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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?

This episode is part 3 of a series.
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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 u