It’s time for school! The Astro101 series will cover some of the most important questions in astronomy. In today’s lesson, we’ll have: What are the different kinds of stars? Why does classification in astronomy make no sense? How the heck do we define a “star”, anyway? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Welcome back, students. Class is in session for our Astro one zero one series. And remember, there will be a quiz on Friday, so be sure to be taking notes. It will not be open book or open podcast. It's just going to rely solely on the contents of your own brain for good or ill.
So just remember that as we start today's lesson on stars. Now stars are kind of a big deal in astronomy. For a very long time, they were the only deal in astronomy. They were just the thing, and, like, it's it's so easy. I love these deceptively simple questions, questions that seem like they should be easy to answer, but it turns out it's kind of hard to answer.
And today's simple question is, what exactly is a star? Like, you know, back in the day, way back in the day, like, thousands of years ago, it was just a tiny point of light in the night sky. And it's definitely not a planet. The planets were also tiny points of light in the night sky, but planets did something else. They did something different.
They wandered around. They had their own thing going on. And they definitely weren't comets. Comets were very, very odd. No one was exactly sure what comets were.
Like, maybe they were some sort of aerial phenomenon, maybe some sort of meteorological event, maybe it had something to do with the with the heavens. Who knows? But the stars are definitely not comets because comets are weird and fuzzy and just just weird. The stars stayed fixed. That was about it.
Like, they were just fixed points of light in the night sky. Like, every year, you'd see the exact same set of stars, the exact same constellations. Yeah. Yeah. Every once in a while, there'd be a random flare up or disappearance, but those were rare enough to be recognized as special occasions and not part of the standard definition of a star.
Like, sometimes, like, a brand new star would appear and and everyone would be like, woah. That's kinda messed up. But then a few weeks later, it would go away, and you would just go about your business. Or there are some stars, like Mira, the the star Mira, that would change in brightness and dim and and get brighter over the course of, like, weeks or months or even years. And people would say, wow, that's really messed up.
That's weird. I wonder what's going on. No idea. Anyway, back to business. The business of stars.
What were they? Were they holes to heaven? Were they distant campfires? You know, every culture had their own answer. And one of the most annoying questions about the stars was how far away are they?
And it's annoying because it's a really, really, really hard question to answer. And figuring out how far away they are can help us understand what they are. Like, if we're trying to understand what the heck is a star, well, at least we should be able to answer some related questions like, how big are they? How far away are they? Do they move around?
Etcetera etcetera. And so that was the program in the late fifteen hundreds, early '16 hundreds is they didn't know what these stars were. You know, some people thought they were like our sun, but just further away. Other people didn't. They really had no basis one way or the other for deciding, so it was just tiny points of light in the night sky.
Now how far away are they? Maybe we can tackle that question first. Maybe we have the technology. And the first person to tackle that was the famous famous astronomer, Tycho Brahe. This is the mentor of Johannes Kepler.
This guy was massively kooky, like, yo, he's the one that had, like, the golden nose because he got chopped off in the duel. He's the one that died from a birth bladder because he was at a king's banquet and didn't wanna get up. So if you need to scare your kids, if they're having trouble, like, remembering to go to the bathroom just so you don't wanna end up like Tycho Brahe because he died because his bladder burst. Now Tycho Brahe did not have a telescope. We oh, he was contemporaneous with the telescope, with Galileo's telescope, but he didn't use one.
He was a little bit earlier on the scene before the telescope gained wide mainstream acceptance, but he had this massive observatory. He was measuring and charting the positions of stars like crazy, all with the naked eye. He was probably the last great naked eye astronomer, the last great pre telescope astronomer, very interesting fellow, and also a very clever fellow because he was living in the time of the Copernican revolution. He was living at the time when people were starting to suggest for various reasons that maybe the earth isn't at the center of the universe with everything wheeling about us. Maybe the sun is at the center of the universe, and we orbit the sun, and the stars are orbiting the sun, and, like, everything it's all focused on the sun.
People were starting like, people like Copernicus and then eventually his student Kepler were starting to make this suggestion. We're starting to make this argument. Galileo was starting to rattle some cages down in Italy about, hey. Maybe the sun is at the center of the solar system and it thereby the center of the universe. Tycho Brahe, though, had a very clever argument.
