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How do we know the mass of stars? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Did you ever eat astronauts ice cream? As a kid,
0:11
Speaker 1: that stuff seemed so exciting, but when you put it
0:15
Speaker 1: in your mouth, it was always kind of gross and disappointing.
0:18
Speaker 1: I remember my brothers and I wondering if it was
0:20
Speaker 1: made four astronauts or maybe like made out of astronauts either.
0:25
Speaker 1: For hey, it makes me wonder about something. You know,
0:27
Speaker 1: how if you're on Earth and you eat too much
0:30
Speaker 1: ice cream, you gain weight? Right, Well, if you're in space,
0:34
Speaker 1: can you eat all the ice cream you want without
0:37
Speaker 1: gaining weight? I think I just invented the world's most
0:41
Speaker 1: expensive diet. For a hundred million dollars, or the price
0:45
Speaker 1: of a ticket to the space station, you can eat
0:47
Speaker 1: all the ice cream you want and still lose weight.
1:06
Speaker 1: I'm Daniel, I'm a particle physicist, and I'm not here
1:10
Speaker 1: to sell you some crazy weight lost scheme. No. Instead,
1:14
Speaker 1: Welcome to the podcast Daniel and Jorge Explain the Universe,
1:18
Speaker 1: a production of I Heart Radio. In our podcast, we
1:22
Speaker 1: try to explain to you everything we do know about
1:24
Speaker 1: the universe and everything we don't know about the universe.
1:28
Speaker 1: We bring you to the forefront of science and introduce
1:31
Speaker 1: you to the questions that scientists are asking today the
1:34
Speaker 1: things that are puzzling physicists that are making them go,
1:37
Speaker 1: what how does that work? Because we think that everybody
1:41
Speaker 1: deserves to be out there on the forefront of knowledge,
1:44
Speaker 1: wondering what the rest of humanity how there's amazing and
1:47
Speaker 1: crazy and bonkers universe is put together. Unraveling that mystery.
1:52
Speaker 1: Revealing the secrets of the universe, to me, seems like
1:55
Speaker 1: the grandest journey of humanity. And it's amazing that it
1:58
Speaker 1: even works, that we can by our minds and figure
2:01
Speaker 1: out the way the universe works. And as we look
2:05
Speaker 1: out into the cosmos, we see incredible stuff out there.
2:09
Speaker 1: And so our podcast tries to explain to you what's
2:12
Speaker 1: going on out there in the center of stars, around
2:15
Speaker 1: the edges of black holes, in all the tiny little particles,
2:18
Speaker 1: And we try to teach you also not just what
2:20
Speaker 1: science does know, but how it knows it, because you know,
2:25
Speaker 1: there is a pattern in the history of science where
2:27
Speaker 1: we think we know something, it's sort of established wisdom
2:30
Speaker 1: for a while, and then it gets overturned and we discover, oh,
2:35
Speaker 1: the universe is actually something different, It works differently from
2:38
Speaker 1: the way that we thought it did. Those are the
2:41
Speaker 1: best moments, if you ask me, in the history of science,
2:44
Speaker 1: when we're peeling back a layer of reality and discovering
2:47
Speaker 1: that the universe is pretty different from the way our
2:50
Speaker 1: intuitive experience led us to believe. And so when that happens,
2:54
Speaker 1: you have to wonder, why do we think the old thing?
2:57
Speaker 1: Why did we get it wrong? So it's important sometimes
3:00
Speaker 1: them to dig into how do we know what we know?
3:03
Speaker 1: And that's what we want to talk about on the
3:05
Speaker 1: podcast today. When we look out at the stars in
3:08
Speaker 1: the sky, we see some of them are bright and
3:10
Speaker 1: some of them are dim. But when scientists talk about stars,
3:14
Speaker 1: they're often not just talking about how bright they are.
3:16
Speaker 1: They're talking about how big they are, how massive they are.
3:21
Speaker 1: Stars with a lot of mass, stars with not so
3:23
Speaker 1: much mass, black holes with a huge amount of mass.
3:26
Speaker 1: But how is it possible to know that? So on
3:29
Speaker 1: today's program, we'll be asking the question, how do you
3:37
Speaker 1: measure the mass of a star? And it's not just me,
3:41
Speaker 1: and it's not just you who's wondering about this question.
3:44
Speaker 1: We actually got this question from a listener. Here's Dustin Hi,
3:48
Speaker 1: Daniel and Horror Hey, I wonder how physicists come up
3:52
Speaker 1: with the weight of planets and stars. Thank you things
3:55
Speaker 1: doesn't very much for sending in your question. And this
3:58
Speaker 1: is a really interesting and the important question. It's not
4:01
Speaker 1: just an academic one, because the mass of a star
4:05
Speaker 1: is really important. How much stuff you gather together to
4:08
Speaker 1: form that burning plasma in the sky determines its fate.
4:13
Speaker 1: Is it going to end up as a white dwarf,
4:15
Speaker 1: is it going to turn into a neutron star? Is
4:17
Speaker 1: it going to collapse into a black hole? All of
4:20
Speaker 1: those outcomes are determined by how much mass it has,
4:24
Speaker 1: and so understanding how much mass there is in stars
4:27
Speaker 1: is really important and turns out not that easy. But
4:31
Speaker 1: before we dig into how scientists actually do it, what
4:35
Speaker 1: they do know, and what they don't know about the
4:37
Speaker 1: mass of stars, I went out there into the wilds
4:40
Speaker 1: of the internet to ask people how they thought scientists
4:43
Speaker 1: did it or how they would do it themselves. So
4:46
Speaker 1: thank you very much to everybody who volunteered their brain
4:49
Speaker 1: to speculate how to measure the mass of something super
4:53
Speaker 1: duper far away that you could never hope to touch.
4:55
Speaker 1: And if you would like to participate in future baseless
4:58
Speaker 1: speculations for our podcast, please don't hesitate right into us
5:02
Speaker 1: two questions at Daniel and Jorge dot com. Here's what
5:06
Speaker 1: people had to say, my guts, and it would be
5:09
Speaker 1: to say that we know by how bright it is.
5:14
Speaker 1: Perhaps it's done by measuring the gravitational lensing effect on
5:19
Speaker 1: light passing around the stars, well, possibly with the help
5:23
Speaker 1: of mass spect traumata readings. So you know what elements
5:28
Speaker 1: you have in the star, and then you try to
5:30
Speaker 1: figure out how much of them you have. I could
5:33
Speaker 1: guess that poluminosity plays a part in it, and probably
5:38
Speaker 1: the orbit, but but I'm not sure, so I would
5:42
Speaker 1: say I don't know. I imagine that if we can
5:45
Speaker 1: tell the distance to a star, we can probably figure
5:47
Speaker 1: out its diameter, and from its light signature we can
5:54
Speaker 1: tell its chemical composition, and from the brightness of the
6:00
Speaker 1: star itself, I would imagine we could tell from the
6:05
Speaker 1: combination of those three things what the mass is somehow
6:09
Speaker 1: backing out from that information. All right. I love hearing
6:12
Speaker 1: people think on their feet, trying to solve this really
6:16
Speaker 1: hard problem from nothing, figuring it out from scratch, and
6:19
Speaker 1: there's a lot of good ideas in there. You hear
6:21
Speaker 1: people talking about how you need to know the mass
6:24
Speaker 1: of other stars nearby or maybe you can connect it
6:27
Speaker 1: to the brightness and hey, hey are on the right track.
