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How old is that star? That planet? That nebula? Figuring out the ages of astronomical objects is surprisingly challenging. Fortunately, astronomers have developed a series of techniques they can use to work out the ages of stuff.
Transcript
Human transcription provided by GMR Transcription
Fraser Cain:
Astronomy Cast Episode 717. How old is that thing in space? Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand, not only what we know, but how we know what we know. I’m Fraser Cain. I’m the publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest.
Hey, Pamela. How you doing?
Dr. Pamela Gay:
I am doing well. It is [inaudible] [00:01:14]. It is spring.
Fraser Cain:
Yep.
Dr. Pamela Gay:
I do have a cool announcement to share out with all of our audience. On the – I should’ve had these dates in front of me – on Friday, May 24th, I’m gonna be doing a meet-and-greet in Baltimore at a cool, old clock restoration place that has now been turned into a bar. And then on May 30th, I’m going to do a meet-and-greet in Orlando at the Eastside Market.
All of this information is going up on my social media and CosmoQuest’s social media, and it will go on Astronomy Cast social media. It’s not there yet. So, if you are going to be in either of those places, come say hi. I’ll be in Baltimore for the Balticon convention. So, if you wanna hear me give talks, that’s your opportunity.
Fraser Cain:
Right on. How old is that star, that planet, that nebula? Figuring out the ages of astronomical objects is surprisingly challenging. But, fortunately, astronomers have developed a series of techniques they can use to work out the ages of stuff in space. So, this is gonna be like a collection of techniques, overlapping in some cases. It’s like the distance ladder.
Dr. Pamela Gay:
Yeah. Yeah.
Fraser Cain:
But it’s the age ladder, which is sort of like a different – I don’t know – totally different perspective in how you think about stuff in space. So, I don’t know, we could talk about stuff in the Solar System, stuff out in space, the beginning of the universe itself. Where would you like to begin this conversation?
Dr. Pamela Gay:
So, the way that we measure time falls into two categories. There is expanding stuff, and we can just work backwards. And then there is all of the stuff that has an age ladder that is usually rooted in atoms and stuff. And I say, why don’t we start with all of the stuff that’s expanding?
Fraser Cain:
Okay. That sounds good.
Dr. Pamela Gay:
Like, Crab Nebula, is that a good place to start?
Fraser Cain:
Yeah, totally. Yeah. Okay. So, this is a great example, right. You look in space. You see this puff cloud of material, and you ask yourself: How old is this thing? Now in this specific case, we know how old it is because people watched it happen.
Dr. Pamela Gay:
And what’s cool is we figured out, yes, these two things exactly match because looking at the expansion rate and working backwards also matches the historic records. And so, to have this double confirmation is kind of awesome. So, with the Crab Nebula, we have photographic evidence of what it’s been doing since the early-1900s. We can look at this. We can see where the details in that it kind of looks like a dead bug pattern of clouds and gas.
Fraser Cain:
Right.
Dr. Pamela Gay:
We can see where it is relative to all the background stars.
Fraser Cain:
Right. And the point is that this is a supernova remnant.
Dr. Pamela Gay:
Exactly.
Fraser Cain: T
hat this is the expanding debris cloud from a supernova that went off.
Dr. Pamela Gay:
And then, because we have these old images, we take new images. And you can superimpose them, lining them up using the stars. Stars, some of them have moved, but in general they haven’t. And you can see all of this dead bug of clouds of gas and dust have moved. And this allows us to work out the rate of expansion. And once you know the rate at which something is expanding – and we have enough images that we can now see it’s also a continuous expansion – once you know the rate at which something is expanding, you know the size that something is. That’s now just a distance equation that all of us have done when we were trying to figure out how long until I get to the place I’m going.
Fraser Cain:
Right, right, right.
Dr. Pamela Gay:
You take the total size –
Fraser Cain:
Two friends get into a car. Yeah, they get in the car.
Dr. Pamela Gay:
Exactly.
Fraser Cain:
They are traveling eastward at 50 kilometers per hour. How long does it take for them to reach their destination. Yeah, yeah, yeah.
Dr. Pamela Gay:
Yeah. So, it’s exactly that math.
Fraser Cain:
Yeah.
Dr. Pamela Gay:
You take the rate they’re moving. You take the distance. And you can figure out the time. It’s easy.
