1 00:00:08,720 --> 00:00:11,520 Speaker 1: Did you ever eat astronauts ice cream? As a kid, 2 00:00:11,960 --> 00:00:15,200 Speaker 1: that stuff seemed so exciting, but when you put it 3 00:00:15,200 --> 00:00:18,200 Speaker 1: in your mouth, it was always kind of gross and disappointing. 4 00:00:18,720 --> 00:00:20,759 Speaker 1: I remember my brothers and I wondering if it was 5 00:00:20,800 --> 00:00:24,919 Speaker 1: made four astronauts or maybe like made out of astronauts either. 6 00:00:25,000 --> 00:00:27,800 Speaker 1: For hey, it makes me wonder about something. You know, 7 00:00:27,840 --> 00:00:30,520 Speaker 1: how if you're on Earth and you eat too much 8 00:00:30,560 --> 00:00:34,440 Speaker 1: ice cream, you gain weight? Right, Well, if you're in space, 9 00:00:34,640 --> 00:00:37,400 Speaker 1: can you eat all the ice cream you want without 10 00:00:37,479 --> 00:00:41,520 Speaker 1: gaining weight? I think I just invented the world's most 11 00:00:41,640 --> 00:00:45,480 Speaker 1: expensive diet. For a hundred million dollars, or the price 12 00:00:45,520 --> 00:00:47,879 Speaker 1: of a ticket to the space station, you can eat 13 00:00:47,960 --> 00:00:51,000 Speaker 1: all the ice cream you want and still lose weight. 14 00:01:06,959 --> 00:01:10,200 Speaker 1: I'm Daniel, I'm a particle physicist, and I'm not here 15 00:01:10,240 --> 00:01:14,080 Speaker 1: to sell you some crazy weight lost scheme. No. Instead, 16 00:01:14,200 --> 00:01:18,479 Speaker 1: Welcome to the podcast Daniel and Jorge Explain the Universe, 17 00:01:18,800 --> 00:01:22,520 Speaker 1: a production of I Heart Radio. In our podcast, we 18 00:01:22,600 --> 00:01:24,839 Speaker 1: try to explain to you everything we do know about 19 00:01:24,880 --> 00:01:28,080 Speaker 1: the universe and everything we don't know about the universe. 20 00:01:28,400 --> 00:01:31,160 Speaker 1: We bring you to the forefront of science and introduce 21 00:01:31,240 --> 00:01:34,480 Speaker 1: you to the questions that scientists are asking today the 22 00:01:34,600 --> 00:01:37,240 Speaker 1: things that are puzzling physicists that are making them go, 23 00:01:37,760 --> 00:01:41,399 Speaker 1: what how does that work? Because we think that everybody 24 00:01:41,480 --> 00:01:43,920 Speaker 1: deserves to be out there on the forefront of knowledge, 25 00:01:44,200 --> 00:01:47,400 Speaker 1: wondering what the rest of humanity how there's amazing and 26 00:01:47,440 --> 00:01:52,360 Speaker 1: crazy and bonkers universe is put together. Unraveling that mystery. 27 00:01:52,400 --> 00:01:54,920 Speaker 1: Revealing the secrets of the universe, to me, seems like 28 00:01:55,000 --> 00:01:58,720 Speaker 1: the grandest journey of humanity. And it's amazing that it 29 00:01:58,840 --> 00:02:01,880 Speaker 1: even works, that we can by our minds and figure 30 00:02:01,880 --> 00:02:05,160 Speaker 1: out the way the universe works. And as we look 31 00:02:05,240 --> 00:02:09,000 Speaker 1: out into the cosmos, we see incredible stuff out there. 32 00:02:09,320 --> 00:02:12,040 Speaker 1: And so our podcast tries to explain to you what's 33 00:02:12,080 --> 00:02:15,080 Speaker 1: going on out there in the center of stars, around 34 00:02:15,080 --> 00:02:18,520 Speaker 1: the edges of black holes, in all the tiny little particles, 35 00:02:18,880 --> 00:02:20,760 Speaker 1: And we try to teach you also not just what 36 00:02:20,919 --> 00:02:25,080 Speaker 1: science does know, but how it knows it, because you know, 37 00:02:25,160 --> 00:02:27,400 Speaker 1: there is a pattern in the history of science where 38 00:02:27,720 --> 00:02:30,520 Speaker 1: we think we know something, it's sort of established wisdom 39 00:02:30,600 --> 00:02:34,880 Speaker 1: for a while, and then it gets overturned and we discover, oh, 40 00:02:35,120 --> 00:02:38,880 Speaker 1: the universe is actually something different, It works differently from 41 00:02:38,919 --> 00:02:41,640 Speaker 1: the way that we thought it did. Those are the 42 00:02:41,680 --> 00:02:44,040 Speaker 1: best moments, if you ask me, in the history of science, 43 00:02:44,040 --> 00:02:47,079 Speaker 1: when we're peeling back a layer of reality and discovering 44 00:02:47,280 --> 00:02:50,079 Speaker 1: that the universe is pretty different from the way our 45 00:02:50,200 --> 00:02:54,320 Speaker 1: intuitive experience led us to believe. And so when that happens, 46 00:02:54,360 --> 00:02:57,120 Speaker 1: you have to wonder, why do we think the old thing? 47 00:02:57,200 --> 00:02:59,919 Speaker 1: Why did we get it wrong? So it's important sometimes 48 00:03:00,000 --> 00:03:02,919 Speaker 1: them to dig into how do we know what we know? 49 00:03:03,440 --> 00:03:05,040 Speaker 1: And that's what we want to talk about on the 50 00:03:05,040 --> 00:03:08,320 Speaker 1: podcast today. When we look out at the stars in 51 00:03:08,360 --> 00:03:10,440 Speaker 1: the sky, we see some of them are bright and 52 00:03:10,480 --> 00:03:14,040 Speaker 1: some of them are dim. But when scientists talk about stars, 53 00:03:14,080 --> 00:03:16,679 Speaker 1: they're often not just talking about how bright they are. 54 00:03:16,919 --> 00:03:20,480 Speaker 1: They're talking about how big they are, how massive they are. 55 00:03:21,000 --> 00:03:23,160 Speaker 1: Stars with a lot of mass, stars with not so 56 00:03:23,280 --> 00:03:25,800 Speaker 1: much mass, black holes with a huge amount of mass. 57 00:03:26,720 --> 00:03:29,520 Speaker 1: But how is it possible to know that? So on 58 00:03:29,560 --> 00:03:37,440 Speaker 1: today's program, we'll be asking the question, how do you 59 00:03:37,520 --> 00:03:41,240 Speaker 1: measure the mass of a star? And it's not just me, 60 00:03:41,280 --> 00:03:43,960 Speaker 1: and it's not just you who's wondering about this question. 61 00:03:44,200 --> 00:03:48,720 Speaker 1: We actually got this question from a listener. Here's Dustin Hi, 62 00:03:48,800 --> 00:03:52,320 Speaker 1: Daniel and Horror Hey, I wonder how physicists come up 63 00:03:52,360 --> 00:03:55,120 Speaker 1: with the weight of planets and stars. Thank you things 64 00:03:55,200 --> 00:03:58,240 Speaker 1: doesn't very much for sending in your question. And this 65 00:03:58,440 --> 00:04:01,440 Speaker 1: is a really interesting and the important question. It's not 66 00:04:01,520 --> 00:04:04,600 Speaker 1: just an academic one, because the mass of a star 67 00:04:05,000 --> 00:04:08,720 Speaker 1: is really important. How much stuff you gather together to 68 00:04:08,840 --> 00:04:12,840 Speaker 1: form that burning plasma in the sky determines its fate. 69 00:04:13,200 --> 00:04:14,840 Speaker 1: Is it going to end up as a white dwarf, 70 00:04:15,120 --> 00:04:17,440 Speaker 1: is it going to turn into a neutron star? Is 71 00:04:17,440 --> 00:04:20,120 Speaker 1: it going to collapse into a black hole? All of 72 00:04:20,160 --> 00:04:23,680 Speaker 1: those outcomes are determined by how much mass it has, 73 00:04:24,080 --> 00:04:26,919 Speaker 1: and so understanding how much mass there is in stars 74 00:04:27,200 --> 00:04:31,560 Speaker 1: is really important and turns out not that easy. But 75 00:04:31,720 --> 00:04:34,960 Speaker 1: before we dig into how scientists actually do it, what 76 00:04:35,080 --> 00:04:37,599 Speaker 1: they do know, and what they don't know about the 77 00:04:37,640 --> 00:04:40,400 Speaker 1: mass of stars, I went out there into the wilds 78 00:04:40,400 --> 00:04:43,240 Speaker 1: of the internet to ask people how they thought scientists 79 00:04:43,279 --> 00:04:46,560 Speaker 1: did it or how they would do it themselves. So 80 00:04:46,640 --> 00:04:49,840 Speaker 1: thank you very much to everybody who volunteered their brain 81 00:04:49,960 --> 00:04:53,000 Speaker 1: to speculate how to measure the mass of something super 82 00:04:53,040 --> 00:04:55,560 Speaker 1: duper far away that you could never hope to touch. 83 00:04:55,920 --> 00:04:58,839 Speaker 1: And if you would like to participate in future baseless 84 00:04:58,839 --> 00:05:02,840 Speaker 1: speculations for our podcast, please don't hesitate right into us 85 00:05:02,839 --> 00:05:06,919 Speaker 1: two questions at Daniel and Jorge dot com. Here's what 86 00:05:06,960 --> 00:05:09,200 Speaker 1: people had to say, my guts, and it would be 87 00:05:09,240 --> 00:05:14,040 Speaker 1: to say that we know by how bright it is. 88 00:05:14,320 --> 00:05:18,800 Speaker 1: Perhaps it's done by measuring the gravitational lensing effect on 89 00:05:19,000 --> 00:05:23,760 Speaker 1: light passing around the stars, well, possibly with the help 90 00:05:23,839 --> 00:05:27,880 Speaker 1: of mass spect traumata readings. So you know what elements 91 00:05:28,279 --> 00:05:30,839 Speaker 1: you have in the star, and then you try to 92 00:05:30,880 --> 00:05:33,800 Speaker 1: figure out how much of them you have. I could 93 00:05:33,960 --> 00:05:38,880 Speaker 1: guess that poluminosity plays a part in it, and probably 94 00:05:38,960 --> 00:05:42,440 Speaker 1: the orbit, but but I'm not sure, so I would 95 00:05:42,440 --> 00:05:45,480 Speaker 1: say I don't know. I imagine that if we can 96 00:05:45,520 --> 00:05:47,799 Speaker 1: tell the distance to a star, we can probably figure 97 00:05:47,800 --> 00:05:54,320 Speaker 1: out its diameter, and from its light signature we can 98 00:05:54,360 --> 00:06:00,479 Speaker 1: tell its chemical composition, and from the brightness of the 99 00:06:00,560 --> 00:06:05,800 Speaker 1: star itself, I would imagine we could tell from the 100 00:06:05,880 --> 00:06:09,760 Speaker 1: combination of those three things what the mass is somehow 101 00:06:09,800 --> 00:06:12,880 Speaker 1: backing out from that information. All right. I love hearing 102 00:06:12,920 --> 00:06:16,000 Speaker 1: people think on their feet, trying to solve this really 103 00:06:16,040 --> 00:06:19,600 Speaker 1: hard problem from nothing, figuring it out from scratch, and 104 00:06:19,640 --> 00:06:21,839 Speaker 1: there's a lot of good ideas in there. You hear 105 00:06:21,839 --> 00:06:24,240 Speaker 1: people talking about how you need to know the mass 106 00:06:24,320 --> 00:06:27,440 Speaker 1: of other stars nearby or maybe you can connect it 107 00:06:27,440 --> 00:06:30,400 Speaker 1: to the brightness and hey, hey are on the right track. 