6:30
Speaker 1: So let's talk about how scientists can measure the mass
6:34
Speaker 1: of stars. Well, in general, if you're looking at an
6:37
Speaker 1: object in space, right, all you're getting is the light
6:41
Speaker 1: from that object. It's basically impossible to know how big
6:46
Speaker 1: or how massive that object is. Remember what mass is.
6:50
Speaker 1: Mass is like how much stuff there is in a star.
6:54
Speaker 1: It affects the stars inertia, like how it responds to forces,
6:58
Speaker 1: and it also affects the stars gravity. You know how
7:01
Speaker 1: hard it pulls on stuff. But none of that can
7:04
Speaker 1: be directly observed from the star itself very easily. Mostly
7:09
Speaker 1: that's the effect of other stuff on the star, right,
7:13
Speaker 1: the inertia or the effect of the star on others stuff.
7:17
Speaker 1: And so the short answer is is that in general,
7:20
Speaker 1: it's impossible to directly measure the mass of an isolated star.
7:25
Speaker 1: We had the same problem when we were measuring the
7:27
Speaker 1: age of isolated stars for example. Right, we talked about
7:30
Speaker 1: how to measure the age of stars, and it turns
7:32
Speaker 1: out that you need to do it in big groups
7:34
Speaker 1: because you're seeing how the stars, which are probably formed
7:37
Speaker 1: together are probably dying together and you can use that
7:40
Speaker 1: as a sort of clock for the population of stars,
7:43
Speaker 1: but individual isolated stars, it's very hard to tell how
7:46
Speaker 1: old they are. It's a similar problem for measuring their mass.
7:50
Speaker 1: If the stars out there and there's nothing near by it,
7:53
Speaker 1: nothing that can probe its gravity or be influenced by
7:56
Speaker 1: its gravity, or give it a push or a pull,
7:59
Speaker 1: it's all most impossible to measure its mass. But that
8:03
Speaker 1: doesn't mean the problem is impossible. What we do in
8:06
Speaker 1: this case is that we find a special category of
8:09
Speaker 1: stars where we can figure out how to measure their
8:12
Speaker 1: mass because they're near something, there's something nearby that's tugging
8:16
Speaker 1: on them or that they are tugging on. We can
8:18
Speaker 1: measure the mass for that sort of special category of stars,
8:22
Speaker 1: and then we can try to fill in the gap
8:24
Speaker 1: and figure out a way to extrapolate to all the
8:26
Speaker 1: other stars. All right, so first let's talk about how
8:30
Speaker 1: to measure it for a special category of stars where
8:34
Speaker 1: you actually can see them doing some tugging or some pulling,
8:37
Speaker 1: where their mass really is important for what we observe.
8:41
Speaker 1: And the most powerful way to do this is to
8:43
Speaker 1: see binary star systems. These are systems where you have
8:48
Speaker 1: two stars nearby, and it seems sort of exotic, like
8:51
Speaker 1: the kind of thing you might see in Star Wars
8:53
Speaker 1: with multiple suns rising over the horizon. And you know,
8:56
Speaker 1: I love that they do that because it's sort of
8:58
Speaker 1: gives you a sense that an alien world. And so
9:02
Speaker 1: we have the idea I think that binary star systems
9:05
Speaker 1: are unusual, that they are weird, and that's just because
9:08
Speaker 1: we're used to looking at one son. It turns out
9:10
Speaker 1: binary star systems are not that rare. A lot of
9:14
Speaker 1: stars are born in pairs, and if you think about it,
9:16
Speaker 1: it makes a lot of sense actually, because how is
9:19
Speaker 1: the star form? Do you have a big collection of
9:21
Speaker 1: gas and dust and stuff that's swirling all around and
9:25
Speaker 1: something happens to trigger its collapse and it rushes down
9:28
Speaker 1: and collapses. But stars are not formed by themselves. You
9:31
Speaker 1: have a huge cloud which usually forms many stars at
9:34
Speaker 1: roughly the same time, and so it makes some sense
9:37
Speaker 1: for those stars to be tugging on each other and
9:40
Speaker 1: even for those stars to end up in orbit around
9:43
Speaker 1: each other. So lucky for us, binary star systems are
9:46
Speaker 1: not that rare. So what you can do is look
9:50
Speaker 1: at the pair of stars, because if they are close
9:53
Speaker 1: to each other, even if they are really really far
9:55
Speaker 1: from us. Then we can see the effect of their
9:58
Speaker 1: gravity on each other, which gives us a clue as
10:02
Speaker 1: to what their mass is. Alright, so let's dig into
10:06
Speaker 1: it and figure out how that actually works. If you're
10:08
Speaker 1: looking at a binary star system, how do you use
10:12
Speaker 1: what you're looking at what you see to actually figure
10:14
Speaker 1: out what the mass of those stars is. Well. The
10:17
Speaker 1: thing to remember about a binary star system, first of all,
10:21
Speaker 1: is that it's not one star orbiting the other. The
10:24
Speaker 1: two are orbiting each other, right, So there's some like
10:27
Speaker 1: point in between them that the two of them are
10:29
Speaker 1: both moving around. That's the center of mass. So both
10:33
Speaker 1: stars are in motion. Right from the point of view
10:37
Speaker 1: one star, the other ones moving, and from the point
10:39
Speaker 1: of view of the second star, the first one is moving.
10:42
Speaker 1: And what that means is that we can measure their
10:44
Speaker 1: relative motion. As the star moves further away from us,
10:48
Speaker 1: it's light gets stretched out, the wavelengths get a little
10:51
Speaker 1: bit longer, and when it star is moving towards us,
10:54
Speaker 1: it's light gets compressed a little bit. The wavelengths get
10:57
Speaker 1: a little bit shifted. This is called red shift. When
11:00
Speaker 1: it gets longer wavelengths as it moves away from us
11:03
Speaker 1: and blue shift as it gets shorter wavelengths when it's
11:06
Speaker 1: moving towards us. This is very similar to how you
11:08
Speaker 1: discover a planet is orbiting around a star, because you
11:12
Speaker 1: see the gravitational effect of the planet makes a star
11:15
Speaker 1: wiggle a little bit and that changes the light that
11:18
Speaker 1: we see. Now, in that case, you only have a
11:20
Speaker 1: single star, and you're deducing the presence of the otherwise
11:24
Speaker 1: invisible planet by looking at these Doppler shifts in the
11:27
Speaker 1: light from the star. In this case, we have both stars.
11:32
Speaker 1: So what you can do is look at the Doppler
11:34
Speaker 1: shift from both stars independently, so you get both of
11:38
Speaker 1: these curves right, and you can see how these curves change.
11:41
Speaker 1: You can see, for example, oh, the star is moving
11:43
Speaker 1: away from us. Oh, now it's moving towards us. So
11:46
Speaker 1: it's moving away from us, and now it's moving towards us.
