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Cosmic Queries: Unraveling Stellar Mysteries In this enlightening Q&A episode of Space Nuts, hosts Andrew Dunkley and Professor Jonti Horner tackle a trio of intriguing questions from listeners. From the complexities of hydrogen fusion to the potential for life in Martian caves and the mysteries of stellar activity, this episode is a deep dive into the cosmos.
Episode Highlights:
- Hydrogen to Helium Fusion: Ken from Maroochydore seeks clarity on the fusion process in stars, questioning why the mass of helium appears greater than the sum of its hydrogen components. Jonty explains the concept of binding energy and how it plays a crucial role in energy production during fusion, demystifying this fundamental stellar process.
- Caves on Mars: Mark from Brisbane wonders about the possibility of limestone caves on Mars and whether they could support life with a stable atmosphere. The hosts discuss the geological differences between Earth and Mars, the challenges of oxygen presence, and the implications for future human habitation in Martian caves.
- Understanding Stellar Activity: Casey from Colorado inquires about the changing activity levels of stars and solar cycles. Jonty elaborates on the magnetic forces driving solar cycles, the variability of different stars, and the fascinating world of asteroseismology, revealing how stars can change over time and what that means for our understanding of the universe.
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- Introduction to Hydrogen Fusion
- The Binding Energy Explained
- Potential for Life in Martian Caves
- The Nature of Stellar Activity
- Understanding Solar Cycles and Variability
00:00:00 --> 00:00:02 Andrew Dunkley: Hi there. Thanks for joining us. This is a Q
00:00:02 --> 00:00:04 and A edition of Space Nuts. My name is
00:00:04 --> 00:00:07 Andrew Dunkley. Thanks for your company. Uh,
00:00:07 --> 00:00:09 coming up, we're going to answer audience
00:00:09 --> 00:00:12 questions. Um, one from Ken. Uh,
00:00:12 --> 00:00:14 I'm going to paraphrase his 500 word
00:00:14 --> 00:00:16 question by saying, why don't the numbers add
00:00:16 --> 00:00:18 up when turning hydrogen into helium?
00:00:19 --> 00:00:22 Also, a question from Mark about caves on
00:00:22 --> 00:00:25 Mars and Casey wants
00:00:25 --> 00:00:28 to talk about changes in stars.
00:00:28 --> 00:00:30 That's all coming up on this Q and A edition
00:00:30 --> 00:00:33 of space nuts. 15 seconds. Guidance is
00:00:33 --> 00:00:36 internal. 10, 9,
00:00:36 --> 00:00:38 ignition sequence start. Space nuts.
00:00:39 --> 00:00:40 Jonti Horner: 5, 4, 3, 2.
00:00:40 --> 00:00:43 Andrew Dunkley: 1. 2, 3, 4, 5, 5, 4, 3,
00:00:43 --> 00:00:44 2, 1.
00:00:44 --> 00:00:45 Jonti Horner: Space nuts.
00:00:45 --> 00:00:47 Andrew Dunkley: Astronauts report it feels good.
00:00:48 --> 00:00:50 I just got a text to say my car's been
00:00:50 --> 00:00:52 serviced, so I'll be back in about 20 minutes
00:00:52 --> 00:00:53 if you just want to. Hang on. I'm kidding.
00:00:54 --> 00:00:56 Uh, joining us to answer all those questions
00:00:56 --> 00:00:58 is Professor Jonty Horner, professor of
00:00:58 --> 00:01:00 Astrophysics at the University of Southern
00:01:00 --> 00:01:01 Queensland. Hi, Jonty. Good day.
00:01:01 --> 00:01:02 Jonti Horner: How are you going to.
00:01:03 --> 00:01:05 Andrew Dunkley: I am m. All right. Good to see you again.
00:01:05 --> 00:01:06 Jonti Horner: It's good to be back. I was going to say the
00:01:06 --> 00:01:08 amount I've been talking too much. You've
00:01:08 --> 00:01:11 probably got time to go and get the car and I
00:01:11 --> 00:01:11 could probably
00:01:11 --> 00:01:14 Andrew Dunkley: ask you a question, bolt down and get the car
00:01:14 --> 00:01:17 and come back just in time to hear the end
00:01:17 --> 00:01:18 of the first sentence.
00:01:18 --> 00:01:20 Jonti Horner: Yes, and I do apologise to listeners if I've
00:01:20 --> 00:01:23 rambled on too much, but it's when you get to
00:01:23 --> 00:01:24 talk about your hobby and people have to
00:01:24 --> 00:01:26 listen, it's, you know, hard not to get
00:01:26 --> 00:01:28 excited and it is, isn't it?
00:01:28 --> 00:01:28 Andrew Dunkley: It is.
00:01:29 --> 00:01:31 Let's get, uh, straight into our first
00:01:31 --> 00:01:34 question. Hi, Andrew and Jonty. I'm going to
00:01:34 --> 00:01:35 say it's not what he wrote, but anyway, it
00:01:35 --> 00:01:38 doesn't matter. Ken, uh, from Maroochydore,
00:01:38 --> 00:01:40 longtime listener and fan and love the way
00:01:40 --> 00:01:43 you make complex issues sound simple. I am
00:01:43 --> 00:01:45 trying to understand the basic fusion
00:01:45 --> 00:01:47 reaction that both me and my accountant are
00:01:47 --> 00:01:50 struggling with. Uh, the basic fusion
00:01:50 --> 00:01:52 reaction in our sun converts hydrogen to
00:01:52 --> 00:01:55 helium. To summarise the reaction, four
00:01:55 --> 00:01:58 hydrogen nuclei, I.e. four protons,
00:01:58 --> 00:02:01 go through two steps to create one
00:02:01 --> 00:02:04 helium nucleus containing two neutrons
00:02:04 --> 00:02:06 and two protons. Additionally, gamma rays,
00:02:06 --> 00:02:09 neutrinos and positrons are released.
00:02:09 --> 00:02:12 My anatomy textbook tells me that about
00:02:12 --> 00:02:14 600 million tonnes of hydrogen convert to
00:02:14 --> 00:02:17 596 million tonnes of helium every
00:02:17 --> 00:02:20 second. The 4 million tonnes is
00:02:20 --> 00:02:23 conveyed to energy as per E equals
00:02:23 --> 00:02:26 MC squared. If neutrons had a lower
00:02:26 --> 00:02:28 mass than protons it would all make perfect,
00:02:28 --> 00:02:31 perfect sense, but they don't. They have a
00:02:31 --> 00:02:34 higher mass. So the mass of the helium
00:02:34 --> 00:02:37 nucleus is higher than the mass of the four
00:02:37 --> 00:02:39 protons. All the explanations I've read
00:02:39 --> 00:02:42 sound pretty dodgy, and my accountant says he
00:02:42 --> 00:02:44 could never get away with such, such
00:02:44 --> 00:02:46 explanations with the tax department.
00:02:47 --> 00:02:49 Could you please explain the devil in the
00:02:49 --> 00:02:52 detail that I'm missing? I, uh, love it.
00:02:52 --> 00:02:54 That's a great question. And thanks for the
00:02:54 --> 00:02:56 research, Ken. Hope all is well in
00:02:56 --> 00:02:58 Maroochydore. Not far from you, just a bit
00:02:58 --> 00:02:59 further up the coast.
00:02:59 --> 00:03:02 Jonti Horner: Yes, out to the coast and up a bit north of
00:03:02 --> 00:03:04 Brisbane, up on the sunshine course, which is
00:03:04 --> 00:03:05 kind of lovely area.
00:03:05 --> 00:03:08 Andrew Dunkley: It's kind of, it's only kind of lovely.
00:03:08 --> 00:03:09 Jonti Horner: Yeah, only kind of lovely. It's getting,
00:03:09 --> 00:03:11 getting very aggressively more and more
00:03:11 --> 00:03:13 touristed. Tracks a slightly different
00:03:13 --> 00:03:15 tourist demographic to the Gold coast, which
00:03:15 --> 00:03:18 is south of Brisbane, um, but is still
00:03:18 --> 00:03:20 a little bit more touristy than you'd like
00:03:20 --> 00:03:22 it. It's a little bit like hippie central,
00:03:22 --> 00:03:25 but not to the level of Byron nuts.
00:03:25 --> 00:03:27 Andrew Dunkley: Yeah, I get you. Yeah, yeah, yeah.