Fraser Cain:
Now, what are we seeing expanding? I mean, are we actually seeing this cloud of debris? Or I know in some cases you’re seeing the light that is leaving the explosion, and so it’s illuminating the surroundings.
Dr. Pamela Gay:
Right. So, we have two different things we have to worry about. With the Crab Nebula supernova remnant, we are actually seeing the stuff move in some cases and the shockwave propagate. In other cases, we’re looking at light echoes.
One of the coolest side effects of the MACHO Project, which looked at the nearby Magellanic Clouds, was they saw these weird, bright streaks through a lot of their images that they initially thought were errors in the optics. But as they went back year after year, they were able to see these bands did not remain in the same place. And as the bands moved, when they ran the geometry, they were able to figure out this is an expanding shell of light. Oh, this is an expanding shell of light.
Fraser Cain:
Wow.
Dr. Pamela Gay:
And the shell of light is moving at the speed of light, and it’s simply hitting gas and dust particles between the stars, getting reflected back at us. So, we’re seeing this expanding shell. And then it’s just a geometry problem to figure out where the center of that is and calculate when the supernova event that triggered the light was let off.
Fraser Cain:
Yeah. The example that we see in the sky more recently is Supernova 1987A –
Dr. Pamela Gay:
Yeah.
Fraser Cain:
– which has that really cool ring structure –
Dr. Pamela Gay:
Yeah.
Fraser Cain:
– and has these weird pearls embedded within the ring itself.
Dr. Pamela Gay:
Yes.
Fraser Cain:
And, in fact, this material was hurled out as the star itself was dying. And that ring that we’re seeing is the light emanating away from the blast zone, illuminating all of the previous stuff that had been thrown out as it’s interacting with the interstellar medium. It’s a phenomenal idea.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
So, we sort of led into this idea that you can calculate back. And so, when astronomers calculated – you know, used the simple geometry problem to figure out how long this expanding gas cloud has been going on with the Crab Nebula, when did they calculate the beginning?
Dr. Pamela Gay:
1054. And there were Chinese records of something in 1054. And it now looks like that’s the match. This is what it is. There’s archeological records in the American Southwest as well that may be dateable because it’s thought that those carvings in rock may be the supernova. And they’re in the right era, so we may also be able to use what we see with the light echo, what was confirmed in Chinese writing, to get at the date of records in the American Southwest.
Fraser Cain:
Wow.
Dr. Pamela Gay:
So, it all slowly allows things to go full circle as we create a time ladder, as you said at the beginning of the show. But in this case, it’s time of one thing and a whole lot of records that get interrelated. And I just wanna say, it isn’t just supernovae that do this. One of my favorite light echoes is V838 Mon.
Fraser Cain:
Yeah.
Dr. Pamela Gay:
This is a star that flashed amazingly two different color flashes. We were able to watch them evolve. And this expanding shell, or pair of shells of light, allowed us to map out the distribution of material around this star, stars, after these nova events took place.
Fraser Cain:
Look, almost everything you see in space is very static.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
You’re looking at this nebula. You’re looking at this galaxy. And I it is gonna be unchanged for tens of thousands of years, millions of years in some cases. But in V838 Mon, it is this –
Dr. Pamela Gay:
Yeah.
Fraser Cain:
The timelapse just from Hubble shows just how much it really looks like this explosion. Yeah, it’s absolutely incredible. And so, you can say, “When did this event take place?” We’ve done a ton of reporting on Universe Today about this stellar archeology that goes on where astronomers will find some supernova remnant. They’ll use that technique that you mentioned; they’ll calculate the age of the blast wave, determine the date when this should have been visible, and then they go looking in historical records for anybody that noticed a bright star in the sky appear at this time.
And it comes up again and again and again in ancient Chinese records, ancient Japanese records, European records, Greek records, things like that. It’s amazing how much there is this correlation because these supernovae must have just been so exciting and scary for the people who saw them. They were there.
Dr. Pamela Gay:
Yeah. Yeah.
Fraser Cain:
And we get to find out that this happened. All right. So, we talked about things that are expanding.
Dr. Pamela Gay:
Yes.
Fraser Cain:
Now let’s try to figure out the ages of more static objects. All right. So, let’s talk about something simple. A star, “How old is that star?” he says.