11:49
Speaker 1: And so from those curves you can get some really
11:52
Speaker 1: interesting information. First of all, you can figure out the
11:55
Speaker 1: period of each star. How long does it take to
11:58
Speaker 1: move around the other one. This is because you can
12:01
Speaker 1: see when the velocity turns around right, it gets red shifted,
12:04
Speaker 1: then it gets blue shifted. What it flips over is
12:07
Speaker 1: that point when it's turning around, and so from those
12:10
Speaker 1: flipover points you can figure out what is the period
12:13
Speaker 1: of this star moving around the other one, And you
12:16
Speaker 1: can also actually measure the velocity of each star around
12:20
Speaker 1: the other one. This is determined by the amount of
12:23
Speaker 1: red shift and blue shift, or at the larger the velocity,
12:26
Speaker 1: the more red shift and blue shift you get as
12:29
Speaker 1: it swings around the other star. So we can just
12:32
Speaker 1: by watching the color of the light from these two
12:35
Speaker 1: stars change figure out what the period is, how long
12:38
Speaker 1: it takes for them to orbit each other, and their velocities.
12:42
Speaker 1: And that's really awesome because then we could just plug
12:45
Speaker 1: it into an ancient equation. Kepler's third law tells you
12:50
Speaker 1: exactly what the combined mass of the system is if
12:54
Speaker 1: you know the period and you know these velocities. So
12:57
Speaker 1: that's pretty cool. Now we know just from the period
13:01
Speaker 1: and those velocities, we know the total mass of this
13:03
Speaker 1: binary star system, but we also know their relative velocities.
13:07
Speaker 1: We know which one is moving faster than the other one. Say,
13:10
Speaker 1: for example, they don't have an equal amount of mass.
13:13
Speaker 1: It's not like two equal binary stars, but instead maybe
13:17
Speaker 1: one of them is much bigger than the other one,
13:20
Speaker 1: the bigger one is going to be moving slower and
13:22
Speaker 1: the smaller one is going to be doing more of
13:24
Speaker 1: the moving around the bigger one. So in the case
13:27
Speaker 1: when the two stars have the same mass, will have
13:29
Speaker 1: the same relative velocity. In the case when the two
13:32
Speaker 1: stars have very unequal masses, when it's very asymmetric, then
13:36
Speaker 1: one of them will be moving faster than the other one.
13:39
Speaker 1: And so from this relative velocity, which again we know,
13:43
Speaker 1: we can figure out how to split up the total
13:45
Speaker 1: mass of the systems into the two masses, and so
13:48
Speaker 1: boom that means from a binary star system, just from
13:52
Speaker 1: watching their light wiggle, we can figure out what is
13:55
Speaker 1: the mass of each star in the system. And one
13:58
Speaker 1: of my favorite things about this is that it doesn't
14:01
Speaker 1: just work for stars that are close by. It also
14:03
Speaker 1: works for stars that are super duper far away, because
14:06
Speaker 1: you just have to look at the light that's coming
14:08
Speaker 1: from the star. It's not like you need to see
14:11
Speaker 1: the gap between the stars or anything. You just need
14:13
Speaker 1: to look at the light pattern. And the cool thing
14:16
Speaker 1: about the Doppler shift is that it doesn't like disappear.
14:19
Speaker 1: If there's a Doppler shift pattern in light that comes
14:22
Speaker 1: to us from something really really far away, there's still
14:25
Speaker 1: that same shift when it gets here. It will persist
14:29
Speaker 1: over billions and billions of light years of space. And
14:32
Speaker 1: so this is powerful because it lets us look at
14:34
Speaker 1: even further away stars. So we get like a larger sample,
14:38
Speaker 1: so we can learn like a more general trend rather
14:41
Speaker 1: than just understanding something that's happening in our neighborhood. And
14:44
Speaker 1: also we can tell whether it's something that's true here
14:48
Speaker 1: and something that's true far away. We always want to
14:51
Speaker 1: be open to surprises when we look out into the universe.
14:54
Speaker 1: We don't want to draw too many conclusions just by
14:56
Speaker 1: looking at our cosmic neighborhood. So it's important a technique
15:00
Speaker 1: that works for nearby and also works for really far
15:03
Speaker 1: away stuff, all right. So that's how we measure the
15:07
Speaker 1: mass of binary stars of special star systems where we
15:11
Speaker 1: have two stars that we can see and we can
15:14
Speaker 1: measure their velocities from the wiggles in their light. But
15:17
Speaker 1: we're interested in all the stars. We want to know
15:19
Speaker 1: what is the mass of any given star we see
15:22
Speaker 1: out there in the universe, even the ones that are
15:24
Speaker 1: not in binary star systems. So how do we do that.
15:28
Speaker 1: We'll talk about that in a moment, but first I
15:30
Speaker 1: want to take a quick break. All right, we are back,
15:47
Speaker 1: and we are blowing your mind by thinking about incredible
15:50
Speaker 1: huge pockets of gas out there that are fusing themselves
15:54
Speaker 1: and radiating photons which zoom across billions of light years
15:58
Speaker 1: of the universe before they it to our eyes and
16:01
Speaker 1: our telescopes and carry with them incredible nuggets of knowledge
16:05
Speaker 1: about what's happening in far away corners of the universe.
16:09
Speaker 1: And we're gonna use that light to figure out the
16:11
Speaker 1: answer to a really interesting question, which is how big
16:15
Speaker 1: is that star? How massive is it? What is its future?
16:19
Speaker 1: Is it a huge blob of hydrogen which will eventually
16:22
Speaker 1: collapse into a black hole or is it going to
16:24
Speaker 1: end up a glowing white dwarf for trillions of years?
16:28
Speaker 1: And that is entirely determined by how much mass it has.
16:32
Speaker 1: So we talked about how to measure the mass of
16:35
Speaker 1: a special category of star stars. Will we identify two
16:39
Speaker 1: of them near each other that are orbiting each other,
16:41
Speaker 1: so we can use their gravitational effect on each other,
16:44
Speaker 1: which determines their relative periods and velocities of their orbits
16:48
Speaker 1: to figure out how much mass there is exactly, But
16:51
Speaker 1: now we want to move beyond that. We want to
16:53
Speaker 1: understand can we talk about any arbitrary star. We see
16:57
Speaker 1: a star in the sky, and we want to know
16:59
Speaker 1: how much mass does it have? How can we figure
17:02
Speaker 1: that out if it doesn't happen to have a big
17:05
Speaker 1: massive object near it that lets us directly measure its mass,
17:09
Speaker 1: And so here we have to be very careful. We
17:11
Speaker 1: have no direct way to measure the mass of those stars.
17:15
Speaker 1: But what we do is we look for a connection
17:18
Speaker 1: between what we can measure, like the brightness of a star,
17:22
Speaker 1: and what we want to know, like the mass of
17:24
Speaker 1: the star. And we look for that connection not in
17:27
Speaker 1: the actual stars out there in the universe, but in
17:30
Speaker 1: the stars here in our computers at home. Because we
17:34
Speaker 1: have an idea for how stars work, We have some
17:37
Speaker 1: theory about it. We know the nuclear fusion that happens
17:41
Speaker 1: inside stars. We think we understand something of the gravitational
17:44
Speaker 1: pressure that's pulling these things down and making these things happen.
