Transcript: Questions Show #8

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Fraser Cain: We’ve been so crazy following our own whims through the universe that we’ve neglected your questions. That ends today. It’s time to dig deep into our overflowing email box to retrieve the puzzling questions our listeners have sent in.
 
Let’s start with what I think is our best question ever.
 
[laughter]
 
This is it, this is the greatest question. Justin Craig sent in an awesome email. He says, “I was wondering, if you had enough anti-matter and you put it into a black hole with an equal mass, would the black hole disappear or just become twice as heavy?�
 
Now, before we go into the actual answer of question, let’s give the listeners background on anti-matter. I don’t think we’ve done a show on anti-matter, so what is it?

Dr. Pamela Gay: Anti-matter is basically the “Spock has a beard� universe of matter if you’ve watched old generation Star Trek. An electron of antimatter has the opposite charge. It has all the opposite physical characteristics that a regular electron has. So you have an electron and a positron, take one and turn it inside out in every way you can (except for the mass – you can never have negative mass, it’s always a positive quality), and it’s exactly the opposite. Opposites often annihilate one another.

Fraser: This isn’t just some crazy, calculated theory. This is real stuff – you can calculate it in the lab, you can smash it together, it annihilates and produces a gigantic amount of energy. This is real stuff.

Pamela: Yeah, we’ve produced positrons – I think they’ve even put together anti-matter hydrogen and helium anti-atoms in various laboratories. You’re just very, very careful to suspend them away from everything else while you’re working with them. But we can create these things.

Fraser: So this isn’t some theoretical concept. Astronomers see the presence of antimatter out in the universe, being produced naturally. In fact, it was recently announced there’s a cloud of antimatter in the Milky Way.

Pamela: There’s a bunch of natural processes that it’s just part of how energy settles out when it’s becoming matter. If you take energy and you say, “okay, let’s change it into matter� you’re going to get a regular matter particle and an antimatter particle. Everything is created in the yin and yang, in terms of you have to have a positive charge and a negative charge. All of these things have to balance out in these energy goes into matter reactions. In some cases it can actually create clouds of antimatter.
 
There’s a cloud here in the Milky Way that we detect because of the very specific gamma ray light it gives off, that has a colour that you pretty much only get when you have these matter/antimatter reactions. We think this is perhaps coming from low-mass x-ray binaries that are creating this cloud of antimatter.

Fraser: All right. We know there’s antimatter, but just creating clouds which are annihilating instantaneously. We’re not actually clumping together gigantic quantities – enough to say, create a black hole. But let’s say we could. We’ve pulled it all together and fashioned it into a ball of antimatter with exactly the same mass as our target black hole. Then we smash them together.

Pamela: Here’s where I said it’s really important that mass is always a positive quantity. If you take a pile of matter and a pile of antimatter, they’re both going to have the same gravitational effects on things, they’re both going to have the same style event horizons… they’re going to have the same everything.
 
The thing is, with a black hole, you can’t tell if it’s matter or antimatter.

Fraser: Whoa.

Pamela: Yeah.

Fraser: Right, of course. So an antimatter black hole would, in all cases, feel identical to a regular black hole, if you’re trying to orbit it or whatever.

Pamela: If you basically went around with some sort of antimatter vacuum cleaner and collected antimatter into a bigger and bigger and bigger pile until the pile had enough mass (regular mass with antimatter characteristics), that it condensed down to a black hole… it’s a black hole. When you take an antimatter black hole and a matter black hole and throw them together… we don’t know what’s going on within the event horizon. From the outside perspective, you just made one really large black hole. Which is kind of cool.

Fraser: Hold on, let’s break this down a little bit. Say we have our antimatter black hole and we’re streaming planets and asteroids at it, they’re just disappearing into the antimatter black hole. Now, there would’ve been an explosion going on as these asteroids are striking the antimatter, right?

Pamela: The problem is you’re starting from the assumption the matter inside a black hole is normal. It’s not – at least, we can’t think of any way that it’s normal. So the whole idea that you have electrons goes away. The whole idea that you have protons goes away.
 
What you have is some surreal quark soup that all the different bits and pieces that make up both matter and antimatter are slammed together and forced into these really small volumes. Things lose their identity in the process.

Fraser: We’ll get that question in a second – we’ve actually got another question on that identity and information. I guess my question – sorry to not let go here – let’s imagine you had your antimatter black hole and your regular matter black hole, wouldn’t your antimatter black hole be the exact same configuration as the regular black hole, just the antimatter version of that/
 
Say it’s some kind of soup of particles which no longer look like protons/electrons/whatever. Wouldn’t they just be anti-versions of whatever’s in the black hole, and wouldn’t that still create the explosion?

Pamela: How do we know, because it’s in the event horizon, that these particles are able to hold on to that level of identity? How do we know they haven’t turned into pure energy as they cram themselves in there?

Fraser: Let’s say we do know. Let’s say they do remain as a mere version of the particles that are in a black hole. What happens then?

Pamela: Well, it’s just energy being released, but that energy can’t get out because it’s a black hole.

Fraser: Well that’s the question isn’t it – the energy can’t get out because it’s a black hole which would stop even energy. So obviously you’re not going to have an explosion of chunks of things because they would just be sucked down. You’re not going to have radiation because it’s going to get sucked down. You’re not going to get sound…. Anything. There may very well be an explosion, but you wouldn’t know it happened. Is that right?

Pamela: Exactly. Basically, what goes in stays in and we can’t find out anything beyond that. Since the antimatter systems have positive mass, you just have a bigger black hole.

Fraser: Right. Wow.
 
All right. I think that’s it – I guess, that’s the question at the heart of it. The hope was maybe the two would cancel each other out and you’d be able to break past black hole-ness, right, and the whole thing would explode and turn into a release of energy. Even energy can’t escape this black hole, so even if there is a release of energy, no one’s the wiser.

Pamela: Exactly.

Fraser: Awesome question. Best question ever.
 
[laughter]
 
Let’s get on to some other best questions ever. Let’s go with that – continuing on the information.
 
We had a question from Maureen Egan, and she wants to know, “in terms of information not being able to escape the gravitational pull of a black hole, what exactly is information? When I imagine information I think of data like that stored on a floppy disk or CD.�
 
I think astronomers do use that term quite loosely – all the information is lost, so who knows what happens to it. But what is information?

Pamela: I’m not sure so much that we use the phrase loosely as we just throw it around a lot without ever telling anyone what it means.

Fraser: Oh, fine – yeah.

Pamela: Which is a little bit more evil on our part.

Fraser: Yeah, no – I just throw around, “information’s gone – moving on�
 
[laughter]
 
“Stop, what does it mean?� So really – what does it mean, information loss?

Pamela: It’s an idea that came out of quantum mechanics. It’s this whole idea that particles have varied states in them. If you take an atom – let’s talk about helium, which is nice and simple. In a helium atom, you can have two different electrons. One is going to have spin-up and the other will have spin-down if they’re both in their lowest energy state. This is something that comes from the poly-exclusion principle, which says you can’t have two electrons with the same spin in the same orbital.
 
The fact that one is spin-up is information. The different states particles take on, or the different wave functions, all of this is different types of information. It’s the quantum states that are tied up in particles that people try and figure out how to take advantage of in building the next generation of hard drives where we store information in the spins of electrons.
 
How do we figure out how to tap into this so that we can build atoms that store the genetic code of the human genome, or something crazy like that. I don’t think you can actually do that one.

Fraser: Maybe we could do an analogy in star trek, like where you hop in a transporter and you’re going to be teleported from where you are to the moon down below. You want to make sure that the teleporter can rebuild you, atom by atom, and for it to be able to do that it’s going to be able to put an atom here with this quantum state, an atom there with that quantum state, etc. It’s got to get it exactly right or you won’t be you anymore. You’ll be somebody else, or even just a mess.

Pamela: All of this can include information such as what the polarization of a photon, what is the orientation of the waving of the electric and magnetic fields of that photon as it passes through space.

