in a minute, or from long exposure cameras which accumulate
• 14:30
  Speaker 1: a lot of photons over time, so you can see
• 14:33
  Speaker 1: things that are dimmer um. People have the impression, I think,
• 14:36
  Speaker 1: from those photographs or from astronomical pictures from space telescopes,
• 14:41
  Speaker 1: that there's a huge number of stars you can see
• 14:43
  Speaker 1: in the sky, but those are not, of course, using
• 14:46
  Speaker 1: the naked eye. The naked eye, you have to get
• 14:48
  Speaker 1: a certain number of photons per second in order to
• 14:51
  Speaker 1: register the star, and so very few stars a tiny
• 14:55
  Speaker 1: fraction of stars in the universe satisfy the criterion of
• 14:58
  Speaker 1: either being close enough or being bright enough that you
• 15:02
  Speaker 1: get enough individual photons and your personal eyeball to see
• 15:06
  Speaker 1: those stars, and people often use as a sort of metric.
• 15:10
  Speaker 1: There's one particularly bright star in the sky. It's called Vega,
• 15:13
  Speaker 1: and the threshold for being able to see a star
• 15:16
  Speaker 1: using the naked eye for most humans is brightness of
• 15:19
  Speaker 1: about zero point to five per cent the brightness of Vega.
• 15:23
  Speaker 1: So anything brighter than that you can see. Anything dimmer
• 15:26
  Speaker 1: than that you just can't see with the naked eye.
• 15:28
  Speaker 1: You just don't get enough photons to even know it's there.
• 15:31
  Speaker 1: I mean, it's there, and it's sending photons into space,
• 15:35
  Speaker 1: but those photons are just not dense enough by the
• 15:37
  Speaker 1: time they get here for you to spot one. Right,
• 15:39
  Speaker 1: maybe you're standing between is like a photon over there
• 15:41
  Speaker 1: and a photon over there, and none of them hit
• 15:43
  Speaker 1: your eye, right, or maybe one of them hits your eye.
• 15:46
  Speaker 1: But you need a certain number of photons. You need
• 15:48
  Speaker 1: several photons for to really register something. Okay, but then
• 15:52
  Speaker 1: you can soup it up. Right, you can say, well,
• 15:54
  Speaker 1: I don't necessarily just need to use the naked eye.
• 15:58
  Speaker 1: Certainly you can see more. We have tell lescopes, and
• 16:00
  Speaker 1: that's exactly what telescopes are for. And even binoculars. Binoculars
• 16:04
  Speaker 1: and telescopes, they don't actually do much zooming. Mostly the
• 16:08
  Speaker 1: power of a telescope or binoculars is in gathering light.
• 16:13
  Speaker 1: In the front of a telescope, you have a large lens,
• 16:15
  Speaker 1: and that lens gathers more light than your eye, right,
• 16:18
  Speaker 1: it's bigger than your eye. And this is why bigger
• 16:22
  Speaker 1: telescopes are more powerful, because they gather more light and
• 16:24
  Speaker 1: they focus it so you have a larger area to
• 16:28
  Speaker 1: receive those photons, which means you're gonna get more photons,
• 16:31
  Speaker 1: which means you're more likely to see something. Imagine you
• 16:34
  Speaker 1: had a telescope the size of the Earth, right, you
• 16:37
  Speaker 1: would gather a huge number of photons per second from
• 16:40
  Speaker 1: some distant star compared to only the photons that fall
• 16:44
  Speaker 1: on the tiny little spot on Earth that is your eyeball.
• 16:47
  Speaker 1: So the power of a telescope or binoculars is frankly
• 16:51
  Speaker 1: just to gather more light so you can see dimmer things, right,
• 16:54
  Speaker 1: because it takes the light from a large area and
• 16:57
  Speaker 1: it puts it onto a small area. So it's like
• 17:00
  Speaker 1: multiplying the number of photons you're seeing from every source
• 17:03
  Speaker 1: by some factor, and that makes a really big difference
• 17:08
  Speaker 1: because it means that you can see sort of a
• 17:09
  Speaker 1: larger sphere away from the Earth and that spear right.
• 17:13
  Speaker 1: The radius of that sphere, the number of stars net
• 17:16
  Speaker 1: fear grows very quickly with the radius, and so if
• 17:19
  Speaker 1: you're multiplying the power of your eyeballs using a telescope,
• 17:23
  Speaker 1: you can actually see up to about three hundred thousand
• 17:26
  Speaker 1: stars in the sky. Right. You can go down to
• 17:29
  Speaker 1: compare to Vega again, you can you can go down
• 17:32
  Speaker 1: to a number of about zero point zero one percent
• 17:35
  Speaker 1: of Vega. Vega is a particularly bright star in the sky,
• 17:38
  Speaker 1: and you can see a lot more stars if you
• 17:41
  Speaker 1: just use a telescope or binoculars. It's a huge multiplication factor. Now,
• 17:45
  Speaker 1: another thing you can do, of course, is you can
• 17:47
  Speaker 1: just put your camera on right, and you can turn
• 17:50
  Speaker 1: it on, and you can leave it open for a while.