17:48
Speaker 1: We compare a lot of these models to what we
17:50
Speaker 1: observe about nearby stars. So we've been spending decades developing
17:54
Speaker 1: these sort of like theory of how stars work, and
17:58
Speaker 1: in that theory, at least in our computational models of stars,
18:02
Speaker 1: we do see a relationship. We see a connection between
18:05
Speaker 1: the mass of the star, which is the thing we
18:08
Speaker 1: want to know but can't directly observe, and the luminosity
18:13
Speaker 1: of the star, the brightness of the star. In fact,
18:16
Speaker 1: we see a pretty direct relationship. While talking a minute
18:19
Speaker 1: about what that relationship is and why we think it
18:22
Speaker 1: makes sense the sort of physics that underlies the connection
18:25
Speaker 1: between the mass of a star and its brightness, but
18:28
Speaker 1: first let's make sure we're understanding the larger strategy. What
18:31
Speaker 1: we're gonna do here is get a connection sort of
18:34
Speaker 1: in our theory or in our simulations, between the mass
18:37
Speaker 1: and the luminosity, and then we're going to calibrate that.
18:40
Speaker 1: We're gonna make sure it's right by looking at binary
18:43
Speaker 1: star systems. So binary star systems give us a way
18:46
Speaker 1: to actually measure both the mass and the luminosity. And
18:50
Speaker 1: then we have these calculations we can do that can
18:53
Speaker 1: connect mass to luminosity, and we'll use the actual measurements
18:57
Speaker 1: to make sure that calculation is correct and various points.
19:01
Speaker 1: So we have like sort of a string that connects
19:04
Speaker 1: mass and luminosity, and then we have various pushpins we're
19:07
Speaker 1: gonna put into it to make sure that it's sort
19:09
Speaker 1: of nailed down by actual measurements in reality, and between
19:14
Speaker 1: those pushpins, between the actual measurements we make for the
19:16
Speaker 1: binary stars were basically extrapolating the little bit of guesswork
19:21
Speaker 1: and a lot of complicated nuclear theory, but it's not
19:24
Speaker 1: something we actually know, and so there's, for example, a
19:27
Speaker 1: gap where somebody could come in later and decide, oh,
19:29
Speaker 1: it turns out our model for stars was actually wrong
19:32
Speaker 1: and these extrapolations didn't quite work. So it's important for
19:35
Speaker 1: you guys to understand while we actually know what we
19:38
Speaker 1: actually can measure, which is the mass of a few
19:40
Speaker 1: binary star systems, and where we get the rest of
19:43
Speaker 1: the information which comes from this nuclear theory, which helps
19:46
Speaker 1: us sort of interpolate between the examples that we can't
19:50
Speaker 1: measure directly. Alright, So we have a connection in our
19:54
Speaker 1: models between the mass and the luminosity, and it's really
19:58
Speaker 1: kind of fascinating. It tells us that the larger the mass,
20:02
Speaker 1: the brighter the star. Right, the bigger your original scoop
20:06
Speaker 1: size of hydrogen, the brighter the star is the faster
20:10
Speaker 1: it's gonna burn, And that means something else really cool,
20:12
Speaker 1: which means that big stars burn bright, they burn hot,
20:16
Speaker 1: but they don't burn for very long. So the big
20:19
Speaker 1: ones are like flashy and exciting and very very bright
20:22
Speaker 1: and shine their love into the universe, but not for
20:26
Speaker 1: that long, whereas the little star that could it's sort
20:28
Speaker 1: of out there pumping not nearly as much light, but
20:32
Speaker 1: he can do it for a much much longer time.
20:35
Speaker 1: These little stars can last for billions or maybe even
20:38
Speaker 1: trillions of years, whereas the really really big stars only
20:43
Speaker 1: last for like a few hundred million years before the
20:46
Speaker 1: party is over. So what does that actually mean? Well,
20:49
Speaker 1: the relationship for stars, like around the mass of our sun,
20:52
Speaker 1: are a little bit less and then up to about
20:54
Speaker 1: fifty times the mass of our sun, the luminosity of
20:58
Speaker 1: a star goes like the mass to the fourth power.
21:02
Speaker 1: That means that a star twice as massive as our
21:05
Speaker 1: sun will be like almost sixteen times as bright as
21:09
Speaker 1: our sun. A star twenty times the mass of our
21:13
Speaker 1: sun will be a hundred and twenty thousand times as bright.
21:18
Speaker 1: That's right, you double the mass of the star, you
21:20
Speaker 1: don't just double the brightness, right, it goes up by
21:23
Speaker 1: the power of four, and so it increases very very quickly.
21:28
Speaker 1: And that gives you a sense for why these really
21:31
Speaker 1: big stars burned out so quickly, because they were incredibly bright.
21:36
Speaker 1: The bigger the star, the brighter the star, and the
21:38
Speaker 1: faster it dies. It also means if you turn your
21:42
Speaker 1: attention the other direction, that stars that are less bright
21:46
Speaker 1: than the Sun are much much dimmer and burn much
21:49
Speaker 1: much longer. For example, there are stars out there that
21:53
Speaker 1: have a fraction of the mass of our sun and
21:56
Speaker 1: they burn like one ten thousands as brightly as our sun.
22:01
Speaker 1: Imagine living on a planet around the star that was
22:04
Speaker 1: one ten thousands the brightness of our star. Right, you
22:08
Speaker 1: could be much much closer to the star without it
22:11
Speaker 1: being brighter than the Sun, which means it would fill
22:14
Speaker 1: up a much bigger area in your sky. Right, so
22:18
Speaker 1: you could have, for example, a nice, warm, toasty day
22:21
Speaker 1: in front of a huge sun in your sky without
22:24
Speaker 1: actually getting burned. So the opportunity is there for like
22:28
Speaker 1: crazy science fiction ideas for what it would be like
22:31
Speaker 1: to have a huge star in your sky seemed pretty
22:34
Speaker 1: wide open to me. And so this relationship, this fourth
22:37
Speaker 1: power relationship, is true for stars that are like half
22:41
Speaker 1: the mass of the Sun up to about fifty times
22:43
Speaker 1: the mass of the Sun, and then there's some kinks
22:45
Speaker 1: in it. Like above fifty times the mass of the Sun,
22:48
Speaker 1: there's a different relationship. It actually becomes linear, doesn't grow
22:52
Speaker 1: as quickly, and below half the mass of the Sun,
22:56
Speaker 1: the relationship changes again. It goes more like mass squared.
23:00
Speaker 1: And so there's some details there, but let's understand sort
23:02
Speaker 1: of the general idea. Why is it that a more
23:06
Speaker 1: massive star would burn brightly and so much more brightly.
23:10
Speaker 1: Why isn't the relationship just linear. Why isn't it that
23:13
Speaker 1: if you have twice as much mass in the star,
23:16
Speaker 1: it burns twice as bright and twice as hot. Well,
23:19
Speaker 1: the answer comes from the details of the nuclear theory.
23:23
Speaker 1: So let's dig into what's going on on the inside
23:26
Speaker 1: of this star, right, Why is the star made at all? Right,
23:30
Speaker 1: a star comes from the gravitational pressure. You have a
23:34
Speaker 1: big collection of gas out there in space, and it's
23:36
Speaker 1: gravity that's tugging it together. But gravity is not the
23:40
Speaker 1: only thing active in a star, right. If it were,
23:43
Speaker 1: then every star and every object would just collapse into
23:46
Speaker 1: a black hole. Now, a star is a pretty steady
23:49
Speaker 1: state object. It can last for millions or billions or
23:53
Speaker 1: maybe trillions of years because there are two forces at play.