00:03:27 --> 00:03:30 Byron is the hippie, uh, capital of the, of
00:03:30 --> 00:03:33 the country, I think. Or to, to
00:03:33 --> 00:03:36 be more like Nimbin a bit further down.
00:03:36 --> 00:03:38 That's, that's very, very hippie.
00:03:39 --> 00:03:41 Anyway, um, so,
00:03:41 --> 00:03:43 yeah, he doesn't understand the balance. It
00:03:43 --> 00:03:46 doesn't. It doesn't. To paraphrase, why don't
00:03:46 --> 00:03:48 the numbers add up when turning hydrogen into
00:03:48 --> 00:03:50 helium? That's the short version of the
00:03:50 --> 00:03:51 question.
00:03:51 --> 00:03:52 Jonti Horner: And I totally get that. Because if you look
00:03:52 --> 00:03:55 at the masses of protons and
00:03:55 --> 00:03:58 neutrons in isolation and add them together,
00:03:58 --> 00:04:01 helium is two protons, two neutrons. Add them
00:04:01 --> 00:04:04 together, taking the mass of a proton and the
00:04:04 --> 00:04:06 mass of a neutron, and you get a given value
00:04:06 --> 00:04:09 for the mass of a helium nucleus. And
00:04:09 --> 00:04:10 then you look at the mass of a helium nucleus
00:04:10 --> 00:04:13 and it isn't that mass. And that doesn't make
00:04:13 --> 00:04:15 sense because if you've got four nucleons
00:04:16 --> 00:04:19 together, surely the mass of the nucleus is
00:04:19 --> 00:04:21 the four nucleons added together. And that's
00:04:21 --> 00:04:23 effectively the fundamental of what's being
00:04:23 --> 00:04:26 said here. Added to which a, uh,
00:04:26 --> 00:04:29 hydrogen nucleus is a proton, a deuterium
00:04:29 --> 00:04:30 nucleus is a proton and a neutron, but
00:04:30 --> 00:04:33 hydrogen nucleus is a proton. Four hydrogen
00:04:33 --> 00:04:36 nuclei go together to make a helium nucleus,
00:04:36 --> 00:04:38 which is 2 protons, 2 neutrons and M. In the
00:04:38 --> 00:04:40 process, you kick a few things out and do a
00:04:40 --> 00:04:43 few weird things, surely. Therefore,
00:04:43 --> 00:04:46 four hydrogen nuclei have the mass of
00:04:46 --> 00:04:49 four protons. One helium nucleus has a mass
00:04:49 --> 00:04:51 of two Protons plus two neutrons. And when
00:04:51 --> 00:04:53 you get the numbers off Wikipedia, that would
00:04:53 --> 00:04:55 suggest that the helium nucleus should be
00:04:55 --> 00:04:57 more massive than hydrogen and you should
00:04:57 --> 00:05:00 lose energy rather than create it, because
00:05:00 --> 00:05:02 you've had to create mass. Fundamentally in
00:05:02 --> 00:05:04 actuality though, the mass of the helium
00:05:04 --> 00:05:07 nucleus is lower than the mass of two
00:05:07 --> 00:05:10 protons plus two neutrons. And that's
00:05:10 --> 00:05:12 where the misunderstandings coming in, or not
00:05:12 --> 00:05:14 really misunderstanding, that's where the
00:05:14 --> 00:05:16 complexity and the confusion comes in
00:05:17 --> 00:05:19 and takes everybody a little bit to get your
00:05:19 --> 00:05:20 head around this when you first come across
00:05:20 --> 00:05:23 it. Helium nucleus, quite rightly is
00:05:23 --> 00:05:25 made of two protons and two neutrons. But
00:05:25 --> 00:05:28 those two protons and neutrons are held
00:05:28 --> 00:05:30 together. They're bound together by the
00:05:30 --> 00:05:32 nuclear force, held in strongly enough that
00:05:32 --> 00:05:34 the repulsion from the two positively charged
00:05:34 --> 00:05:37 things don't blow it apart. So there is
00:05:37 --> 00:05:38 something going on called the binding energy.
00:05:39 --> 00:05:41 And the binding energy is the amount of
00:05:41 --> 00:05:43 energy you would have to throw at a helium
00:05:43 --> 00:05:46 nucleus to separate the two protons and
00:05:46 --> 00:05:48 the two neutrons and make them fly through
00:05:48 --> 00:05:51 space separately. Again with the energy and
00:05:51 --> 00:05:54 mass equivalence. If you were to do that,
00:05:54 --> 00:05:57 you would then have 4, 4 nucleons
00:05:57 --> 00:05:58 independently of each other, which would have
00:05:58 --> 00:06:01 the mass we've just calculated. But you've
00:06:01 --> 00:06:03 had to add energy and energy is equivalent to
00:06:03 --> 00:06:06 mass. So what it's saying is that the mass of
00:06:06 --> 00:06:09 a helium nucleus is lower than the
00:06:09 --> 00:06:11 mass you would expect from the four nucleons
00:06:11 --> 00:06:13 because of the effect of this binding energy
00:06:13 --> 00:06:15 that's in there. And that binding energy is
00:06:15 --> 00:06:18 something we can calculate. It's helium is
00:06:18 --> 00:06:20 remarkably high compared to the things on
00:06:20 --> 00:06:22 either side of it. Helium 3 hydrogen, 3
00:06:22 --> 00:06:25 lithium, and that is incredibly tightly
00:06:25 --> 00:06:27 bound. That binding energy
00:06:28 --> 00:06:30 is what leads to the little bit of mass
00:06:30 --> 00:06:32 deficit with the helium atom being lighter
00:06:32 --> 00:06:34 than the four nucleons that went to make it.
00:06:35 --> 00:06:37 And it's that energy that's released. Now
00:06:37 --> 00:06:40 this is why nuclear fusion can work, because
00:06:40 --> 00:06:43 if you put four nucleons
00:06:43 --> 00:06:45 together from hydrogen atoms and make ah,
00:06:45 --> 00:06:48 from hydrogen nuclei to make a helium nuclei
00:06:48 --> 00:06:50 with the binding energy, it means you get
00:06:50 --> 00:06:53 more energy out than you get in. You produce
00:06:53 --> 00:06:56 energy. And that's true if you fuse
00:06:56 --> 00:06:58 helium. Helium is difficult. You can't fuse
00:06:58 --> 00:07:01 it to anything until carbon because
00:07:01 --> 00:07:04 lithium, beryllium, boron have a
00:07:04 --> 00:07:06 lower binding energy per nucleon than helium
00:07:06 --> 00:07:09 does. So you actually have to put energy in
00:07:09 --> 00:07:11 to fuse helium to those things rather than
00:07:11 --> 00:07:13 getting energy out. So that doesn't happen.
00:07:13 --> 00:07:16 Helium can fuse to carbon, but you need
00:07:16 --> 00:07:18 three helium nuclei to collide at once.
00:07:19 --> 00:07:22 Which is hard. From then on, from carbon
00:07:22 --> 00:07:23 upwards, you can get a little bit of energy
00:07:24 --> 00:07:25 from fusing heavier and heavier things
00:07:25 --> 00:07:28 together until you get to iron. Iron 56
00:07:28 --> 00:07:31 has the highest amount of binding energy per
00:07:31 --> 00:07:34 nucleon. So if you try and fuse
00:07:34 --> 00:07:36 hydrogen, fuse iron atoms to make a heavier
00:07:36 --> 00:07:39 atom than iron, you have to put more energy
00:07:39 --> 00:07:42 in than you get out. And that's what causes
00:07:42 --> 00:07:44 stars fundamentally to go supernova, is that
00:07:44 --> 00:07:46 they fuse heavier and heavier things to iron
00:07:46 --> 00:07:48 and then the fuel sources cut off suddenly,
00:07:49 --> 00:07:51 they then collapse. You get a boom because of
00:07:51 --> 00:07:54 the shockwave going bouncy, bouncy. Some of
00:07:54 --> 00:07:56 the energy from that supernova,
00:07:56 --> 00:07:59 uh, gets taken up in the fusion of iron to
00:07:59 --> 00:08:01 make heavier elements and gets sunk into
00:08:01 --> 00:08:03 that, which is where we get all the elements
00:08:03 --> 00:08:05 heavier than iron, everything up to uranium
00:08:05 --> 00:08:08 and beyond those things heavier than
00:08:08 --> 00:08:11 iron, the binding energy per nucleon gets
00:08:11 --> 00:08:13 lower and lower the further up you go. Which
00:08:13 --> 00:08:16 is why for things heavier than iron,
00:08:16 --> 00:08:19 nuclear fusion costs energy, but nuclear
00:08:19 --> 00:08:22 fission liberates energy because you're going
00:08:22 --> 00:08:24 back up the slope again. So uranium
00:08:24 --> 00:08:26 fissioning to be become lighter elements
00:08:26 --> 00:08:28 gives off energy because of that binding
00:08:28 --> 00:08:30 energy difference. It's all part of the same
00:08:30 --> 00:08:32 thing. Now I, I understand that
00:08:32 --> 00:08:35 intuitively this really isn't a
00:08:35 --> 00:08:37 satisfying answer because this binding energy
00:08:37 --> 00:08:39 sounds a bit like your accountant thinking
00:08:39 --> 00:08:41 you've got a tax dodge. It's a bit like
00:08:41 --> 00:08:43 capital gains tax or fringe benefits or, or
00:08:43 --> 00:08:46 uh, what is it? Negative gearing? Binding
00:08:46 --> 00:08:48 energy may well be the negative gearing of
00:08:48 --> 00:08:50 the cosmos because it's when you put four
00:08:50 --> 00:08:51 nucleons together, they're wear less than
00:08:51 --> 00:08:53 they would do on their own.