108 00:06:30,800 --> 00:06:34,800 Speaker 1: So let's talk about how scientists can measure the mass 109 00:06:34,880 --> 00:06:37,880 Speaker 1: of stars. Well, in general, if you're looking at an 110 00:06:37,920 --> 00:06:41,240 Speaker 1: object in space, right, all you're getting is the light 111 00:06:41,279 --> 00:06:45,679 Speaker 1: from that object. It's basically impossible to know how big 112 00:06:46,040 --> 00:06:50,560 Speaker 1: or how massive that object is. Remember what mass is. 113 00:06:50,720 --> 00:06:54,159 Speaker 1: Mass is like how much stuff there is in a star. 114 00:06:54,560 --> 00:06:58,480 Speaker 1: It affects the stars inertia, like how it responds to forces, 115 00:06:58,800 --> 00:07:01,640 Speaker 1: and it also affects the stars gravity. You know how 116 00:07:01,680 --> 00:07:04,480 Speaker 1: hard it pulls on stuff. But none of that can 117 00:07:04,520 --> 00:07:09,160 Speaker 1: be directly observed from the star itself very easily. Mostly 118 00:07:09,520 --> 00:07:13,120 Speaker 1: that's the effect of other stuff on the star, right, 119 00:07:13,200 --> 00:07:16,960 Speaker 1: the inertia or the effect of the star on others stuff. 120 00:07:17,480 --> 00:07:19,920 Speaker 1: And so the short answer is is that in general, 121 00:07:20,440 --> 00:07:24,440 Speaker 1: it's impossible to directly measure the mass of an isolated star. 122 00:07:25,000 --> 00:07:27,080 Speaker 1: We had the same problem when we were measuring the 123 00:07:27,160 --> 00:07:30,520 Speaker 1: age of isolated stars for example. Right, we talked about 124 00:07:30,560 --> 00:07:32,720 Speaker 1: how to measure the age of stars, and it turns 125 00:07:32,720 --> 00:07:34,680 Speaker 1: out that you need to do it in big groups 126 00:07:34,720 --> 00:07:37,400 Speaker 1: because you're seeing how the stars, which are probably formed 127 00:07:37,440 --> 00:07:40,400 Speaker 1: together are probably dying together and you can use that 128 00:07:40,520 --> 00:07:42,880 Speaker 1: as a sort of clock for the population of stars, 129 00:07:43,120 --> 00:07:46,480 Speaker 1: but individual isolated stars, it's very hard to tell how 130 00:07:46,480 --> 00:07:50,280 Speaker 1: old they are. It's a similar problem for measuring their mass. 131 00:07:50,520 --> 00:07:53,160 Speaker 1: If the stars out there and there's nothing near by it, 132 00:07:53,240 --> 00:07:56,680 Speaker 1: nothing that can probe its gravity or be influenced by 133 00:07:56,720 --> 00:07:59,320 Speaker 1: its gravity, or give it a push or a pull, 134 00:07:59,640 --> 00:08:03,360 Speaker 1: it's all most impossible to measure its mass. But that 135 00:08:03,400 --> 00:08:06,680 Speaker 1: doesn't mean the problem is impossible. What we do in 136 00:08:06,680 --> 00:08:09,600 Speaker 1: this case is that we find a special category of 137 00:08:09,680 --> 00:08:12,440 Speaker 1: stars where we can figure out how to measure their 138 00:08:12,480 --> 00:08:16,240 Speaker 1: mass because they're near something, there's something nearby that's tugging 139 00:08:16,280 --> 00:08:18,920 Speaker 1: on them or that they are tugging on. We can 140 00:08:18,960 --> 00:08:21,880 Speaker 1: measure the mass for that sort of special category of stars, 141 00:08:22,200 --> 00:08:24,080 Speaker 1: and then we can try to fill in the gap 142 00:08:24,120 --> 00:08:26,760 Speaker 1: and figure out a way to extrapolate to all the 143 00:08:26,920 --> 00:08:30,800 Speaker 1: other stars. All right, so first let's talk about how 144 00:08:30,800 --> 00:08:33,960 Speaker 1: to measure it for a special category of stars where 145 00:08:34,000 --> 00:08:37,080 Speaker 1: you actually can see them doing some tugging or some pulling, 146 00:08:37,160 --> 00:08:40,360 Speaker 1: where their mass really is important for what we observe. 147 00:08:41,120 --> 00:08:43,640 Speaker 1: And the most powerful way to do this is to 148 00:08:43,760 --> 00:08:47,800 Speaker 1: see binary star systems. These are systems where you have 149 00:08:48,000 --> 00:08:51,560 Speaker 1: two stars nearby, and it seems sort of exotic, like 150 00:08:51,600 --> 00:08:53,640 Speaker 1: the kind of thing you might see in Star Wars 151 00:08:53,720 --> 00:08:56,760 Speaker 1: with multiple suns rising over the horizon. And you know, 152 00:08:56,800 --> 00:08:58,640 Speaker 1: I love that they do that because it's sort of 153 00:08:58,840 --> 00:09:01,839 Speaker 1: gives you a sense that an alien world. And so 154 00:09:02,000 --> 00:09:05,319 Speaker 1: we have the idea I think that binary star systems 155 00:09:05,320 --> 00:09:08,200 Speaker 1: are unusual, that they are weird, and that's just because 156 00:09:08,240 --> 00:09:10,679 Speaker 1: we're used to looking at one son. It turns out 157 00:09:10,720 --> 00:09:14,040 Speaker 1: binary star systems are not that rare. A lot of 158 00:09:14,120 --> 00:09:16,920 Speaker 1: stars are born in pairs, and if you think about it, 159 00:09:16,920 --> 00:09:19,400 Speaker 1: it makes a lot of sense actually, because how is 160 00:09:19,400 --> 00:09:21,880 Speaker 1: the star form? Do you have a big collection of 161 00:09:21,920 --> 00:09:24,840 Speaker 1: gas and dust and stuff that's swirling all around and 162 00:09:25,000 --> 00:09:28,240 Speaker 1: something happens to trigger its collapse and it rushes down 163 00:09:28,280 --> 00:09:31,360 Speaker 1: and collapses. But stars are not formed by themselves. You 164 00:09:31,400 --> 00:09:34,719 Speaker 1: have a huge cloud which usually forms many stars at 165 00:09:34,800 --> 00:09:37,880 Speaker 1: roughly the same time, and so it makes some sense 166 00:09:37,880 --> 00:09:39,920 Speaker 1: for those stars to be tugging on each other and 167 00:09:40,240 --> 00:09:43,200 Speaker 1: even for those stars to end up in orbit around 168 00:09:43,280 --> 00:09:46,840 Speaker 1: each other. So lucky for us, binary star systems are 169 00:09:46,880 --> 00:09:50,080 Speaker 1: not that rare. So what you can do is look 170 00:09:50,120 --> 00:09:52,960 Speaker 1: at the pair of stars, because if they are close 171 00:09:53,040 --> 00:09:55,400 Speaker 1: to each other, even if they are really really far 172 00:09:55,480 --> 00:09:58,200 Speaker 1: from us. Then we can see the effect of their 173 00:09:58,280 --> 00:10:02,280 Speaker 1: gravity on each other, which gives us a clue as 174 00:10:02,320 --> 00:10:06,040 Speaker 1: to what their mass is. Alright, so let's dig into 175 00:10:06,080 --> 00:10:08,959 Speaker 1: it and figure out how that actually works. If you're 176 00:10:08,960 --> 00:10:11,800 Speaker 1: looking at a binary star system, how do you use 177 00:10:12,000 --> 00:10:14,600 Speaker 1: what you're looking at what you see to actually figure 178 00:10:14,640 --> 00:10:17,760 Speaker 1: out what the mass of those stars is. Well. The 179 00:10:17,800 --> 00:10:20,959 Speaker 1: thing to remember about a binary star system, first of all, 180 00:10:21,240 --> 00:10:24,120 Speaker 1: is that it's not one star orbiting the other. The 181 00:10:24,200 --> 00:10:27,400 Speaker 1: two are orbiting each other, right, So there's some like 182 00:10:27,520 --> 00:10:29,880 Speaker 1: point in between them that the two of them are 183 00:10:29,920 --> 00:10:33,520 Speaker 1: both moving around. That's the center of mass. So both 184 00:10:33,760 --> 00:10:37,120 Speaker 1: stars are in motion. Right from the point of view 185 00:10:37,200 --> 00:10:39,360 Speaker 1: one star, the other ones moving, and from the point 186 00:10:39,400 --> 00:10:41,640 Speaker 1: of view of the second star, the first one is moving. 187 00:10:42,200 --> 00:10:44,400 Speaker 1: And what that means is that we can measure their 188 00:10:44,400 --> 00:10:47,880 Speaker 1: relative motion. As the star moves further away from us, 189 00:10:48,120 --> 00:10:51,120 Speaker 1: it's light gets stretched out, the wavelengths get a little 190 00:10:51,160 --> 00:10:54,120 Speaker 1: bit longer, and when it star is moving towards us, 191 00:10:54,559 --> 00:10:57,240 Speaker 1: it's light gets compressed a little bit. The wavelengths get 192 00:10:57,240 --> 00:11:00,400 Speaker 1: a little bit shifted. This is called red shift. When 193 00:11:00,440 --> 00:11:03,000 Speaker 1: it gets longer wavelengths as it moves away from us 194 00:11:03,240 --> 00:11:06,199 Speaker 1: and blue shift as it gets shorter wavelengths when it's 195 00:11:06,240 --> 00:11:08,920 Speaker 1: moving towards us. This is very similar to how you 196 00:11:08,960 --> 00:11:12,120 Speaker 1: discover a planet is orbiting around a star, because you 197 00:11:12,160 --> 00:11:15,640 Speaker 1: see the gravitational effect of the planet makes a star 198 00:11:15,720 --> 00:11:18,280 Speaker 1: wiggle a little bit and that changes the light that 199 00:11:18,320 --> 00:11:20,600 Speaker 1: we see. Now, in that case, you only have a 200 00:11:20,640 --> 00:11:24,079 Speaker 1: single star, and you're deducing the presence of the otherwise 201 00:11:24,160 --> 00:11:27,800 Speaker 1: invisible planet by looking at these Doppler shifts in the 202 00:11:27,920 --> 00:11:31,720 Speaker 1: light from the star. In this case, we have both stars. 203 00:11:32,160 --> 00:11:34,120 Speaker 1: So what you can do is look at the Doppler 204 00:11:34,160 --> 00:11:38,080 Speaker 1: shift from both stars independently, so you get both of 205 00:11:38,120 --> 00:11:41,640 Speaker 1: these curves right, and you can see how these curves change. 206 00:11:41,679 --> 00:11:43,800 Speaker 1: You can see, for example, oh, the star is moving 207 00:11:43,840 --> 00:11:46,120 Speaker 1: away from us. Oh, now it's moving towards us. So 208 00:11:46,240 --> 00:11:49,079 Speaker 1: it's moving away from us, and now it's moving towards us. 209 00:11:49,440 --> 00:11:52,199 Speaker 1: And so from those curves you can get some really 210 00:11:52,240 --> 00:11:55,360 Speaker 1: interesting information. First of all, you can figure out the 211 00:11:55,520 --> 00:11:58,480 Speaker 1: period of each star. How long does it take to 212 00:11:58,600 --> 00:12:01,120 Speaker 1: move around the other one. This is because you can 213 00:12:01,160 --> 00:12:04,800 Speaker 1: see when the velocity turns around right, it gets red shifted, 214 00:12:04,800 --> 00:12:07,680 Speaker 1: then it gets blue shifted. What it flips over is 215 00:12:07,720 --> 00:12:10,280 Speaker 1: that point when it's turning around, and so from those 216 00:12:10,280 --> 00:12:13,680 Speaker 1: flipover points you can figure out what is the period 217 00:12:13,720 --> 00:12:16,400 Speaker 1: of this star moving around the other one, And you 218 00:12:16,400 --> 00:12:20,200 Speaker 1: can also actually measure the velocity of each star around 219 00:12:20,240 --> 00:12:23,280 Speaker 1: the other one. This is determined by the amount of 220 00:12:23,360 --> 00:12:26,440 Speaker 1: red shift and blue shift, or at the larger the velocity, 221 00:12:26,679 --> 00:12:29,000 Speaker 1: the more red shift and blue shift you get as 222 00:12:29,040 --> 00:12:32,240 Speaker 1: it swings around the other star. So we can just 223 00:12:32,280 --> 00:12:35,120 Speaker 1: by watching the color of the light from these two 224 00:12:35,160 --> 00:12:38,920 Speaker 1: stars change figure out what the period is, how long 225 00:12:38,960 --> 00:12:42,360 Speaker 1: it takes for them to orbit each other, and their velocities. 