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We learned how to figure out the ages of objects in the Solar System, now we push out into the deeper Universe. What about stars, galaxies, and even the Universe itself? How old is it?
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Show Notes
Gyrochronology
Gyrochronology – Astrobiology Magazine
Measuring gravity’s pull at the surface of distant stars
Surface gravity along the main sequence. – SAO/NASA ADS
Transcript
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Professor Cain: Astronomy Cast Episode 524: Age and Origins, Part 3: Beyond the Solar System. Welcome to Astronomy Cast our weekly fast paced journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Professor Cain, publisher of Universe Today, with me as always Dr. Pamela Gay a senior scientist for the Planetary Science Institute and the director of Cosmo Quest. Hi, Pamela how you doin’?
Dr. Gay: I’m doing well. How are you doing Fraser?
Professor Cain: Doing great. I have like literally no news.
Dr. Gay: That says something considering you’re the publisher of Universe Today.
Professor Cain: Okay, possibly we have some news there. And if you wanna find out what that news is, you should sign up for my weekly e-mail newsletter at UniverseToday.com/newsletter, which I just sent out the door. Maybe that’s part of it, is that whenever I write my new newsletter each week, it just completely empties out of my brain. Oh, Sky & Telescope is for sale, so somebody should buy that and take good care of it because –
Dr. Gay: It’s true, please.
Professor Cain: Yeah, because Sky & Telescope was like the most meaningful piece of magazine periodical growing up. I found a pile of Sky & Telescope magazines at a garage sale on Hornby Island and I must have gotten my hands on like 40 or 50 of them and so then –
[Crosstalk]
Dr. Gay: Oh, wow.
Professor Cain: Yeah, so it was this huge collection that covered like a decade. And someone was obviously a collector and so I just went through them. That’s what taught the fine details of Astronomy, what object we up, how to find them, techniques, and things like that. And so, I know for a lot of people who are listening to the show Sky & Telescope is like – was the Bible for Astronomy, like really good, hard science observing guide. And it’s too bad that it’s sort of gone through some tough times.
People keep saying, “Will Universe Today put in an offer?” No, I don’t think I can afford it. I don’t even know what to do with it. We don’t kill trees at Universe Today, we just push electrons around. But anyways, but is someone does have money and wants to take care of it, it should be up for sale in the next couple months.
Dr. Gay: Okay. On that cheerful note.
Professor Cain: We learned how to figure out the ages of objects in the solar system. Now we push out into the deeper universe, what about stars, galaxies, and even the universe itself, how old is it all? All right, so we kind of half taught people how to figure out how old stars are but we have not taught people how to figure out how old stars that aren’t the sun are very well.
Dr. Gay: That is fair.
Professor Cain: So, let’s start there with how do we figure out how old stars are in the wider universe.
Dr. Gay: Well, there are a bunch of different ways and the newest way that scientist have come up with is to actually look at how fast stars are rotating, and this was something that I never knew would be a thing. And one of the factors I loved most was the press release actually said how to pronounce this technique gyrochronology, –
Professor Cain: I love that term.
Dr. Gay: – so just like gyroscope. The idea is that stars change their speed over time as they undergo mass loss and as that mass is carried away, so too is the angular momentum of the system. So, it’s super difficult to measure the rotational velocities of stars, but if you can do it this is the cool new way that they cool kids with the best instruments are measuring the velocities of stars and their ages.
Professor Cain: So, I’m trying to think about how you would measure that, right? How do you measure the rotation of a star and then how do you know what the rotation tells you about the age of the star? So, how do you measure the rotation of a star first?
Dr. Gay: The most accurate way to do it is to look at stars that have sunspots and measure how long it takes for that sunspot to across the face of the star. So, just like we measured the rotation rate of our sum, the first order by watching those little sunspots march across the front, we can look at changes in brightness of distant stars that are tied to changes in whether or not we’re seeing sunspots on that distant star.
Professor Cain: That’s amazing. Right? That you can see sunspots moving across the face of a star, but by guess how much light they’re putting out and then you can use that as a way to say, “Okay, that’s probably a sunspot,” and then when it returns to that same level of brightness just a couple of weeks later, then that’s probably that same group of sunspots is moving across the surface again. Mind-bending. Now I would also assume that there’s some way sort of with the Doppler Effect that you can measure the sides of stars to sort of get a sense of how quickly they’re turning. One part of the star is moving away from you and one part of the star is moving towards you.