Fraser: So there’s any number of ways you could measure an atom or a photon or a particle or anything and that’s the information that is thought to be destroyed when it goes into a black hole.

Pamela: It’s the most basic way of putting this is what are the quantum states of the particles – that’s the information the particles carry.

Fraser: So the thinking is that if you could somehow pull that stuff back out of the black hole, there would be no way to re-create that information. No way to ever know what its quantum state was.

Pamela: Yeah.

Fraser: Why is that bad?

Pamela: Well, we like to think that no information is ever lost. Ever particle in some way, holds its entire past inside of it. It couldn’t exist if a whole series of different things hadn’t existed. You take energy and that energy has to split into a positive and negative charge, different spins that are conserved… you have all these different things that have to get conserved in the creation of matter and reactions and there are very specific processes that are the only allowed atomic processes, as particles decay from one to another through time.
 
If this information could get lost, it’s sort of like erasing the history of the particle, which is kind of sad and also implies that there’s information about our universe that gets lost forever.

Fraser: Right, but astronomers aren’t boo-hooing about lost information. This information loss breaks something, right?

Pamela: Well, one of the tenets we start with is no information can ever be lost or destroyed. If black holes can lose or destroy information, there’s one of our basic tenets gone, and that makes people uncomfortable.

Fraser: Okay, I think we could talk about this all day. Hopefully that gives you the information you were looking for, Maureen, and we’ll come back around and talk about information in black holes… that’s a whole show, I’m sure.

Pamela: Yeah. The short answer is, as near as we can tell, black holes don’t eat information – it has ways of escaping. But that’s for another entire episode.

Fraser: Right, right. Okay. But when they’re talking about information, that’s what they’re talking about – the quantum state of the stuff that gets consumed.
 
Let’s move on and go to our next question. This is from Mark Maultby. “If gravity is the force of interaction between objects, what is the smallest object that could noticeably be said to have gravitational attraction?�
 
I just want to set some scale here. If I have the Earth, with the Sun… Here we are on the Earth, feeling its gravity. If I go across the universe, to the other side of the universe, I’m still feeling the effect of gravity from the Sun, right?

Pamela: Oh yeah.

Fraser: Now, not much, obviously.

Pamela: Not noticeably.

Fraser: It’s so miniscule you can’t even have numbers to describe it, but it is there. Every piece of matter in the whole universe is interacting gravitationally with every other piece of matter in the whole universe. That’s true?

Pamela: That is exactly true.

Fraser: Okay. All right, and then that doesn’t matter for any size – for a planet, a moon, a proton, an electron, a neutrino… anything, there’s still a gravitational force that’s being done across the universe. I guess the question is, is there some point where that doesn’t happen anymore?

Pamela: No. It’s either you have mass – and if you have mass, then you affect things with gravity. Or you have no mass, in which case you can fly across the universe at the speed of light.

Fraser: Is there a minimum amount of mass you can have/

Pamela: Nope.

Fraser: But, we had this conversation just a couple of weeks ago about the Higgs-boson. I know there’s the concept of gravitons. Is there some number where, if you’re smaller than the Higgs-boson, then you won’t have mass? Like, you need to have one Higgs-boson to have mass – I’m speaking gibberish, right?

Pamela: That’s one of the crazy things. Higgs-bosons have a fair amount of mass.

Fraser: Theoretically.

Pamela: Or at least, they have a fair amount of energy (and energy and mass are kind of interchangeable, which makes the way we talk kind of confusing). The real question comes down to what is the least massive particle that we know about? That’s probably quarks. Three quarks combine to make a proton.
 
I think the real question is what the particle is with the smallest mass out there. Here you have to start remembering there’s quarks, and they combine to create protons and basically have mass (in a sort of weird kind of way). Electrons have mass, neutrinos have mass. Then there’s this stuff called dark matter that we don’t know what the heck it is. It has mass. It gravitationally effects things.
 
I’m not sure we know, yet, at this point in time, exactly what the smallest particle out there is, because we’re still discovering particles. We’re still trying to figure out what this weird stuff called dark matter is. But it’s basically, when you start getting down to these… this is a single lepton, a single boson, a quark… these individual units have slightly different masses, but these are the smallest things (smaller than atoms), that are capable of gravitationally affecting other things in the cosmos.

Fraser: But that kind of feels like you’re not quite answering the question.
 
[laughter]
 
You’re kind of saying that these are the smallest particles that we know about – that we know exist for sure – and we know that those particles have mass, and therefore they can gravitationally attract. If you had one quark on one side of the universe and another on the other side of the universe, if they weren’t being expanded away from each other, they would eventually come together, over a long time.
 
But the question is, is there some theoretical limit where you just can’t have any less mass?

Pamela: When defining the smallest possible things, we often say, “there’s the Planck unit of time� (the smallest discernable unit of time), or there’s the Planck length… You’d think there’d also be a Planck mass, which would be a limit to how small mass could get. But there’s not.
 
As far as we know, there may not be a limit to how small something can get in terms of mass, but we’re still figuring out the particle world. We still haven’t found the Higgs-boson (if it exists). We still haven’t found the graviton (if it exists). There’s this whole realm (potentially) of different particles that don’t interact via the electromagnetic force like electrons and protons do, that are making up dark matter. For all we know, the least massive particle out there is also the most common particle out there and happens to be whatever it is that makes up dark matter.
 
We’re still learning. Particle physics, the standard model, these are things we’re still working to define. As far as we know, no – there is no mandated-by-the-cosmos boundary on how small a mass we can get. We’re still exploring.

Fraser: I guess the question will maybe help to be answered by upcoming work with the Large Hadron Collider.

Pamela: Yes.

Fraser: We don’t really know. That was a good question too.
 
[laughter]

Pamela: These are the ones that stump me.

Fraser: I know, I know. Let’s move on. Something I think is a little simpler – this comes from Sabre Rosewood and the question is: “since the tides on Earth are part of what causes our moon to slowly move away, what will happen once the oceans are gone? Will the Moon stop moving away from the Earth?�
 
We talked about this in our show “Where Does the Moon Come From?� and discussed that – the Moon is slowly moving away from the Earth. What’s the cause of that?

Pamela: It boils down to conservation of angular momentum. The Earth isn’t a perfect sphere: it has mountains, it deforms itself due to the gravitational pull of the Moon. As the planet rotates, it bulges out so that part of it bulges toward the Moon and part of it bulges in the opposite direction because the gravity is not so strong over there.
 
This deformity basically gives the Moon a gravitational handle to hold on to our planet and say, “no – don’t rotate past me, keep the bulge pointed this direction!� The rotation of the Earth is constantly trying to carry the bulge past the Moon. Gravity grabs that bulge and pulls it back. This pulling back on the bulge that’s trying to rotate past the Moon is slowly, slowly, slowly, slowing the rotation of the planet.

Fraser: You’re talking about a bulge that’s coming from mountains or one side of the Earth is a little more bulged than the rest of it. Oceans move huge distances – more than the mountains ever move. There’s a gigantic amount of ocean on the planet, so does that play a significant role in this?

Pamela: It plays a significant role, but it’s not the only role. So, 50 million years from now, when our oceans start to evaporate away, we’re still going to have these tidal effects. We’re still going to have this planetary flexing that prevents us from becoming a perfect sphere ever. This planetary flexing is going to continue to slow the rotation of the planet until eventually we’re completely locked so the same face of our world is always facing the same face of our moon.

Fraser: And the moon will stop moving away.

Pamela: It will stop moving away.

Fraser: Right, so the oceans are part of the bulge on the Earth, but they’re not the whole thing. Eventually, even when the oceans boil away, the Earth and the moon will still go through this dance until they figure it out – until the Earth and moon are facing the same side toward each other forever and always. Which I think would be longer than the lifetime of the Sun, right?

Pamela: Yeah, that’s what we think right now at least.

Fraser: It’ll be a red giant before it happens.
 
Okay, cool question. Let’s move on. Paul Barnett asks “Since the universe is expanding and we believe that matter cannot be created or destroyed but only changed from one form to another, I’m curious to know where the new matter comes from to occupy the new space that’s created. Is there new matter being spontaneously created?â€?
 