226 00:12:42,760 --> 00:12:45,520 Speaker 1: And that's really awesome because then we could just plug 227 00:12:45,559 --> 00:12:50,000 Speaker 1: it into an ancient equation. Kepler's third law tells you 228 00:12:50,400 --> 00:12:54,160 Speaker 1: exactly what the combined mass of the system is if 229 00:12:54,200 --> 00:12:57,680 Speaker 1: you know the period and you know these velocities. So 230 00:12:57,880 --> 00:13:01,120 Speaker 1: that's pretty cool. Now we know just from the period 231 00:13:01,160 --> 00:13:03,720 Speaker 1: and those velocities, we know the total mass of this 232 00:13:03,840 --> 00:13:07,760 Speaker 1: binary star system, but we also know their relative velocities. 233 00:13:07,800 --> 00:13:10,800 Speaker 1: We know which one is moving faster than the other one. Say, 234 00:13:10,840 --> 00:13:13,640 Speaker 1: for example, they don't have an equal amount of mass. 235 00:13:13,679 --> 00:13:17,800 Speaker 1: It's not like two equal binary stars, but instead maybe 236 00:13:17,800 --> 00:13:19,760 Speaker 1: one of them is much bigger than the other one, 237 00:13:20,000 --> 00:13:22,319 Speaker 1: the bigger one is going to be moving slower and 238 00:13:22,400 --> 00:13:24,400 Speaker 1: the smaller one is going to be doing more of 239 00:13:24,440 --> 00:13:27,320 Speaker 1: the moving around the bigger one. So in the case 240 00:13:27,360 --> 00:13:29,880 Speaker 1: when the two stars have the same mass, will have 241 00:13:29,960 --> 00:13:32,760 Speaker 1: the same relative velocity. In the case when the two 242 00:13:32,840 --> 00:13:36,679 Speaker 1: stars have very unequal masses, when it's very asymmetric, then 243 00:13:36,720 --> 00:13:39,120 Speaker 1: one of them will be moving faster than the other one. 244 00:13:39,520 --> 00:13:42,680 Speaker 1: And so from this relative velocity, which again we know, 245 00:13:43,200 --> 00:13:45,200 Speaker 1: we can figure out how to split up the total 246 00:13:45,280 --> 00:13:48,280 Speaker 1: mass of the systems into the two masses, and so 247 00:13:48,440 --> 00:13:52,080 Speaker 1: boom that means from a binary star system, just from 248 00:13:52,080 --> 00:13:55,440 Speaker 1: watching their light wiggle, we can figure out what is 249 00:13:55,480 --> 00:13:58,560 Speaker 1: the mass of each star in the system. And one 250 00:13:58,600 --> 00:14:00,960 Speaker 1: of my favorite things about this is that it doesn't 251 00:14:01,040 --> 00:14:03,560 Speaker 1: just work for stars that are close by. It also 252 00:14:03,600 --> 00:14:06,880 Speaker 1: works for stars that are super duper far away, because 253 00:14:06,920 --> 00:14:08,840 Speaker 1: you just have to look at the light that's coming 254 00:14:08,880 --> 00:14:11,480 Speaker 1: from the star. It's not like you need to see 255 00:14:11,520 --> 00:14:13,840 Speaker 1: the gap between the stars or anything. You just need 256 00:14:13,920 --> 00:14:16,760 Speaker 1: to look at the light pattern. And the cool thing 257 00:14:16,760 --> 00:14:19,600 Speaker 1: about the Doppler shift is that it doesn't like disappear. 258 00:14:19,960 --> 00:14:22,840 Speaker 1: If there's a Doppler shift pattern in light that comes 259 00:14:22,840 --> 00:14:25,760 Speaker 1: to us from something really really far away, there's still 260 00:14:25,840 --> 00:14:29,080 Speaker 1: that same shift when it gets here. It will persist 261 00:14:29,160 --> 00:14:32,600 Speaker 1: over billions and billions of light years of space. And 262 00:14:32,640 --> 00:14:34,720 Speaker 1: so this is powerful because it lets us look at 263 00:14:34,760 --> 00:14:38,080 Speaker 1: even further away stars. So we get like a larger sample, 264 00:14:38,400 --> 00:14:41,440 Speaker 1: so we can learn like a more general trend rather 265 00:14:41,480 --> 00:14:44,760 Speaker 1: than just understanding something that's happening in our neighborhood. And 266 00:14:44,800 --> 00:14:47,800 Speaker 1: also we can tell whether it's something that's true here 267 00:14:48,000 --> 00:14:51,360 Speaker 1: and something that's true far away. We always want to 268 00:14:51,400 --> 00:14:54,240 Speaker 1: be open to surprises when we look out into the universe. 269 00:14:54,560 --> 00:14:56,800 Speaker 1: We don't want to draw too many conclusions just by 270 00:14:56,840 --> 00:15:00,640 Speaker 1: looking at our cosmic neighborhood. So it's important a technique 271 00:15:00,640 --> 00:15:03,560 Speaker 1: that works for nearby and also works for really far 272 00:15:03,600 --> 00:15:07,480 Speaker 1: away stuff, all right. So that's how we measure the 273 00:15:07,600 --> 00:15:11,680 Speaker 1: mass of binary stars of special star systems where we 274 00:15:11,720 --> 00:15:14,040 Speaker 1: have two stars that we can see and we can 275 00:15:14,080 --> 00:15:16,920 Speaker 1: measure their velocities from the wiggles in their light. But 276 00:15:17,000 --> 00:15:19,680 Speaker 1: we're interested in all the stars. We want to know 277 00:15:19,720 --> 00:15:22,120 Speaker 1: what is the mass of any given star we see 278 00:15:22,200 --> 00:15:24,680 Speaker 1: out there in the universe, even the ones that are 279 00:15:24,800 --> 00:15:28,000 Speaker 1: not in binary star systems. So how do we do that. 280 00:15:28,240 --> 00:15:30,400 Speaker 1: We'll talk about that in a moment, but first I 281 00:15:30,440 --> 00:15:47,280 Speaker 1: want to take a quick break. All right, we are back, 282 00:15:47,400 --> 00:15:50,800 Speaker 1: and we are blowing your mind by thinking about incredible 283 00:15:50,920 --> 00:15:54,600 Speaker 1: huge pockets of gas out there that are fusing themselves 284 00:15:54,640 --> 00:15:58,520 Speaker 1: and radiating photons which zoom across billions of light years 285 00:15:58,520 --> 00:16:01,280 Speaker 1: of the universe before they it to our eyes and 286 00:16:01,400 --> 00:16:05,640 Speaker 1: our telescopes and carry with them incredible nuggets of knowledge 287 00:16:05,880 --> 00:16:09,600 Speaker 1: about what's happening in far away corners of the universe. 288 00:16:09,960 --> 00:16:11,840 Speaker 1: And we're gonna use that light to figure out the 289 00:16:11,840 --> 00:16:15,280 Speaker 1: answer to a really interesting question, which is how big 290 00:16:15,440 --> 00:16:19,400 Speaker 1: is that star? How massive is it? What is its future? 291 00:16:19,640 --> 00:16:22,480 Speaker 1: Is it a huge blob of hydrogen which will eventually 292 00:16:22,480 --> 00:16:24,680 Speaker 1: collapse into a black hole or is it going to 293 00:16:24,840 --> 00:16:28,040 Speaker 1: end up a glowing white dwarf for trillions of years? 294 00:16:28,520 --> 00:16:32,400 Speaker 1: And that is entirely determined by how much mass it has. 295 00:16:32,680 --> 00:16:35,040 Speaker 1: So we talked about how to measure the mass of 296 00:16:35,080 --> 00:16:39,240 Speaker 1: a special category of star stars. Will we identify two 297 00:16:39,240 --> 00:16:41,440 Speaker 1: of them near each other that are orbiting each other, 298 00:16:41,680 --> 00:16:44,200 Speaker 1: so we can use their gravitational effect on each other, 299 00:16:44,400 --> 00:16:48,360 Speaker 1: which determines their relative periods and velocities of their orbits 300 00:16:48,520 --> 00:16:51,800 Speaker 1: to figure out how much mass there is exactly, But 301 00:16:51,960 --> 00:16:53,800 Speaker 1: now we want to move beyond that. We want to 302 00:16:53,880 --> 00:16:57,840 Speaker 1: understand can we talk about any arbitrary star. We see 303 00:16:57,840 --> 00:16:59,400 Speaker 1: a star in the sky, and we want to know 304 00:16:59,640 --> 00:17:02,520 Speaker 1: how much mass does it have? How can we figure 305 00:17:02,560 --> 00:17:05,439 Speaker 1: that out if it doesn't happen to have a big 306 00:17:05,640 --> 00:17:09,680 Speaker 1: massive object near it that lets us directly measure its mass, 307 00:17:09,760 --> 00:17:11,800 Speaker 1: And so here we have to be very careful. We 308 00:17:11,880 --> 00:17:15,359 Speaker 1: have no direct way to measure the mass of those stars. 309 00:17:15,400 --> 00:17:17,840 Speaker 1: But what we do is we look for a connection 310 00:17:18,320 --> 00:17:22,000 Speaker 1: between what we can measure, like the brightness of a star, 311 00:17:22,359 --> 00:17:24,760 Speaker 1: and what we want to know, like the mass of 312 00:17:24,800 --> 00:17:27,159 Speaker 1: the star. And we look for that connection not in 313 00:17:27,240 --> 00:17:30,359 Speaker 1: the actual stars out there in the universe, but in 314 00:17:30,400 --> 00:17:34,679 Speaker 1: the stars here in our computers at home. Because we 315 00:17:34,760 --> 00:17:37,760 Speaker 1: have an idea for how stars work, We have some 316 00:17:37,920 --> 00:17:41,240 Speaker 1: theory about it. We know the nuclear fusion that happens 317 00:17:41,280 --> 00:17:44,679 Speaker 1: inside stars. We think we understand something of the gravitational 318 00:17:44,760 --> 00:17:47,960 Speaker 1: pressure that's pulling these things down and making these things happen. 