• 17:52
  Speaker 1: And if you just leave a camera fixed and looking
• 17:55
  Speaker 1: at the night sky, then you'll see these traces because
• 17:58
  Speaker 1: stars move right there with its hours don't move, but
• 18:00
  Speaker 1: the earth moves right the earth rotates relative to the
• 18:03
  Speaker 1: star field. And so you take a picture and you
• 18:06
  Speaker 1: just leave your camera fixed, and you'll notice the stars
• 18:09
  Speaker 1: making this circular trail across the film. And so you
• 18:13
  Speaker 1: see like this photon hit and then that photon hit,
• 18:15
  Speaker 1: and then that photon hit. Each of those is a photon.
• 18:18
  Speaker 1: It's taken its journey across the cosmos from the star
• 18:22
  Speaker 1: to your film, from the star to your eyeballs. And
• 18:26
  Speaker 1: so the power of the photography there is to again
• 18:28
  Speaker 1: accumulate photons over time. And if you'd like to see
• 18:32
  Speaker 1: a really crisp picture of the night sky, what you
• 18:34
  Speaker 1: need to do is get a camera that moves with
• 18:37
  Speaker 1: the stars right, that tracks the stars. Usually finds one
• 18:40
  Speaker 1: guide star. It's got a little motor on it which
• 18:42
  Speaker 1: will turn the camera so that it captures photons from
• 18:46
  Speaker 1: the same star in the same place and sort of
• 18:48
  Speaker 1: adds them up. And that's how you can do this
• 18:50
  Speaker 1: long exposure photography that gives you the sense of seeing
• 18:53
  Speaker 1: something really deep in space, because that's exactly what you're doing.
• 18:57
  Speaker 1: The longer you look at the night sky, the more
• 18:59
  Speaker 1: you accumulate the those photons, the more you're seeing stuff
• 19:02
  Speaker 1: that's far away right where the photons are spread out
• 19:05
  Speaker 1: more by the time they get here, so it takes
• 19:07
  Speaker 1: longer to gather enough photons from them in order to
• 19:10
  Speaker 1: see them in your eye or to see them in
• 19:12
  Speaker 1: the camera. All right, but let's talk about what we
• 19:15
  Speaker 1: can do with crazy telescopes and how deep we can
• 19:18
  Speaker 1: see into the night sky using technology. But first let's
• 19:22
  Speaker 1: take another break. All right, So we're talking about how
• 19:37
  Speaker 1: many stars there are are in the sky and how
• 19:39
  Speaker 1: many of them we can see. Now, of course, we're
• 19:42
  Speaker 1: not limited to looking at the night sky with our eyeballs,
• 19:45
  Speaker 1: and we're not limited to looking at the night sky
• 19:47
  Speaker 1: with binoculars or dinky telescopes that we have in our backyard.
• 19:51
  Speaker 1: As a species. We have invested incredible technology which floats
• 19:55
  Speaker 1: out in space, so it's not even limited by the
• 19:57
  Speaker 1: resolution of the atmosphere. It floats in space and it
• 20:00
  Speaker 1: looks deep out into the universe to give us pictures.
• 20:03
  Speaker 1: And of course I'm talking about the Hubble space telescope
• 20:06
  Speaker 1: and a whole battery of other space telescopes which have
• 20:09
  Speaker 1: given us amazing insights into what's going on in the universe.
• 20:12
  Speaker 1: So how does that work, Well, it's the same story.
• 20:15
  Speaker 1: All you need to do to see deeper into the
• 20:17
  Speaker 1: universe is gather more light, and your options there are
• 20:20
  Speaker 1: make your telescope bigger, and so Hubble is huge, right
• 20:23
  Speaker 1: compared to your back backyard telescope. It's got a really
• 20:26
  Speaker 1: big light gathering lens that gives it a multiplication. But
• 20:31
  Speaker 1: Hubble also can point it stuff and you can just
• 20:33
  Speaker 1: stay pointed at it for a while, and so we
• 20:36
  Speaker 1: can gather enough light to see really distant objects. Imagine
• 20:41
  Speaker 1: some galaxy from very early in the universe. It's super
• 20:44
  Speaker 1: far away, right, it's billions and billions of light years,
• 20:46
  Speaker 1: say ten billion light years away. Then the intensity of
• 20:50
  Speaker 1: the light from that star here ten billion light years
• 20:53
  Speaker 1: away is a tiny factor compared to the intensity. If
• 20:58
  Speaker 1: you were like you know, one a like earth distance
• 21:01
  Speaker 1: from that star, something like one over ten billion squared.