Now, let me try and rephrase the question, because I think he made a couple of mistakes there. We talk about the universe expanding, the expansion of the universe, both from the big bang but also from the additional dark energy that’s helping to push the universe apart. We’re getting more space in between the galaxies and galaxy clusters and interstellar space. But we’re not necessarily getting any space in between the galaxies because they’re held together.
 
I guess the question is, let’s look way out into the most unpopulated part of the universe where space is expanding apart and we can measure the density of how many atoms per cubic kilometre there are out there. As the space is expanding from dark energy, is there any more matter coming into existence?

Pamela: No, that’s the cool thing. The universe is basically diluting itself over time.

Fraser: So it’s like you’re pouring water into something that was quite thick, and it’s just making it more and more dilute – more and more thinned out.

Pamela: Or, the way I like to think about it, if you imagine blowing up a balloon, the balloon has very thick walls when it’s small, but the more and more you blow it up, the thinner those walls get, the fewer atoms there are per square centimetre of area on the surface of that balloon.

Fraser: Until it pops.

Pamela: Until it pops.

Fraser: Our universe isn’t going to pop, is it?

Pamela: No.

Fraser: Okay.

Pamela: But it’s going to get pretty empty.

Fraser: Right, and that’s it – there could be some point in the far, far future where everywhere you look, there’s no atoms around. Right now, I forget – did you mention how dense space is?

Pamela: It’s on the order of nothing per cubic meter?

Fraser: Right, okay. The occasional particle per cubic meter, but you could eventually get to the point where there’s one particle per light year.

Pamela: What’s weird though is this is true of atoms of normal matter. There’s this thing called dark energy, and near as we can tell, dark energy is constant at all times. When we look at how much energy there is per cubic meter of space, it works out to a few protons worth of energy at all points in time, even though the total volume of the universe has increased.
 
That means the amount of energy, the amount of dark energy in the entire universe, is somehow increasing as the universe gets larger, because its staying constant as a function of volume. This gets confusing.

Fraser: I’ve got a zinger for you now, then. We always talk about the fact that matter and energy are interchangeable. So, is dark energy interchangeable with matter? Could you freeze it into matter?

Pamela: As far as we know, no. Dark energy is this weird enigma, currently. We don’t know what causes it. As near as any theorist that I can understand has gotten, dark energy is basically a field of energy that permeates all of space and time. If you can imagine this mesh of energy that is everywhere, all at once, and not getting all metaphysical on me… if you can imagine this lowest possible energy state (that isn’t zero) it permeates everywhere. One of the fears is something will come along and trigger that wave, that field that permeates everywhere to crash down to zero and no one knows what will happen.

Fraser: Now you’re freaking people out here.
 
[laughter]

Pamela: Theorists do scary things with their mathematics sometimes. Like I said, we don’t really understand it right now. So, give us a few years.

Fraser: Okay. To summarize then, with the expansion of the big bang and the addition of dark energy, the universe is growing but the amount of matter in the universe isn’t changing, so it’s really just getting diluted. So, back to the question where’s the matter coming from, it’s not coming from anywhere – there’s no additional matter. All the matter in the entire universe was created in the big bang, and that’s all we’ve got.

Pamela: That’s all we’ve got.

Fraser: All right. Speaking of all we’ve got, there’s one more question. This is a good one too, in fact this is a question I was going to ask you about and I never got around to it.
 
This one comes from the forum, the Bad Astronomy & Universe Today forum. “I can’t wrap my head around the physics of gravity assist. Why does travel in the same direction of an object’s orbit speed something up, while travel in the opposite direction slows it down? I keep thinking the approach push and depart pull would cancel each other out either way and not change the speed at all.�
 
This is great – I’ve thought about it too. You’ve got a spaceship going toward Jupiter and it’s going to get a gravitational assist to pick up velocity and go much faster. As it approaches Jupiter, Jupiter is speeding it up. I get that – it’s velocity might be changing as it’s falling into Jupiter’s gravity well. As it does its fly past, and starts to move away from Jupiter again, now Jupiter’s pulling back on it. It should be slowing back down. Shouldn’t you just end up with the same velocity? It’s like going down a hill and then back up it on the other side, shouldn’t you end up going the same speed you were going before?

Pamela: That would be exactly right if the object you were having the gravity assist from wasn’t moving.
 
The key is you are gravitationally falling into the gravity hole of some object in motion. If it’s not in motion, you go in, come back out and your energy hasn’t changed at all. If you imagine a completely frictionless, gently curved valley in the road. You go down a hill, up a hill, no friction occurs so you’re going the same speed on both sides of the hill even though you speed up going in and slow down going up… it all cancels out in the end.
 
The catch is, if the object’s moving, the amount of time that it is either able to gravitationally pull on you to speed you up or gravitationally pull on you to slow you down changes. If you’re moving in the same direction as the object that’s giving you the gravitational assist, as you’re moving toward it, it’s saying “yes! Catch up with me!� and pulling on you to get you to catch up to it. So the whole time, you’re approaching it, it’s running away from you. As it’s running away, it’s pulling on you to help you catch up.
 
Once you catch up to it, you’ve gained all this velocity, so you’re able to zip away from it with extra velocity you didn’t have before, because the extra time you had catching up with it lead to you getting some of its velocity and spending extra time falling in and not extra time falling out.

Fraser: So, you’re slowing down Jupiter by a teeny-tiny little bit, to slow it down in its orbit, and its speeding you up to pull you up to its speed.

Pamela: Yes.

Fraser: Right. So the amount that you get falling into it and the moving back away from it do cancel out, but it’s that process where it’s pulling you up to its speed in the orbit which is what adds to your velocity.

Pamela: If you’re going in opposite directions, then you end up putting the extra effort into slowing down to meet its speed. Then you end up going slower on the other side. Same thing.

Fraser: Right, and I know the MESSENGER space craft is using that method to be able to go into orbit around Mercury.

Pamela: Yes.

Fraser: They’re using this process to be able to slow themselves down, as well – if you just go the opposite direction, you slow yourself down.
 
That totally makes sense. I honestly didn’t have it thought through, so thank you.

Pamela: It’s a really cool affect.

Fraser: You know what, I said that was the last one, but we’ve enough time for one last, quick little question. I think this is a quick one. Rich from New York wants to know “if the Sun were to suddenly vanish, would we feel the effects of gravity instantaneously, or would it take approximately 8 minutes, just like light?�

Pamela: It would take 8 minutes, just like light. Fast enough?

Fraser: The speed of gravity is the speed of light. If the Sun disappeared, we would see the light disappear and we’d also suddenly feel the gravity disappear.

Pamela: It would appear as if all of a sudden all the stars became visible and they’re moving in the wrong way. That’s kind of cool.

Fraser: Yeah.
 
And that effect works the same for us moving around the Milky Way, the Moon going around the Earth… it waits for the speed of gravity. Cool.
 
I think that plays into our recent show about gravity waves, that’s what the whole trick is about. You’re watching as waves of gravity are released from objects as they wash over the planet. That’s it – that was quick.

Pamela: Cool.

Fraser: Perfect. I think that covers everything.

Transcript: Globular Clusters

Download the transcript
 

Fraser Cain: This week, we’re going to study some of the most ancient objects in the entire universe: globular clusters. These relics of the early universe contain hundreds of thousands of stars held together by their mutual gravity. In fact, when I get a telescope out to show my friends and family, the great globular cluster in Hercules is one of the first things I’ll point out. It just looks like a fuzzy ball through a telescope, but in my mind I can see all the stars.
 
Let’s just talk a bit about globular clusters. What are they?

Dr. Pamela Gay: They are, at the most simplistic level, they’re collections of 10 thousand to hundreds of thousands of stars gravitationally bound together that formed in some cases 12 billion years ago. They’re out, orbiting on the edges of our galaxy, and on the edges of most of the galaxies we observe out there.