00:08:53 --> 00:08:56 Um, it's ultimate waste loss plan. But it is
00:08:56 --> 00:08:58 this bind that leads to the mass you
00:08:58 --> 00:09:01 would measure for a helium nucleus being less
00:09:01 --> 00:09:03 than the mass you would measure for hydrogen
00:09:03 --> 00:09:06 nuclei. And it's a mass as you measure, not
00:09:06 --> 00:09:08 the masses of the individual components. If
00:09:08 --> 00:09:10 you took them out and put them on their own,
00:09:10 --> 00:09:13 that is the important thing. This is usually
00:09:13 --> 00:09:15 skipped in the explanations. And this is
00:09:15 --> 00:09:17 where the challenges come in. Because in the
00:09:17 --> 00:09:20 explanations you just say a helium atom is
00:09:20 --> 00:09:22 less massive than four hydrogen atoms.
00:09:23 --> 00:09:25 Ergo some mass has been lost. Therefore
00:09:25 --> 00:09:28 energy is produced by equals MC squared. And
00:09:28 --> 00:09:30 it skips all this discussion of the particle
00:09:30 --> 00:09:32 physics underpinning it on um, this binding
00:09:32 --> 00:09:35 energy. As always, there is a fairly
00:09:35 --> 00:09:37 detailed discussion of this in the wikipedia
00:09:37 --> 00:09:39 page for Helium 4. Talking about the
00:09:39 --> 00:09:41 stability and that's got the binding energy
00:09:41 --> 00:09:43 curve in. There is also discussions of
00:09:43 --> 00:09:45 binding energy and that out there. And
00:09:45 --> 00:09:48 particle Physics out there. It is basically
00:09:48 --> 00:09:50 though, that the binding energy causes this
00:09:50 --> 00:09:52 mass deficit. And it's that mass deficit that
00:09:52 --> 00:09:55 has been converted to energy that it's
00:09:55 --> 00:09:58 in liberated in fusion. So I
00:09:58 --> 00:10:00 appreciate it is not the most satisfying
00:10:00 --> 00:10:03 answer, but that's our
00:10:03 --> 00:10:06 understanding of the why behind all of this.
00:10:06 --> 00:10:08 And the proof is in the pudding. Fusion
00:10:08 --> 00:10:11 happens. It produces energy at the
00:10:11 --> 00:10:13 right level that we calculate that all, all
00:10:13 --> 00:10:15 of our models suggest it should do. So it
00:10:15 --> 00:10:17 seems that this is a very accurate
00:10:18 --> 00:10:21 representation of how the world works, even
00:10:21 --> 00:10:23 if it doesn't immediately feel
00:10:23 --> 00:10:26 satisfying and commonsensical. And uh,
00:10:26 --> 00:10:28 part of that is that common sense we've
00:10:28 --> 00:10:31 developed based on the experience of the
00:10:31 --> 00:10:33 world around us at our scales, at macroscopic
00:10:33 --> 00:10:34 scales. And the further you go from the
00:10:34 --> 00:10:37 conditions in this room, the less accurate
00:10:37 --> 00:10:39 modern sense, common sense is at predicting
00:10:39 --> 00:10:42 the outcomes of things. And that's why
00:10:42 --> 00:10:44 it's hard to work these things out.
00:10:45 --> 00:10:48 Andrew Dunkley: Yeah, it's like, I mean, listening
00:10:48 --> 00:10:50 to your explanation would be the same as me
00:10:50 --> 00:10:53 trying to explain to a kangaroo how to use
00:10:53 --> 00:10:55 a pedestrian crossing. So, you know,
00:10:55 --> 00:10:58 it's. I can understand Ken's
00:10:58 --> 00:11:01 frustration. Um, but
00:11:02 --> 00:11:05 if I understand your explanation thoroughly,
00:11:05 --> 00:11:08 um, Ken, what, what Jonty was saying
00:11:08 --> 00:11:09 was that shift happens.
00:11:11 --> 00:11:14 That's basically it, I think. But thanks for
00:11:14 --> 00:11:17 the question. Great to hear from you. This is
00:11:17 --> 00:11:18 a Q and A edition of Space Nuts with Andrew
00:11:18 --> 00:11:20 Dunkley and Professor Johnty Horner.
00:11:25 --> 00:11:26 Jonti Horner: Space Nuts.
00:11:26 --> 00:11:29 Andrew Dunkley: Okay, Jonty, our next question says,
00:11:29 --> 00:11:31 uh, Fred Watson was talking about caves on
00:11:31 --> 00:11:34 Mars and that got me thinking. The caves on
00:11:34 --> 00:11:36 Earth are, uh, primarily made of limestone,
00:11:36 --> 00:11:39 which has high concentrations of CO2
00:11:39 --> 00:11:42 that was locked in by millions of years of
00:11:42 --> 00:11:44 sea creatures popping into the sea, creating
00:11:44 --> 00:11:46 a layer on the seabed that could be,
00:11:47 --> 00:11:50 uh, um, seabed. Could it be the same effect
00:11:50 --> 00:11:53 on Mars? No. Punctuation caught me out
00:11:53 --> 00:11:55 there. If so, I need to ask, ah,
00:11:56 --> 00:11:58 the question. Um, you get,
00:11:58 --> 00:12:01 uh. If
00:12:01 --> 00:12:04 so, I don't need to ask the next question.
00:12:04 --> 00:12:06 Um, you get what I mean. But the next
00:12:06 --> 00:12:09 question is, if caves are deep enough,
00:12:09 --> 00:12:12 could it be possible that the atmosphere at
00:12:12 --> 00:12:15 the base of these caves could be dense enough
00:12:15 --> 00:12:17 to create a stable atmosphere with higher
00:12:17 --> 00:12:19 concentrations of oxygen for life? And could
00:12:19 --> 00:12:22 that life be looking at Earth with
00:12:22 --> 00:12:23 envious eyes?
00:12:23 --> 00:12:24 Jonti Horner: Yeah, very funny.
00:12:24 --> 00:12:27 Andrew Dunkley: Hope not. Uh, Mark? Uh, thank you, Mark.
00:12:27 --> 00:12:30 So, um, caves on Earth, we. Yeah, not all of
00:12:30 --> 00:12:31 them, but uh, quite a lot of them are
00:12:31 --> 00:12:34 limestone. Um, could it be the
00:12:34 --> 00:12:37 same way as caves were created
00:12:37 --> 00:12:40 on Mars? I think that's the initial question.
