 I found a mystery. Oh, good morning. Today I thought I'd try something a little different. And honestly it's something that I have kind of pushed against with this channel, something that I've resisted. And that is telling you about my research. I've had a lot of comments from people on this channel or like feedback, people saying, why don't you talk about astronomy, talk about introductory science. It's not that I don't think those are great things to do. And in fact, I'll link below several awesome channels that do just that. So for example, Space with Sarah, great channel where you can learn some basics about astronomy and the universe. David Kipping's Cool Worlds channel. I love that channel. They talk about all their latest papers. They're much more consistent about talking about astronomy, the topic. I have largely pushed away from that both to sort of distinguish myself and this channel, but also because it's not what I always want to talk about. It's too hot. It's too hot. For those who are new or maybe you're just wondering, the theme has always been I want to show what it's like to be an astronomer. That's why I try to show and discuss different aspects of the craft of doing astronomy because I think that's something unique. And so it's not that I don't want to talk about the science, it's just that I feel like this is an opportunity to tell a different story. But today, I'm going to talk a little bit about the science and it's not the big grand, where is the universe coming from mystery, but it's like a nice little mystery. The story starts really with the Gaia Sprints. A couple of years ago, during the very first Gaia Sprint, I couldn't travel. And so I did a remote sprint just in my basement. I spent the week writing a paper and it was just following up stars from Kepler that we had already observed had rotation periods from the Kepler light curves, these little sinusoidal modulations from star spots rolling in and out of view. And I just matched that sample to the original release from Gaia. Fast forward to Gaia Sprint 2, i.e. Gaia DR2 comes out and blows our collective minds because it's so incredible. And my plan was just to write the same paper. In fact, before the Gaia Sprint, I was already working on this, putting the little figures together, making the exact same graphs and the exact same measurements that I did from Gaia DR1, and just updating it for Gaia DR2. And so I'm working on this paper and I'm working on this kind of rehashing of what I had done a year earlier. And I noticed a little detail, like a small detail that kind of seemed insignificant. Let me show you. Okay, this is the figure that surprised me. Now this black line represents what we call the main sequence, the normal stars like the sun, like here. On the x-axis we have the color, so these are blue stars, these are red stars, i.e. these are hot stars, these are cold stars. And we have faint and bright in wacky astronomer units. So faint, red, dim stuff, bright, blue, hot stuff. Cool. This is what we call the Hertzsprung-Russell diagram, or in observational units we call it the color, magnitude, diagram. And this is like bread and butter astrophysics right here. This is like what I spent a bunch of my PhD looking at. There's actually two groups of stars here, the main sequence, and right above it, a secondary line, which we call the equal mass binary main sequence, i.e. there's two stars that are the exact same brightness. And so they bring the brightness up right here. And here I've colored each of the dots by the rotation period we measure from Kepler's. So we know that these red ones are the slow rotators, the M dwarfs, the low mass stars rotate slowly, the big hot stars rotate fast. And there's reasons why, but that's not the point. If you look very carefully, there is a little yellow valley here, a little yellow trough. This is my mystery. These are stars that are brighter, so higher than the normal main sequence stars, and yet rotate a little slower. That is not expected. Okay, here's a zoomed in version of the same stuff. Now I'm using a better color scheme, red and blue. Blue are the rapid and red are the slow rotators. I've drawn these thick lines, which show the average color versus magnitude, i.e. the average track for slow rotators in red and rapid rotators in blue. And you can see that there is a clear offset. So you're thinking, okay, dude, so you've got this diagram, there's a line, there's slower stars above the line. Who cares? And this is kind of like classic science. My job is to try to convey to you that this is actually profoundly unexpected, profoundly rather, rather unexpected. See, stars do change in brightness and in color, but they tend over their lifetime to move sort of along that main sequence track, they get brighter, and they get bluer. The reason stars sit on this line in the first place is that this is the space where things are stable, where inward pressure of gravity is balanced by the outward pressure of nuclear fusion in the core. That balance, we call it hydrostatic equilibrium, that balance, that equilibrium, exists along this line. That's the line where equilibrium is happy. So the sun very slightly expands. There's a whole long story about why it expands and gets brighter and gets bluer, hotter over time, just a little bit. And despite what some people have said, this is not the cause of global warming. Okay, the point being, if stars lose angular momentum, i.e. they slow down over time, that as a star gets brighter, it will be slowing down, but you expect it to get brighter and bluer, not brighter and redder or brighter and the same color, i.e. straight up. And so this is the mystery. Why do we see the slow rotators sitting above the rapid rotators? Why are they distinguished? Every theorist I've talked to, every stellar model that I've consulted, that's not what they produce. They don't produce this spread away from the main sequence over time. What's great is I discovered this feature while making the figures to just repeat my old paper and I didn't know what to do with it. I didn't know if this was, if I was just naive, if I had forgotten some basic. And so I turned to my colleagues and my friends on Twitter. Nobody can come up with an explanation for it. And so we published a paper. And so this year, I published a paper with my postdoc advisor, Kevin Covey. So Davenport and Covey 2018, where we show the updated versions of paper one, but we show this mystery and we say we don't know what it is. It's great. So what is it? Okay, here's my best guess. I think it's binary stars. Remember, I said there's that second track that lies above the first one? Well, that's when you have two stars that are the exact same brightness. But what if you have one star that's brighter than the other? Well, then they don't quite lie all the way up. They lie sort of a little red and a little bright. By the time you get down to the secondary star being half as big as the primary star, which should happen very often, you are right smack in the middle of this spread. Binary stars, as far as I know, are the only thing that could possibly cause an offset in this direction. So now the question is, why are we systematically seeing binary stars rotating slower? And I think the answer is tides. The same reason the moon always shows the same face to the earth is tides is that gravitational interplay between the moon and the earth. The same reason the moon causes high tide and low tide. This same force, I think, is at work in these binaries. And so for the first time, we have a handle not just on the rotation of tens of thousands of stars, but now we might have the tidal and orbital dynamics of tens of thousands of binary stars for the very first time. And we didn't even know they were just sitting there waiting for us. And so the proposal we wrote to the National Science Foundation, to the NSF, was let's match theory about how tides and binary stars work with these observations and new ones that are coming soon. Let's combine our observational and our theory, and let's bring them together and see if we can solve this mystery, see if we can explain why these stars spread out. And in doing so, can we learn something very basic and fundamental about how stars are born, how often they are in binaries, and what those binaries look like? It's a simple observation, a kind of a cute little mystery, which I think will have a big impact on how we understand stars and binary stars. Okay, and what I love about it is that binary stars aren't in vogue, they aren't really in fashion in astronomy anymore. Binary stars are like which your grandfather's astronomers were studying. And so I love that we found this mystery, which might have profound impact. And it's these classical things, these binary stars which might be at work. So there you go, there's my treat on my latest paper, Davenport and Covi 2018, you can go read it, it's open source, it's put online. This is why we launch new telescopes and why we get new data and why we search in regions of parameter space that we've never explored before, because we find new things which will go back and break all of our assumptions about old things. You never know when you'll take a new data set and make a graph that you've made 100 times before with other data and something new will pop up. In the astro vlog tradition of showing what it's like to be a scientist, that is what it's like to be an astronomer.