Dr. Gay: If only you could separately measure the light coming from either side of the star. But we don’t quite have the capacity to do that. So, what we do instead is we look at the line broadening, but there’s a complexity to this that can add a lot of error to the measurements, and that complexity is gravity. So, the surface gravity of a star also effects the thickness of the spectral lines of a star. So, if you can accurately figure out what kind of star it is, what kind of mass it likely has, you can make assumptions about what its surface gravity will be and make assumptions about how that gravity will affect the width of the lines, and then you can assume that whatever’s left behind is line-thickening due to rotation.
But, it’s a much less precise method, although to be fair we’re looking at sunspots on distant stars where we’re going, “Ah-hah, it’s brightness dipped this many percentages and then came back up in a non-periodic way. Therefore, this is a sunspot.” So, this is one of those techniques that is sort of like wow there’s a lot of error, but this is cool.
Professor Cain: That’s amazing. Okay, so that’s your method for a star. So, now we are measuring the speed that a star is turning, how do we then use that to figure out how old the star is?
Dr. Gay: Well, this is where you have to couple different methods and just like we have a distance ladder for measuring the distances of stars, we kind of have an age ladder for measuring the ages of stars. And this age ladder is based on our understandings of stellar evolution that is then grounded in radioisotopes and then we extend it out now with the rotations of stars. So, the science teams that did this, they were looking primarily at main sequence stars, these are stars like our sun that are burning primarily Hydrogen and Helium in their core. And they were looking at late F, G, K, and M stars, so these are the smaller kinds of stars.
And in systems where you have a main sequence, where you have stars that are still in the process of evolving, this means that you may have your biggest stars have already well, stopped being main sequence stars, they’ve already evolved away. And because predictably from larges to smallest evolve off the main sequence, finish burning that Hydrogen and Helium in their core, other processes for bigger stars.
As they evolve off that point says okay everything more massive than this is done burning, everything below this is still burning, and from radioisotope – which we talked about before – we can get precise ages for that point for stars that are close enough to get high enough resolution spectroscopy. And we use stellar evolution models for systems that are further away that we can’t accurately measure. So, through a combination of stellar evolution models and actually getting to measure things from radioisotopes, we can say, “Okay, when we see this turn off, it mean this age. When we see this turn off, it means this age.”
Professor Cain: Now again that technique of measuring the spins of things can tell you how some degenerate objects, how old they are. And I think the best example of this is pulsars, neutron stars, that whole process, right? By measuring the spin rate, you can tell kind of how old the object is.
Dr. Gay: And this is also reliant on them being in isolated systems. If you have a massive star that goes supernova, collapses down to something tiny, it’s initially going to have a much larger rotation rate. Over time its rotation’s gonna slow for a whole variety of different reasons, but this starts to give you relative ages of different systems. Now the reason I say that this only works with isolated stars is it’s possible to transfer angular momentum between stars in a binary system. So, any time you’re looking at the rotation rate of something, it needs to be the non-influenced rotation rate.
We can see this in our own Earth we’re, to some estimations, a binary planet with our own moon, and our rotation rate is slowing as the moon moves further away, and it’s this title locking of our two systems that is in the process of heading towards being completely title-locked. That we’re changing the rotation rates of both world as we evolve their separation.
Professor Cain: Right. When a star goes supernova, if it’s more massive than –five times more massive than the sun, then you get a neutron star as the outcome and they start out having a ton of that angular momentum, they’re spinning very quickly. This is a pulsar and the fastest millisecond pulsars are the freshest, and then overtime they are losing their energy. How? Right, what is causing them to slow down?
Dr. Gay: So, you can actually end up with the stars deforming themselves, filling the rush lobes essentially, where the side of each star that’s pointed towards the other deforms and gravity tugs extra on that, and so you have this slowing because the stars aren’t perfect spheres because they’re pulling on each other. So, this is a title effect, you also have mass loss effect, mass transfer effects – which can speed up one star while slowing down the other –
Professor Cain: Well, even – I was checking more like an individual star, not necessarily one that’s in a binary system. How does an individual star, slow –
[Crosstalk]
Dr. Gay: Mass loss.