00:12:40 --> 00:12:43 Jonti Horner: So I'd stress here, I'm Not a geophysicist,
00:12:43 --> 00:12:45 but the limestone we get on Earth is stuff
00:12:45 --> 00:12:47 that is now above sea level that was once
00:12:47 --> 00:12:49 below sea level. You had all these calcium
00:12:49 --> 00:12:52 shelled creatures die and fall to the bottom
00:12:52 --> 00:12:54 of the, uh, ocean and then get compacted over
00:12:54 --> 00:12:57 millions of years to form this rock. And then
00:12:57 --> 00:12:59 plate tectonics lifted the rock above the
00:12:59 --> 00:13:01 surface of the ocean. I guess on Mars you'd
00:13:01 --> 00:13:03 probably argue the oceans went away. There's
00:13:03 --> 00:13:05 not plate tectonics to lift things up, but
00:13:05 --> 00:13:07 the places that were ocean now no longer
00:13:08 --> 00:13:10 are. Ah, this is
00:13:11 --> 00:13:13 predisposed on the idea that you develop
00:13:13 --> 00:13:16 things with enough calcium
00:13:16 --> 00:13:18 to be able to make shells and stuff like
00:13:18 --> 00:13:21 that. Um, I believe, and I stand to
00:13:21 --> 00:13:23 be corrected on this, that the things that
00:13:23 --> 00:13:25 make calcium are typically
00:13:25 --> 00:13:28 oxygen breathers, not carbon dioxide
00:13:28 --> 00:13:30 breathers. But I may be wrong on that. Um,
00:13:30 --> 00:13:32 they die, they precipitate stuff out. Now,
00:13:32 --> 00:13:34 the first point is whether they could be
00:13:34 --> 00:13:36 limestone caves on Mars. Ah, now that would
00:13:36 --> 00:13:39 be predisposed on the appropriate
00:13:39 --> 00:13:42 evolution of life to get to the point where
00:13:42 --> 00:13:43 you have things that could leave fossils,
00:13:43 --> 00:13:45 that could leave shells and stuff. And we
00:13:45 --> 00:13:48 haven't yet found any fossils of such life on
00:13:48 --> 00:13:51 Mars. And without such life, you couldn't get
00:13:51 --> 00:13:53 limestone. I think think will be an open
00:13:53 --> 00:13:54 question. I suspect if we got Earth, uh,
00:13:54 --> 00:13:56 scientists and stuff like that in on the
00:13:56 --> 00:13:59 show, they'd have good reasons why limestone
00:13:59 --> 00:14:01 will be unlikely to be common on Mars.
00:14:01 --> 00:14:04 Because, yes, you did have oceans and lakes,
00:14:04 --> 00:14:07 but was there, ah, enough time for
00:14:07 --> 00:14:09 enough deposits to be made of shelled
00:14:09 --> 00:14:12 creatures which we don't even know evolved?
00:14:12 --> 00:14:14 So there's a lot of complexity there. It's
00:14:14 --> 00:14:16 obviously something, I don't know for
00:14:16 --> 00:14:19 definite whether you could have limestone
00:14:19 --> 00:14:22 on Mars. Um, I haven't heard of it being
00:14:22 --> 00:14:25 detected on Mars, but absence of evidence
00:14:25 --> 00:14:26 is not evidence of absence. But I don't
00:14:26 --> 00:14:28 believe it is common, otherwise we'd be
00:14:28 --> 00:14:30 fairly well aware of it. But there are other
00:14:30 --> 00:14:32 ways you can get caves on Earth. And I mean,
00:14:32 --> 00:14:35 we've got lava tubes and lava caves up
00:14:35 --> 00:14:37 in North Queensland in Uladulla. We've got
00:14:37 --> 00:14:39 similar things have been found on the Moon
00:14:39 --> 00:14:42 and Mars. There are skylights and lava tubes
00:14:42 --> 00:14:44 on Mars that people have even suggested could
00:14:44 --> 00:14:47 be suitable places for humans to
00:14:47 --> 00:14:49 go and live. Because if you're underground,
00:14:49 --> 00:14:51 you're shielded from radiation. And of
00:14:51 --> 00:14:53 course, if you're in a cave, you can seal the
00:14:53 --> 00:14:56 entrance and fill it with air, which would be
00:14:56 --> 00:14:58 good. Now that kind of links to the second
00:14:58 --> 00:15:01 part of the question here from Mark, which is
00:15:01 --> 00:15:02 if you have caves deep enough, could you have
00:15:02 --> 00:15:04 enough atmosphere in those caves to have
00:15:05 --> 00:15:08 atmospheric pressure? I think that's
00:15:08 --> 00:15:10 unlikely here because those cave
00:15:10 --> 00:15:13 systems would probably be connected to the
00:15:13 --> 00:15:16 surface and air would diffuse out of
00:15:16 --> 00:15:18 them. So you equilibriate and you don't
00:15:18 --> 00:15:21 go into the cave in the lava tube on Earth,
00:15:21 --> 00:15:24 and suddenly it's 3 atmospheres. Because if
00:15:24 --> 00:15:25 it was 3 atmospheres, the air would be pushed
00:15:25 --> 00:15:28 out the entrance. So I don't
00:15:28 --> 00:15:30 think you get to atmospheric pressure in
00:15:30 --> 00:15:33 these caves unless they were very, very deep
00:15:33 --> 00:15:35 and were sealed and the air was sealed in
00:15:35 --> 00:15:37 from when the atmospheric pressure was higher
00:15:37 --> 00:15:39 and it hadn't escaped. But with the porosity
00:15:39 --> 00:15:40 of the rocks, I think that would be very
00:15:40 --> 00:15:43 unlikely. The next thing is about there being
00:15:43 --> 00:15:46 oxygen in those caves. And I think of all of
00:15:46 --> 00:15:48 these things, that is the least likely
00:15:48 --> 00:15:50 because Mars
00:15:51 --> 00:15:54 doesn't have much, if any free oxygen
00:15:54 --> 00:15:56 in the atmosphere, because oxygen reacts with
00:15:56 --> 00:15:59 everything. And on Mars, there are bits of
00:15:59 --> 00:16:00 methane being produced. We're not quite sure
00:16:00 --> 00:16:03 what's going on there, but the oxygen, free
00:16:03 --> 00:16:06 oxygen from Mars has been very
00:16:06 --> 00:16:08 effectively absorbed into the surface through
00:16:08 --> 00:16:11 chemistry. Um, big part of why Mars looks
00:16:11 --> 00:16:12 red, of course, is that you can effectively
00:16:12 --> 00:16:15 say the surface is rusted iron
00:16:15 --> 00:16:18 oxide. The oxygen in the air has reacted with
00:16:18 --> 00:16:19 the iron in the surface and
00:16:19 --> 00:16:22 Andrew Dunkley: been locked up like a lot of Australia.
00:16:22 --> 00:16:25 Jonti Horner: Yes. Um, now you can produce some oxygen in
00:16:25 --> 00:16:28 Mars's atmosphere. If you get water into the
00:16:28 --> 00:16:30 atmosphere, and there is a very small amount,
00:16:30 --> 00:16:32 traces of ox of water in Mars's atmosphere,
00:16:32 --> 00:16:35 we do get water clouds there. Without an
00:16:35 --> 00:16:37 ozone layer, some of that water, particularly
00:16:37 --> 00:16:38 the water that gets highest in the
00:16:38 --> 00:16:41 atmosphere, will get dissociated. It will get
00:16:41 --> 00:16:43 broken into hydrogen and oxygen by
00:16:43 --> 00:16:46 ultraviolet radiation and the hydrogen will
00:16:46 --> 00:16:48 then escape because hydrogen atoms travel so
00:16:48 --> 00:16:50 quickly that Mars gravity can't hold them,
00:16:50 --> 00:16:52 which means the hydrogen goes away and the
00:16:52 --> 00:16:55 oxygen is left behind. So you will be
00:16:55 --> 00:16:57 producing small trace amounts of oxygen in
00:16:57 --> 00:16:59 Mars's atmosphere all the time. But then the
00:16:59 --> 00:17:02 oxygen would then be used up in chemistry and
00:17:02 --> 00:17:04 removed. So in order to have large amounts of
00:17:04 --> 00:17:06 oxygen in one of these caves, you'd need a
00:17:06 --> 00:17:09 source of oxygen, and you'd need that source
00:17:09 --> 00:17:11 to provide enough oxygen that the oxygen can
00:17:11 --> 00:17:13 overcome everything that's trying to remove
00:17:13 --> 00:17:16 it. Now, on Earth, it took a
00:17:16 --> 00:17:18 huge fraction of Earth's life before you got
00:17:18 --> 00:17:20 the great oxidation event for life to
00:17:20 --> 00:17:21 actually get to the point where it could
00:17:21 --> 00:17:23 produce more oxygen than the Earth, uh,
00:17:23 --> 00:17:26 system could absorb. So to have abundant
00:17:26 --> 00:17:28 oxygen on Mars, uh, strikes me as Very
00:17:28 --> 00:17:31 unlikely. Maybe possible that in the future
00:17:31 --> 00:17:32 that will change though, because if we went
00:17:32 --> 00:17:35 to Mars, then one of the ways people are
00:17:35 --> 00:17:37 thinking the first human habitats will be
00:17:37 --> 00:17:39 built will be to go to the caves in the lava
00:17:39 --> 00:17:42 tunnels and live there. And then you can make
00:17:42 --> 00:17:44 a sealed environment that you can then pump
00:17:44 --> 00:17:46 with an artificial atmosphere. So it could be
00:17:46 --> 00:17:49 that if you ask that question in 20 years
00:17:49 --> 00:17:52 time, Mark, the answer would be yes. There
00:17:52 --> 00:17:53 are caves with high enough atmospheric
00:17:53 --> 00:17:56 concentrations for life. We've put them
00:17:56 --> 00:17:58 there. Would they be looking back at Earth
00:17:58 --> 00:18:00 with anxious eyes, with envious eyes? I guess
00:18:00 --> 00:18:02 it depends on the person who's emigrated
00:18:02 --> 00:18:05 there. There's these fabulous ideas of what
00:18:05 --> 00:18:07 humanity will look like when we're a multi
00:18:07 --> 00:18:09 planet species. But the thing that stuck with
00:18:09 --> 00:18:11 me more than anything else was this amazing
00:18:11 --> 00:18:14 Talk from a doctor in 2012, 2013,
00:18:14 --> 00:18:16 2014 at one of our space research conferences
00:18:17 --> 00:18:19 who basically talked about the difficulty
00:18:19 --> 00:18:22 humans have reproducing when you go even
00:18:22 --> 00:18:24 very slightly away from standard temperature
00:18:24 --> 00:18:26 pressure at sea level. Talked about the
00:18:26 --> 00:18:29 challenges at invaders had into South
00:18:29 --> 00:18:30 America when they reached the high Andes.