Dr. Pamela Gay:
I was gonna start with like cratering, which is a whole lot easier. Okay. Let’s go to the hard stuff first. I’m with you. I’m with you.
Fraser Cain:
Yeah. Okay. All right. All right. Yeah.
Dr. Pamela Gay:
So, with stars, we use nucleo-cosmochemistry, which is just a really fun word to say. The idea is there are a whole lot of atomic nuclei that are not entirely stable. And when a star forms, they will have a certain ratio given the supernova material that went into them of the radioactive material and the dotter particles that come off when that material decays into the dotter particles.
And so, when we look at some stars with just the right atmospheric conditions, we are able to see the ratios of the atoms that have the half-lives and do the decaying and the dotter particles they get decayed into. And by looking at these ratios, we start to be able to say this object appears to be this amount of time old.
Fraser Cain:
Right.
Dr. Pamela Gay:
Now, this is an imperfect science because stars can eat their neighbors. They can eat their friends. They can eat their planets.
Fraser Cain:
Right.
Dr. Pamela Gay:
So, there’s always the potential for contamination. It’s also an imperfect science because if a star’s small enough you have confection. If a star is big enough, it’s just gonna have a much more weird atmosphere to deal with. And if it’s a main sequence star, its surface gravity is just gonna make things harder. It’s complicated.
Fraser Cain:
But to make lives easier, if it is in a globular star cluster, then you’ve got another vector to try and triangulate the age.
Dr. Pamela Gay:
Yes. So, this is where we start getting into the, do astronomers actually understand the lives of stars as a way of calculating the age?
So, we know in general that stars over time go from burning hydrogen in their core to burning more and more advanced atoms. Throughout this process, they change in color. They change in size. And when you make a plot of the color of the stars and the luminosity of the stars, they group up in different places in this plot according to what’s going on in their centers.
And the first thing we see is, because the biggest stars run out of hydrogen to burn in their cores first, they leave the line that represents the main sequence of hydrogen burning first. And it’s more complicated than that. There are things other than hydrogen –
Fraser Cain:
Right.
Dr. Pamela Gay:
– being burned by the biggest stars on the main sequence. Do not at us.
Fraser Cain:
Right.
Dr. Pamela Gay:
But by measuring where this turnoff is as you go to smaller and smaller star, you can first rank, okay, this is definitely older than this.
Fraser Cain:
Right, yeah.
Dr. Pamela Gay:
And as our modeling gets more and more advanced, we start to be able to say we are pretty sure that stars that have gone through all these different processes and have this combination of atoms, this metallicity scientifically, are going to be this age at this point.
Fraser Cain:
Yeah.
Dr. Pamela Gay:
And so, this is how we went from when I was a graduate student being very confused that the globular clusters appear to be older than the universe.
Fraser Cain:
Right.
Dr. Pamela Gay:
We just didn’t quite have our stellar evolution nailed down. We’re better now.
Fraser Cain:
Well, right. And I think that goes to the challenge of any of these techniques.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
We’ll get into this in a second. But really until the last couple of years, you would be off by billions of years.
Dr. Pamela Gay:
Occasionally. Yeah, that was a thing.
Fraser Cain:
It was not very accurate to know the age of that star. You’d say like, “It’s a [inaudible] [00:15:38] star. It’s probably at this phase. It’s two to four billion years old.” Right.
Dr. Pamela Gay:
Yeah. Yeah.
Fraser Cain:
Which isn’t the kind of level of accuracy. But there’s been a technique developed fairly recently, asteroseismology.
Dr. Pamela Gay:
Yes.
Fraser Cain:
Which is giving us much more accurate measurements of stellar ages.
Dr. Pamela Gay:
And this is because asteroseismology, like making anything resonate, allows us to get at the density and size of that cavity in the outer atmosphere of the star. And one of my earliest things of research was actually looking at how [inaudible] for instance over time their periods will evolve as the density of the star changes with nucleosynthesis –
Fraser Cain:
Right.
Dr. Pamela Gay:
– and stellar mixing, and all these other things going on.
And so, by being able to get this check on the conditions in the out layers of the star, getting this check on how the different forms of energy transfer taking place – the convective region, the radiative transfer region – we are able to start saying, from the data, we know these are our boundary conditions. And knowing boundary conditions, that reduces so much of the error.