Fraser: How many are we going to find in a typical galaxy like the Milky Way?

Pamela: It’s all a function of how big the galaxy they’re attached to is. Our own galaxy seems to have well over a hundred different globular clusters. We’re finding new ones every day as satellites like the Spitzer Infrared Observatory peer through the dust and gas and are able to find new globular clusters in places we hadn’t been able to look before.
 
They basically form a spherical distribution all the way around our galaxy, orbiting in some cases in two different directions. There’s two different populations. They’re old, metal-poor, and everywhere we look. They’re the ancient stewards of our galaxy.

Fraser: Okay. When we talk about ancient, how ancient are they?

Pamela: One of the great mysteries for a long time was, we looked at them and they seemed to be older than the universe. It turned out we had miscalculated how old the universe was and we had miscalculated how old the stars were. Around the year 2000, once we got everything put together, it began to show up that our universe is 13.7 billion years old and these clusters of stars are 12 billion years old.

Fraser: I love that. I love that up until the year 2000, astronomers knew there were stars that were older than the estimates of the age of the universe, and that bugged them, but they were able to just kind of deal with it – “yeah, we have our estimate for the age of the universe wrong, and we probably have our estimate for the age of the stars wrong, but for now this is the best we can do.�
 
[laughter]
 
I think that’s great.

Pamela: I’ve had this moment at the chalkboard before. You start off at the upper-left-hand corner of three chalkboards and you start deriving equations, and you keep going and going and get to the very end and look at the number on the board and it doesn’t match the number you calculated in the quiet of the privacy of your own office. You know somewhere on those three chalkboards, there was a mistake. And you don’t know where.
 
Now, at the chalkboard, I can usually go through and my students are more than willing to help me find where I dropped the one-half or squared something that should’ve been cubed. But when making calculations of the age of globular clusters, you’re not talking about three chalkboards of calculations. You’re talking about thousands of lines of computer code going through and trying to calculate stellar evolution models, saying, “a star spends this long on the main sequence doing these things�
 
In all those thousands of lines of code, in all of the mathematics that go into the simulations to write those thousands of lines of code, there are so may places where our approximations might not be right, or where we might be missing a term in our calculations. It took us a long time to figure out what was going on and to get computers powerful enough that we didn’t have to make as many approximations.
 
Then, when it came to measuring the age of the universe, it was an observational challenge that was pretty much unsettled until the WMAP results came in. There, we just had to build the bigger, better microwave telescope.

Fraser: Okay, fine. So they’re not older than the universe. That’s still plenty old. What kind of forces came together to build these globular clusters in the first place?

Pamela: A large, dense, glob of stuff all by its lonesome settled into forming dense, rich stars. Over time, the stars segregated themselves by mass.

Fraser: Why did they form all these different stars and not just one big, supermassive black hole?

Pamela: As the cloud of material collapses, it ends up fragmenting. It turns out that you don’t generally have one nice, completely smooth cloud of gas. Rather, you have a cloud of gas with a few knots in it. Those individual knots, those individual places that are a little bit more dense than other locations, as the entire cloud collapses those little knots end up collecting gas to themselves, hogging it and forming individual stars out of this large clump of gas and dust.
 
It’s through the fragmentation that you end up with these populations of tens or hundreds of thousands of stars all clumped together.

Fraser: Do they form as separate clumps as the galaxy is forming, almost like planets inside a solar system might form around a star? Or did they form as kind of mini-galaxies and get absorbed into galaxies through collisions later on?

Pamela: One of the large mysteries we’re trying to sort out is why we have globular clusters with very specific geometries and star distributions that are roughly the same size as dwarf galaxies. What is it that made one clump of dust and gas form a globular cluster, and another clump of dust and gas form a dwarf galaxy? We’re still working to figure that out.
 
We think part of it might be globular clusters form in the halos of pre-existing giant galaxies. Dwarf spheroidal galaxies tend to form in isolation all by themselves. Some how, the kinematics involved ends up with two different things forming. Part of this might be the dark matter involved. Globular clusters don’t have the same dark matter halos associated with them that you get with little tiny dwarf galaxies. If you take a dark matter halo and throw a globular cluster’s worth of mass inside of it, you can get a dwarf galaxy.
 
If instead you just take a clump of dust and gas and embed it inside the much larger dark matter halo of a giant galaxy like the milky way, then you seem to get globular clusters.

Fraser: I didn’t realise that the amount of matter in a globular cluster could be the same amount as in a dwarf galaxy. That’s quite interesting.

Pamela: It’s one of those weird things. This is only true for the smallest of the dwarf galaxies and the largest of the globular clusters.

Fraser: What about composition? What kinds of stars are they? You called them metal-poor – why’s that?

Pamela: Stars come in a lot of different compositions. Our Sun tends to have, for a star, a lot of things like iron – a lot of heavier elements (like silicon). We can look at it’s spectrum and say, “look at all those rich titanium lines, those rich strontium lines in the spectrum of the star.�
 
Instead, if I start looking at the elements found in the stars of a globular cluster, I’ll see a lot of those elements just aren’t present. These stars can have a hundred or even a thousand less metal than our Sun has in it. We call these stars metal-poor because compared to the Sun, they have only a percent or a fraction of a percent of the same number of heavy atoms in their atmosphere.

Fraser: I know that stars get their heavier elements through successive generations of stars living, exploding as supernova, releasing their material which gets sucked into a new star-forming cloud so you get recycling going on and on. Do they just not get a chance to go through very many generations before they formed?

Pamela: 12 billion years ago, there just wasn’t that much heavy metal hanging out waiting to be eaten into the newly forming stars. One the really cool things about globular clusters is pretty much all of the stars in the globular cluster formed in one violent period of star formation.
 
When I look at a sample of a hundred different stars in say, M13 (the Hercules cluster you mentioned), all those stars are going to be basically the exact same age. They’re going to have formed out of the same cloud of material (so they have the same composition). The only thing that varies from star to star in these systems is their mass.

Fraser: That was going to be my next question. We learned early on that the heaviest stars burn their fuel quickly and then detonate at supernova, while the smaller – the Sun-sized stars and smaller can live on for billions and billions of years as main sequence or white dwarf stars. Is there some kind of mass limit where you just doesn’t see a certain size of star in those clusters anymore?

Pamela: That’s right. You look at these things and none of the large stars are left any longer. You’re down to stars smaller than the Sun hanging out on the main sequence. Then, you have remnants of the stars. You have whit dwarfs, neutron stars, all hanging out going, “hey! We used to be big!� these are stars that shed their mass, exploded as supernova and went through planetary nebula formation. Those planetary nebula have, in many cases, been largely destroyed just by the passing of time. Globular clusters are systems rich in ancient stars and stellar remnants – nothing young or big.

Fraser: Are there any forces that will take a globular cluster apart? They’ve been around for 12 billion years – there must be some really serious forces keeping them together.

Pamela: They’re one of the most tightly bound objects we know of (in terms of large populations of stars). Open clusters, in the disk of our own galaxy are much smaller – hundreds of stars in some cases. They get shredded by gravity over time. Globular clusters are tightly bound systems that are able to, in general, sustain orbiting our galaxy.
 
As we look around we do see instances of globular clusters that are elongated or a little bit mis-formed, that have gone through gravitational interactions with our galaxy or with other galaxies. That’s the cool thing: we can observe globular clusters around our galaxy, around some of the dwarf galaxies (the Fornax dwarf has its own globular clusters, we see them in the Large Magellenic Cloud). We can see them in all different environments, and in some cases the environments are rather hostile and destructive.

Fraser: Why do astronomers find globular clusters so interesting? Do they use them as a tool for some of the science they’re working on?

Pamela: They’re laboratories. Because you can look at M3 and get several thousand stars made out of the same stuff, you can see “if I change this variable involving mass, I get this difference in outcome. If I create a binary system, I get this difference in outcome� We can use them to say “I’ve now controlled for age and composition, all I’m going to vary is whether a star is in a binary or not, and what the mass of that star is.� I can then see the outcome in the star’s evolution.
 