00:18:30 --> 00:18:32 They couldn't colonise there because they
00:18:32 --> 00:18:35 weren't able to reproduce. Talking about very
00:18:35 --> 00:18:37 slight changes in conditions being enough to
00:18:37 --> 00:18:39 render our ability to have children
00:18:40 --> 00:18:42 null and void. On Mars you've got one third
00:18:42 --> 00:18:45 gravity. Unless you get dystopian science
00:18:45 --> 00:18:48 fiction future where women enter
00:18:48 --> 00:18:50 centrifuges for nine months in order to carry
00:18:50 --> 00:18:52 a child to term, in order to simulate one
00:18:52 --> 00:18:55 ghost. The perspective is at least in the
00:18:55 --> 00:18:58 relatively near future, humanity on the
00:18:58 --> 00:19:00 moon, humanity on Mars will be
00:19:01 --> 00:19:03 not self sustaining. We won't be able to have
00:19:03 --> 00:19:05 children there. And so it may well be that
00:19:05 --> 00:19:08 Mars becomes a interplanetary retirement
00:19:08 --> 00:19:11 home. People sow their wild oats, live their
00:19:11 --> 00:19:13 lives and then go to Mars later in life for
00:19:13 --> 00:19:15 the adventure. And then would they look back
00:19:15 --> 00:19:17 with envy? Well, they'd probably look back
00:19:17 --> 00:19:18 with a little bit of longing, but also they'd
00:19:18 --> 00:19:20 have the excitement of where they are are.
00:19:20 --> 00:19:22 So it could be that in 20 years time, 30
00:19:22 --> 00:19:25 years time, there will be life in the caves
00:19:26 --> 00:19:28 with artificially enhanced oxygen levels.
00:19:29 --> 00:19:31 Looking back at Earth, watching the news and
00:19:31 --> 00:19:33 all the rest of it, but I don't think at the
00:19:33 --> 00:19:34 minute that that's the case.
00:19:35 --> 00:19:38 Andrew Dunkley: Okay, fair enough. Uh, and uh, just one
00:19:38 --> 00:19:40 more point, Mark. I just did a quick search
00:19:40 --> 00:19:42 about could there be limestone on Mars? And
00:19:42 --> 00:19:44 according to an article in Science
00:19:44 --> 00:19:46 AstroDailyPod which dates back nearly 20
00:19:46 --> 00:19:49 years now, uh, yes, limestone, specifically
00:19:49 --> 00:19:51 carbonate minerals likely exist on Mars, but
00:19:53 --> 00:19:55 probably not in massive thick sedimentary
00:19:55 --> 00:19:58 beds found On Earth, while early Mars was
00:19:58 --> 00:20:00 warmer and wetter, supporting the potential
00:20:00 --> 00:20:03 for carbonate formation, the planet lacked
00:20:03 --> 00:20:05 the extensive oceans and tectonic plate
00:20:05 --> 00:20:07 activity required to build large limestone
00:20:07 --> 00:20:10 deposits. So there you are. They
00:20:10 --> 00:20:13 think there possibly is limestone
00:20:13 --> 00:20:16 on Mars, but not uh, enough to do
00:20:16 --> 00:20:18 what we've seen on Earth. Uh,
00:20:18 --> 00:20:21 but great question and uh, certainly food for
00:20:21 --> 00:20:24 thought, uh, and appreciate it. Mark, thanks
00:20:24 --> 00:20:25 for sending it in.
00:20:25 --> 00:20:27 This is Space Nuts. Andrew Dunkley here with
00:20:27 --> 00:20:28 Professor Jonty Horner.
00:20:33 --> 00:20:34 Jonti Horner: M. Space Nuts.
00:20:34 --> 00:20:37 Andrew Dunkley: And you're listening to a Q and A edition.
00:20:37 --> 00:20:39 We've got one more question to tackle. Hello,
00:20:39 --> 00:20:42 this is Casey from Colorado, she's one of our
00:20:42 --> 00:20:45 regular contributors. I have some questions
00:20:45 --> 00:20:47 about stars. Why do the activity levels of
00:20:47 --> 00:20:50 stars change? What causes solar cycles?
00:20:50 --> 00:20:53 Does every type of star go through solar
00:20:53 --> 00:20:56 cycling? Love the show, Hope you're both
00:20:56 --> 00:20:58 well. Uh, thanks Casey from Colorado.
00:20:59 --> 00:21:01 Um, it's a good question because we don't
00:21:01 --> 00:21:03 really talk about this sort of thing much,
00:21:03 --> 00:21:06 but uh, we're just about to go out of
00:21:06 --> 00:21:09 uh, the peak of solar activity
00:21:09 --> 00:21:12 in our own Solar System, the 11 year cycle
00:21:12 --> 00:21:15 that they talk about. Uh, so that's probably
00:21:15 --> 00:21:17 where we should start. We know the sun goes
00:21:17 --> 00:21:20 through an 11 year cycle and um,
00:21:22 --> 00:21:24 and we witness different things during those
00:21:24 --> 00:21:27 11 years because it's constantly changing.
00:21:27 --> 00:21:29 Jonti Horner: Yeah, it's worth stressing Shreya, that
00:21:29 --> 00:21:32 uh, effectively all stars are uh,
00:21:32 --> 00:21:35 inherently variable to some degree. And
00:21:35 --> 00:21:37 our sun is actually incredibly low
00:21:37 --> 00:21:39 variability compared to many stars.
00:21:40 --> 00:21:43 The solar cycle we observe is
00:21:43 --> 00:21:45 arguably a 22ish year cycle with
00:21:45 --> 00:21:48 two peaks and two minima. And the subtlety
00:21:48 --> 00:21:50 there is we get solar maximum when there are
00:21:50 --> 00:21:52 lots of sunspots, lots of activity, more
00:21:52 --> 00:21:55 aurora and solar minimum when we've got fewer
00:21:55 --> 00:21:57 sunspots, less activity, fewer aurora.