319 00:17:48,280 --> 00:17:50,639 Speaker 1: We compare a lot of these models to what we 320 00:17:50,680 --> 00:17:54,560 Speaker 1: observe about nearby stars. So we've been spending decades developing 321 00:17:54,560 --> 00:17:58,120 Speaker 1: these sort of like theory of how stars work, and 322 00:17:58,560 --> 00:18:01,960 Speaker 1: in that theory, at least in our computational models of stars, 323 00:18:02,320 --> 00:18:05,919 Speaker 1: we do see a relationship. We see a connection between 324 00:18:05,960 --> 00:18:08,240 Speaker 1: the mass of the star, which is the thing we 325 00:18:08,320 --> 00:18:13,080 Speaker 1: want to know but can't directly observe, and the luminosity 326 00:18:13,119 --> 00:18:15,960 Speaker 1: of the star, the brightness of the star. In fact, 327 00:18:16,040 --> 00:18:19,440 Speaker 1: we see a pretty direct relationship. While talking a minute 328 00:18:19,440 --> 00:18:22,000 Speaker 1: about what that relationship is and why we think it 329 00:18:22,040 --> 00:18:25,040 Speaker 1: makes sense the sort of physics that underlies the connection 330 00:18:25,440 --> 00:18:28,320 Speaker 1: between the mass of a star and its brightness, but 331 00:18:28,359 --> 00:18:31,359 Speaker 1: first let's make sure we're understanding the larger strategy. What 332 00:18:31,480 --> 00:18:34,040 Speaker 1: we're gonna do here is get a connection sort of 333 00:18:34,200 --> 00:18:37,439 Speaker 1: in our theory or in our simulations, between the mass 334 00:18:37,640 --> 00:18:40,520 Speaker 1: and the luminosity, and then we're going to calibrate that. 335 00:18:40,560 --> 00:18:43,639 Speaker 1: We're gonna make sure it's right by looking at binary 336 00:18:43,680 --> 00:18:46,560 Speaker 1: star systems. So binary star systems give us a way 337 00:18:46,600 --> 00:18:50,520 Speaker 1: to actually measure both the mass and the luminosity. And 338 00:18:50,600 --> 00:18:52,920 Speaker 1: then we have these calculations we can do that can 339 00:18:53,040 --> 00:18:57,520 Speaker 1: connect mass to luminosity, and we'll use the actual measurements 340 00:18:57,560 --> 00:19:01,080 Speaker 1: to make sure that calculation is correct and various points. 341 00:19:01,600 --> 00:19:04,240 Speaker 1: So we have like sort of a string that connects 342 00:19:04,400 --> 00:19:07,440 Speaker 1: mass and luminosity, and then we have various pushpins we're 343 00:19:07,440 --> 00:19:09,960 Speaker 1: gonna put into it to make sure that it's sort 344 00:19:09,960 --> 00:19:14,320 Speaker 1: of nailed down by actual measurements in reality, and between 345 00:19:14,359 --> 00:19:16,960 Speaker 1: those pushpins, between the actual measurements we make for the 346 00:19:16,960 --> 00:19:21,080 Speaker 1: binary stars were basically extrapolating the little bit of guesswork 347 00:19:21,160 --> 00:19:23,919 Speaker 1: and a lot of complicated nuclear theory, but it's not 348 00:19:24,040 --> 00:19:26,960 Speaker 1: something we actually know, and so there's, for example, a 349 00:19:27,040 --> 00:19:29,760 Speaker 1: gap where somebody could come in later and decide, oh, 350 00:19:29,800 --> 00:19:32,200 Speaker 1: it turns out our model for stars was actually wrong 351 00:19:32,240 --> 00:19:35,359 Speaker 1: and these extrapolations didn't quite work. So it's important for 352 00:19:35,359 --> 00:19:38,000 Speaker 1: you guys to understand while we actually know what we 353 00:19:38,080 --> 00:19:40,720 Speaker 1: actually can measure, which is the mass of a few 354 00:19:40,760 --> 00:19:43,399 Speaker 1: binary star systems, and where we get the rest of 355 00:19:43,440 --> 00:19:46,480 Speaker 1: the information which comes from this nuclear theory, which helps 356 00:19:46,560 --> 00:19:50,000 Speaker 1: us sort of interpolate between the examples that we can't 357 00:19:50,160 --> 00:19:54,760 Speaker 1: measure directly. Alright, So we have a connection in our 358 00:19:54,880 --> 00:19:58,560 Speaker 1: models between the mass and the luminosity, and it's really 359 00:19:58,640 --> 00:20:02,639 Speaker 1: kind of fascinating. It tells us that the larger the mass, 360 00:20:02,680 --> 00:20:06,520 Speaker 1: the brighter the star. Right, the bigger your original scoop 361 00:20:06,640 --> 00:20:10,199 Speaker 1: size of hydrogen, the brighter the star is the faster 362 00:20:10,320 --> 00:20:12,800 Speaker 1: it's gonna burn, And that means something else really cool, 363 00:20:12,880 --> 00:20:16,480 Speaker 1: which means that big stars burn bright, they burn hot, 364 00:20:16,560 --> 00:20:19,800 Speaker 1: but they don't burn for very long. So the big 365 00:20:19,840 --> 00:20:22,920 Speaker 1: ones are like flashy and exciting and very very bright 366 00:20:22,960 --> 00:20:25,960 Speaker 1: and shine their love into the universe, but not for 367 00:20:26,080 --> 00:20:28,880 Speaker 1: that long, whereas the little star that could it's sort 368 00:20:28,880 --> 00:20:31,919 Speaker 1: of out there pumping not nearly as much light, but 369 00:20:32,000 --> 00:20:35,119 Speaker 1: he can do it for a much much longer time. 370 00:20:35,560 --> 00:20:38,760 Speaker 1: These little stars can last for billions or maybe even 371 00:20:38,960 --> 00:20:43,160 Speaker 1: trillions of years, whereas the really really big stars only 372 00:20:43,240 --> 00:20:46,119 Speaker 1: last for like a few hundred million years before the 373 00:20:46,160 --> 00:20:49,040 Speaker 1: party is over. So what does that actually mean? Well, 374 00:20:49,280 --> 00:20:52,920 Speaker 1: the relationship for stars, like around the mass of our sun, 375 00:20:52,960 --> 00:20:54,879 Speaker 1: are a little bit less and then up to about 376 00:20:54,960 --> 00:20:58,520 Speaker 1: fifty times the mass of our sun, the luminosity of 377 00:20:58,520 --> 00:21:01,920 Speaker 1: a star goes like the mass to the fourth power. 378 00:21:02,880 --> 00:21:05,680 Speaker 1: That means that a star twice as massive as our 379 00:21:05,720 --> 00:21:09,840 Speaker 1: sun will be like almost sixteen times as bright as 380 00:21:09,880 --> 00:21:13,160 Speaker 1: our sun. A star twenty times the mass of our 381 00:21:13,200 --> 00:21:17,959 Speaker 1: sun will be a hundred and twenty thousand times as bright. 382 00:21:18,520 --> 00:21:20,520 Speaker 1: That's right, you double the mass of the star, you 383 00:21:20,520 --> 00:21:23,879 Speaker 1: don't just double the brightness, right, it goes up by 384 00:21:23,920 --> 00:21:28,360 Speaker 1: the power of four, and so it increases very very quickly. 385 00:21:28,640 --> 00:21:31,640 Speaker 1: And that gives you a sense for why these really 386 00:21:31,680 --> 00:21:35,720 Speaker 1: big stars burned out so quickly, because they were incredibly bright. 387 00:21:36,040 --> 00:21:38,520 Speaker 1: The bigger the star, the brighter the star, and the 388 00:21:38,600 --> 00:21:42,280 Speaker 1: faster it dies. It also means if you turn your 389 00:21:42,280 --> 00:21:46,040 Speaker 1: attention the other direction, that stars that are less bright 390 00:21:46,080 --> 00:21:49,440 Speaker 1: than the Sun are much much dimmer and burn much 391 00:21:49,640 --> 00:21:53,439 Speaker 1: much longer. For example, there are stars out there that 392 00:21:53,520 --> 00:21:56,399 Speaker 1: have a fraction of the mass of our sun and 393 00:21:56,440 --> 00:22:00,840 Speaker 1: they burn like one ten thousands as brightly as our sun. 394 00:22:01,359 --> 00:22:04,240 Speaker 1: Imagine living on a planet around the star that was 395 00:22:04,359 --> 00:22:08,280 Speaker 1: one ten thousands the brightness of our star. Right, you 396 00:22:08,280 --> 00:22:11,760 Speaker 1: could be much much closer to the star without it 397 00:22:11,880 --> 00:22:14,200 Speaker 1: being brighter than the Sun, which means it would fill 398 00:22:14,320 --> 00:22:18,320 Speaker 1: up a much bigger area in your sky. Right, so 399 00:22:18,560 --> 00:22:21,160 Speaker 1: you could have, for example, a nice, warm, toasty day 400 00:22:21,440 --> 00:22:24,840 Speaker 1: in front of a huge sun in your sky without 401 00:22:24,880 --> 00:22:28,320 Speaker 1: actually getting burned. So the opportunity is there for like 402 00:22:28,480 --> 00:22:31,800 Speaker 1: crazy science fiction ideas for what it would be like 403 00:22:31,960 --> 00:22:34,800 Speaker 1: to have a huge star in your sky seemed pretty 404 00:22:34,800 --> 00:22:37,879 Speaker 1: wide open to me. And so this relationship, this fourth 405 00:22:37,920 --> 00:22:41,200 Speaker 1: power relationship, is true for stars that are like half 406 00:22:41,200 --> 00:22:43,639 Speaker 1: the mass of the Sun up to about fifty times 407 00:22:43,680 --> 00:22:45,920 Speaker 1: the mass of the Sun, and then there's some kinks 408 00:22:45,920 --> 00:22:48,600 Speaker 1: in it. Like above fifty times the mass of the Sun, 409 00:22:48,840 --> 00:22:52,639 Speaker 1: there's a different relationship. It actually becomes linear, doesn't grow 410 00:22:52,760 --> 00:22:55,640 Speaker 1: as quickly, and below half the mass of the Sun, 411 00:22:56,080 --> 00:22:59,440 Speaker 1: the relationship changes again. It goes more like mass squared. 412 00:23:00,040 --> 00:23:02,800 Speaker 1: And so there's some details there, but let's understand sort 413 00:23:02,840 --> 00:23:05,880 Speaker 1: of the general idea. Why is it that a more 414 00:23:06,160 --> 00:23:10,440 Speaker 1: massive star would burn brightly and so much more brightly. 415 00:23:10,760 --> 00:23:13,720 Speaker 1: Why isn't the relationship just linear. Why isn't it that 416 00:23:13,800 --> 00:23:16,080 Speaker 1: if you have twice as much mass in the star, 417 00:23:16,520 --> 00:23:19,960 Speaker 1: it burns twice as bright and twice as hot. Well, 418 00:23:19,960 --> 00:23:23,560 Speaker 1: the answer comes from the details of the nuclear theory. 419 00:23:23,920 --> 00:23:26,840 Speaker 1: So let's dig into what's going on on the inside 420 00:23:26,880 --> 00:23:30,520 Speaker 1: of this star, right, Why is the star made at all? Right, 421 00:23:30,560 --> 00:23:33,960 Speaker 1: a star comes from the gravitational pressure. You have a 422 00:23:34,000 --> 00:23:36,760 Speaker 1: big collection of gas out there in space, and it's 423 00:23:36,840 --> 00:23:40,439 Speaker 1: gravity that's tugging it together. But gravity is not the 424 00:23:40,520 --> 00:23:43,359 Speaker 1: only thing active in a star, right. If it were, 425 00:23:43,560 --> 00:23:46,840 Speaker 1: then every star and every object would just collapse into 426 00:23:46,840 --> 00:23:49,800 Speaker 1: a black hole. Now, a star is a pretty steady 427 00:23:49,880 --> 00:23:53,240 Speaker 1: state object. It can last for millions or billions or 428 00:23:53,320 --> 00:23:57,160 Speaker 1: maybe trillions of years because there are two forces at play. 429 00:23:57,400 --> 00:24:00,400 Speaker 1: There are two things they're pushing against each other, which 430 00:24:00,440 --> 00:24:02,800 Speaker 1: is what keeps the star stable. So there has to 431 00:24:02,800 --> 00:24:05,359 Speaker 1: be some sort of outward pressure to keep a star 432 00:24:05,480 --> 00:24:08,280 Speaker 1: from collapsing. In the case of the most stars, that 433 00:24:08,359 --> 00:24:12,640 Speaker 1: pressure comes from the fusion, from the nuclear reaction that's 434 00:24:12,680 --> 00:24:15,960 Speaker 1: powering the star itself. And it's fascinating to me because 435 00:24:15,960 --> 00:24:18,760 Speaker 1: it's such a give and take, like gravity pushes on 436 00:24:18,800 --> 00:24:22,880 Speaker 1: the star, squeezes on the star. That's what actually causes fusion, 437 00:24:23,240 --> 00:24:26,960 Speaker 1: and that fusion releases a lot of radiation. That radiation, 438 00:24:27,000 --> 00:24:30,720 Speaker 1: that brightness, that energy pushes the star back out right. 