His argument relied on the concept of parallax. A parallax is what happens when you change viewpoints and objects seem to shift around. So if you if you hold up your finger, I know I've done this before in this podcast, but it never hurts to do it again. I don't care right now if you're driving or if you're sitting in a subway car or you're going for a run or you're just sitting in a room minding your own business, I want you to put your finger a few inches in front of your face and then close one eye and then alternate, close the other, go back and forth. Camera one, camera two, camera one, camera two.
What your finger you will wiggle. You'll see it wiggle relative to the background. And if you extend your arm all the way out as far as you can and close your eyes left and right, your finger will still wigger wiggle, but slightly less so. And then if you pick a point somewhere in the far distance, wiggle, close your eyes back and forth, back and forth, back and forth, you'll see a lot less wiggle. Congratulations.
You have discovered the concept of parallax where the apparent position of objects changes depending on your point of view. Now the cool thing about Paillax is that you can measure that angle of the wiggle. You can measure how much an object wiggles. And then if you know the distance between your two viewpoints, like, say, the width between your eyes, you have the angle, the wiggle, you have the distance between your eyes. You can build a little triangle.
You can do a little trigonometry, and out pops a distance. So it's a tool for measuring distance. Tycho Brahe did not invent parallax. People have been using parallax for centuries to measure distances to distant things. Trigonometry goes back just a little bit in the history of human civilization, But Tycho was the one to apply to the stars.
And he said, look, if the Earth is orbiting around the sun, then as the months go by, we are occupying different positions in, what do you call this, the The solar system. Okay. Whatever you call it, that's what's happening. Like, we're over here. And then, you know, six months later, we're tens of millions of miles away from that point.
So it's like we took our eyeballs and spread them really, really far apart. That's a really big triangle. So if I look at the position of distant stars, I should be able to measure a parallax. I can count measure the little wiggle, and I can calculate the distance to the stars. He did that, and then he didn't do that because he he attempted to, but what turned out is that he couldn't get a single distance to a single star.
He couldn't measure any parallax wiggle over the course of months. That meant and Taeko was very, very quick to make this argument. He's like, look. Either the Earth is really at the center of the solar system, in which case we're not moving around, there's no parallax wiggle, and we can't use this technique to measure the distance of the stars. Or based on my known limitations in observing, the stars could be so far away that I can't measure their wiggle.
But in order, they would have to be at least 700 times further away than Saturn, which just sounds absolutely ridiculous. That was the number one criticism against the Sun centered universe idea. And you know what? No one was able to argue against him. Eventually, the Sun centered universe became came into fashion.
That's the the way I like to phrase it. It's just slowly over the course of decades, more and more people adopted it, mostly through the work of Kepler, mostly because it made horoscopes easier to predict. True story in the history of science. Here we are. It wasn't until the eighteen hundreds that people were able to finally get a distance to a star.
And it was a German astronomer, Friedrich Bessel, famous for lots of other things, like a self taught mathematician and astronomer and overall genius. He was the first person over two hundred years after Tycho Brahe made his famous argument. No one could rebut that argument until 1838 when Friedrich Bessel measured the distance to the star 61 Cygni. That distance, by the way, turns out to be around 10 light years, and it's one of our nearest neighbor stars. Once Friedrich Bessel did that, once he was able to get a parallax distance measurement to a star, astronomers went nuts over the course of the eighteen hundreds, the nineteen hundreds, and and to be perfectly honest, the '2 thousands.
We're still doing it. We still use parallax to measure the distances to all sorts of stars. The latest and greatest at the time of this recording is the IS spacecraft measurement. Did parallax measures to, I don't know, like, hundreds of thousands of stars? It's crazy, crazy big number.
And being able to get the distance to a star is the tool number one for unlocking what stars are. Because now that you have a distance, you can start to get more information like you can get a star's brightness. It's true brightness. Like some stars are bright because they're bright, some stars are bright because they're close. And you can't tell the difference until you actually know the distances, and once you know the distances, then you can get the true brightness, this number that astronomers call the luminosity.
So that is tool number one, getting the distances, getting their luminosities. Tool number two came a couple decades later in the eighteen hundreds with the invention of spectroscopy, and I've discussed it before, and I'll discuss it now. Spectroscopy is so critical to astronomy. Spectroscopy allows you to figure out what stars are made of. So you can see why this is such an essential tool.