00:21:58 --> 00:22:00 And we get those on and about an 11 year
00:22:00 --> 00:22:02 cycle from one peak to the next, sometimes a
00:22:02 --> 00:22:05 bit shorter, sometimes a bit longer. But the
00:22:05 --> 00:22:08 origin of the solar cycles with the
00:22:08 --> 00:22:10 sun is the Sun's magnetic field. The sun
00:22:11 --> 00:22:13 has a magnetic field that runs from the North
00:22:13 --> 00:22:15 Pole to the South Pole. And it
00:22:15 --> 00:22:18 rotates in such a way that the rotation
00:22:18 --> 00:22:20 period at the equator and the rotation period
00:22:20 --> 00:22:22 at the poles are different rotates as a fluid
00:22:22 --> 00:22:25 body. So the time it takes the Sun's equator
00:22:25 --> 00:22:27 to rotate is a couple of days quicker than
00:22:27 --> 00:22:30 the poles. The magnetic field lines running
00:22:30 --> 00:22:32 from the North Pole to the South Pole get
00:22:32 --> 00:22:34 hooked up in the material and gradually get
00:22:34 --> 00:22:36 wound up a bit like an elastic band. And so
00:22:36 --> 00:22:38 the Sun's magnetic field gets more and more
00:22:38 --> 00:22:41 complicated through the 11 years, starts to
00:22:41 --> 00:22:42 get kinks and the kinks break through the
00:22:42 --> 00:22:44 surface. So you get locations where the
00:22:44 --> 00:22:46 magnetic field comes out of the surface,
00:22:46 --> 00:22:48 loops up and goes back down. And in the
00:22:48 --> 00:22:50 places where it's nearly vertical, it cools
00:22:50 --> 00:22:52 the surface of the sun because it suppresses
00:22:52 --> 00:22:53 convection of new energy from, from
00:22:53 --> 00:22:55 underneath, leading to cooler spots which
00:22:55 --> 00:22:58 look darker and therefore are sunspots. And
00:22:58 --> 00:22:59 gradually the sun gets more and more wound
00:22:59 --> 00:23:02 up. Sunspot activity begins at high latitudes
00:23:02 --> 00:23:04 and works its way down towards the equator
00:23:04 --> 00:23:07 and eventually around solar maximum, you get
00:23:07 --> 00:23:09 the field line starting to snap and break and
00:23:09 --> 00:23:12 you get a polar reversal happen. The north
00:23:12 --> 00:23:14 pole becomes a south pole and the south pole
00:23:14 --> 00:23:16 becomes a north pole and then it all begins
00:23:16 --> 00:23:18 again. So the reason we talk about a 22 year
00:23:18 --> 00:23:21 cycle is you get solar maximum
00:23:21 --> 00:23:23 with north pole to the north, well, north
00:23:23 --> 00:23:25 pole to the top, and then a minimum, then
00:23:25 --> 00:23:27 solar maximum with the south pole to the top,
00:23:27 --> 00:23:29 then a minimum, then you're back to where you
00:23:29 --> 00:23:32 started from. Roughly. Those
00:23:32 --> 00:23:35 cycles are driven by the magnetic activity of
00:23:35 --> 00:23:37 the sun and they vary the brightness of our
00:23:37 --> 00:23:39 star, um, by a vanishingly small amount. It's
00:23:39 --> 00:23:42 an incredibly stable star. I think you're
00:23:42 --> 00:23:44 talking about variability on the level, about
00:23:44 --> 00:23:47 one part in 2, 10, something
00:23:47 --> 00:23:49 like that. Now that is such a low level of
00:23:49 --> 00:23:51 variability that if we were observing an
00:23:51 --> 00:23:54 other star, we probably wouldn't be able to
00:23:54 --> 00:23:56 pick up the variability in the total
00:23:56 --> 00:23:58 brightness, but we would be able to pick up
00:23:58 --> 00:24:00 the magnetic activity. And this is magnetic
00:24:00 --> 00:24:03 activity. And star spots are uh, one of the
00:24:03 --> 00:24:05 challenges for people trying to find
00:24:05 --> 00:24:08 exoplanets because a star spot can mimic as
00:24:08 --> 00:24:10 an exoplanet and stellar activity like the
00:24:10 --> 00:24:13 Sun's magnetic cycle and the star. So SAR
00:24:13 --> 00:24:16 spots that go with it can actually be mimic
00:24:16 --> 00:24:17 a radial velocity planet. So there's a lot of
00:24:17 --> 00:24:20 work done in when we think we've got a signal
00:24:20 --> 00:24:22 confirming that it is actually a planet and
00:24:22 --> 00:24:24 not a star spot. So you've got that kind of
00:24:24 --> 00:24:25 stellar activity.
00:24:25 --> 00:24:28 Now most stars are significantly, uh, more
00:24:28 --> 00:24:29 variable than the sun. And there's a lot of
00:24:29 --> 00:24:31 other ways stars can vary. We think that
00:24:32 --> 00:24:35 most sun like stars will have sunspot cycles
00:24:35 --> 00:24:36 like the sun, and it's due to the structure
00:24:36 --> 00:24:39 of the convective and the radiative zones and
00:24:39 --> 00:24:41 all the rest of it. The internal structure of
00:24:41 --> 00:24:42 the sun, a bit like the Earth, has a crust, a
00:24:42 --> 00:24:45 mantle and a car. The magnetic field in the
00:24:45 --> 00:24:47 top layer of the sun can get tangled up.
00:24:49 --> 00:24:50 Stars of different masses have a bit of a
00:24:50 --> 00:24:52 different structure, but there's A lot of
00:24:52 --> 00:24:55 other ways that stars can be variable. And so
00:24:55 --> 00:24:57 we have a very wide variety of different
00:24:57 --> 00:24:59 types of variable stars. There are flare
00:24:59 --> 00:25:01 stars like Proxima Centauri, which have
00:25:02 --> 00:25:04 stellar activity that can occasionally be a
00:25:04 --> 00:25:06 super flare that can cause the star's
00:25:06 --> 00:25:07 brightness to change by almost a factor of
00:25:07 --> 00:25:10 100. And I normally talk about Proxima
00:25:10 --> 00:25:12 Centauri being 100 times too fancy with the
00:25:12 --> 00:25:14 naked eye, but in one mega flare it had a few
00:25:14 --> 00:25:16 years ago, it almost reached the edge of
00:25:16 --> 00:25:18 naked eye visibility. That flare was that
00:25:18 --> 00:25:21 intense. You've then got a, ah, large number
00:25:21 --> 00:25:23 of stars that vary in brightness. Bixa
00:25:23 --> 00:25:26 pulsate Bixa size changes fundamentally
00:25:26 --> 00:25:29 and usually these are stars coming towards
00:25:29 --> 00:25:30 the end of the life entering a bit of
00:25:30 --> 00:25:33 instability for various reasons. Some of them
00:25:33 --> 00:25:35 are only just moving off the main sequence or
00:25:35 --> 00:25:38 are very young. Others are super giant stars
00:25:38 --> 00:25:41 with different kinds of variability. But
00:25:41 --> 00:25:43 that variability causes their
00:25:43 --> 00:25:46 diameter to change, causes them to pulsate.
00:25:46 --> 00:25:47 And that's because they're just slightly out
00:25:47 --> 00:25:49 of equilibrium. When they're at their
00:25:49 --> 00:25:52 smallest they get hotter, they're putting out
00:25:52 --> 00:25:54 more energy because they're a bit hotter.
00:25:55 --> 00:25:57 Um, the surface layers therefore are pushed
00:25:57 --> 00:26:00 outwards with more force than gravity can
00:26:00 --> 00:26:02 push them in and they start to expand, they
00:26:02 --> 00:26:04 go through the equilibrium point but because
00:26:04 --> 00:26:06 they're still expanding they keep going. As
00:26:06 --> 00:26:08 the stars outer layers get bigger and bigger,
00:26:09 --> 00:26:12 the star cools. Because when you
00:26:12 --> 00:26:14 take a gas and you increase its volume,
00:26:14 --> 00:26:16 you lower the pressure and you lower the
00:26:16 --> 00:26:18 temperature so that material cools
00:26:19 --> 00:26:21 and eventually is giving out less energy
00:26:22 --> 00:26:25 than gravity pulling in would cause and it
00:26:25 --> 00:26:27 starts to collapse again. So instead of like
00:26:27 --> 00:26:29 the sun staying at that very fixed radius
00:26:29 --> 00:26:31 because gravity and radiation are balanced
00:26:31 --> 00:26:33 perfectly, you can get this oscillating
00:26:33 --> 00:26:34 behaviour where you overshoot in both
00:26:34 --> 00:26:37 directions. And um, sometimes that's fairly
00:26:37 --> 00:26:39 small, sometimes that's fairly large. And it
00:26:39 --> 00:26:41 happens at different phases of stars lives in
00:26:41 --> 00:26:43 different ways. One of the most famous types
00:26:43 --> 00:26:45 of variable stars are known as the Cepheid
00:26:45 --> 00:26:48 variable stars. And um, these are somewhat
00:26:48 --> 00:26:50 evolved stars coming towards the end of their
00:26:50 --> 00:26:52 life where the pulsation period
00:26:53 --> 00:26:56 is directly linked to how luminous a star is.