439 00:24:30,760 --> 00:24:34,000 Speaker 1: So gravity causes fusion, which creates this sort of like 440 00:24:34,119 --> 00:24:37,640 Speaker 1: back reaction which pushes out on the star to keep 441 00:24:37,760 --> 00:24:41,200 Speaker 1: it from collapsing. And there's a whole fascinating rabbit hole 442 00:24:41,240 --> 00:24:43,880 Speaker 1: you could go down there, right because then that fusion 443 00:24:43,920 --> 00:24:47,320 Speaker 1: creates heavier elements, which increases the gravity dot dot dot. 444 00:24:47,600 --> 00:24:49,880 Speaker 1: But there's a whole other podcast episode about the life 445 00:24:49,880 --> 00:24:52,800 Speaker 1: cycle of stars. Let's focus right now and just what's 446 00:24:52,840 --> 00:24:55,560 Speaker 1: happening inside a star and why if a star gets 447 00:24:55,600 --> 00:24:59,959 Speaker 1: more massive, it also gets brighter. So the reason simply 448 00:25:00,200 --> 00:25:03,520 Speaker 1: is that a more massive star has more gravity. Right, 449 00:25:03,920 --> 00:25:06,880 Speaker 1: there's a bigger helping of mass, and each of those 450 00:25:06,920 --> 00:25:10,240 Speaker 1: little particles has a little gravitational force on it. Each 451 00:25:10,280 --> 00:25:12,800 Speaker 1: of them is then squeezing the center of the star. 452 00:25:13,400 --> 00:25:15,359 Speaker 1: So if you take a star the mass of our 453 00:25:15,400 --> 00:25:18,439 Speaker 1: sun and you wrap it in another helping of hydrogen, 454 00:25:18,720 --> 00:25:23,840 Speaker 1: another solar mass of hydrogen, there's gonna be additional gravitational forces. Right, 455 00:25:24,000 --> 00:25:26,919 Speaker 1: It's going to increase the pressure on the center of 456 00:25:26,960 --> 00:25:29,679 Speaker 1: the star. And so in order for the star to 457 00:25:29,720 --> 00:25:32,640 Speaker 1: be stable, for it to avoid collapsing into a black hole, 458 00:25:33,080 --> 00:25:36,600 Speaker 1: it needs more pressure outwards. Right. It needs to be 459 00:25:36,680 --> 00:25:40,639 Speaker 1: sending out more radiation. It needs to be brighter. And 460 00:25:40,680 --> 00:25:42,720 Speaker 1: you might think, well, just because it needs to be 461 00:25:42,760 --> 00:25:45,760 Speaker 1: brighter doesn't mean that it does. Right. Well, stars that 462 00:25:46,040 --> 00:25:48,879 Speaker 1: don't get brighter, that don't provide those pressures, they do 463 00:25:48,960 --> 00:25:52,280 Speaker 1: collapse into black holes. Right, So if there's a mechanism 464 00:25:52,320 --> 00:25:55,400 Speaker 1: to provide that pressure, that's what keeps the star is stable, 465 00:25:55,840 --> 00:25:58,919 Speaker 1: and in fact there is because increasing the pressure on 466 00:25:58,960 --> 00:26:02,720 Speaker 1: the core of the star increases the temperature. And when 467 00:26:02,760 --> 00:26:06,080 Speaker 1: you increase the temperature at the core. The nuclear theory 468 00:26:06,119 --> 00:26:09,520 Speaker 1: tells us something happens the fusion that's happening at the 469 00:26:09,520 --> 00:26:12,880 Speaker 1: core of the star, the merging of hydrogen into helium 470 00:26:12,880 --> 00:26:16,119 Speaker 1: and then helium into heavier elements to release more energy. 471 00:26:16,480 --> 00:26:22,080 Speaker 1: The rate of that fusion depends very sensitively on the temperature, 472 00:26:22,440 --> 00:26:25,959 Speaker 1: and so here's where that nonlinearity effect comes in. Here's 473 00:26:26,000 --> 00:26:29,000 Speaker 1: why doubling the mass of a star doesn't just make 474 00:26:29,040 --> 00:26:32,320 Speaker 1: it twice as bright, because the nuclear fusion at its 475 00:26:32,320 --> 00:26:35,760 Speaker 1: core is very very sensitive to the temperatures. That's the 476 00:26:35,840 --> 00:26:39,439 Speaker 1: nonlinear response. If you crank up the temperature of the 477 00:26:39,440 --> 00:26:41,760 Speaker 1: core of the star by just a little bit, you 478 00:26:41,800 --> 00:26:44,920 Speaker 1: can lead to a very large increase in the reaction rate. 479 00:26:45,240 --> 00:26:47,560 Speaker 1: And this is something that's very complicated and difficult to 480 00:26:47,560 --> 00:26:49,640 Speaker 1: wrap your mind around. But just because there's so many 481 00:26:49,760 --> 00:26:52,199 Speaker 1: particles involved, I mean, you have to sort of like 482 00:26:52,480 --> 00:26:54,879 Speaker 1: take your mind and go deep into the center of 483 00:26:54,880 --> 00:26:58,280 Speaker 1: the star and imagine all of those hygrogen atoms pushing 484 00:26:58,280 --> 00:27:01,879 Speaker 1: against each other. Because remember, hydrogen doesn't like to fuse. 485 00:27:02,280 --> 00:27:05,000 Speaker 1: It's nuclei are positively charged. You've got a bunch of 486 00:27:05,000 --> 00:27:08,040 Speaker 1: protons in there, and protons push away from each other. 487 00:27:08,440 --> 00:27:11,080 Speaker 1: In order to get the hydrogen to fuse and helium, 488 00:27:11,280 --> 00:27:14,280 Speaker 1: you've got to really squeeze it down. You have to 489 00:27:14,320 --> 00:27:16,720 Speaker 1: have a lot of pressure at the same time, you 490 00:27:16,760 --> 00:27:18,280 Speaker 1: have to have a lot of energy. You need a 491 00:27:18,359 --> 00:27:21,159 Speaker 1: high temperature because you need those hygen atoms to be 492 00:27:21,200 --> 00:27:24,560 Speaker 1: banging into each other, and so you can have conditions 493 00:27:24,600 --> 00:27:27,440 Speaker 1: at a very high pressure and temperature but without fusion, 494 00:27:27,680 --> 00:27:30,200 Speaker 1: and then all of a sudden fusion happens. You've sort 495 00:27:30,200 --> 00:27:34,320 Speaker 1: of overcome a threshold. And then once that threshold happens, 496 00:27:34,560 --> 00:27:36,879 Speaker 1: more fusion leads to more fusion because more of it 497 00:27:36,920 --> 00:27:39,359 Speaker 1: gets to that high pressure and temperature that you need. 498 00:27:39,760 --> 00:27:43,280 Speaker 1: So there's a very nonlinear effect there, because hydrogen doesn't 499 00:27:43,320 --> 00:27:46,240 Speaker 1: want to fuse at all, and once you've created the 500 00:27:46,240 --> 00:27:48,840 Speaker 1: conditions for fusion, it can lead to sort of a 501 00:27:48,920 --> 00:27:53,359 Speaker 1: runaway process. So that's the basic idea. The reason that 502 00:27:53,640 --> 00:27:57,440 Speaker 1: brighter stars are more massive is because there's a higher 503 00:27:57,480 --> 00:28:02,240 Speaker 1: temperature at their core created by this incredible gravitational pressure 504 00:28:02,480 --> 00:28:05,679 Speaker 1: from the additional mass, and that higher temperature leads to 505 00:28:05,720 --> 00:28:09,119 Speaker 1: a faster nuclear reaction. And of course that faster nuclear 506 00:28:09,119 --> 00:28:11,680 Speaker 1: reaction is not just gonna make the star brighter which 507 00:28:11,720 --> 00:28:14,360 Speaker 1: provides the pressure you need to keep the star alive. 508 00:28:14,800 --> 00:28:18,040 Speaker 1: It's also going to eventually kill the star because that 509 00:28:18,160 --> 00:28:22,520 Speaker 1: faster nuclear reaction uses up the fuel. These stars are 510 00:28:22,560 --> 00:28:24,679 Speaker 1: made out of a lot of hydrogen, but there's not 511 00:28:24,720 --> 00:28:28,240 Speaker 1: an infinite supply of it, right, Eventually the star is 512 00:28:28,280 --> 00:28:31,600 Speaker 1: gonna fuse so much hydrogen into helium that's gonna get 513 00:28:31,600 --> 00:28:34,600 Speaker 1: a helium core, and that helium is gonna fuse and 514 00:28:34,600 --> 00:28:37,479 Speaker 1: you're gonna get something denser and denser and denser, And 515 00:28:37,560 --> 00:28:40,000 Speaker 1: so that's just a ticking clock. Right. You have a 516 00:28:40,080 --> 00:28:43,240 Speaker 1: fixed amount of fuel. Once you've formed a star, it's 517 00:28:43,280 --> 00:28:46,520 Speaker 1: difficult for it to like create more matter unless it 518 00:28:46,560 --> 00:28:50,320 Speaker 1: has some like very close binary you can steal matter from. 519 00:28:50,360 --> 00:28:52,480 Speaker 1: So then it's just a question of like how many 520 00:28:52,520 --> 00:28:55,960 Speaker 1: fusion cycles can it have. If it's a really big star, 521 00:28:56,040 --> 00:28:58,960 Speaker 1: it will burn through those really really quickly and eventually 522 00:28:59,000 --> 00:29:01,440 Speaker 1: get to the stage where collapses. Or if it's a 523 00:29:01,480 --> 00:29:04,160 Speaker 1: little star and it's burning in a much lower temperature 524 00:29:04,160 --> 00:29:07,120 Speaker 1: and brightness, then it's sort of like biding its time 525 00:29:07,120 --> 00:29:09,960 Speaker 1: and it can last a lot lot longer. You might 526 00:29:10,120 --> 00:29:12,240 Speaker 1: expect it to be the opposite, right, You might expect 527 00:29:12,240 --> 00:29:14,800 Speaker 1: that big stars have more fuel and so they can 528 00:29:14,920 --> 00:29:18,640 Speaker 1: last longer into the deep future of the universe. But 529 00:29:18,720 --> 00:29:21,719 Speaker 1: it's sort of surprisingly the opposite. Those big stars do 530 00:29:21,840 --> 00:29:24,240 Speaker 1: have more fuel, but they also burn more fuel, and 531 00:29:24,240 --> 00:29:28,360 Speaker 1: they burn more fuel per second because they're burning hotter. 