Transcript

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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