These things, while they’re all more metal-poor than our Sun (at least the ones around our own galaxy), they’re all slightly different ages. They’re ancient – but they’re slightly different versions of ancient. It’s sort of like going from a 70 year old to a 90 year old. They’re all grandparent-age, but there are differences between a 70 year old and a 90 year old biologically. With these systems, they’re all ancient, but there are differences in stellar evolution that we’re able to observe.
 
They’re one of the most fascinating tools for studying stellar evolution that we have, because you can see so many stars and control what you’re looking at so carefully.

Fraser: I guess with a hundred thousand (or more) stars in a cluster, you can see every single mass of star from the smallest white dwarf, or the smallest red dwarf, all the way up to the largest star that hasn’t died yet. I guess you can see, in some clusters that line falling off. In some cases, the bigger stars have died, and in other clusters they’re younger and the biggest stars haven’t died yet.

Pamela: Yeah, no. All the big stars are dead. That’s the funky thing about them: there are no big stars. You’re left looking at strictly solar-sized type stars and smaller in most cases.

Fraser: Do we see any clusters that are younger than this 12 billion years old? Do we see any that are just forming anywhere?

Pamela: Not locally, but as we look out at other galaxies, we do start to be able to see them around other galaxies, particularly in star-forming regions and in areas where galaxies still have chunks of basically, virgin gas waiting to get used. We did, starting in 2000, start to discover newly-forming globular clusters. That was kind of cool. Up until then, we had no clue where these buggers came from, we just knew they were out there. We didn’t know which came first: the galaxy or the globular cluster. Now we know that they form together.

Fraser: They form together. I know there’s a relationship between supermassive black holes and galaxies themselves. Is there a relationship between globular clusters and the galaxies they live in?

Pamela: This is something we’re still trying to work and figure out. One of the problems is we can steal globular clusters from other galaxies when we eat them. It’s hard to sort out the naturally born, biological globular clusters (to use a bad analogy) and the adopted children.

Fraser: How would we tell the difference?

Pamela: That’s the problem. With the Milky Way Galaxy, we have these two populations of globular clusters. One is orbiting around the galaxy in the same direction the galaxy is rotating. The other population seems to either not be rotating relative to the Milky Way or it’s going in the wrong direction.
 
With these two different kinematic populations, we also find differences in the composition of the stars. One population has even fewer metals than the other population. Astronomers are left thinking this is probably because we ate another galaxy and stole its globular clusters, but there’s also the possibility that maybe one group of these systems just formed a little later on, a little further out. We’re not really sure.
 
We need to keep studying, and keep looking at other galaxies with high-resolution images. If I watch a galaxy that’s just starting to form (and we’re just starting to find occasional examples of galaxies still forming today), how is it the globular clusters form with them?

Fraser: There was actually a really interesting piece of research that came out in just the last couple of weeks, where scientists were using a globular cluster as a laboratory. In this case they wanted to look at the distribution of regular stars and white dwarfs. Their assumption was the stars will sink down to a certain point in the cluster, depending on their mass – the heavier stars will sink to the middle and the lighter stars would be pushed up. They found a lot of white dwarfs were higher up than they were expected to be. Did you read that?

Pamela: Yeah – that was a really neat case. With globular clusters, the stars do stratify themselves, where the really bloated bigger stars, even the really ancient red stars (the really giant ones that aren’t too much bigger than the Sun necessarily, and they aren’t main sequence stars, but they’re still bigger than little tiny baby red dwarfs). These bigger stars sink to the centre of the globular cluster, but the lighter stars end up floating to the surface. They actually pick up kicks from gravitational interactions – the bigger stars get less of a kick and the littler stars get more of a kick and are able to move outward in the system.
 
They looked at white dwarfs. White dwarfs come in a small variety of masses. They expected to find the heavier weight white dwarfs in the centre and the lighter weight white dwarfs further out. When they looked at really old white dwarfs (ones that had started to cool and weren’t quite as blue in colour), that was true: the bigger ones were further in and the lighter ones were further out. When they looked at really young white dwarfs, from stars that just finished going to the white dwarf phase, all of the young white dwarfs were on the outer edges (that they found) of these globular clusters.
 
They think what’s happening is these stars, during their last stages of life as they puff off their atmosphere, somehow this puffing off isn’t completely symmetric. Just like someone going out into space with a little air can, when you spray your air can you might end up rocketing yourself slightly in one direction. These stars, by throwing off more mass in one direction than another, are able to rocket themselves to the outer edges of the globular cluster.

Fraser: Oh I see, so when they turn into a white dwarf, it’s almost like a natural rocket. They push themselves outside the cluster and then over time, when they no longer have that kick, gravity asserts itself again and they get sorted back into where they belong in the cluster.

Pamela: In open clusters of stars in the plane of the Milky Way, this kick is enough that white dwarfs leave the open cluster behind. In the higher gravity environment of globular clusters, these stars are kind of stuck hanging out on the outskirts and don’t actually get to escape. Overtime, through various interactions with other stars, they do end up sinking to where they belong due to their mass.

Fraser: I know there’s been a revolution over the last decade or so: with a lot of the new infrared observatories they’re finding a lot of brand new star clusters, some of them are even quite close.

Pamela: As I said, globular clusters form a sphere around our galaxy. This is actually how we were originally able to figure out where in our galaxy our solar system is located. We looked out and could say, “I see this number of globular clusters in this direction, this number in this direction and I know they form a sphere.� We were able to pinpoint how far from the centre of the sphere we were located.
 
There were certain areas of the sky where we weren’t able to look through the density of stars, gas and dust in the plane of our galaxy. We weren’t able to see that population of globular clusters in that outer sphere, just because they were blocked from our view completely – just like if you threw a hula-hoop around your head, there would be sections of the area around you that you’d never be able to see.
 
With the Spitzer Space Telescope, we’re able to start peering through the gas and dust. Really long wavelengths of light (really red infrared wavelengths) are able to skirt through gas and dust like no big deal. By using this other colour that’s invisible to the eye, Spitzer is able to see through the crowded disk of the galaxy to see objects that had previously been hidden from us.

Fraser: Including clusters.

Pamela: Including clusters.

Fraser: Did that change our understanding at all about the distribution of them, or was it always suspected that they would be there and look, they found them?

Pamela: It was always suspected they would be there. This was one of those times we were able to say “Aha!� we knew what the distribution should look like. We couldn’t prove it, because there were these areas we couldn’t see in. Now, as we peer into these new areas we can say, “yep – we had it all right all along.� It’s always good to be proven right.

Fraser: What do you think the future holds for globular clusters? Will they just hang on forever, or will they just get chipped away and eventually dissipate into a galaxy?

Pamela: I think they’re going to hang on for a long time.

Fraser: Define long – we’re talking astronomical terms here, so you can use some big numbers.

Pamela: Ohhh… they’re going to be around for billions and billions of years to come. They’re going to survive the collision of Andromeda and the Milky Way and end up orbiting whatever new system we end up forming. I’m sure a few will be lost along the line, but there will be survivors out there.
 
What’s kind of cool is someday far in the future, someday these clusters of stars are going to be clusters of white dwarfs, neutron stars and the occasional peppering of black holes. We’re looking at future compact mass object clusters instead of star clusters.

Fraser: They’ve got to be the perfect place to look for black holes and neutron stars. Are many found there?

Pamela: Finding neutron stars is always hard. You end up having to look for them by looking for binary companion systems. Globular clusters are extremely dense, so it becomes hard to do the detailed observations necessary to find neutron stars.
 
We are finding black holes in these systems. They’re able to make themselves apparent by eating things now and then. As gas and dust pours into a happily feeding black hole, it gets heated up and excited into giving off x-ray light.
 
By pointing telescopes like the XMN Newton at globular clusters, we’re able to go “Ooo! X-ray emission – there must be a black hole there.� We’re finding the theorized population of intermediate-mass black holes can in some cases be found lurking in the centres of globular clusters. This is brand new information (fresh from last January).
 