532 00:29:28,680 --> 00:29:31,240 Speaker 1: So that's an idea for why there's this connection between 533 00:29:31,240 --> 00:29:33,440 Speaker 1: the mass of the star and the brightness of the star, 534 00:29:33,840 --> 00:29:36,120 Speaker 1: and that helps us sort of connect the dots between 535 00:29:36,160 --> 00:29:39,920 Speaker 1: the things that we actually can observe. Those binary star systems, 536 00:29:40,160 --> 00:29:42,760 Speaker 1: the ones where we see the masses have an effect 537 00:29:42,800 --> 00:29:44,920 Speaker 1: on each other, and that tells us whether we get 538 00:29:44,920 --> 00:29:47,880 Speaker 1: in these calculations correct and it gives us confidence that 539 00:29:47,960 --> 00:29:50,240 Speaker 1: we can interpolate between the things that we can see. 540 00:29:50,640 --> 00:29:53,480 Speaker 1: But people are always working to improve these measurements. People 541 00:29:53,480 --> 00:29:56,120 Speaker 1: are always looking for other ways to double check it, 542 00:29:56,400 --> 00:29:59,440 Speaker 1: to figure out if this is wrong, to find another 543 00:29:59,520 --> 00:30:01,880 Speaker 1: way to measure these things. The best thing you can 544 00:30:01,880 --> 00:30:05,880 Speaker 1: do in sciences have two totally independent measurements, ones that 545 00:30:05,960 --> 00:30:09,440 Speaker 1: make maybe different mistakes or different assumptions that cross check 546 00:30:09,480 --> 00:30:12,000 Speaker 1: each other. And so there are a couple of other 547 00:30:12,120 --> 00:30:15,200 Speaker 1: ways we can get a handle on the mass of stars, 548 00:30:15,240 --> 00:30:18,920 Speaker 1: but they're even more specialized than the binary star system. Method. 549 00:30:19,400 --> 00:30:23,040 Speaker 1: One of them is called gravitational micro lensing. This is 550 00:30:23,040 --> 00:30:25,680 Speaker 1: a super fun one. It's when a star acts like 551 00:30:25,720 --> 00:30:28,880 Speaker 1: a lens. Because remember that the gravity of a star 552 00:30:29,080 --> 00:30:32,320 Speaker 1: isn't just a force pulling on things. It actually is 553 00:30:32,480 --> 00:30:36,640 Speaker 1: bending the shape of the space around the star. This 554 00:30:36,840 --> 00:30:39,600 Speaker 1: is Einstein's idea of how gravity works. Rather than being 555 00:30:39,640 --> 00:30:43,360 Speaker 1: a force, it's a change in the way space is curved. 556 00:30:43,680 --> 00:30:47,080 Speaker 1: In Einstein's idea of gravity, space is not just like 557 00:30:47,160 --> 00:30:50,560 Speaker 1: a backdrop. It's a thing that bends as mass gets 558 00:30:50,600 --> 00:30:53,080 Speaker 1: around it. And there's a complicated dance there, right, because 559 00:30:53,480 --> 00:30:56,479 Speaker 1: mass tells space how to bend, and then the bending 560 00:30:56,520 --> 00:30:59,440 Speaker 1: of space tells mass and other things how to move 561 00:30:59,680 --> 00:31:02,840 Speaker 1: in hooting light. And so a star out there in 562 00:31:02,840 --> 00:31:06,080 Speaker 1: the universe bends the space around it, and so a 563 00:31:06,120 --> 00:31:09,880 Speaker 1: photon coming to us from for example, a background galaxy 564 00:31:10,480 --> 00:31:13,840 Speaker 1: might get bent as it passes around that star, causing 565 00:31:13,880 --> 00:31:16,920 Speaker 1: a distortion. So if you're looking at a galaxy really 566 00:31:16,920 --> 00:31:20,240 Speaker 1: really far away, for example, and a star passes right 567 00:31:20,360 --> 00:31:23,680 Speaker 1: in front of that galaxy, it will distort that galaxy 568 00:31:23,680 --> 00:31:26,880 Speaker 1: in a very particular way, and in a particular way 569 00:31:27,040 --> 00:31:30,600 Speaker 1: that depends on its mass. So this is one way 570 00:31:30,640 --> 00:31:33,640 Speaker 1: we can measure the mass of an object without anything 571 00:31:33,680 --> 00:31:37,360 Speaker 1: else necessarily being near it by seeing how it distorts 572 00:31:37,520 --> 00:31:41,240 Speaker 1: light from background galaxies, because the distortion of that light 573 00:31:41,320 --> 00:31:44,160 Speaker 1: depends on the mass. The higher the mass of the star, 574 00:31:44,560 --> 00:31:47,200 Speaker 1: the more it will distort that light. And then of 575 00:31:47,240 --> 00:31:49,200 Speaker 1: course we can look at the brightness of that star 576 00:31:49,280 --> 00:31:50,560 Speaker 1: and we can look it up on our chart and 577 00:31:50,560 --> 00:31:52,880 Speaker 1: we can say, oh, does this follow the pattern that 578 00:31:52,920 --> 00:31:55,920 Speaker 1: we expect. Does this help us understand that the connections 579 00:31:55,960 --> 00:31:58,920 Speaker 1: between the luminosity and the mass of the star. And 580 00:31:58,960 --> 00:32:01,320 Speaker 1: so this is more difficult to do because it's not 581 00:32:01,360 --> 00:32:03,880 Speaker 1: something that happens very often. It's not something you can 582 00:32:03,920 --> 00:32:07,080 Speaker 1: necessarily predict. Is a sort of a chance encounter a 583 00:32:07,160 --> 00:32:10,600 Speaker 1: star wandering in front of another system that we were 584 00:32:10,640 --> 00:32:13,000 Speaker 1: already looking at. But when it does happen, it's a 585 00:32:13,120 --> 00:32:16,240 Speaker 1: very nice calibration and lets us look at a different 586 00:32:16,280 --> 00:32:19,760 Speaker 1: population of stars and get a direct measurement of their 587 00:32:19,800 --> 00:32:23,160 Speaker 1: mass as well. And then there's one extra cool, sort 588 00:32:23,200 --> 00:32:26,800 Speaker 1: of crazy idea for how to measure the massive stars, 589 00:32:27,160 --> 00:32:31,959 Speaker 1: and that's using astro seismology. These days, we can study 590 00:32:32,000 --> 00:32:35,280 Speaker 1: in real detail the light coming from stars, and some 591 00:32:35,320 --> 00:32:38,840 Speaker 1: astronomers think that variations in the light that comes from 592 00:32:38,880 --> 00:32:43,360 Speaker 1: a star might be due to seismic activity on the 593 00:32:43,400 --> 00:32:47,360 Speaker 1: surface of the star. That's right. These stars have sort 594 00:32:47,400 --> 00:32:50,120 Speaker 1: of like crusts, and you know how our son, for example, 595 00:32:50,240 --> 00:32:54,719 Speaker 1: sometimes blows out big coronal ejections or flares up and 596 00:32:54,760 --> 00:32:57,400 Speaker 1: flares down. If you're studying the light from that star 597 00:32:57,480 --> 00:32:59,720 Speaker 1: from really far away, you can tell when one of 598 00:32:59,720 --> 00:33:03,000 Speaker 1: these events happens because it affects the light that you see. 599 00:33:03,360 --> 00:33:06,440 Speaker 1: Because the star is spinning, there's a variable effect on 600 00:33:06,480 --> 00:33:09,400 Speaker 1: the light. So there's like a hot spot on one 601 00:33:09,520 --> 00:33:12,880 Speaker 1: side of a star. You'll see it as the star spins, 602 00:33:12,920 --> 00:33:14,920 Speaker 1: and then you won't see it as it spins away 603 00:33:14,920 --> 00:33:17,080 Speaker 1: from you. So you can see this sort of pattern 604 00:33:17,280 --> 00:33:20,320 Speaker 1: in the light from the star because of crazy effects 605 00:33:20,360 --> 00:33:23,200 Speaker 1: on the surface of the star. And there's some models 606 00:33:23,240 --> 00:33:25,920 Speaker 1: that tell us that there's a connection between the sort 607 00:33:25,920 --> 00:33:29,480 Speaker 1: of rate of these effects, how often this happens seismic 608 00:33:29,520 --> 00:33:32,880 Speaker 1: events in the crusts of stars, and the mass of 609 00:33:32,920 --> 00:33:37,520 Speaker 1: those stars. And so this astro seismology looking at the 610 00:33:37,600 --> 00:33:41,000 Speaker 1: pulsation on the surface of stars might also be able 611 00:33:41,040 --> 00:33:44,320 Speaker 1: to give us clues as to the mass of those stars. 612 00:33:44,960 --> 00:33:47,440 Speaker 1: All right, So we've been talking about how we measured 613 00:33:47,480 --> 00:33:49,880 Speaker 1: the mass of stars. I want to dig into what 614 00:33:50,040 --> 00:33:53,520 Speaker 1: we've learned, what is the range of mass of stars 615 00:33:53,600 --> 00:33:56,160 Speaker 1: and why that's important and what it tells us about 616 00:33:56,200 --> 00:33:59,120 Speaker 1: the history and the future of our universe. But first, 617 00:33:59,320 --> 00:34:15,279 Speaker 1: let's take another quick break. Okay, we're back, and we're 618 00:34:15,320 --> 00:34:18,480 Speaker 1: talking about how we know what we do know about 619 00:34:18,480 --> 00:34:21,560 Speaker 1: the universe and how well we know it, And specifically 620 00:34:21,600 --> 00:34:24,080 Speaker 1: today we're digging into the question of how do we 621 00:34:24,200 --> 00:34:26,960 Speaker 1: know the mass of a star, how do we measure that, 622 00:34:27,200 --> 00:34:29,319 Speaker 1: how do we figure it out when we can't measure it, 623 00:34:29,520 --> 00:34:32,239 Speaker 1: and how well do we actually know that? And so 624 00:34:32,320 --> 00:34:34,560 Speaker 1: we've been talking about how we measure it using binary 625 00:34:34,680 --> 00:34:38,640 Speaker 1: star systems and gravitational micro lensing, and how we interpolate 626 00:34:38,719 --> 00:34:43,000 Speaker 1: between those measurements using nuclear theory models about how a 627 00:34:43,120 --> 00:34:46,520 Speaker 1: star works, and now let's talk about what that means. 628 00:34:46,960 --> 00:34:49,400 Speaker 1: It turns out that the mass of a star is 629 00:34:49,440 --> 00:34:53,480 Speaker 1: basically the most important thing about it. The whole future 630 00:34:53,520 --> 00:34:55,440 Speaker 1: of the star, what's going to happen to it, its 631 00:34:55,440 --> 00:34:58,680 Speaker 1: whole life cycle, it's life span in fact, and then 632 00:34:58,719 --> 00:35:01,440 Speaker 1: the future of any civilized Asian living around that star 633 00:35:01,760 --> 00:35:06,040 Speaker 1: depends entirely on how much stuff went into the star, 634 00:35:06,160 --> 00:35:10,000 Speaker 1: what the mass serving was for the material that ended 635 00:35:10,080 --> 00:35:13,000 Speaker 1: up in that star. And there's a few outcomes that 636 00:35:13,000 --> 00:35:16,080 Speaker 1: are possible for a star. But this is awesome stellar 637 00:35:16,200 --> 00:35:19,319 Speaker 1: life cycle chart, which you encourage everybody to google and 638 00:35:19,400 --> 00:35:21,600 Speaker 1: take a look at. But the end points of this 639 00:35:21,680 --> 00:35:25,560 Speaker 1: are super fascinating. Basically, once you become a star, you 640 00:35:25,640 --> 00:35:29,120 Speaker 1: have a few possible outcomes. You can either become a 641 00:35:29,200 --> 00:35:32,440 Speaker 1: brown dwarf, which is like a failed star that never 642 00:35:32,520 --> 00:35:34,840 Speaker 1: really takes off and has the same kind of bright 643 00:35:34,880 --> 00:35:38,839 Speaker 1: fusion that's for very low mass stuff. Or you can 644 00:35:38,880 --> 00:35:41,080 Speaker 1: become like a normal star, which ends up as a 645 00:35:41,120 --> 00:35:45,040 Speaker 1: white dwarf actor blows out its outer layers and then eventually, 646 00:35:45,080 --> 00:35:48,759 Speaker 1: after trillions of years, cools to a black dwarf, so 647 00:35:48,800 --> 00:35:53,040 Speaker 1: you've got brown dwarves, white dwarves, black dwarves. Or if 648 00:35:53,040 --> 00:35:55,799 Speaker 1: you're even bigger, you can end up as a neutron star, 649 00:35:56,360 --> 00:35:59,279 Speaker 1: which might eventually turn into a pulsar. Or you can 650 00:35:59,400 --> 00:36:02,480 Speaker 1: end up a lack hole, and in some scenarios you 651 00:36:02,560 --> 00:36:05,000 Speaker 1: end up is just like this big super and over remnant. 