And we’ve also had another really cool check on a lot of this. Which is, as our telescopes get better, we’re able to see more and more white dwarfs in these clusters. And we know from how nuclear burning works what temperature the core of a star was when it decided to get rid of its atmosphere, and that gives the starting temperature of a white dwarf star.
And then we look at them, and we can see what temperatures are the white dwarfs as they cool off. And that starts to tell us, okay, this distribution of white dwarf temperatures, in combination with this combination of evolved stars in the rest of the cluster means it has to be a given age.
Fraser Cain:
Right.
Dr. Pamela Gay:
So, there’s lots of checksums coming into place.
Fraser Cain:
Yeah, it’s really cool, right. We look at the chemicals in the stars of our atmosphere.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
We look at its neighbors. We look at the tiny variations in its brightness, the wobbles of seismic waves passing through the star, and you can triangulate on the age of that star.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
Three interlap – and there are more.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
But if you’ve got other things that it’s interacting with, other clues and hints, it feels very much like a detective working on a very complex case.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
And if you notice something nearby, then you can draw that in as a hint to the age of this object. So, let’s talk about just the ages of surfaces on worlds.
Dr. Pamela Gay:
And this comes down to looking at cratering. And we can also get a ground truth by cheating and going there and grabbing rocks. So, the moon is the best example. When we look at the moon, we see areas that are extremely cratered. We see areas that are quite smooth. And it all comes down to how old is that surface. If a region in the moon is sufficiently cratered such that every time a new rock comes in and hits it it’s just erasing other craters – it’s basically at an equilibrium for the distribution of craters in that region – that is a very old region. Adding new craters does nothing.
Fraser Cain:
Right.
Dr. Pamela Gay:
If I’m looking at a region that has progressively fewer craters than that saturated region, I can then order the ages by this is fewer, so it’s younger; this is even fewer, so it’s even younger than that. Bare naked crater list, that’s a brand-new section.
And craters get erased through a variety of different means on the moon. Once upon a time, there was volcanism. Today, the biggest way to erase a region is you hit it with something sufficiently large that it flattens an area, probably melts the entire area as well, which is just a different form of lava flow. It’s no longer volcanic; still have a crater, just not a volcanic crater. Flame craters, that’s the moral, I guess, for the moon. So, you basically erase an area. And then we know the rate at which rocks in space attack as a function of size.
Fraser Cain:
Right.
Dr. Pamela Gay:
There’s this regular onslaught of tiny stuff. There’s a less regular but frequent enough that we can actually see bright spots when we look at the crescent moon where the bright spots appear on the dark part of the moon regularly in imagery, which is super cool. That’s impacts taking place. And by knowing the rate at which cratering occurs as a function of size of the thing doing the impacting, we can say what is the difference in age between different areas.
Now, what we know with the moon, we can put on a this is the actual time scale because the Apollo astronauts landed in a variety of different places. The lunar samples taken from the Soviet Union were taken from a variety of places. And by saying this region where this rock came from is related to this amount of cratering, and this rock from over here that came from this amount of cratering has this actual age using radio dating, radioactive material dating, then we can correlate all of the crater densities to actual ages.
And what’s cool is we can then also take this and expand it out to the rest of the solar system. Now, it’s a little bit more complex. We have to make assumptions about things closer to the sun are gonna get hammered a lot more. Things further out from the sun are generally going to get hit less. Exceptions – get too close to Jupiter, all bets are off.
Fraser Cain:
Right.
Dr. Pamela Gay:
And by combining models, ground truth with lunar samples, and what we hope to someday get with ground truth from places like Mars and asteroids that aren’t rubble piles, we will be able to work out the cratering rate throughout our solar system and, thus, get the age of various surfaces that aren’t being affected by weather and atmospheric conditions. Here on Earth, it’s a completely different story.
Fraser Cain:
Right.
Dr. Pamela Gay:
We use sedimentation and radio carbon dating and other atoms, depending on the ages you’re going towards. But cratering is a great way to understand the rest of the solar system. And it’s from cratering that we know the surface of Pluto has been around for less time than insects belonging to the family of bees have been on the planet Earth.
Fraser Cain:
Right.
Dr. Pamela Gay:
Bees have existed as bees longer than the surface of Pluto has existed based on cratering rates, and that’s amazing.