We started to get hints about 10-15 years ago that there was observational, certain evidence that there are super-massive black holes in the centres of certain galaxies. We’ve looked for them in the centres of dwarf galaxies thinking we’d find smaller ones and that’s been a really hard journey.
 
It was theorized that since globular clusters have similar numbers of stars, perhaps they had intermediate-mass black holes in their centres. In the very second globular cluster they looked in (unfortunately not the first), there was that x-ray emission that said, “yes, there’s an intermediate black hole here and it’s eating something right now!�

Fraser: Good chances – that’s amazing.

Pamela: It’s always nice when the needle decides to sit on the top of the haystack.

Fraser: Yeah, exactly. There’s a classic science fiction story by Isaac Asimov, called Nightfall, where the planet happens to exist in a binary system or in a star cluster. It’s always light – it’s never dark. The astronomer predicts a bunch of the suns are going to be down and there’s going to be an eclipse and things are going to be dark… and predicts everyone’s going to go crazy.
 
I can just imagine… what would it look like to be on a planet in a cluster like that, and to see the sky? What would it look like?

Pamela: Here’s one of the random factoids that helps eliminate it (to use a bad pun).
 
Our nearest star of noticeable brightness is a little more than three light years away. There are 100 thousand or more stars crammed into 100 light years’ diameter sphere in a globular cluster.
 
From within the globular cluster, you have hundreds of stars that are amazingly bright every where you look in the sky, many of which are visible during the day as you orbit whatever star you’re able to orbit. It would be amazingly bright.
 
The only way I can imagine getting sufficient eclipses is if you’re surrounded by an asteroid cloud – and that wouldn’t be a good way to be a planet.

Fraser: Maybe it wasn’t a star cluster, maybe it was just a system with a bunch of stars, but a globular star cluster would take that to the next level.

Pamela: Yeah.
 
So there are some space artists out there who have done amazing artwork where they’ve sat down and worked with the scientists to work out what it would look like to orbit a brown dwarf, to orbit in a star cluster. There’s some amazing work out there if you just Google hard enough and look around. Lots of different folks are working on this.
 
Life in a globular cluster: it’s like if you imagine having a spaghetti colander above you with a floodlight. A hundred thousand stars near by shining down on your little world.

Fraser: I would love to see that.

Pamela: For now, these are objects that you can go out and see what they look like with a pair of binoculars and in some cases just your eyes. Omega Centauri (if you’re lucky enough to live in the right part of the planet) is bright enough to see with your naked eye. M13 is out there waiting to be discovered. All of these systems make excellent small telescope opportunities.

Fraser: I highly recommend it – if you have a small telescope or a friend with a small telescope and you can say, “show me a globular cluster!� They’ll know where to point it. It’s quite surprising – you’ll see a nice, little…. Well, it’s a fuzzy ball. But in your imagination, think about that distant world with a spaghetti strainer series of holes with floodlights above you. It’s amazing.
 
[laughter]
 
But yeah, I really like looking at the clusters. That’s one of the first ones I’ll point to, after Saturn.

Fraser Cain: First I’d like to let everyone know that Pamela and I were interviewed on The Skeptic’s Guide to the Universe last week on their episode 95. Skeptic’s Guide … if you haven’t heard of it, it’s one of my favourite podcasts dealing with science and scepticism. It’s a completely different show from us, with this great collaborative group and they hash out various topics on science and scepticism. They interviewed us for about 45 minutes about dark matter, astrobiology, scepticism in astronomy and a bunch of other topics. Kind of like a 45 minute episode of Astronomy Cast. You can find their site at www.theskepticsguide.org and we’ll probably be doing more stuff with them in the future, so stay tuned.
 
On to the show. This week, we’re talking about some of the most bizarre objects in the Universe: neutron stars and some of their even weirder cousins: pulsars and magnetars. You’ve heard the terms, now learn the science.
 
Okay Pamela, what’s a neutron star?

Dr. Pamela Gay: A neutron star is a star made of neutrons.

Fraser: Oh, come on!
 
[laughter]

Pamela: It was just too tempting!

Fraser: I know. Okay, more!

Pamela: So seriously, you take a four to eight solar mass star, let it evolve and eventually it’s going to build up an iron core. An iron core isn’t capable of generating more energy, and when a star runs out of the ability to generate energy it explodes as a type II supernova.
 
The stuff that’s left (if there’s enough of it left – if there’s somewhere between 1.4 and 3.2ish solar masses) is going to start to collapse. As it collapses, the electrons and the protons aren’t going to be able to push each other apart. They’re going to collapse down and end up, for every electron and proton, producing one neutron and one neutrino. In this process, roughly 10^57 neutrinos are going to carry away a whole lot of energy, and what’s left is going to collapse down into a neutron star.
 
These objects are tiny. They’re about 5-20km in radius and they get smaller the more mass you throw on them because they squish down. When they first form, they’re about a million degrees, which makes them x-ray objects. Some of them are spinning at as much as 1000 Hz – they’re spinning 1000 times per second.
 
How’s that for an explanation?

Fraser: All right, that’s good.
 
[laughter]
 
Now, if I remember my supernova conversation with you, if you get a star that’s too big, 50 times the mass of the Sun, it just detonates and goes kaboom, right? And there’s nothing left.

Pamela: Right. Yes, exactly.

Fraser: But if it’s smaller, as you say, if it’s in this four to eight solar mass range, it explodes as a supernova but something’s left. If it gets any bigger, is that where you get a black hole?

Pamela: That’s where you get a black hole.

Fraser: Okay, it has more mass than our Sun – which is just going to become a white dwarf, right?

Pamela: Right.

Fraser: So if it’s in that in between range, four to eight solar masses, it doesn’t have enough mass to become a black hole, but I guess the process is kind of the same, right? It’s collapsing down and then it just goes beyond a neutron star and turns into a black hole. So we’re talking about those lucky stars in the four to eight solar mass range that can turn into a neutron star.

Pamela: The really, really big stars… they don’t even quite get to the iron core part. They just blow themselves apart. The slightly smaller stars will actually end up becoming black holes, and it’s the even smaller ones (that are still on the huge side of the stellar mass distribution) that become the neutron stars. So there’s a whole continuum of ways that stars can blow themselves to smithereens.

Fraser: And we’ve got gravity overcoming the strong nuclear force, is that right?

Pamela: Yes.

Fraser: So normally you’ve got a proton and an electron and it just gets mashed together into a neutron?

Pamela: It gets mashed together into the neutron. This is actually the weak force at play when you’re transforming the arrangement of the quarks, where you’re going from one configuration of up and down quarks to another configuration of up and down quarks and flying off neutrinos.

Fraser: So if I could see a neutron star, what would it look like?

Pamela: They are just hot objects, so when we look at them we don’t see anything in particular that’s different about them because they’re primarily neutrinos. In fact, they have an outer metallic shell around the neutron core. They glow really, really hot and we see them as what’s called a black body.
 
A typical example of a black body is a rock. If you take a rock and you heat it up, it will eventually start to glow red. This is sort of like what Captain Kirk likes to do with phasers when they’re abandoned on strange alien planets. If you keep heating the rock up with Capt. Kirk’s phaser, it’s eventually going to glow white. If you keep going, the rock will probably vaporize. If you keep going and you’re dealing with something that doesn’t vaporize, it will eventually be hot enough that it’s giving off light in the x rays.
 
With neutron stars you have an object so hot that they’re emitting light in the x rays. They’re also giving off light in other colours, but it’s in x rays that they particularly stand out all across the sky. There’s actually some pretty neat Chandra images that look like you’re looking at Christmas tree lights through fog because all the slightly different shades of x ray they artificially make different colours. It’s a quick and dirty way to find lots of neutron stars quickly – with x rays.

Fraser: So they artificially turn low energy x rays into red and high energy x rays into blue?

Pamela: …Yeah, something like that. They make some decision on this is this, this is this.

Fraser: Right, so they’re blazing in the x rays, but they’re not necessarily blazing in the lower spectrum at all.