652 00:36:05,040 --> 00:36:06,960 Speaker 1: The whole thing just sort of blows up and spreads 653 00:36:07,000 --> 00:36:09,719 Speaker 1: out into the universe. But the one that you end up. 654 00:36:09,920 --> 00:36:13,319 Speaker 1: Your eventual faith if you're a star, depends just on 655 00:36:13,440 --> 00:36:16,719 Speaker 1: how much stuff you have, because it's the amount of 656 00:36:16,800 --> 00:36:19,960 Speaker 1: stuff that drives the whole process, right, that makes you 657 00:36:20,200 --> 00:36:23,320 Speaker 1: burn hot and fast, or makes you burn slow and cool. 658 00:36:23,719 --> 00:36:27,120 Speaker 1: So the mass of the star is actually really, really important, 659 00:36:27,400 --> 00:36:30,279 Speaker 1: and it's not just important for understanding the fate of 660 00:36:30,320 --> 00:36:33,799 Speaker 1: an individual star. It's also really important for understanding the 661 00:36:33,840 --> 00:36:36,719 Speaker 1: whole collection of stars. Right. We want to know the 662 00:36:36,760 --> 00:36:39,400 Speaker 1: mass of stars because we want to understand, for example, 663 00:36:39,680 --> 00:36:42,560 Speaker 1: how the galaxy is rotating. Think about one of the 664 00:36:42,600 --> 00:36:45,840 Speaker 1: deepest and most amazing discoveries in astronomy in the last 665 00:36:45,920 --> 00:36:50,040 Speaker 1: hundred years, which was that galaxies are mostly not made 666 00:36:50,040 --> 00:36:52,840 Speaker 1: out of stars and gas and dust, that they're made 667 00:36:52,880 --> 00:36:56,000 Speaker 1: out of something else. This is an observation that came 668 00:36:56,040 --> 00:36:59,359 Speaker 1: out of just looking at the rotations of those galaxies 669 00:36:59,400 --> 00:37:03,600 Speaker 1: and asking how fast are those galaxies rotating? And is 670 00:37:03,640 --> 00:37:08,120 Speaker 1: there enough mass in the galaxy to explain how fast 671 00:37:08,120 --> 00:37:11,120 Speaker 1: it's rotating? Right? If you, for example, put a bunch 672 00:37:11,120 --> 00:37:13,000 Speaker 1: of ping pong balls on a Merry Go round and 673 00:37:13,040 --> 00:37:16,200 Speaker 1: spun it, they would fly off into space. The reason 674 00:37:16,239 --> 00:37:19,160 Speaker 1: that doesn't happen for stars in the galaxy, which is 675 00:37:19,239 --> 00:37:22,600 Speaker 1: basically a huge cosmic merry go round, is because there's 676 00:37:22,640 --> 00:37:25,840 Speaker 1: something holding them in. But you can measure the speed 677 00:37:26,120 --> 00:37:29,480 Speaker 1: of our galactic merry go round and ask is there 678 00:37:29,600 --> 00:37:32,520 Speaker 1: enough mass in the galaxy to provide the gravity you 679 00:37:32,560 --> 00:37:35,719 Speaker 1: need to hold the stars in place? Well, how do 680 00:37:35,760 --> 00:37:38,600 Speaker 1: you do that calculation? All you can do is look 681 00:37:38,680 --> 00:37:40,799 Speaker 1: at the stuff. You see. You look at all those 682 00:37:40,880 --> 00:37:43,720 Speaker 1: bright points of light in the sky, and you add 683 00:37:43,880 --> 00:37:47,759 Speaker 1: up their individual masses, And that's exactly the point. You 684 00:37:47,800 --> 00:37:50,839 Speaker 1: need to know the mass of all those stars in 685 00:37:50,960 --> 00:37:54,360 Speaker 1: order to do that calculation. If we had no idea 686 00:37:54,760 --> 00:37:57,799 Speaker 1: what the masses of stars were, we never would have 687 00:37:57,880 --> 00:38:00,759 Speaker 1: discovered dark matter because we would have looked at all 688 00:38:00,800 --> 00:38:03,200 Speaker 1: the stars in the sky and said, well, we have 689 00:38:03,280 --> 00:38:05,759 Speaker 1: no idea how much mass there is, so maybe there's 690 00:38:05,840 --> 00:38:08,400 Speaker 1: enough to provide the gravity we need to hold the 691 00:38:08,440 --> 00:38:12,319 Speaker 1: galaxy together despite how fast it's spinning. But no, we 692 00:38:12,440 --> 00:38:14,680 Speaker 1: do know the mass of those stars, and so we 693 00:38:14,680 --> 00:38:18,520 Speaker 1: were able to calculate how much mass there is in stars. 694 00:38:19,080 --> 00:38:21,080 Speaker 1: And we looked at that and we said, well, is 695 00:38:21,120 --> 00:38:24,239 Speaker 1: there enough mass to provide the gravity needs to hold 696 00:38:24,239 --> 00:38:26,680 Speaker 1: the galaxy together? And of course you probably know by 697 00:38:26,719 --> 00:38:30,759 Speaker 1: now the answer is not even close. The galaxy is 698 00:38:30,800 --> 00:38:33,920 Speaker 1: spinning at a crazy fast rate, and the mass of 699 00:38:33,960 --> 00:38:37,520 Speaker 1: the stars we can see does not provide enough gravity 700 00:38:37,560 --> 00:38:40,759 Speaker 1: to hold it together. That was the essential clue, the 701 00:38:40,840 --> 00:38:44,360 Speaker 1: first crack that told us that there was something else 702 00:38:44,440 --> 00:38:47,400 Speaker 1: out there in the universe. So doing these kinds of 703 00:38:47,440 --> 00:38:50,839 Speaker 1: studies things that might seem a little boring, like our 704 00:38:50,880 --> 00:38:53,279 Speaker 1: stars as bright as we expect them to be, how 705 00:38:53,360 --> 00:38:55,920 Speaker 1: much mass do they have? Can we explain it doesn't 706 00:38:56,000 --> 00:39:00,000 Speaker 1: make sense, seems sometimes like sort of scientific busy world, 707 00:39:00,360 --> 00:39:03,560 Speaker 1: But sometimes the answers don't add up, and they reveal 708 00:39:03,640 --> 00:39:08,040 Speaker 1: a huge cosmic mystery. So that's why it's important to 709 00:39:08,080 --> 00:39:10,560 Speaker 1: know the massive stars. That's why the mass of the 710 00:39:10,600 --> 00:39:13,800 Speaker 1: stars determined not just the fate of the individual objects, 711 00:39:13,840 --> 00:39:17,279 Speaker 1: but potentially how everything works. And now we know that 712 00:39:17,280 --> 00:39:20,319 Speaker 1: it's not actually the mass of stars that's determining like 713 00:39:20,600 --> 00:39:24,240 Speaker 1: the whole shape and future of our universe. It's actually 714 00:39:24,280 --> 00:39:27,560 Speaker 1: all of that dark matter, because there is five times 715 00:39:27,560 --> 00:39:30,279 Speaker 1: as much dark matter in the universe as there is 716 00:39:30,440 --> 00:39:33,280 Speaker 1: hydrogen and helium and the kind of stuff that makes stars, 717 00:39:33,800 --> 00:39:36,600 Speaker 1: and it's that dark matter that controls like why there's 718 00:39:36,600 --> 00:39:39,719 Speaker 1: a galaxy here and not over there. The reason is 719 00:39:39,760 --> 00:39:41,759 Speaker 1: that there was a big blob of dark matter and 720 00:39:41,920 --> 00:39:45,200 Speaker 1: it attracted all the hydrogen and helium, and it helped 721 00:39:45,239 --> 00:39:48,160 Speaker 1: compress all of that stuff, and it started the fire 722 00:39:48,280 --> 00:39:51,560 Speaker 1: in those stars. So we wouldn't even have stars and 723 00:39:51,600 --> 00:39:54,120 Speaker 1: galaxies the way we do today if it weren't for 724 00:39:54,200 --> 00:39:56,680 Speaker 1: that dark matter. And we wouldn't know about the dark 725 00:39:56,680 --> 00:40:00,239 Speaker 1: matter if we didn't have a careful measurement a way 726 00:40:00,239 --> 00:40:04,160 Speaker 1: to figure out the mass of individual stars. Alright, so 727 00:40:04,200 --> 00:40:06,560 Speaker 1: then let's talk about what the mass of the stars 728 00:40:06,640 --> 00:40:09,680 Speaker 1: actually are. Right, we look at our Sun. That's a 729 00:40:09,680 --> 00:40:12,880 Speaker 1: pretty standard candle. We think, let's use the Sun as 730 00:40:12,920 --> 00:40:15,759 Speaker 1: an example star and measure everything in terms of like 731 00:40:16,200 --> 00:40:19,440 Speaker 1: one solar mass. Well, right off the bat, we discover 732 00:40:19,760 --> 00:40:22,360 Speaker 1: the Sun is kind of unusual, is sort of a 733 00:40:22,440 --> 00:40:25,200 Speaker 1: large star. The most common star out there in our 734 00:40:25,239 --> 00:40:30,000 Speaker 1: galaxy is one called a red dwarf. Is significantly smaller 735 00:40:30,120 --> 00:40:33,239 Speaker 1: and colder than the Sun, and it's hard to see them, right. 736 00:40:33,560 --> 00:40:37,160 Speaker 1: These stars are much much dimmer than the Sun. Something 737 00:40:37,200 --> 00:40:39,600 Speaker 1: that's half as bright as the Sun is going to 738 00:40:39,680 --> 00:40:43,000 Speaker 1: be one six as dim. And there are a lot 739 00:40:43,040 --> 00:40:45,279 Speaker 1: of stars out there that are smaller than half the 740 00:40:45,320 --> 00:40:48,160 Speaker 1: mass of the Sun, and so they're much much dimmer, 741 00:40:48,200 --> 00:40:51,160 Speaker 1: which makes them hard to see. So we don't even 742 00:40:51,239 --> 00:40:54,040 Speaker 1: really have a complete catalog of all of those stars, 743 00:40:54,480 --> 00:40:56,960 Speaker 1: but none of the stars that are very nearby are 744 00:40:57,040 --> 00:40:59,640 Speaker 1: much larger. In fact, none of the stars that are 745 00:40:59,680 --> 00:41:03,240 Speaker 1: within thirty light years of the Sun is very much bigger, 746 00:41:03,320 --> 00:41:06,680 Speaker 1: Like the biggest star in our neighborhood is about four 747 00:41:06,719 --> 00:41:09,800 Speaker 1: times the mass of the Sun. And as you explore 748 00:41:09,880 --> 00:41:12,680 Speaker 1: out there into the universe, you discover something really interesting, 749 00:41:13,000 --> 00:41:14,960 Speaker 1: which is that there seems to be sort of a 750 00:41:15,000 --> 00:41:18,600 Speaker 1: limit to how big stars can be. And we discover 751 00:41:18,760 --> 00:41:21,920 Speaker 1: this just because there aren't stars that are much much bigger. 