23:57
Speaker 1: There are two things they're pushing against each other, which
24:00
Speaker 1: is what keeps the star stable. So there has to
24:02
Speaker 1: be some sort of outward pressure to keep a star
24:05
Speaker 1: from collapsing. In the case of the most stars, that
24:08
Speaker 1: pressure comes from the fusion, from the nuclear reaction that's
24:12
Speaker 1: powering the star itself. And it's fascinating to me because
24:15
Speaker 1: it's such a give and take, like gravity pushes on
24:18
Speaker 1: the star, squeezes on the star. That's what actually causes fusion,
24:23
Speaker 1: and that fusion releases a lot of radiation. That radiation,
24:27
Speaker 1: that brightness, that energy pushes the star back out right.
24:30
Speaker 1: So gravity causes fusion, which creates this sort of like
24:34
Speaker 1: back reaction which pushes out on the star to keep
24:37
Speaker 1: it from collapsing. And there's a whole fascinating rabbit hole
24:41
Speaker 1: you could go down there, right because then that fusion
24:43
Speaker 1: creates heavier elements, which increases the gravity dot dot dot.
24:47
Speaker 1: But there's a whole other podcast episode about the life
24:49
Speaker 1: cycle of stars. Let's focus right now and just what's
24:52
Speaker 1: happening inside a star and why if a star gets
24:55
Speaker 1: more massive, it also gets brighter. So the reason simply
25:00
Speaker 1: is that a more massive star has more gravity. Right,
25:03
Speaker 1: there's a bigger helping of mass, and each of those
25:06
Speaker 1: little particles has a little gravitational force on it. Each
25:10
Speaker 1: of them is then squeezing the center of the star.
25:13
Speaker 1: So if you take a star the mass of our
25:15
Speaker 1: sun and you wrap it in another helping of hydrogen,
25:18
Speaker 1: another solar mass of hydrogen, there's gonna be additional gravitational forces. Right,
25:24
Speaker 1: It's going to increase the pressure on the center of
25:26
Speaker 1: the star. And so in order for the star to
25:29
Speaker 1: be stable, for it to avoid collapsing into a black hole,
25:33
Speaker 1: it needs more pressure outwards. Right. It needs to be
25:36
Speaker 1: sending out more radiation. It needs to be brighter. And
25:40
Speaker 1: you might think, well, just because it needs to be
25:42
Speaker 1: brighter doesn't mean that it does. Right. Well, stars that
25:46
Speaker 1: don't get brighter, that don't provide those pressures, they do
25:48
Speaker 1: collapse into black holes. Right, So if there's a mechanism
25:52
Speaker 1: to provide that pressure, that's what keeps the star is stable,
25:55
Speaker 1: and in fact there is because increasing the pressure on
25:58
Speaker 1: the core of the star increases the temperature. And when
26:02
Speaker 1: you increase the temperature at the core. The nuclear theory
26:06
Speaker 1: tells us something happens the fusion that's happening at the
26:09
Speaker 1: core of the star, the merging of hydrogen into helium
26:12
Speaker 1: and then helium into heavier elements to release more energy.
26:16
Speaker 1: The rate of that fusion depends very sensitively on the temperature,
26:22
Speaker 1: and so here's where that nonlinearity effect comes in. Here's
26:26
Speaker 1: why doubling the mass of a star doesn't just make
26:29
Speaker 1: it twice as bright, because the nuclear fusion at its
26:32
Speaker 1: core is very very sensitive to the temperatures. That's the
26:35
Speaker 1: nonlinear response. If you crank up the temperature of the
26:39
Speaker 1: core of the star by just a little bit, you
26:41
Speaker 1: can lead to a very large increase in the reaction rate.
26:45
Speaker 1: And this is something that's very complicated and difficult to
26:47
Speaker 1: wrap your mind around. But just because there's so many
26:49
Speaker 1: particles involved, I mean, you have to sort of like
26:52
Speaker 1: take your mind and go deep into the center of
26:54
Speaker 1: the star and imagine all of those hygrogen atoms pushing
26:58
Speaker 1: against each other. Because remember, hydrogen doesn't like to fuse.
27:02
Speaker 1: It's nuclei are positively charged. You've got a bunch of
27:05
Speaker 1: protons in there, and protons push away from each other.
27:08
Speaker 1: In order to get the hydrogen to fuse and helium,
27:11
Speaker 1: you've got to really squeeze it down. You have to
27:14
Speaker 1: have a lot of pressure at the same time, you
27:16
Speaker 1: have to have a lot of energy. You need a
27:18
Speaker 1: high temperature because you need those hygen atoms to be
27:21
Speaker 1: banging into each other, and so you can have conditions
27:24
Speaker 1: at a very high pressure and temperature but without fusion,
27:27
Speaker 1: and then all of a sudden fusion happens. You've sort
27:30
Speaker 1: of overcome a threshold. And then once that threshold happens,
27:34
Speaker 1: more fusion leads to more fusion because more of it
27:36
Speaker 1: gets to that high pressure and temperature that you need.
27:39
Speaker 1: So there's a very nonlinear effect there, because hydrogen doesn't
27:43
Speaker 1: want to fuse at all, and once you've created the
27:46
Speaker 1: conditions for fusion, it can lead to sort of a
27:48
Speaker 1: runaway process. So that's the basic idea. The reason that
27:53
Speaker 1: brighter stars are more massive is because there's a higher
27:57
Speaker 1: temperature at their core created by this incredible gravitational pressure
28:02
Speaker 1: from the additional mass, and that higher temperature leads to
28:05
Speaker 1: a faster nuclear reaction. And of course that faster nuclear
28:09
Speaker 1: reaction is not just gonna make the star brighter which
28:11
Speaker 1: provides the pressure you need to keep the star alive.
28:14
Speaker 1: It's also going to eventually kill the star because that
28:18
Speaker 1: faster nuclear reaction uses up the fuel. These stars are
28:22
Speaker 1: made out of a lot of hydrogen, but there's not
28:24
Speaker 1: an infinite supply of it, right, Eventually the star is
28:28
Speaker 1: gonna fuse so much hydrogen into helium that's gonna get
28:31
Speaker 1: a helium core, and that helium is gonna fuse and
28:34
Speaker 1: you're gonna get something denser and denser and denser, And
28:37
Speaker 1: so that's just a ticking clock. Right. You have a
28:40
Speaker 1: fixed amount of fuel. Once you've formed a star, it's
28:43
Speaker 1: difficult for it to like create more matter unless it
28:46
Speaker 1: has some like very close binary you can steal matter from.
28:50
Speaker 1: So then it's just a question of like how many
28:52
Speaker 1: fusion cycles can it have. If it's a really big star,
28:56
Speaker 1: it will burn through those really really quickly and eventually
28:59
Speaker 1: get to the stage where collapses. Or if it's a
29:01
Speaker 1: little star and it's burning in a much lower temperature
29:04
Speaker 1: and brightness, then it's sort of like biding its time
29:07
Speaker 1: and it can last a lot lot longer. You might
29:10
Speaker 1: expect it to be the opposite, right, You might expect
29:12
Speaker 1: that big stars have more fuel and so they can
29:14
Speaker 1: last longer into the deep future of the universe. But
29:18
Speaker 1: it's sort of surprisingly the opposite. Those big stars do
29:21
Speaker 1: have more fuel, but they also burn more fuel, and
29:24
Speaker 1: they burn more fuel per second because they're burning hotter.