Professor Cain: – its rotational rate? But also –
Dr. Gay: Expansion. So –
Professor Cain: And gravitational waves, right? Or not gravitational waves, but can’t they lose momentum just like into space itself? I forget the method, maybe it’s only in a binary system.
[Crosstalk]
Dr. Gay: So, that is again – you can have gravitational radiation and that is something that we worry about with compact stars, compact star binaries. For your average binary system that is less of a concern and it’s more like you shed mass, you slow down. Unlike humans, who shed mass and speed up, but stars have their own way of working.
Professor Cain: So, what about white dwarfs?
Dr. Gay: Now white dwarfs, they are awesome insofar as they start out supper hot, it’s a leftover stellar core from a star like our sun that has poffed off its outer atmosphere to form a planetary nebula. And that diamond in the sphere that is left behind is white hot initially, but it’s not generating new heat over time, it’s simply radiating a heat away to outer space and this means that over time it is cooling off, it’s becoming less luminous, and it should be doing this in a way that is completely predictable. The question is are our theories correct, and this is the modern challenge, not whether or not this technique can be used but how well do our theories match reality.
Because there’s phase transitions that go on as the star cools that our new thinking is that these phase transitions within the star may cause them to linger at one particular apparent luminosity for longer than they might remain at other apparent luminosities.
Professor Cain: But over time, you know give it billions of years, trillions of years, they should cool down to the background temperature of the university. And so, you should be able –
Dr. Gay: Black stars.
Professor Cain: Yeah. And so you should be able to measure the temperature of a white dwarf and know roughly how old it is knowing what temperature it probably started at.
Dr. Gay: And this is where reading really old Astronomy papers and literature becomes fun, is you have Astronomers that were talking about Black Dwarfs, Black Stars and they’re simply talking about really cold stars, not some euphemism for Black Holes. So, that White Dwarf over the fullness of time is gonna become invisible to the human eyeball.
Professor Cain: That’s really cool. All right, so we have a way of trying to figure out how old a lot of stars are, if they’re in binary systems, then that messes things up. What about galaxies themselves, can we get a sense of how old the galaxy is and when it came together?
Dr. Gay: We can get a limit on it. So, if you look at a star system that doesn’t have on-going star formation going on and you measure what the general population of the stars, red and dead is the phrase that’s generally used, that tells you that that galaxy is at least old enough to have formed and evolved all those stars, so it puts a limit. And then the other things is, we’re looking back in time with most galaxies, so when I look at the light of a galaxy that has been traveling to reach me for 12 billion years, because light doesn’t move instantaneously, so the further away something is, the further back in time I get to look.
When I look at these 12 billion light year away systems and I see stars that are millions to maybe even billions years old – well not billions at that point – hundreds of millions of years old, that tells me that system must have formed at least if there’s 500 million year old stars and I’m looking at something 12 billion lightyears away that means it has to have formed at least 12.5 billion years ago. And it’s these limits that are really exciting to us as we look back at the furthest systems, because we get to see galaxies that we know are in the process of forming, because they haven’t been there long enough for this to be their fourth generation of dying stars.
If I look at a nearby galaxy and it’s all red stars, it might have gone through seven generations of star formation and formed at the beginning of the universe. So, looking at galaxies I have to combine the red shift information that tells me loosely how far way it is, supernova information if I’ve got that, and then combine it with how old are the oldest stars that I’m able to tell are there by the combined spectrum of light from that system.
Professor Cain: There are actually a couple of other objects that I just realize that are connected to that. One is to know how old globular clusters are, because they’re often used as a way to age –
Dr. Gay: Our own system.
Professor Cain: Yeah, our own galaxy, right? So, when we think of these globular clusters, these gigantic balls of hundreds of thousands of stars, how do we know how old they are?
Dr. Gay: It goes back to that main sequence turnoff that I spoke about earlier in the show. Globular clusters are these fabulous things that we don’t fully understand the formation mechanism for, where all the stars in the system are made out of one kind of material, one mixture of star dust and gas, and all the stars were formed out of one epoc of star formation. And since they’re all made of the some stuff they’re all going to evolve in similar ways, since they were all formed at the exact same time that tells me that the largest stars I see on the main sequence are indicative of the age of that globular cluster.