Fraser Cain:
Right. Right. And so, the gist being that some active processes happen to resurface the surface of Pluto.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
And it’s the same. We look at the surface of Europa, and it’s surprisingly smooth, not a lot of craters there.
Dr. Pamela Gay:
Yes.
Fraser Cain:
So, some process is actively smoothing it out while we look at Mercury, we look at the moon.
Dr. Pamela Gay:
Ganymede.
Fraser Cain:
They look a lot older.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
And, again, it’s back to this overlapping methods of measurement that you’ve got. On the one hand, you’ve got the actual samples that were brought home to Earth to allow you to sample to figure out within the closest tens of millions of years when those samples – like when these regions were formed on the surface of the moon.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
Was it this lava flow? Was it that lava flow? Was it an ancient hilly terrain? Whatever. And then you count up the crater count, and now you start to realize how often these crater, these impacts were happening, and you can measure the ages with incredible precision across the surface. And that gets used on Mars as well. It’s kind of amazing. This crater happened before that crater. You can tell –
Dr. Pamela Gay:
Yes.
Fraser Cain:
– when this impact happened because of the amount of sub-craters inside of it, which is bonkers.
Dr. Pamela Gay:
It’s really amazing. And it works in so many different ways, even when you do have weathering. Here on Earth, it’s from our lack of craters that we’re able to start to get at how young the surface of our planet has to be. When we look at Mars, we can see this region of Mars must have been wildly changed due to some factor because it’s just dunes as far as the eye can see, and you don’t have craters. Now, admittedly, dunes are probably eating many craters.
Our geology and our search for how worlds are changing is driven by what we do and don’t see with cratering, and that’s just a really cool dichotomy. And it really starts to get fascinating when you look at things like the radar data of what Venus looks like beneath all that cloud because, yes, there are craters, but not as many as you would expect if it was a dead world. Highlight, Venus was not a dead world; it was very interesting until recently.
Fraser Cain:
Yeah. Yeah. Yeah, that you don’t see the kinds of cratering that you would expect to see. And it all feels like it got a refresh at a very specific point in time. And so, you get this theory that, in fact, the entire crust of Venus turned itself inside out at some specific point in time, which is mind bending –
Dr. Pamela Gay:
Yeah.
Fraser Cain:
– to think that’s how a planet can evolve geologically. It sounds scary.
Well, that was very cool, Pamela. I love this idea. Is there a name for this, the age ladder? I don’t know if there’s a term for this? I wonder.
Dr. Pamela Gay:
There is not. I don’t think I’ve ever really heard anyone put it as age ladder before.
Fraser Cain:
Yeah.
Dr. Pamela Gay:
And I love that.
Fraser Cain:
Yeah. Yeah, all right.
Dr. Pamela Gay:
And I think I’m going to give you credit and use that whenever I can.
Fraser Cain:
That sounds good.
Dr. Pamela Gay:
So, thank you for making my life happier.
Fraser Cain:
That sounds good. Thanks, Pamela.
Dr. Pamela Gay:
And thank you not just Fraser, but thank you everyone who’s out there supporting the show, allowing us to do this week after week.
This week, I would like to thank Jordan Young, BogieNet, Stephen Veit, Jeanette Wink, Børre Andre Lysvoll, Kristian Magersholt, Siggi Kemmler, Andrew Poelstra, Brian Cagle, David Truog, Ed, David, Gerhard Schwarzer, Buzz Parsec, Zero Chill, Laura Kittleson, Robert Palsma, Joe Hollstein, Richard Drumm, Les Howard, Gordon Dewis, Adam Annis-Brown, Alexis, Brenda, Conrad Halling, Kim Barron, Astrosetz – I think this person made up their name – Klombadrov loves science, WandererM101, Felix Gutt, William Andrews, Gold, Jeff Collins, Masa Hiryuu, Simon Parton, Jeremy Kerwin, Kellianne and David Parker, Slug, Harald Bardenhagen, Alex Cohen, Claudia Mastroianni, Kseniya Panfilenko, Matt Rucker, Abraham Cottrill, Mark Steven Rasnake, Anitusar, Alex Raine.
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Fraser Cain:
Awesome. Thanks, everyone. And we’ll see you next week.
Dr. Pamela Gay:
Bye-bye.