Once you know how far away the stars are, you can get start to measure some of the properties of theirs of the stars. You can try to get a handle on their mass or their width or their brightness once you know how far away they are. And then with spectroscopy, you can start to get a handle on what they're made of. And you can put all this together to try to figure out what is a star? What are these points of light?
Spectroscopy is what happens when you split the light from a distant object or any object really and look in detail at all the different bits of light that make up all the light coming from that thing. So, like, okay. How many blues at this wavelength? How many reds at this wavelength? How many yellows at that wavelength?
Etcetera, etcetera, etcetera. You have built a spectrum. Interesting features will appear in spectra. If there's a lot of an l a particular element and it's glowing brightly, you'll get, special lines like a little fingerprint that appears in the spectrum due to that particular element. If there's the element is absorbing a lot of light, you'll get, like, dark lines.
You'll get, or like a reverse fingerprint appearing in that spectrum. So the spectrum tell can tell you what you're made of what it's made of. And the most hilarious thing about this of astronomy in the late eighteen hundreds is that they were using this tool of spectroscopy like gangbusters. Like, they were taking the spectra, especially in the late eighteen hundreds of every star that they could. This is before we had any idea of what a spectrum really was.
Like, why does hydrogen, when you make it glow, why does it have this particular fingerprint? We don't know. Well, they didn't know. Now we know it's because of quantum mechanics, but they didn't know, but they still used it. They and because you could make a hydrogen lamp in your laboratory, see its spectral fingerprint, and then like stare sun and see the exact same fingerprint and guess that maybe the sun has a lot of hydrogen in it.
They did all this without knowing exactly how or why it worked, but they just knew that it worked. That is science for you, and that is especially astronomy for you. They had no theory of the atom. They had no concept of quantum mechanics. They had no idea what they were doing, but dang it, they were gonna do it.
The long and short of it is by the late eighteen hundreds, we started to have some interesting numbers to attach to various stars. We could have their distances. We could have their luminosities. We could have a listing of some of their elements. We could get a handle on what color they were.
The end result was a big, giant, confusing mess. Like, we were collecting all this data, and we didn't know how to thread it. We didn't know how to sort it. We didn't know how to categorize it. In typical astronomy fashion, the numbers were tabulated, put into spreadsheets.
Everybody made their own classification scheme, and nobody had any idea what was going on. And the typical, at this time, classification schemes were based on the spectra. So and and they would name stars based on their spectral type, and different astronomers were doing different things like, oh, this line of hydrogen is slightly thicker, so that should be that kind of spectral type. Or, oh, there are a bunch of elements in this one that don't appear in other kinds of stars, so that's gonna be that kind of spectral type. Or, oh, this star is slightly redder than average, so it should be its own spectral type, you know, etcetera, etcetera, etcetera.
Just trying to find clever ways to divide and classify the different kinds of stars that they were observing. Usually, they were classified with a letter of alpha of the alphabet. So, okay, this is spectral type a. This is spectral type b. This is spectral type c, etcetera, etcetera for the rest of the alphabet.
It was a big mess. Nobody agreed. The classification schemes were all over the place until the early nineteen hundreds, and an astronomer extraordinaire named Annie Jump Cannon. This is a woman with more patience than I can ever summon in my entire lifetime. She was begrudgingly allowed to work at Harvard.
She was hired as a computer back then. Computers were that was a job description. But she ended up being really good at this whole astronomy game and too good to ignore and just brush under the rug even though this was not something that women do. Fascinating story in its own right. Happy to do an an entire episode just on Annie Jump Cannon's career.
Feel free to ask. She classified in her lifetime about 350,000 stars. I mean, come on. This is all by hand, by eye. She would look the telescopes, the observatories would stare at a star, collect its spectra, record it on a photographic print.
She would look at the print, classify the scar. This is type a. This is type b. This is type c. This is a boom.
Boom. Boom. Boom. Boom. She was like a machine for classification.
She was able to amass so much data that she was able to propose a classification scheme. One of the hard parts of the late eighteen hundreds was that because we didn't have a lot of spectra, it was hard to find, like, common groups and common threads and what were the exceptions and and what seemed to be a a general trend here or there, like but with Annie Jump Cannon's work, with 350,000 stars, you start to see some interesting patterns. So she proposed a classification scheme that made sense to her based on the strength of a particular set of spectral lines. I mean, it was worth a shot. Like, okay.