Pamela: They’re just too small to give off a whole lot of light that we’re going to notice above the background of everything else. You have something that’s basically the size of Manhattan. It’s giving off vast quantities of light per square metre, but it only has so many square metres. When you look out across the sky, you can have a neutron star happily emitting light, but it’s competing against things like Sirius, Betelgeuse, Rigel – all these bright objects – against nebulas, background galaxies… so they’re going to get lost.
 
If you instead turn Chandra at the sky and start looking around the sky in x rays there’s not a lot of stuff out there that’s hot enough to be emitting x rays. All of a sudden neutron stars start springing up against the background sky.

Fraser: Now can neutron stars be part of a binary object, is that another way you can see them yanking a star around with it’s gravity?

Pamela: You can see them yanking stars around with their gravity, and they can also build up accretion disks. If one of these high mass objects gets too close to their companion star, they can start stripping material off of the companion star. This will make them what’s called a cataclysmic variable: a star that strips off material, forms an accretion disk and periodically the accretion disk undergoes it’s own related nuclear reactions and flares up into a nova event.

Fraser: So you might have mini pieces of stars – reactions like fusion reactions going on outside the neutron star as it’s dragging this material in.

Pamela: Exactly. You get this pancake disk where within the disk the densities and temperatures become identical to the centre of the star. So you have the nuclear fusion process going on within the accretion disk.

Fraser: I guess if the neutron star gets too much mass it might tip over the limit and turn into a black hole.

Pamela: That is also a problem to be considered, and when you get two neutron stars that end up orbiting together and eventually collapsing together, that’s another way to form a black hole. In that case you get a short duration gamma ray burst in the process.

Fraser: Right, which we talked about a couple of weeks back.

Pamela: Exactly.

Fraser: All right, so what is a pulsar and how is that different from a neutron star?

Pamela: Well, all pulsars are neutron stars, but not all neutron stars are pulsars.
 
If you have a generally young, just formed neutron star that has a fairly strong magnetic field and is rotating quickly, that magnetic field can channel materials through the field. Electrons, other charged particles… they’re going to follow the magnetic field lines. Because pulsars are rotating and because rotating charged particles generate magnetic fields, there’s all sorts of complicated, scary physics going on… you can end up with very strong magnetic fields that create basically jets of particles.
 
These jets of particles appear in radio observations. So when you look at a pulsar in the radio, you can use them as some of the most accurate clocks in the Universe. We look at them and every time we hear a beat of the clock, what’s happening is the beam of the pulsar is passing in front of our telescope.
 
One of the neat things about this is we could be looking at a neutron star and never know it’s a pulsar because it’s pointed away from us. We only see pulsars as pulsars when that magnetic field’s poles are pointed at the planet Earth and our radio telescopes are able to intercept the radio signal.

Fraser: So it’s kind of like a lighthouse turning and so we will only know there’s a lighthouse over there when the beam from the lighthouse passes over our boat.

Pamela: Exactly, and this is one of the weird cases where you have to imagine a lighthouse where one side of the two-sided spotlight points slightly up to the sky, and the other part points slightly down toward the water. If you’re a person at the foot of the lighthouse looking up at the lighthouse, you’re only going to see one of those two beams: the one that’s pointed down toward the water.
 
But if you’re flying over the lighthouse you’re only going to see the one that’s pointed up at the sky. So in the case of pulsars, the magnetic fields’ pulls aren’t necessarily (and in fact we don’t think they are) lined up with the rotational axis of the star.

Fraser: How fast can they be rotating?

Pamela: This is where they’re going 1000 Hz – 1000 rotations per second. They slow down over time, though.

Fraser: How can you have… Our Sun, doesn’t it take days to rotate?

Pamela: It rotates on roughly a one month schedule.

Fraser: Right, so how can a neutron star rotate thousands of times a second?

Pamela: Well… anyone who’s watched an ice skater spin in any of the competitions (the worlds, the Olympics), they hold their arms out and gracefully, slowly spin around smiling, and then they pull their arms in and start whipping into vastly faster speeds. Now, they’re just going from having arms extended to arms in. That’s not a huge difference in the grand scheme of things.
 
We’re looking at a four to eight [solar] mass object that ends up admittedly being only 1.4 to 3.2 solar mass object, that’s going from normal star size down to the size of Manhattan. So in the process of squishing down, it takes all of its angular momentum, it takes all of its rotation and the rotation speeds up as the object compresses smaller and smaller.
 
The process of the supernova itself can also give the star a kick and increase its rotation rate. It also sometimes kicks it out of the supernova remnant, which means we’re sometimes finding pulsars that are sort of roaming freely, and we also find supernova remnants that don’t have a neutron star or black hole in their centre.

Fraser: Now, you mentioned briefly that a pulsar slows down. How does that happen?

Pamela: The energy it’s giving off in the process of pulsating has to go somewhere. As the material is getting streamed out, it’s carrying the rotational energy away and turning it into kinetic energy. We’re just changing the type of energy and as the energy goes away from the pulsar, then you also end up with the pulsar slowing down with time.

Fraser: Right, okay.

Pamela: It’s all about the energy. Think of it as… you’re standing there throwing rocks while you’re rotating. As those rocks go out in straight lines, your rotation is going to slow down.

Fraser: I’m just surprised that astronomers would be able to detect that slowing down of speed. It must be tiny.

Pamela: Well, they’re such precise pulsaters, that we can detect changes on the order of fractions of a millisecond and some of them have been observed for enough decades that we can start to see noticeable changes in their pulsation rate. So in the course of a year they’re as accurate as an atomic clock. Looked at over the course of years, those fractional changes build up, and we can see the changing period.

Fraser: Wow. Okay, so then what is a magnetar?

Pamela: A magnetar is this new thing that we’re just starting to understand. It was first put forward theoretically back in 1992 by Robert Duncan and Chris Thompson. Rob Duncan was actually at the University of Texas where I was. I got to hear some of the early talks in the mid-90s, which was kind of cool.
 
These are a special type of neutron star that has an especially strong magnetic field. Not all stars are created equal, that’s what makes the Universe exciting. Some stars start off with an intrinsically large magnetic field, and when you collapse down the star you also collapse down the magnetic field, confine it to a smaller area, and it gets stronger.
 
I have to admit understanding what’s called magneto-hydro-dynamics, the study of how magnetic fields arise in stars and all that sort of stuff is a bit beyond my ability to get at the mathematics of. They say in astronomy there’s two ways to confuse anyone: one is you ask how does this magnetic field affect that and the other is asking how does that look in three-dimensions. Understanding magnetars requires that you understand both magnetic fields and three-dimensional fluid modelling of degenerate mass, which is just a lot of scary words which means: it’s hard.
 
These folks, they figured out, using complicated modelling, that you can talk a star with a strong magnetic field and create a neutron star that initially has an uber-strong magnetic field. They estimate that perhaps only one in ten neutron stars, when first formed, has the potential to be a magnetar, and of those that actually become magnetars, they’ll only last for about 10 thousand years, which on cosmic scales is a really short period of time.
 
So you have a neutron star, that has an anomalously high magnetic field for a short period of time and they do weird things. Some magnetars are considered to be what are called anomalous x ray pulsars. These are slow rotating. They rotate every 5-12 seconds instead of the 100 or 1000 times per second you’ll see in some of the fast rotating ones. They have extremely high B-fields, and periodically they’ll give off bursts of x ray energy.

Fraser: Sorry, what’s a B-field?

Pamela: Sorry – magnetic field. I slipped into astronomy jargon. They’ll have an extremely high magnetic field, which physicists call a B-field for no obvious reason that I know. We’re starting to find these. So we have these anomalous x ray pulsars.
 
We also have things called soft gamma ray repeaters; there’s only four of these known. These are objects that give off low-energy x rays randomly, and repeatedly. So, here we are: we have a sub-class of neutron stars, and then we have two sub-classes of that. All of these are short-lived, very special events that can be very dangerous.
 