00:26:58 --> 00:27:00 So two similar stars, but one's brighter than
00:27:00 --> 00:27:03 the other intrinsically, put them at the same
00:27:03 --> 00:27:04 distance, one's more luminous, looks
00:27:04 --> 00:27:06 brighter, they will pulsate with different
00:27:06 --> 00:27:08 periods. And if you can measure the period,
00:27:08 --> 00:27:11 you can measure how luminous that star is.
00:27:11 --> 00:27:13 And that makes Cepheid variables an excellent
00:27:13 --> 00:27:16 step on our distance ladder. Because you see
00:27:16 --> 00:27:17 a star that's a certain Brightness, you don't
00:27:17 --> 00:27:19 really know how far away it is. But if you
00:27:19 --> 00:27:21 can measure the period of the Cepheid
00:27:21 --> 00:27:23 variable pulsating, that tells you
00:27:23 --> 00:27:26 intrinsically how luminous that star is,
00:27:26 --> 00:27:28 which means we can work out its distance. So
00:27:28 --> 00:27:29 they're really useful.
00:27:30 --> 00:27:32 Some stars are astonishingly variable.
00:27:33 --> 00:27:35 Um, among the stars with the biggest
00:27:35 --> 00:27:37 variation in brightness from brightest to
00:27:37 --> 00:27:40 faintest, um, are known as the Myra stars.
00:27:40 --> 00:27:43 Ah, Myra, the archetypal one is known as
00:27:43 --> 00:27:45 Myra, the wonderful Myra at its brightest
00:27:45 --> 00:27:48 is easily visible with a naked eye at
00:27:48 --> 00:27:50 its fantasy, you need a telescope to see it.
00:27:50 --> 00:27:52 And a few of these stars have
00:27:53 --> 00:27:54 amplitudes, the difference in brightness
00:27:54 --> 00:27:57 between the brightest and faintest that are
00:27:57 --> 00:27:58 such that they vary in brightness by more
00:27:58 --> 00:28:01 than a factor of 10. Um, a couple of
00:28:01 --> 00:28:04 examples here, Chi Cygni, which at its
00:28:04 --> 00:28:06 brightness is a magnitude 3.3 star, so
00:28:06 --> 00:28:08 comfortable with the naked eye, but not that
00:28:08 --> 00:28:10 bright at, ah, its Faintest is magnitude
00:28:10 --> 00:28:13 14.2. Um, that is
00:28:13 --> 00:28:16 a factor of about 25 in brightness
00:28:16 --> 00:28:18 between brightest and faintests. That is
00:28:19 --> 00:28:21 so long a period that that was discovered,
00:28:21 --> 00:28:24 that variability back in 1686.
00:28:25 --> 00:28:27 You've got a wide variety of mirror type
00:28:27 --> 00:28:29 stars dominating the stars with the biggest
00:28:29 --> 00:28:31 variability. But there are other type ones in
00:28:31 --> 00:28:34 there. Arcarona Borealis is a famous one in
00:28:34 --> 00:28:36 the bowl of the northern crown, normally only
00:28:36 --> 00:28:39 barely visible with the naked eye. So carbon
00:28:39 --> 00:28:42 star, where the mirror stars
00:28:42 --> 00:28:44 have a periodic variation, they're pulsating
00:28:44 --> 00:28:47 in a broadly periodic way with periods of a
00:28:47 --> 00:28:49 year or more. Acherona
00:28:49 --> 00:28:51 Borealis is different. It shines along,
00:28:51 --> 00:28:52 shines along, shines along and then suddenly
00:28:52 --> 00:28:54 it's like somebody drops a curtain in front
00:28:54 --> 00:28:57 of it and its brightness plunges and then it
00:28:57 --> 00:28:59 gradually brightens up again. And that is not
00:28:59 --> 00:29:01 exactly periodic. It's known as a carbon
00:29:01 --> 00:29:03 star. And what's happening is that, ah,
00:29:03 --> 00:29:05 occasionally it's got a huge amount of carbon
00:29:05 --> 00:29:08 in its atmosphere. Occasionally the carbon
00:29:08 --> 00:29:10 condenses into soot, blocking the light from
00:29:10 --> 00:29:12 underneath. That cools the outer layers,
00:29:12 --> 00:29:14 which gets this runaway condensation of
00:29:14 --> 00:29:16 carbon carbon into soot. That traps the
00:29:16 --> 00:29:19 radiation from inside, so the heat builds up
00:29:19 --> 00:29:20 inside until eventually the carbon gets
00:29:20 --> 00:29:22 turned back into a gas again. The clouds
00:29:22 --> 00:29:24 clear and the star brightens again.
00:29:24 --> 00:29:26 Andrew Dunkley: So that's, I was, I was just going to
00:29:26 --> 00:29:28 stupidly suggest that it was soot.
00:29:28 --> 00:29:31 Jonti Horner: Yeah, it is, it's a carbon star. And a Corona
00:29:31 --> 00:29:34 Borealis is the archetypal, most
00:29:34 --> 00:29:36 famous one. It's described as a low mass
00:29:36 --> 00:29:38 yellow supergiant. So again, it's a star
00:29:38 --> 00:29:39 coming towards the end of its life,
00:29:40 --> 00:29:43 um, every so often can be after just
00:29:43 --> 00:29:45 a few months, or it can be a few years. It
00:29:45 --> 00:29:48 can dim by as much as a factor of 10
00:29:48 --> 00:29:51 because it kind of suits up, clouds up.
00:29:51 --> 00:29:53 And if you look at the light curve of that.
00:29:53 --> 00:29:54 If you're bored, have a look at the light
00:29:54 --> 00:29:57 curve on Wikipedia, because it's really
00:29:57 --> 00:29:59 head scratching. It shows you how random this
00:29:59 --> 00:30:01 is and how hard it must have been for people
00:30:01 --> 00:30:04 to understand. Um, there was an incredible
00:30:04 --> 00:30:06 deep minimum that happened in the, the
00:30:06 --> 00:30:09 mid-2010s, I think it was, where it
00:30:09 --> 00:30:11 dimmed and then it stayed dim for ages.
00:30:12 --> 00:30:13 Normally it's bright and then it dims for a
00:30:13 --> 00:30:15 bit and then it brightens up fairly quickly.
00:30:16 --> 00:30:19 But fundamentally, there's a huge variety of
00:30:19 --> 00:30:21 ways in which stars can vary intrinsically
00:30:21 --> 00:30:24 themselves. Their brightness can vary. It
00:30:24 --> 00:30:27 is known from discussions with the
00:30:27 --> 00:30:28 traditional owners of the land here in
00:30:28 --> 00:30:30 Australia that the variability of
00:30:30 --> 00:30:32 Beetlejuice, Nal, Deborah, and two bright red
00:30:32 --> 00:30:34 giant, giant red supergiant stars in our
00:30:34 --> 00:30:37 summer sky, Northern Hemisphere winter sky,
00:30:37 --> 00:30:39 are variable. We saw that with the great
00:30:39 --> 00:30:41 dimming of Beetlejuice about a decade ago.
00:30:41 --> 00:30:44 Now, that kind of variability has been known
00:30:44 --> 00:30:46 for hundreds, if not thousands of years among
00:30:46 --> 00:30:48 traditional owners around the world who look
00:30:48 --> 00:30:50 at the night sky so that stars
00:30:50 --> 00:30:53 varying intrinsically in brightness, the star
00:30:53 --> 00:30:56 itself varying. And, um, it's a wonderful
00:30:56 --> 00:30:57 rabbit hole for people to wander down. It's
00:30:57 --> 00:30:59 another area of astronomy where amateur
00:30:59 --> 00:31:02 astronomy contribute a lot because there's
00:31:02 --> 00:31:04 very active variable star observers who will
00:31:04 --> 00:31:05 go out there and measure the brightness of
00:31:05 --> 00:31:08 stars repeatedly to track when they vary.
00:31:08 --> 00:31:10 We get a lot of that knowledge. You've then
00:31:10 --> 00:31:12 got a second type of stellar variability
00:31:12 --> 00:31:15 which is not intrinsic, but is extrinsic.