We'll look at these spectral lines from the element hydrogen. And if they're relatively weak, they'll be this kind of type. And if they're medium weak, they're this kind of type. If they're medium strong, they're this kind of type. And if they're super strong, they're this kind of type.
It was worth a shot. Nobody had any better ideas. She took an older alphabetical classification scheme and reordered it to fit her new template. She removed some redundancies. She changed around the ordering, etcetera.
So she had taken an existing system that went a, b, c, d, e, f, g, etcetera, etcetera, and then got rid of duplicates, merged some together, flipped some order around to make sense for this, her plan of looking at the spectral lines. She wanted a system as simple as possible, and this is what she came up with. And the result and I'm gonna tell you this because it's basically what astronomers still use today. There are seven kinds of stars in the universe, seven types, seven spectral types of stars that astronomers use to classify these. They are labeled o, b, a, f, g, k, and m.
I'll say that again. O b a f g k and m. There's a mnemonic for this if you feel like memorizing o b a f g k f m, which is o b a fine girl or guy kiss me. O b a f g k m. It's better it's a better mnemonic than o b a fine astronomer and pick a classification scheme that doesn't require a mnemonic, which is the one I would have suggested.
Why aren't these type one, type two, type three, or type a, type b, type c? Because history of astronomy. That's why Annie Jump Cannon had to work with an existing system that she walked into, rearranged it to make it better, and it kind of stuck. At the time, and I'll say this again because it's both funny and important, Annie Jump Cannon's and, yes, I will say her full name every single time. Annie Jump Cannon's classification scheme worked.
It was simple. It was based on something we could easily measure, the strength of these hydrogen lines in the spectrum, and we had no idea what was going on. She that her whole reason for doing this was like, yeah, the strength of this this hydrogen spectral line. What is hydrogen? I don't know.
Why do spectral lines exist? I don't know. But, hey, it's working. It gives us a nice simple system with OBAFGKM depending on the strength of this hydrogen line. You get sorted into one of seven categories.
Her classification scheme is still used today because it turned out to match some very important things that we know about stars. Over time, we realized a few things. For example, what Annie Jump Cannon used to describe our classification scheme just so happened to line up with a few other important things about stars like their colors and their surface temperatures. And then we discovered that a star's color and surface temperature is connected to a star's mass, and then we discovered that a star's mass is connected to its lifetime and what's happening in its core. So it just worked out that the classification scheme that made intuitive sense to Annie Jump Cannon made real deep physical sense, and that's why this scheme of seven types labeled o, b, a, f, g, k, and m stuck.
I'll give you the spoiler now and then fill in the gaps because this is a podcast and not a movie. I'm gonna talk I'll I'll tell you about these seven types of stars and what this represents when it comes to a star. Like, if I say, I am looking at an a type star. I we I can tell you what that means. There is a caveat here.
This only means this when the star is in the main part of its life cycle. I'll get to life cycles, either today or in the next episode. So this doesn't work always, but I I will talk about those caveats. But for the vast majority of a star's life, like, over 90% of a star's life, this holds that an o type star has a temperature of over 30,000 Kelvin, and it will look blue. And it will be more than 16 times the mass of the sun.
It will be greater than 6.6 times its radius. Like, boom. Just based on its classification on this hydrogen line, on the like, how strong this hydrogen line is, then you'll apply any jump cannon's classification scheme. It's an o type star. Boom.
It tells you this star is has a temperature of over 30,000 degrees. She didn't know this. Nobody knew it, but it turned out to be true. It turned out that her classification scheme, which made sense to her, can tell us about stars. These o type stars, by the way, are very rare.
They represent point o o o o o 3% of all the stars in the universe, but because they're big and bright, you tend to see a lot of them on the night sky. The next type down is type b. Type b have a temperature between ten and thirty thousand Kelvin. They look blueish. They'll have a mass anywhere between two and sixteen times that of the sun, a radius anywhere between one point eight and six point six that of the sun, and they're about point 1% of the stars.