29:28
Speaker 1: So that's an idea for why there's this connection between
29:31
Speaker 1: the mass of the star and the brightness of the star,
29:33
Speaker 1: and that helps us sort of connect the dots between
29:36
Speaker 1: the things that we actually can observe. Those binary star systems,
29:40
Speaker 1: the ones where we see the masses have an effect
29:42
Speaker 1: on each other, and that tells us whether we get
29:44
Speaker 1: in these calculations correct and it gives us confidence that
29:47
Speaker 1: we can interpolate between the things that we can see.
29:50
Speaker 1: But people are always working to improve these measurements. People
29:53
Speaker 1: are always looking for other ways to double check it,
29:56
Speaker 1: to figure out if this is wrong, to find another
29:59
Speaker 1: way to measure these things. The best thing you can
30:01
Speaker 1: do in sciences have two totally independent measurements, ones that
30:05
Speaker 1: make maybe different mistakes or different assumptions that cross check
30:09
Speaker 1: each other. And so there are a couple of other
30:12
Speaker 1: ways we can get a handle on the mass of stars,
30:15
Speaker 1: but they're even more specialized than the binary star system. Method.
30:19
Speaker 1: One of them is called gravitational micro lensing. This is
30:23
Speaker 1: a super fun one. It's when a star acts like
30:25
Speaker 1: a lens. Because remember that the gravity of a star
30:29
Speaker 1: isn't just a force pulling on things. It actually is
30:32
Speaker 1: bending the shape of the space around the star. This
30:36
Speaker 1: is Einstein's idea of how gravity works. Rather than being
30:39
Speaker 1: a force, it's a change in the way space is curved.
30:43
Speaker 1: In Einstein's idea of gravity, space is not just like
30:47
Speaker 1: a backdrop. It's a thing that bends as mass gets
30:50
Speaker 1: around it. And there's a complicated dance there, right, because
30:53
Speaker 1: mass tells space how to bend, and then the bending
30:56
Speaker 1: of space tells mass and other things how to move
30:59
Speaker 1: in hooting light. And so a star out there in
31:02
Speaker 1: the universe bends the space around it, and so a
31:06
Speaker 1: photon coming to us from for example, a background galaxy
31:10
Speaker 1: might get bent as it passes around that star, causing
31:13
Speaker 1: a distortion. So if you're looking at a galaxy really
31:16
Speaker 1: really far away, for example, and a star passes right
31:20
Speaker 1: in front of that galaxy, it will distort that galaxy
31:23
Speaker 1: in a very particular way, and in a particular way
31:27
Speaker 1: that depends on its mass. So this is one way
31:30
Speaker 1: we can measure the mass of an object without anything
31:33
Speaker 1: else necessarily being near it by seeing how it distorts
31:37
Speaker 1: light from background galaxies, because the distortion of that light
31:41
Speaker 1: depends on the mass. The higher the mass of the star,
31:44
Speaker 1: the more it will distort that light. And then of
31:47
Speaker 1: course we can look at the brightness of that star
31:49
Speaker 1: and we can look it up on our chart and
31:50
Speaker 1: we can say, oh, does this follow the pattern that
31:52
Speaker 1: we expect. Does this help us understand that the connections
31:55
Speaker 1: between the luminosity and the mass of the star. And
31:58
Speaker 1: so this is more difficult to do because it's not
32:01
Speaker 1: something that happens very often. It's not something you can
32:03
Speaker 1: necessarily predict. Is a sort of a chance encounter a
32:07
Speaker 1: star wandering in front of another system that we were
32:10
Speaker 1: already looking at. But when it does happen, it's a
32:13
Speaker 1: very nice calibration and lets us look at a different
32:16
Speaker 1: population of stars and get a direct measurement of their
32:19
Speaker 1: mass as well. And then there's one extra cool, sort
32:23
Speaker 1: of crazy idea for how to measure the massive stars,
32:27
Speaker 1: and that's using astro seismology. These days, we can study
32:32
Speaker 1: in real detail the light coming from stars, and some
32:35
Speaker 1: astronomers think that variations in the light that comes from
32:38
Speaker 1: a star might be due to seismic activity on the
32:43
Speaker 1: surface of the star. That's right. These stars have sort
32:47
Speaker 1: of like crusts, and you know how our son, for example,
32:50
Speaker 1: sometimes blows out big coronal ejections or flares up and
32:54
Speaker 1: flares down. If you're studying the light from that star
32:57
Speaker 1: from really far away, you can tell when one of
32:59
Speaker 1: these events happens because it affects the light that you see.
33:03
Speaker 1: Because the star is spinning, there's a variable effect on
33:06
Speaker 1: the light. So there's like a hot spot on one
33:09
Speaker 1: side of a star. You'll see it as the star spins,
33:12
Speaker 1: and then you won't see it as it spins away
33:14
Speaker 1: from you. So you can see this sort of pattern
33:17
Speaker 1: in the light from the star because of crazy effects
33:20
Speaker 1: on the surface of the star. And there's some models
33:23
Speaker 1: that tell us that there's a connection between the sort
33:25
Speaker 1: of rate of these effects, how often this happens seismic
33:29
Speaker 1: events in the crusts of stars, and the mass of
33:32
Speaker 1: those stars. And so this astro seismology looking at the
33:37
Speaker 1: pulsation on the surface of stars might also be able
33:41
Speaker 1: to give us clues as to the mass of those stars.
33:44
Speaker 1: All right, So we've been talking about how we measured
33:47
Speaker 1: the mass of stars. I want to dig into what
33:50
Speaker 1: we've learned, what is the range of mass of stars
33:53
Speaker 1: and why that's important and what it tells us about
33:56
Speaker 1: the history and the future of our universe. But first,
33:59
Speaker 1: let's take another quick break. Okay, we're back, and we're
34:15
Speaker 1: talking about how we know what we do know about
34:18
Speaker 1: the universe and how well we know it, And specifically
34:21
Speaker 1: today we're digging into the question of how do we
34:24
Speaker 1: know the mass of a star, how do we measure that,
34:27
Speaker 1: how do we figure it out when we can't measure it,
34:29
Speaker 1: and how well do we actually know that? And so
34:32
Speaker 1: we've been talking about how we measure it using binary
34:34
Speaker 1: star systems and gravitational micro lensing, and how we interpolate
34:38
Speaker 1: between those measurements using nuclear theory models about how a
34:43
Speaker 1: star works, and now let's talk about what that means.
34:46
Speaker 1: It turns out that the mass of a star is
34:49
Speaker 1: basically the most important thing about it. The whole future
34:53
Speaker 1: of the star, what's going to happen to it, its
34:55
Speaker 1: whole life cycle, it's life span in fact, and then
34:58
Speaker 1: the future of any civilized Asian living around that star
35:01
Speaker 1: depends entirely on how much stuff went into the star,
35:06
Speaker 1: what the mass serving was for the material that ended
35:10
Speaker 1: up in that star. And there's a few outcomes that
35:13
Speaker 1: are possible for a star. But this is awesome stellar
35:16
Speaker 1: life cycle chart, which you encourage everybody to google and
35:19
Speaker 1: take a look at. But the end points of this
35:21
Speaker 1: are super fascinating. Basically, once you become a star, you
35:25
Speaker 1: have a few possible outcomes. You can either become a
35:29
Speaker 1: brown dwarf, which is like a failed star that never
35:32
Speaker 1: really takes off and has the same kind of bright
35:34
Speaker 1: fusion that's for very low mass stuff. Or you can
35:38
Speaker 1: become like a normal star, which ends up as a
35:41
Speaker 1: white dwarf actor blows out its outer layers and then eventually,
35:45
Speaker 1: after trillions of years, cools to a black dwarf, so
35:48
Speaker 1: you've got brown dwarves, white dwarves, black dwarves. Or if
35:53
Speaker 1: you're even bigger, you can end up as a neutron star,
35:56
Speaker 1: which might eventually turn into a pulsar. Or you can
35:59
Speaker 1: end up a lack hole, and in some scenarios you
36:02
Speaker 1: end up is just like this big super and over remnant.