• 21:06
  Speaker 1: In order to see that star, then you need to
• 21:09
  Speaker 1: gather photons from it, and there just aren't very many.
• 21:12
  Speaker 1: If you imagine a sphere centered at that star, and
• 21:14
  Speaker 1: the radius of that sphere is our distance from it,
• 21:17
  Speaker 1: we're sort of sitting out on a sphere that's that
• 21:20
  Speaker 1: distance from the star. Then all the light from that
• 21:23
  Speaker 1: star is spread out across this enormous sphere right that
• 21:26
  Speaker 1: radius is ten billion light years. Then the photons are
• 21:30
  Speaker 1: really dispersed, right, Not very many of them land in
• 21:33
  Speaker 1: any particular area. So what you have to do is
• 21:36
  Speaker 1: you have to wait a long time. You have to
• 21:37
  Speaker 1: point Hubble at it for a while and just gather light.
• 21:41
  Speaker 1: And this is why Hubble time is so valuable, because
• 21:43
  Speaker 1: if you point Hubble at any random speck of space,
• 21:46
  Speaker 1: the Hubble deep field, for example, is a tiny little
• 21:49
  Speaker 1: patch of space that they pointed the camera at for
• 21:51
  Speaker 1: a long time, long enough to gather light from super
• 21:54
  Speaker 1: distant galaxies. And of course the longer they look, the
• 21:58
  Speaker 1: more they see. Because the longer they look, the deeper
• 22:01
  Speaker 1: into space they can see, and there's really no limit, right,
• 22:04
  Speaker 1: Hubble can just point at something basically as long as
• 22:07
  Speaker 1: we want. And since the universe has a finite age,
• 22:10
  Speaker 1: is a maximum distance that anything can be that we
• 22:14
  Speaker 1: could see. And there's really no limit on the brightness
• 22:17
  Speaker 1: of something that we can see with Hubble, right, because
• 22:20
  Speaker 1: we could just keep gathering light and wait until eventually
• 22:23
  Speaker 1: photons hit us. But there is a limit. There is
• 22:25
  Speaker 1: a limit to what Hubble can see. And that's because
• 22:29
  Speaker 1: the universe is expanding. Things that are far away in
• 22:32
  Speaker 1: the universe are moving away from us, and that expansion
• 22:35
  Speaker 1: is accelerating. And all this means that that the light
• 22:38
  Speaker 1: from those objects, because it's moving away from us, the
• 22:41
  Speaker 1: wavelength of the light from those objects is shifted in color,
• 22:45
  Speaker 1: called this red shifting, and things that are further away
• 22:48
  Speaker 1: are moving away more quickly. So you can think about
• 22:51
  Speaker 1: the red shift as a sort of measure of distance.
• 22:54
  Speaker 1: And this is how astronomers talk about the distance to something.
• 22:57
  Speaker 1: They say red shift four, red shift five, or ever.
• 23:00
  Speaker 1: And this talks about how much the light is changed
• 23:03
  Speaker 1: in color from where when it was admitted to by
• 23:06
  Speaker 1: the time it gets here on Earth, and along the
• 23:08
  Speaker 1: way it gets shifted down into the red end of
• 23:11
  Speaker 1: the spectrum because things that are moving away from us
• 23:14
  Speaker 1: have their wavelength stretched. And Hubble of course has a
• 23:17
  Speaker 1: certain frequency of light that it can see. It can't
• 23:20
  Speaker 1: just see photons throughout the entire electromagnetic spectrum. It's got
• 23:24
  Speaker 1: a certain range of photons that can see because of
• 23:27
  Speaker 1: the you know, the material that the lens is made
• 23:29
  Speaker 1: out of, and the camera is sensitive. Right, There's no
• 23:32
  Speaker 1: device that is sensitive across the entire electromagnetic spectrum. So
• 23:36
  Speaker 1: there are some things that are so red shifted that
• 23:39
  Speaker 1: Hubble can't see them. They're basically invisible to Hubble. No
• 23:42
  Speaker 1: matter how long you point Hubble at these things, you
• 23:45
  Speaker 1: just will never see them because the light that's getting
• 23:48
  Speaker 1: here is at the wrong frequency. And that's why we
• 23:52
  Speaker 1: have other space telescopes, right. We have the Spitzer Infrared Telescope,
• 23:56
  Speaker 1: for example. It works in the infrared and specifically to
• 23:59
  Speaker 1: see things that are super duper red shifted, right, because
• 24:03
  Speaker 1: those are the things that are super duper far away. Now,
• 24:06
  Speaker 1: the furthest thing ever seen by Hubble is a galaxy
• 24:10
  Speaker 1: at red shift called seven point seven, which means twenty
• 24:14
  Speaker 1: nine billion light years away. This is something that's so
• 24:17
  Speaker 1: old that when it was made, this galaxy was made,
• 24:21
  Speaker 1: the whole universe was just about six hundred million years old.