The next type down is type a. These are mostly white stars with a temperature between 7,510,000 Kelvin, between one point four and two point one solar masses, 1.4, one point eight solar radii, point 6% of the population. F type, these are just plain white between six and seventy five hundred kelv 13,500 Kelvin. Little over the mass of the sun, little bit bigger, little bit more common, around 3%. Our sun is a g type star.
A g type star, which is a temperature between 5,206,000 Kelvin, looks slightly yellowish light white, like, mostly white with just a tiny tiny bit of yellow, not as much yellow as we see in the daytime sky here on Earth. Mass range anywhere between 8% the mass of the sun and, like, a little bit more around the same size of the sun. Pretty common. 8%. Eight % of all the stars in the Milky Way are g type stars.
Next type down are the k types. These are pale yellowish, maybe even orangish depending on their size. Temperature between 3,752 Kelvin. No smaller than half the mass of the sun, about 70% of the radius, and about 12% of all the stars in the Milky Way. By far, the most common kind of star are the m type stars.
These are the stars that look light orange all the way to red. No smaller than about a tenth the mass of the sun. That's, like, the smallest a star can be in our present day universe to about half the mass of our sun. 76% of all the stars in the Milky Way galaxy are thought to be m type. These are 2,400 to 3,700 Kelvin, the most common kind of star.
And so it's like they're these are the kind these are the seven categories of stars. Based on Annie Jump Cannon's classification scheme reveals and turns out to have this very cool connection to its temperature, to its color, to its mass, to its radius, to all these facts about stars. And she didn't know it. She didn't know it. If she got it wrong, if, like, if if her classification scheme wasn't based turned out to be based on something really meaningful and physical, then, we wouldn't use it today because it wouldn't have these interesting connections to, actual real life properties of stars.
And, yes, of course, it's been refined over the years, but it's basically been that been that main story for a hundred years now. Do I need to mention again? I think I will. That the time we're making these revelations that Annie Jump Cannon was figuring out her classification scheme that we were realizing that there's this deep connection to properties of the star like its color, its surface temperature. We had no idea what stars were.
We weren't even there yet. We were getting a lot of clues. We were beginning to realize that they were like the sun just really far away, but we had no idea what the sun was. The final linchpin, observationally, came with something we call the Hertzsprung Russell diagram, developed independently by Einar Hertzsprung and Henry Norris Russell, who were doing it on their own, and then someone pointed out, like, hey. You're doing the same thing.
I'm like, oh, cool. So now we call it the Hertzsprung Russell diagram or HR diagram because who has the time to say Hertzsprung Russell? Annie Jump Cannon's revolution, her classification scheme was based on data. You can't get that kind of classification scheme just by looking at the stars. You need to study them in hundreds of thousands of them and and acquire as much data as possible.
The HR diagram is the same story. This portrait that I'm about to unveil requires the boring work of science. The not the mind bending theories of space and time, not the giant leaps in intuitive understanding, just grunt work and labeling and surveys and spreadsheets. This is, like, where the real science happens. Or a kind of real science where you just need to collect mountains of data so that you can label things and figure it out and find interesting connections.
Around the same time that Annie Jump Cannon was doing her work, people were starting to make plots of star properties. After all, stars have several properties that we could study, like temperature and brightness, and it's natural to wonder if there are any interesting relationships between any of those properties, if there's some sort of connection, and there is. The connection is between you and Patreon. That's patreon.com/pmsudder to help you help me help this show help you. That is the ultimate connection.
Thank you for all your generous support over all these years. The real answer is that there's a connection between temperature and loo luminosity. Like, if you take a bunch of stars and you record for each star its luminosity, which is its true brightness, and its temperature, its surface temperature, you find that stars don't just have any random combination of surface temperature and luminosity. There's a a diagonal strip in this diagram, in this plot that you can make of all sorts of stars. There's a strip.
Most stars live on this strip. And this strip goes from one end, which is red and dim, which means cool temperature and not very bright. So there's a lot of stars there. And then there's a lot of stars that are on the blue and hot and bright side. So those are like opposite corners of their die of this diagram, and then you have a diagonal strip connecting where the vast majority of stars that you can study live on this strip where there is a direct relationship between a star's temperature and its luminosity.