So, if there are stars on the main sequence that would die in – say it’s an open cluster instead of a globular cluster – that would die in 500 million years, if those stars are still on the main sequence that means it’s less than 500 million years. Now if I look a bit bigger, and I’m like oh there’s nothing there that would live for 200 million years, all those things have gone supernova that starts to give me this narrow window of how old that system is. Now I can keep doing that, keep going further and further down the main sequence until I get to stars that are billions of years old.
Now the complexity here is these are theoretical ages based on how long do we think it takes to go through all these different stages of evolution, and again when we can it’s great to be able to get high resolution spectroscopy and start to nail down these ages not just with the main sequence turn-off period but also by looking at the compositions with high resolution spectrographs and measuring through nuclear cosmochronography what is the age due to radio isotopes.
Professor Cain: Right. I mean I love this idea that you look at a globular cluster and you don’t see any stars that can live for 10 billion years. Therefore, you know – or that all the 10 billion year old stars have died, therefore the globular cluster is older than 10 billion years. Which is just – but then you can say oh but there are some stars that can live 12 billion years, therefore it’s possible that that cluster is less than 12 billion years old, which is an amazing idea. One more sort of astronomical thing and then I wanna move to sort of the big question. So, we see – we report on this all the time on Universe Today, like we see a supernova remnant and we’ll say this supernova remnant happened 5000 years ago. How do we know that?
Dr. Gay: That comes down to there isn’t anything to stop a bullet in space. And when that bullet is part of the outer layer of a star, it just keeps expanding out at a fairly constant rate. With some supernova remnants like the Crab Nebula, we can look out and over decades we can actually see in the images – and this goes back to talking about Sky & Telescope at the beginning of this episode. Sky & Telescope had a fabulous collection of these images that they used to distribute to schools where you can measure with a ruler on the images how the outer layers of the nebula are moving outwards relative to the stars.
And by knowing the dates that these different images where taken and the distance to the system, you could measure the expansion rate. Now if you know the expansion rate you work it backwards to when was this thing compressed down to the size of star and that gives you the how long has it been expanding. Now we are lucky with some of these supernova that we actually have archaeological records so we can check our work.
And it’s amazing to be able to look out and say, “Yes, that particular supernova was observed by Kepler, by Brahe, by the Chinese, by these indigenous people, by Heck space telescope in 1987,” and see the remnants expanding over time and be able to look at other things where we see these light echoes traveling through space and then we can track them back and say there was a supernova here and even though no one noticed it we know when it happened.
Professor Cain: All right, so we got enough time to ask the big question. Which is how do we know how old the universe itself is?
Dr. Gay: It actually builds on how we know how old supernova remnants are. When we look at the expansion rate of our universe, we run it backwards. Now there are checks and balances, there are other ways that we’re like does all this make sense. We look at the fine detailed structure in the cosmic microwave background and that helps us understand oh expletive, the cosmic microwave background had to have gone through a massive inflationary period. Therefore, from this massive inflationary period things – it was a non-smooth expansion.
But we can work things out to get at detailed theories that replicate what we see to model the first 300 – 400 thousand years of the universe to replicate the observed cosmic microwave background. From there we can measure the expansion rates as a function of time and see the gradual changes that are taking place. And then we just do an integration, it’s all calculus all the way down.
Professor Cain: I did some research in this on an article one time or a video and if you could see the cosmic microwave background, this is that is moment when the universe became transparent and light was able to escape out into the universe for the first time and if you could be there to see it when it happened you would see like a reddish glow, would be the color that you would see, but we know we see it as a the cosmic microwave background. And so, it’s that redshift that’s happened over time as those regions that were once red have now been pushed away that the wavelengths have been stretched out to the point that we see them in microwave. And that’s your math, right?
That tells you what age – how much expansion time would it require to take what was reddish light in the beginning to make it look like it was microwaved like today.