• 24:24
  Speaker 1: I mean the universe basically a baby when this thing
• 24:27
  Speaker 1: was born. This thing has been around for a super
• 24:30
  Speaker 1: long time, and now it's really far away. That's the
• 24:33
  Speaker 1: furthest thing the Hubble has ever seen. But remember Hubble
• 24:36
  Speaker 1: can't see things that are super red shifted because it's
• 24:39
  Speaker 1: out of the range of frequency that Hubble is sensitive to.
• 24:43
  Speaker 1: So we use other cameras and we can see things
• 24:45
  Speaker 1: that are out to redshift eleven point nine, which means
• 24:48
  Speaker 1: more than thirty two billion light years away. The universe
• 24:53
  Speaker 1: was a mere four hundred million years old when this
• 24:56
  Speaker 1: thing was formed, So that's the record, a red shift
• 24:59
  Speaker 1: eleven point nine, thirty two billion light years away. That's
• 25:03
  Speaker 1: the furthest thing we've seen. And with our telescopes and
• 25:08
  Speaker 1: with our cameras, and we can look really deep into
• 25:10
  Speaker 1: the universe and we can see a huge number of objects.
• 25:13
  Speaker 1: But even if we see these things that, first of all,
• 25:15
  Speaker 1: we can't see the entire sky this way because looking
• 25:18
  Speaker 1: this far into space seeing things that are this dim
• 25:21
  Speaker 1: takes time, and Hubble just hasn't existed long enough to
• 25:25
  Speaker 1: scan the entire sky at this resolution to spend enough
• 25:29
  Speaker 1: time looking at every little patch of the sky in
• 25:32
  Speaker 1: order to see this, So most of the night sky
• 25:36
  Speaker 1: is unexamined. What we think of as the Hubble deep field,
• 25:39
  Speaker 1: if you look it up, is actually a tiny little
• 25:41
  Speaker 1: patch of the sky. Most of the night sky has
• 25:44
  Speaker 1: photons coming to us from distant, old, ancient objects which
• 25:49
  Speaker 1: might not even exist anymore, and nobody's looking at them
• 25:52
  Speaker 1: right because we don't have enough Hubbles and it would
• 25:54
  Speaker 1: take forever for Hubble to scan the entire sky. So
• 25:58
  Speaker 1: most of the things that are out there, uncountably huge
• 26:01
  Speaker 1: numbers of stars out there, nobody can see them, and
• 26:04
  Speaker 1: nobody's even looking. But maybe one day we'll build enough telescopes,
• 26:09
  Speaker 1: or we'll spend enough time to look all the way
• 26:11
  Speaker 1: through the entire sky, we will be able to see
• 26:14
  Speaker 1: all of those uncountably many objects. But even if you've
• 26:18
  Speaker 1: made a pile of bananas, one banana per per star
• 26:21
  Speaker 1: in the sky, I would still not be able to
• 26:23
  Speaker 1: get my mind around the huge number of objects out there.
• 26:27
  Speaker 1: So next time you're out there camping, look up at
• 26:29
  Speaker 1: the night sky and try to count how many stars
• 26:32
  Speaker 1: can you see? I think you'll be surprised to discover
• 26:35
  Speaker 1: it's not actually that many. It's hard to count up
• 26:37
  Speaker 1: to thousands and thousands of stars, and maybe you get
• 26:40
  Speaker 1: bored before you get there. But remember it's a tiny
• 26:43
  Speaker 1: fraction of the stars that are out there, and we're
• 26:45
  Speaker 1: lucky that as humans we can build devices that let
• 26:48
  Speaker 1: us peer deeper and deeper into the universe and unravel
• 26:51
  Speaker 1: the secrets that those distant objects hold. Thanks for listening
• 26:55
  Speaker 1: and tune in next time for more crazy facts about
• 26:58
  Speaker 1: the universe. If you still have a question after listening
• 27:09
  Speaker 1: to all these explanations, please drop us a line. We'd
• 27:12
  Speaker 1: love to hear from you. You can find us at Facebook, Twitter,
• 27:15
  Speaker 1: and Instagram at Daniel and Jorge That's one word, or
• 27:19
  Speaker 1: email us at Feedback at Daniel and Jorge dot com.
• 27:23
  Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain
• 27:26
  Speaker 1: the Universe is a production of I Heart Radio. For
• 27:29
  Speaker 1: more podcast from my Heart Radio, visit the I Heart
• 27:32
  Speaker 1: Radio app, Apple Podcasts, or wherever you listen to your
• 27:35
  Speaker 1: favorite shows.