36:05
Speaker 1: The whole thing just sort of blows up and spreads
36:07
Speaker 1: out into the universe. But the one that you end up.
36:09
Speaker 1: Your eventual faith if you're a star, depends just on
36:13
Speaker 1: how much stuff you have, because it's the amount of
36:16
Speaker 1: stuff that drives the whole process, right, that makes you
36:20
Speaker 1: burn hot and fast, or makes you burn slow and cool.
36:23
Speaker 1: So the mass of the star is actually really, really important,
36:27
Speaker 1: and it's not just important for understanding the fate of
36:30
Speaker 1: an individual star. It's also really important for understanding the
36:33
Speaker 1: whole collection of stars. Right. We want to know the
36:36
Speaker 1: mass of stars because we want to understand, for example,
36:39
Speaker 1: how the galaxy is rotating. Think about one of the
36:42
Speaker 1: deepest and most amazing discoveries in astronomy in the last
36:45
Speaker 1: hundred years, which was that galaxies are mostly not made
36:50
Speaker 1: out of stars and gas and dust, that they're made
36:52
Speaker 1: out of something else. This is an observation that came
36:56
Speaker 1: out of just looking at the rotations of those galaxies
36:59
Speaker 1: and asking how fast are those galaxies rotating? And is
37:03
Speaker 1: there enough mass in the galaxy to explain how fast
37:08
Speaker 1: it's rotating? Right? If you, for example, put a bunch
37:11
Speaker 1: of ping pong balls on a Merry Go round and
37:13
Speaker 1: spun it, they would fly off into space. The reason
37:16
Speaker 1: that doesn't happen for stars in the galaxy, which is
37:19
Speaker 1: basically a huge cosmic merry go round, is because there's
37:22
Speaker 1: something holding them in. But you can measure the speed
37:26
Speaker 1: of our galactic merry go round and ask is there
37:29
Speaker 1: enough mass in the galaxy to provide the gravity you
37:32
Speaker 1: need to hold the stars in place? Well, how do
37:35
Speaker 1: you do that calculation? All you can do is look
37:38
Speaker 1: at the stuff. You see. You look at all those
37:40
Speaker 1: bright points of light in the sky, and you add
37:43
Speaker 1: up their individual masses, And that's exactly the point. You
37:47
Speaker 1: need to know the mass of all those stars in
37:50
Speaker 1: order to do that calculation. If we had no idea
37:54
Speaker 1: what the masses of stars were, we never would have
37:57
Speaker 1: discovered dark matter because we would have looked at all
38:00
Speaker 1: the stars in the sky and said, well, we have
38:03
Speaker 1: no idea how much mass there is, so maybe there's
38:05
Speaker 1: enough to provide the gravity we need to hold the
38:08
Speaker 1: galaxy together despite how fast it's spinning. But no, we
38:12
Speaker 1: do know the mass of those stars, and so we
38:14
Speaker 1: were able to calculate how much mass there is in stars.
38:19
Speaker 1: And we looked at that and we said, well, is
38:21
Speaker 1: there enough mass to provide the gravity needs to hold
38:24
Speaker 1: the galaxy together? And of course you probably know by
38:26
Speaker 1: now the answer is not even close. The galaxy is
38:30
Speaker 1: spinning at a crazy fast rate, and the mass of
38:33
Speaker 1: the stars we can see does not provide enough gravity
38:37
Speaker 1: to hold it together. That was the essential clue, the
38:40
Speaker 1: first crack that told us that there was something else
38:44
Speaker 1: out there in the universe. So doing these kinds of
38:47
Speaker 1: studies things that might seem a little boring, like our
38:50
Speaker 1: stars as bright as we expect them to be, how
38:53
Speaker 1: much mass do they have? Can we explain it doesn't
38:56
Speaker 1: make sense, seems sometimes like sort of scientific busy world,
39:00
Speaker 1: But sometimes the answers don't add up, and they reveal
39:03
Speaker 1: a huge cosmic mystery. So that's why it's important to
39:08
Speaker 1: know the massive stars. That's why the mass of the
39:10
Speaker 1: stars determined not just the fate of the individual objects,
39:13
Speaker 1: but potentially how everything works. And now we know that
39:17
Speaker 1: it's not actually the mass of stars that's determining like
39:20
Speaker 1: the whole shape and future of our universe. It's actually
39:24
Speaker 1: all of that dark matter, because there is five times
39:27
Speaker 1: as much dark matter in the universe as there is
39:30
Speaker 1: hydrogen and helium and the kind of stuff that makes stars,
39:33
Speaker 1: and it's that dark matter that controls like why there's
39:36
Speaker 1: a galaxy here and not over there. The reason is
39:39
Speaker 1: that there was a big blob of dark matter and
39:41
Speaker 1: it attracted all the hydrogen and helium, and it helped
39:45
Speaker 1: compress all of that stuff, and it started the fire
39:48
Speaker 1: in those stars. So we wouldn't even have stars and
39:51
Speaker 1: galaxies the way we do today if it weren't for
39:54
Speaker 1: that dark matter. And we wouldn't know about the dark
39:56
Speaker 1: matter if we didn't have a careful measurement a way
40:00
Speaker 1: to figure out the mass of individual stars. Alright, so
40:04
Speaker 1: then let's talk about what the mass of the stars
40:06
Speaker 1: actually are. Right, we look at our Sun. That's a
40:09
Speaker 1: pretty standard candle. We think, let's use the Sun as
40:12
Speaker 1: an example star and measure everything in terms of like
40:16
Speaker 1: one solar mass. Well, right off the bat, we discover
40:19
Speaker 1: the Sun is kind of unusual, is sort of a
40:22
Speaker 1: large star. The most common star out there in our
40:25
Speaker 1: galaxy is one called a red dwarf. Is significantly smaller
40:30
Speaker 1: and colder than the Sun, and it's hard to see them, right.
40:33
Speaker 1: These stars are much much dimmer than the Sun. Something
40:37
Speaker 1: that's half as bright as the Sun is going to
40:39
Speaker 1: be one six as dim. And there are a lot
40:43
Speaker 1: of stars out there that are smaller than half the
40:45
Speaker 1: mass of the Sun, and so they're much much dimmer,
40:48
Speaker 1: which makes them hard to see. So we don't even
40:51
Speaker 1: really have a complete catalog of all of those stars,
40:54
Speaker 1: but none of the stars that are very nearby are
40:57
Speaker 1: much larger. In fact, none of the stars that are
40:59
Speaker 1: within thirty light years of the Sun is very much bigger,
41:03
Speaker 1: Like the biggest star in our neighborhood is about four
41:06
Speaker 1: times the mass of the Sun. And as you explore
41:09
Speaker 1: out there into the universe, you discover something really interesting,
41:13
Speaker 1: which is that there seems to be sort of a
41:15
Speaker 1: limit to how big stars can be. And we discover
41:18
Speaker 1: this just because there aren't stars that are much much bigger.