We call this the main sequence. There are other regions on the HR diagram too, which we'll get to in another episode, But the vast majority of stars maintain this connection. If you know a star's luminosity, you know its temperature and vice versa. This only works on the main sequence. And, you know, that that the classification I gave earlier about this is what an o type star looks like.
This is what a k type I'm mocking myself. Type star looks like. Those relationships only work for stars on the main sequence, but the vast majority of stars live on the main sequence. So once you have the work of Annie Jump Cannon and the work of Hertzsprung and Russell, you have the major players in place. You have a decent classification scheme that connects something easy to measure, you know, the hydrogen spectrum coming from these stars, connects it to something hard to measure like a star's temperature.
And then you have another relationship between temperature and luminosity. So you can see there's a connection between, say, hydrogen, temperature, luminosity. There's a relationship, but but what is it? Astronomers long suspected that we are seeing different masses of stars in different parts of their life cycles. Like, there is just star.
Obviously, stars can be different masses because why not? Stars are gonna live their lives. We don't think they live forever. So we suspect this connection between, say, spectral type, temperature, luminosity has something to do with the star's mass and something to do with where a star is in its life cycle. The earliest proposals out there had stars starting out as hot bright blue giants then slowly contracting and releasing heat, and then they would end their lives as small dwarf stars.
This gave lifetimes of stars in the millions or hundreds of millions of years. If you're being generous, this was a problem, especially in the early nineteen hundreds because biologists and geologists were coming to the conclusion that the Earth was billions of years old and everyone thought that astronomers were wrong, and they were right. No. No. Not Dante astronomers.
The astronomers were wrong. The people who thought the astronomers were wrong were right. The astronomers the astronomers were definitely wrong. It took a couple more decades to figure it out. The answer was nuclear fusion.
That was the key that unlocked it all. Once we figured out how atoms work, how nuclei work, how quantum mechanics works, We're able to figure out nuclear fusion. We're able to realize that the crushing pressures and temperatures and densities inside a star are what power it because it can enable nuclear fusion, which can release the energy. We realize that stars form. They have some mass, whatever it is, which starts them somewhere on the main sequence.
As they age, as they're burning hydrogen in their cores, they move up the main sequence slowly becoming brighter and hotter. How quickly they move up the main sequence depends on its mass. Small stars barely budge on the main sequence. Giant stars just, like, move right on by through the main sequence. When nuclear fusion ends, they move off the main sequence and enter a different region of the HR diagram, which I will talk about next episode.
And that's the story. And the linchpin to all this was Annie Jump Cannon's classification scheme, which allowed us to measure things about the stars and then figure out other things about the stars, and then the Hertzsprung Russell diagram. This the combination of her classification scheme and their diagram created an observational reality that must be explained by theory. There is a connection between the strength of the hydrogen line, the temperature of a star, and its luminosity. There is a connection there.
It it's it's right there in the data. After you measure a few hundred thousand stars, you start to see it. It must be explained by theory. Nuclear power is our theory to explain any jump cannon's classifications and the features in the Hertzsprung Russell diagram. That is our theory.
So with this theory, we can finally answer the question of what is a star. It's a giant ball of plasma held together by gravity with nuclear fusion happening in its core. That is the definition of a star. That is our theoretical definition of star that best explains all the available data. A giant ball of plasma.
So if anyone's like, if anyone says, like, hey. What's a star? You can say, boom. You know what? It's a giant ball of plasma held together by gravity with nuclear fusion happening in its core.
I'm out. And, yes, that means they eventually run out of hydrogen fuel and die, but that's for another day. Class dismissed. Thank you for all the contributions via Patreon. Especially, I'd like to thank my top Patreon contributors this month, Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Nooner Dude, Robert m, Nate h, Andrew f, Chris l, Cameron L, Nalia, Aaron s, Tom b, Scott m, and Rob h.
It is their contributions and more that make this show possible. I I I'm still surprised every month that this happens, but I appreciate it nonetheless. Hey. Why don't you check out my book, How to Die in Space? It's available on Amazon, Barnes and Noble.
You can also get autograph copies of my website, pmstarter.com/book. Go ahead and leave some iTunes reviews if you can. And hit me up with more questions, hashtag ask a space man, to get your questions in. I absolutely love doing this show, and I hope you enjoy it too. And I will see you next time for more complete knowledge of time and space.