There was an object that went off December 27 of 2004. This particular object, SGR 1806-20 gave off enough energy that it sort of went through the sides of some space telescopes and still managed to get detected. So the energy didn’t go in the front of the telescope, where energy should enter. It instead went through the side, side-hit the detector, and still managed to blind the detectors. It was a gamma ray event that was very determined to be seen. These things can give off as much energy in a tenth of a second as our Sun released in 100 thousand years.
 
This particular object (which was just the other side of the centre of the Milky Way, only about 50 thousand light years away), gave off so much energy that our ionosphere actually bloated up. It changed the ionosphere of our entire planet from 50 thousand light years away. If that same object had been ten light years away, the Earth’s ozone layer would’ve been completely destroyed. It would’ve been an event similar to 12 kilo tonnes of TNT nuclear blast at 7.5km.

Fraser: Everywhere.

Pamela: Yeah.

Fraser: So that’s similar to like, we talked about the gamma ray burst going off and your neighbourhood just disappearing and the ozone layer letting in the radiation.

Pamela: Yes. It’s just frighteningly huge amounts of energy in very, very short amounts of time. If we’d been able to see in gamma ray light, it would’ve been the largest explosion observed by humans since Kepler’s supernova back in 1604. This object would’ve been several times brighter than the full Moon. If you look at it in terms of total energy, it was brighter than the full Moon. We just don’t see in gamma rays, instead several satellites took it for us. From the blighting their detectors temporarily incurred, we were able to know what happened.

Fraser: Now you brought this up, not me.
 
[laughter]
 
I’ll run with it. That’s one way these things can kill us. Now, what if we had a pulsar reasonably close, firing out its jets? Would that be dangerous?

Pamela: It would be kind of dangerous. There’s worse things in life that could happen. Being near two of these things that are coalescing is much more dangerous. The actual pulses coming off of a pulsar, in the grand scheme of things, are fairly low energy. It’s radio lights. Our atmosphere can protect us from radio fairly well, so that’s not too huge of a danger. Luckily, there’s no magnetars that are known to be near us, so we don’t have to worry about it too much.
 
The real problem is if one of the magnetars does get too close, just its magnetic field can start to tear stuff apart. What you have to worry about is the rogue, high-magnetic field more than the pulse of the pulsar. Your typical magnetar might have a magnetic field of about 10^11 teslas, or 10^15 gauss. That’s a huge magnetic field. For comparison, the Earth’s magnetic field is about 30-60 micro-teslas, and just 10 giga-teslas at the distance of the Moon would wipe out everybody’s credit cards. So we’re talking that one of these in our solar system and all of our plastic money no longer works.

Fraser: Catastrophic!

Pamela: Yeah.

Fraser: But I’m sure like, your computers don’t work, your television doesn’t work, your radios don’t work, your car doesn’t work…

Pamela: More importantly, the magnetic field of a magnetar at a distance of about 1000 km would start tearing apart human tissue because of the magnetic properties of water.
 
So, yeah. Dangerous magnetic field.

Fraser: Yeah, but the chances of a neutron star getting within 1000km …

Pamela: Yeah, not going to happen, but it’s cool to think about.

Fraser: It’s cool to think about!
 
Now I wanted to talk a bit about how pulsars are used for predictions. Aren’t pulsars being used to help confirm relativity?

Pamela: Yes. We look at them in all sorts of different ways. So, first of all there’s the basic idea where you have a pulsar in a binary system. It’s a high mass system, you start to get relativistic effects into the orbits. You have to take these into account in looking at the timing of the pulses.
 
As the object is orbiting, there’s two different things going on: first of all, its distance is changing, and because the distance is changing, the amount of time that different pulses take to reach the Earth changes. So we see some change in the frequency of the pulses just due to the changes in the distance. The other thing that happens is you also end up with relativistic effects caused by the velocities involved. So we can use these to say how the timing is being effected in different ways that prove and (so far) don’t disprove relativity.

Fraser: So this is the situation, I think, didn’t Einstein call it “frame-dragging”?

Pamela: So, frame-dragging you get from fast rotating objects. These are fast rotating objects, so they’re something we can pay attention to, to look at frame-dragging. We can also look at them in terms of if you have a pulsar and a black hole orbiting one another, you can start to see things like gravitational radiation. As gravity radiates away from the system and the orbits change over time, and you can get very accurate orbital measurements using pulsars to time the measurements.

Fraser: I have one last piece of research. Weren’t planets discovered around a pulsar?

Pamela: That’s actually one of the first ways that first we incorrectly and then did correctly find planets around another star. The first object that was a real set of planets was B1257+12, and this was a discovery made in 1992. I actually remember the day that the discovery was announced because I was working at Haystack Observatory in Massachusetts was all excited and wanted to buy me a beer, but I was 18 so he left.

Fraser: You were working in an observatory when you were 18??

Pamela: I’m a freak.
 
[laughter]

Fraser: Just dedicated. I think I was working in a comic book store when I was 18.
 
[more laughter]

Pamela: Yeah, I’ve been a freak for a long time, but it’s fun to be a freak. Yeah, no I was working at Haystack Observatory in my home town, Westford Massachusetts doing basic data reduction of T Tauri stars. I was sitting there, minding my own business, measuring different parameters and great excitement broke out and I was too young to celebrate. It was sad. But, I remember the discovery and it doesn’t have just one planet, but it has three planets associated with it and potentially comets. They’re still sorting that out.
 
What’s neat here is we’re looking at the smallest known planet-like things. In one case, the mass, compared to Jupiter, is 0.000063 Jupiter masses – tiny, tiny thing.

Fraser: Is that smaller than Pluto?

Pamela: I don’t know, but we can put that in the show notes. It’s tiny. I don’t think it’s quite smaller than Pluto, but I could be wrong and we’ll put the answer to that question in the show notes.
 
The other two objects have more realistic masses. They’re 0.014 and 0.012 Jupiter masses. Now what’s cool about these objects is their potential histories. These are either things that survived a supernova explosion, in which case they’re probably the rocky cores of former gas giants. So take Jupiter, blast it with a supernova explosion and the core that’s left behind might be what one of these three objects are. The other possibility is the material that was given off during the supernova explosion re-coalesced to form a new generation of planets out of the freshest recycled material you can imagine.

Fraser: Sort of like a change of life baby.

Pamela: Yeah, exactly.

Fraser: But it must be a pretty awful existence, you can imagine the poor planet that’s just being bathed in x ray radiation and pulsating radio waves. Wouldn’t be a good place to live.

Pamela: Yeah, it’s not exactly… yeah, DNA wouldn’t survive real well, but it’s still neat to think that planets really can be found just about anywhere.

Fraser: Even around a neutron star.

Pamela: Even around a neutron star.

Fraser: What would you say are the biggest unknowns right now, what needs learning more about?

Pamela: We’re still trying to sort out the soft gamma ray repeaters and the x ray pulsars. What causes these different subclasses to form, what is their life expectancy, what is their frequency? It’s just something new, some new idea to follow up on and learn about.
 
What are the mechanisms necessary to form these huge magnetic fields? We’re still trying to fully understand the source behind the magnetic field in our Sun. We’re pretty sure it has to do with a dynamo that originates somewhere between the radiative and convective layers, but we’re not positive.
 
Here we are, working to piece together how magnetic fields are generated in a completely foreign type of object, something that’s made out of matter that we can’t even really create here on the planet (degenerate neutron material), and figure out how are magnetic fields generated with it, how do they evolve with time, how does the star evolve and cool with time?
 
So there’s a lot of interesting stuff to figure out evolving. The first few thousands and tens of thousands of years of life after a pulsar, after a neutron star has been formed and how it changes, and then of course there’s the fate of how the magnetic field damps with time, the rotation damps with time… what happens when the neutron star is cold and dead?

Fraser: So I guess anyone looking for a real challenge in research can go into this three-dimensional magnetic fields.

Pamela: Magneto-hydro-dynamics.

Fraser: (laughing) Magneto-hydro-dynamics. If you want to pick a hard topic in school, there you go.