752 00:41:22,200 --> 00:41:25,920 Speaker 1: The limit seems to be somewhere around a hundred two hundred, 753 00:41:26,040 --> 00:41:29,360 Speaker 1: maybe up to two hundred and fifty times the mass 754 00:41:29,440 --> 00:41:32,759 Speaker 1: of the Sun. The biggest stars out there are just 755 00:41:32,840 --> 00:41:35,480 Speaker 1: about that big. And the reason is that stars that 756 00:41:35,600 --> 00:41:40,440 Speaker 1: have huge amounts of mass burn really really hot and bright, 757 00:41:40,880 --> 00:41:43,439 Speaker 1: and they push so hard from the inside that they're 758 00:41:43,480 --> 00:41:47,400 Speaker 1: basically unstable and blow themselves apart. Anything above that it 759 00:41:47,440 --> 00:41:50,840 Speaker 1: basically blows the excess off, So you just can't have 760 00:41:50,960 --> 00:41:53,759 Speaker 1: a stable blob of matter that's more than like two 761 00:41:53,880 --> 00:41:56,680 Speaker 1: hundred times the mass of the Sun. But we're gonna 762 00:41:56,719 --> 00:41:59,400 Speaker 1: dig into that in a whole other podcast episode about 763 00:41:59,400 --> 00:42:02,640 Speaker 1: the biggest stars in the universe. It's also actually fun 764 00:42:02,680 --> 00:42:05,280 Speaker 1: to think about the history of the universe. It turns 765 00:42:05,280 --> 00:42:09,440 Speaker 1: out that the massive stars changes as the universe gets older, 766 00:42:09,760 --> 00:42:12,920 Speaker 1: and the very very early universe. We think that the 767 00:42:13,000 --> 00:42:16,319 Speaker 1: stars that were first born, the first population of stars 768 00:42:16,360 --> 00:42:19,840 Speaker 1: that came together out of that hydrogen, were actually typically 769 00:42:19,960 --> 00:42:23,520 Speaker 1: much much bigger than the stars we see today. Those 770 00:42:23,560 --> 00:42:28,000 Speaker 1: are these so called population three stars. They're called population 771 00:42:28,040 --> 00:42:31,960 Speaker 1: three stars because our stars today are called population one, 772 00:42:32,040 --> 00:42:34,439 Speaker 1: and then the stars that they came from are called 773 00:42:34,480 --> 00:42:37,680 Speaker 1: population to sort of sort of counting back to the 774 00:42:37,719 --> 00:42:40,560 Speaker 1: beginning of the universe, it might make more sense to 775 00:42:40,600 --> 00:42:43,439 Speaker 1: you to think the first generation of stars are population one, 776 00:42:43,719 --> 00:42:46,760 Speaker 1: and we should be population three, But for whatever reason, 777 00:42:46,880 --> 00:42:51,799 Speaker 1: astronomers are counting backwards. So population three stars are the 778 00:42:51,880 --> 00:42:55,319 Speaker 1: first ones to form in the universe, and those were 779 00:42:55,400 --> 00:42:58,840 Speaker 1: much much bigger because there wasn't very much metal around 780 00:42:58,880 --> 00:43:01,720 Speaker 1: in the universe right after the Big Bang. The universe 781 00:43:01,800 --> 00:43:04,480 Speaker 1: was mostly hydrogen, with a little bit of helium and 782 00:43:04,520 --> 00:43:09,000 Speaker 1: a tiny bit of lithium. But overwhelmingly hydrogen, and hydrogen 783 00:43:09,040 --> 00:43:12,359 Speaker 1: doesn't clump as well as heavier metals, right, it makes 784 00:43:12,360 --> 00:43:16,200 Speaker 1: it harder to collapse into a small sample because molecular 785 00:43:16,280 --> 00:43:18,920 Speaker 1: hydrogen doesn't collapse as well. It's harder for it to 786 00:43:19,040 --> 00:43:22,120 Speaker 1: cool Or you get this big blob of gas in 787 00:43:22,160 --> 00:43:23,680 Speaker 1: which you need to form a star, is for it 788 00:43:23,760 --> 00:43:27,560 Speaker 1: to collapse gravitationally, And it turns out that that's easier 789 00:43:27,600 --> 00:43:30,800 Speaker 1: to do in smaller clumps when you have little blobs 790 00:43:30,800 --> 00:43:34,040 Speaker 1: of metal. That's sort of like seeds, smaller pieces. If 791 00:43:34,080 --> 00:43:36,640 Speaker 1: you just have a huge mass of hydrogen, then it's 792 00:43:36,680 --> 00:43:39,320 Speaker 1: harder for it to collapse into smaller clumps. It tends 793 00:43:39,360 --> 00:43:43,160 Speaker 1: to collapse into these much bigger objects because it's harder 794 00:43:43,160 --> 00:43:45,080 Speaker 1: for it to cool off. And so in the very 795 00:43:45,080 --> 00:43:49,560 Speaker 1: early universe we had really really huge stars, much more 796 00:43:49,600 --> 00:43:53,040 Speaker 1: massive stars than we typically see today. And then the 797 00:43:53,120 --> 00:43:56,600 Speaker 1: second generation of stars had more metal in them. Right, 798 00:43:56,640 --> 00:44:00,680 Speaker 1: because the first generation of stars burned and fueled helium 799 00:44:00,800 --> 00:44:03,520 Speaker 1: production of heavier stuff, and then that sea did the 800 00:44:03,560 --> 00:44:06,920 Speaker 1: next generation of stars. And because there was more helium 801 00:44:06,960 --> 00:44:10,600 Speaker 1: and heavier metals around when the second generation formed, you 802 00:44:10,719 --> 00:44:14,520 Speaker 1: tended to get smaller clumps. So that second generation of stars. 803 00:44:14,760 --> 00:44:17,240 Speaker 1: Some of them are still around, They are still burning 804 00:44:17,239 --> 00:44:19,640 Speaker 1: in the universe. We can see them. Check out our 805 00:44:19,680 --> 00:44:22,799 Speaker 1: podcast episode about the oldest stars in the Universe, and 806 00:44:22,800 --> 00:44:24,879 Speaker 1: you'll see that we've found something that we think our 807 00:44:25,040 --> 00:44:28,560 Speaker 1: second generation stars that are burning for billions and billions 808 00:44:28,640 --> 00:44:31,960 Speaker 1: of years. Those first generation, we think only lasted a 809 00:44:31,960 --> 00:44:35,600 Speaker 1: few hundred million years, if at all. And the crazy 810 00:44:35,640 --> 00:44:38,319 Speaker 1: thing is that we've never even really seen one of them. 811 00:44:38,640 --> 00:44:42,080 Speaker 1: Nobody's ever seen a population three star, and that's, of 812 00:44:42,120 --> 00:44:44,120 Speaker 1: course because they're all burned out, so there are none 813 00:44:44,120 --> 00:44:46,319 Speaker 1: of them in our neighborhood. In order to see when 814 00:44:46,320 --> 00:44:49,000 Speaker 1: you need to look really really far away, so that 815 00:44:49,200 --> 00:44:52,240 Speaker 1: light from that population three star would just be arriving 816 00:44:52,280 --> 00:44:55,719 Speaker 1: to our eyeballs and now, billions of years after it 817 00:44:55,840 --> 00:44:58,800 Speaker 1: already died out. The problem, of course, is that those 818 00:44:58,840 --> 00:45:01,680 Speaker 1: galaxies are super duper far away. We're talking about things 819 00:45:01,680 --> 00:45:05,600 Speaker 1: that are thirteen billion years ago, so they're basically at 820 00:45:05,600 --> 00:45:08,759 Speaker 1: the edge of the observable universe, and those galaxies are 821 00:45:08,760 --> 00:45:11,359 Speaker 1: hard to spot. We can see the galaxy, we know 822 00:45:11,480 --> 00:45:14,640 Speaker 1: it's there. We're getting light from that galaxy. We suspect 823 00:45:14,719 --> 00:45:17,759 Speaker 1: it's filled with population three stars. But we haven't ever 824 00:45:17,840 --> 00:45:22,560 Speaker 1: identified an individual population three star from the early universe, 825 00:45:22,800 --> 00:45:25,239 Speaker 1: and it would be super fascinating if we could. We 826 00:45:25,280 --> 00:45:27,560 Speaker 1: had an understanding of like how big were these things? 827 00:45:27,560 --> 00:45:30,520 Speaker 1: How much mass did they have? All Right, so today 828 00:45:30,560 --> 00:45:33,600 Speaker 1: we've dug into the details of how scientists know the 829 00:45:33,640 --> 00:45:36,040 Speaker 1: mass of these stars. It turns out to be really 830 00:45:36,040 --> 00:45:39,160 Speaker 1: important to the life of a star. Completely controls what's 831 00:45:39,200 --> 00:45:40,960 Speaker 1: going to happen to it, whether it's going to be 832 00:45:41,000 --> 00:45:43,240 Speaker 1: a black hole or end up as a white dwarf. 833 00:45:43,280 --> 00:45:46,040 Speaker 1: And eventually a black dwarf is just turned by the 834 00:45:46,080 --> 00:45:49,480 Speaker 1: original helping of hydrogen and other stuff that it got 835 00:45:49,640 --> 00:45:52,439 Speaker 1: when it was formed. And we can figure that out 836 00:45:52,440 --> 00:45:55,719 Speaker 1: by looking at binary star systems, by looking at gravitational 837 00:45:55,800 --> 00:45:59,000 Speaker 1: micro lensing, and then sort of extrapolating in between what 838 00:45:59,080 --> 00:46:01,960 Speaker 1: we do know to guess about what we don't quite know. 839 00:46:02,440 --> 00:46:05,520 Speaker 1: But there's still a lot of uncertainty because these calculations 840 00:46:05,560 --> 00:46:08,400 Speaker 1: we do they're hard, there's a lot of approximations we 841 00:46:08,400 --> 00:46:11,440 Speaker 1: have to make. We think they're probably not completely accurate. 842 00:46:11,680 --> 00:46:14,560 Speaker 1: We have some confidence that they're not way off because 843 00:46:14,600 --> 00:46:17,200 Speaker 1: we can calibrate them using the stars that we do see, 844 00:46:17,280 --> 00:46:20,200 Speaker 1: where we can measure their mass, but there's always room 845 00:46:20,280 --> 00:46:23,000 Speaker 1: for improvements, and so it's important to think about the 846 00:46:23,040 --> 00:46:26,880 Speaker 1: knowledge and also the uncertainty, because hey, most of the 847 00:46:26,960 --> 00:46:29,920 Speaker 1: universe is uncertain. Most of what we will learn about 848 00:46:30,040 --> 00:46:33,080 Speaker 1: nature and stars and the universe is ahead of us. 849 00:46:33,520 --> 00:46:36,200 Speaker 1: So thanks for coming on this ride to explore what 850 00:46:36,280 --> 00:46:38,480 Speaker 1: we do know and what we don't know, and our 851 00:46:38,520 --> 00:46:42,920 Speaker 1: speculation about what we will eventually know. Thanks everyone, tune 852 00:46:42,960 --> 00:46:53,480 Speaker 1: in next time. Thanks for listening, and remember that Daniel 853 00:46:53,480 --> 00:46:56,000 Speaker 1: and Jorge Explain the Universe is a production of My 854 00:46:56,239 --> 00:46:59,680 Speaker 1: heart Radio. Or more podcast from my heart Radio visit 855 00:46:59,719 --> 00:47:03,600 Speaker 1: the heart Radio app, Apple Podcasts, or wherever you listen 856 00:47:03,680 --> 00:47:04,800 Speaker 1: to your favorite shows.