Dr. Joy: And that gives you a limit. So, we know how fast the cosmic microwave background is moving away from us. Now the question is, has the rate that the universe is expanding changed and it’s sort of like if you know that I was going 90 mph for the first hour of a journey and you know that I’m going somewhere between 70 and 100 at the end of the journey. You can guesstimate based on knowing the distance. How long it took me to travel if you just assume I went 90 the whole time. But if my speed varied at a constant rate – which is what we think now is the acceleration – is the recent acceleration term, it gives you a finer detail. This is where we keep adjusting our numbers over time.
Professor Cain: And we look at say – when we started Astronomy Cast the estimate was 13.5 billion years old, then it went to 13.7 billion years old, and not we’re at 13.77 billion years old and the error spots are getting narrower and narrower at this point. Are you looking at the exact most recent estimate?
Dr. Gay; Yeah, because in my head – yeah it’s 13.772 according – yeah those – Plank measure at 13.82.
Professor Cain: There you go, so plank is the newest measurement, the most accurate measurement of the cosmic microwave back ground gave us this incredibly accurate estimate. Why? Why does a more accurate measure of the cosmic microwave background, gives us a more accurate understanding of how old the universe it.
Dr. Gay: This is where it starts to come down to matching the fine grain structure and the background which starts to tell us how long it was before the big bang. So, when I was in grad school, I remember learning it was 300 years from zero to cosmic microwave background, when now tend to teach it to more like 400, so that gets you part of the difference. And then it’s just a more –
Professor Cain: Thousands, 300 thousand yeah.
Dr. Gay: Yeah, it’s now it’s –
Professor Cain: I guess 3080 thousand, I forget the exact number. Yeah.
Dr. Gay: It’s now also a more accurate color measurement, so when you combine the more accurate color, the more accurate understanding of how long it took, and it also gives us a more accurate understanding of the geometry of the universe. Because how you expand and how the geometry is, it all factors together. So, there are multiple different things at play. There isn’t a simple baby answer. But it all comes down to knowing how long it took to get from 0 to cosmetic microwave background? How long it took to get from cosmic microwave background to now based on changing expansion rates. So, yeah.
Professor Cain: Amazing. So, there you go that’s how you know how old everything is, if anyone asks. Pamela do you have some names to read this week?
Dr. Gay: I do. So, this is the part of the show where we’re working our way through thanking the wonderful patrons that help us pay Susie. Without you Susie would not be paid, so thank you, thank you, thank you. This week we want to thank someone with a complicated name, Kensliea Ponsneviko, Shannon Humbard, David Gates, Eric Franger, Rachel Fry, Fredrick Shorgy, Claudia Masteroni, Tyrion VonNueterdoot, Darcia Daniels, Kristen Brooks, Thomas Tubman, Jimmy Burkeson, Arthur Lashall, Omar Delrivriaro, Chad Collopie, Mark Steven Rasnack, J4184, and William Lauer. Thank you for being part of our Patreon crew.
To those of you who have not yet said Astronomy Cast is worth five dollars a month, instead of getting that extra Starbucks I’m gonna support this podcast I love, don’t make me send – well we can’t lean on Ira Glass, so don’t make me send your friends to have me mock you. Please consider giving it doesn’t take a lot to make a huge difference and we are grateful and we try and make it worthwhile for all of you we have Patreon office hours, most Sundays we have special rewards, you can get your name on our website, and most of all you have my complete thanks by name. I will thank you as I read your names looking at our Patreon, Let me thank you.
Professor Cain: Patreon allows us to make this information. 524 episodes of Astronomy information available to anyone who wants to listen to it and learn about the universe. We don’t have to put it behind a pay wall, this is our way to make it available to as many people as possible and we couldn’t do this without the support of the patrons, so if this is a model that you support, if you like this, then definitely get involved, go to patreon.com/AstronomyCast and pitch in. All right, thanks Pamela we’ll see you next week.
Dr. Gay: Thank you Fraser, see you later.
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Voiceover: Thank you for listening to Astronomy Cast, a non-profit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can e-mail us at info@astronomycast.com. Tweet us @AstronomyCast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3 p.m. Eastern/12 p.m. Pacific or 1900 UTC. Our intro music was provided by David Joseph Wesley, the outro music is by Travis Siro and the show was edited by Susie Murph.
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Duration: 34 minutes
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