41:22
Speaker 1: The limit seems to be somewhere around a hundred two hundred,
41:26
Speaker 1: maybe up to two hundred and fifty times the mass
41:29
Speaker 1: of the Sun. The biggest stars out there are just
41:32
Speaker 1: about that big. And the reason is that stars that
41:35
Speaker 1: have huge amounts of mass burn really really hot and bright,
41:40
Speaker 1: and they push so hard from the inside that they're
41:43
Speaker 1: basically unstable and blow themselves apart. Anything above that it
41:47
Speaker 1: basically blows the excess off, So you just can't have
41:50
Speaker 1: a stable blob of matter that's more than like two
41:53
Speaker 1: hundred times the mass of the Sun. But we're gonna
41:56
Speaker 1: dig into that in a whole other podcast episode about
41:59
Speaker 1: the biggest stars in the universe. It's also actually fun
42:02
Speaker 1: to think about the history of the universe. It turns
42:05
Speaker 1: out that the massive stars changes as the universe gets older,
42:09
Speaker 1: and the very very early universe. We think that the
42:13
Speaker 1: stars that were first born, the first population of stars
42:16
Speaker 1: that came together out of that hydrogen, were actually typically
42:19
Speaker 1: much much bigger than the stars we see today. Those
42:23
Speaker 1: are these so called population three stars. They're called population
42:28
Speaker 1: three stars because our stars today are called population one,
42:32
Speaker 1: and then the stars that they came from are called
42:34
Speaker 1: population to sort of sort of counting back to the
42:37
Speaker 1: beginning of the universe, it might make more sense to
42:40
Speaker 1: you to think the first generation of stars are population one,
42:43
Speaker 1: and we should be population three, But for whatever reason,
42:46
Speaker 1: astronomers are counting backwards. So population three stars are the
42:51
Speaker 1: first ones to form in the universe, and those were
42:55
Speaker 1: much much bigger because there wasn't very much metal around
42:58
Speaker 1: in the universe right after the Big Bang. The universe
43:01
Speaker 1: was mostly hydrogen, with a little bit of helium and
43:04
Speaker 1: a tiny bit of lithium. But overwhelmingly hydrogen, and hydrogen
43:09
Speaker 1: doesn't clump as well as heavier metals, right, it makes
43:12
Speaker 1: it harder to collapse into a small sample because molecular
43:16
Speaker 1: hydrogen doesn't collapse as well. It's harder for it to
43:19
Speaker 1: cool Or you get this big blob of gas in
43:22
Speaker 1: which you need to form a star, is for it
43:23
Speaker 1: to collapse gravitationally, And it turns out that that's easier
43:27
Speaker 1: to do in smaller clumps when you have little blobs
43:30
Speaker 1: of metal. That's sort of like seeds, smaller pieces. If
43:34
Speaker 1: you just have a huge mass of hydrogen, then it's
43:36
Speaker 1: harder for it to collapse into smaller clumps. It tends
43:39
Speaker 1: to collapse into these much bigger objects because it's harder
43:43
Speaker 1: for it to cool off. And so in the very
43:45
Speaker 1: early universe we had really really huge stars, much more
43:49
Speaker 1: massive stars than we typically see today. And then the
43:53
Speaker 1: second generation of stars had more metal in them. Right,
43:56
Speaker 1: because the first generation of stars burned and fueled helium
44:00
Speaker 1: production of heavier stuff, and then that sea did the
44:03
Speaker 1: next generation of stars. And because there was more helium
44:06
Speaker 1: and heavier metals around when the second generation formed, you
44:10
Speaker 1: tended to get smaller clumps. So that second generation of stars.
44:14
Speaker 1: Some of them are still around, They are still burning
44:17
Speaker 1: in the universe. We can see them. Check out our
44:19
Speaker 1: podcast episode about the oldest stars in the Universe, and
44:22
Speaker 1: you'll see that we've found something that we think our
44:25
Speaker 1: second generation stars that are burning for billions and billions
44:28
Speaker 1: of years. Those first generation, we think only lasted a
44:31
Speaker 1: few hundred million years, if at all. And the crazy
44:35
Speaker 1: thing is that we've never even really seen one of them.
44:38
Speaker 1: Nobody's ever seen a population three star, and that's, of
44:42
Speaker 1: course because they're all burned out, so there are none
44:44
Speaker 1: of them in our neighborhood. In order to see when
44:46
Speaker 1: you need to look really really far away, so that
44:49
Speaker 1: light from that population three star would just be arriving
44:52
Speaker 1: to our eyeballs and now, billions of years after it
44:55
Speaker 1: already died out. The problem, of course, is that those
44:58
Speaker 1: galaxies are super duper far away. We're talking about things
45:01
Speaker 1: that are thirteen billion years ago, so they're basically at
45:05
Speaker 1: the edge of the observable universe, and those galaxies are
45:08
Speaker 1: hard to spot. We can see the galaxy, we know
45:11
Speaker 1: it's there. We're getting light from that galaxy. We suspect
45:14
Speaker 1: it's filled with population three stars. But we haven't ever
45:17
Speaker 1: identified an individual population three star from the early universe,
45:22
Speaker 1: and it would be super fascinating if we could. We
45:25
Speaker 1: had an understanding of like how big were these things?
45:27
Speaker 1: How much mass did they have? All Right, so today
45:30
Speaker 1: we've dug into the details of how scientists know the
45:33
Speaker 1: mass of these stars. It turns out to be really
45:36
Speaker 1: important to the life of a star. Completely controls what's
45:39
Speaker 1: going to happen to it, whether it's going to be
45:41
Speaker 1: a black hole or end up as a white dwarf.
45:43
Speaker 1: And eventually a black dwarf is just turned by the
45:46
Speaker 1: original helping of hydrogen and other stuff that it got
45:49
Speaker 1: when it was formed. And we can figure that out
45:52
Speaker 1: by looking at binary star systems, by looking at gravitational
45:55
Speaker 1: micro lensing, and then sort of extrapolating in between what
45:59
Speaker 1: we do know to guess about what we don't quite know.
46:02
Speaker 1: But there's still a lot of uncertainty because these calculations
46:05
Speaker 1: we do they're hard, there's a lot of approximations we
46:08
Speaker 1: have to make. We think they're probably not completely accurate.
46:11
Speaker 1: We have some confidence that they're not way off because
46:14
Speaker 1: we can calibrate them using the stars that we do see,
46:17
Speaker 1: where we can measure their mass, but there's always room
46:20
Speaker 1: for improvements, and so it's important to think about the
46:23
Speaker 1: knowledge and also the uncertainty, because hey, most of the
46:26
Speaker 1: universe is uncertain. Most of what we will learn about
46:30
Speaker 1: nature and stars and the universe is ahead of us.
46:33
Speaker 1: So thanks for coming on this ride to explore what
46:36
Speaker 1: we do know and what we don't know, and our
46:38
Speaker 1: speculation about what we will eventually know. Thanks everyone, tune
46:42
Speaker 1: in next time. Thanks for listening, and remember that Daniel
46:53
Speaker 1: and Jorge Explain the Universe is a production of My
46:56
Speaker 1: heart Radio. Or more podcast from my heart Radio visit
46:59
Speaker 1: the heart Radio app, Apple Podcasts, or wherever you listen
47:03
Speaker 1: to your favorite shows.