00:31:15 --> 00:31:18 What I mean by that is an intrinsically
00:31:18 --> 00:31:20 variable star is a star itself changing. An
00:31:20 --> 00:31:23 extrinsic variation is something else causing
00:31:23 --> 00:31:25 the brightness of the star to change. You
00:31:25 --> 00:31:27 know, put your hand in front of the star. The
00:31:27 --> 00:31:28 stars dissipate because your hand's
00:31:28 --> 00:31:31 absorbing. All the lights disappeared. We
00:31:31 --> 00:31:33 have multiple star systems where we have
00:31:33 --> 00:31:36 eclipsing binaries. Algol is possibly the
00:31:36 --> 00:31:38 most famous of these. The winking demon star,
00:31:38 --> 00:31:40 whose brightness drops by about a factor of
00:31:40 --> 00:31:43 three every couple of days. And that
00:31:43 --> 00:31:46 star is bright enough to be easily visible
00:31:46 --> 00:31:48 with the naked eye. And, um, the
00:31:48 --> 00:31:50 variability in its brightness is sufficiently
00:31:50 --> 00:31:52 large that it's easily noticeable with the
00:31:52 --> 00:31:55 naked eye. So its brightness varies. I
00:31:55 --> 00:31:57 think it's every 70 hours or so. I just want
00:31:57 --> 00:31:59 to cheque it out. Um,
00:32:01 --> 00:32:03 no, it's less often Than that. Algol's
00:32:03 --> 00:32:06 brightness varies every 2.86 days.
00:32:07 --> 00:32:10 So every 2.86 days, the brightness of
00:32:10 --> 00:32:13 the star drops from magnitude 2.1 to 3.4.
00:32:13 --> 00:32:15 That's a brightness change of about a factor
00:32:15 --> 00:32:18 of three times, roughly. And, uh, it dims for
00:32:18 --> 00:32:20 about 10 hours and then brightens up again.
00:32:20 --> 00:32:23 Became known as a winking demon star that has
00:32:23 --> 00:32:26 been known to be variable since prehistory.
00:32:26 --> 00:32:29 In reality, there's allegations
00:32:29 --> 00:32:31 that perhaps an Egyptian calendar that talked
00:32:31 --> 00:32:32 about unlucky days may have been linked to
00:32:32 --> 00:32:35 that. That's questionable. Where it is
00:32:35 --> 00:32:37 really interesting, though, is the
00:32:37 --> 00:32:40 variability of Algol was first explained
00:32:40 --> 00:32:43 by John Goodrich, who is one of
00:32:43 --> 00:32:45 those heroes of astronomy you don't hear
00:32:45 --> 00:32:47 about very often, mainly because he lived a
00:32:47 --> 00:32:50 very short life. He presented findings in May
00:32:50 --> 00:32:53 1783 to suggest that the
00:32:53 --> 00:32:55 variability, the periodic variability of
00:32:55 --> 00:32:58 Algol was caused by a dark body or a dimmer
00:32:58 --> 00:33:01 body passing in front of it every 2.86 days.
00:33:01 --> 00:33:04 He was awarded a medal for this, I believe.
00:33:04 --> 00:33:06 He never got to receive the medal because he
00:33:06 --> 00:33:09 died, as I say, very, very young. Died at the
00:33:09 --> 00:33:12 age of 21. Got the Copley Medal in
00:33:12 --> 00:33:14 1783, I think he was. Uh, passed away
00:33:14 --> 00:33:17 three years after that. So in just 21 years
00:33:17 --> 00:33:20 old, he contributed hugely to our modern
00:33:20 --> 00:33:22 knowledge of variable stars. And, um, you
00:33:22 --> 00:33:24 know, it's very unfortunate that he passed
00:33:24 --> 00:33:26 away at such a young age, but he was able to
00:33:26 --> 00:33:28 explain this variability that had been
00:33:28 --> 00:33:30 clearly known for a very long time. It's
00:33:30 --> 00:33:32 obvious to the naked eye that this star gets
00:33:32 --> 00:33:35 dimmer. It's not a subtle effect, but he
00:33:35 --> 00:33:37 was the one who was able to explain it.
00:33:38 --> 00:33:41 Despite his challenges. He was someone with
00:33:41 --> 00:33:44 certain physical disabilities. He was someone
00:33:44 --> 00:33:46 who had a very difficult life. But he had
00:33:46 --> 00:33:48 such a visionary intellect at the time to
00:33:48 --> 00:33:50 come up with the explanation that this
00:33:50 --> 00:33:52 periodic variability and the style of it and
00:33:52 --> 00:33:55 the frequency and the depth of it being so
00:33:55 --> 00:33:57 repeatable was because this was actually
00:33:57 --> 00:34:00 two objects going around each other. Ties in
00:34:00 --> 00:34:02 with the exoplanet chat we had earlier,
00:34:02 --> 00:34:04 because in a way, this is the indirect
00:34:04 --> 00:34:07 discovery of the binarity of Algol.
00:34:07 --> 00:34:09 You don't know that there are two stars there
00:34:09 --> 00:34:10 because you t two stars separately. They're
00:34:10 --> 00:34:13 circles close together. You can't separate
00:34:13 --> 00:34:16 them with telescopes, modular, hugely massive
00:34:16 --> 00:34:18 interferometers we have today. But you can
00:34:18 --> 00:34:20 infer the two stars there by the
00:34:20 --> 00:34:23 extrinsic variability, the variability of the
00:34:23 --> 00:34:25 light we receive because one blocks light
00:34:25 --> 00:34:28 from the other, fundamentally. So that also
00:34:28 --> 00:34:31 causes cyclical variations in
00:34:31 --> 00:34:33 brightness. But it's not the star in this
00:34:33 --> 00:34:36 case varying. It's a result of the
00:34:36 --> 00:34:38 environment around the star blocking some of
00:34:38 --> 00:34:38 the light.
00:34:39 --> 00:34:42 Andrew Dunkley: Yeah. Okay, so the answer to
00:34:42 --> 00:34:44 Casey's questions are, uh, fundamentally,
00:34:44 --> 00:34:46 yes, um,
00:34:46 --> 00:34:49 stars. Most stars probably have some sort of
00:34:49 --> 00:34:52 variability, some more than others. Uh, the
00:34:52 --> 00:34:55 solar cycles are caused by the buildup
00:34:55 --> 00:34:57 of, um, activity
00:34:59 --> 00:35:01 and um. Yeah. Does every type of
00:35:01 --> 00:35:03 star go through solar cycling?
00:35:03 --> 00:35:05 Jonti Horner: Probably. Does it? It's one of. Like I said,
00:35:05 --> 00:35:06 it's one of the big challenges for us with
00:35:06 --> 00:35:08 our radial velocity work. Looking for planet
00:35:08 --> 00:35:11 trans is filtering out the stellar
00:35:11 --> 00:35:12 cycles and that's particularly a problem for
00:35:12 --> 00:35:14 finding planets like Jupiter on a Jupiter
00:35:14 --> 00:35:17 like orbit. Jupiter goes around the sun every
00:35:17 --> 00:35:19 11.86 years. The solar cycle is about
00:35:19 --> 00:35:22 11 years. It's m hard to disentangle the two.
00:35:23 --> 00:35:25 There's also a fascinating branch of science
00:35:25 --> 00:35:27 and one of my colleagues at Uni SQ is one of
00:35:27 --> 00:35:29 the world's experts in this. Um, Professor
00:35:29 --> 00:35:32 Simon Murphy. This is a discipline called
00:35:32 --> 00:35:35 asteroseismology. We know about the
00:35:35 --> 00:35:37 Earth's interior because of earthquakes. We
00:35:37 --> 00:35:39 can figure out the crust, the core, the
00:35:39 --> 00:35:41 mantle by how different types of seismic
00:35:41 --> 00:35:43 waves pass through the Earth's interior. So
00:35:43 --> 00:35:44 we'll listen to the Earth ringing like a bell
00:35:44 --> 00:35:46 after an earthquake and we can use that
00:35:46 --> 00:35:48 information to sense what the interior
00:35:48 --> 00:35:51 structure is and how it varies. The science
00:35:51 --> 00:35:53 of astroseismology is doing the same kind of
00:35:53 --> 00:35:55 thing with stars, looking at how they wibble
00:35:55 --> 00:35:58 and wobble to map out their interior and
00:35:58 --> 00:35:59 understand it. And that's fundamentally tied
00:35:59 --> 00:36:02 to the variability. Now Simon's got a couple
00:36:02 --> 00:36:04 of PhD students working with him and doing
00:36:04 --> 00:36:06 some fabulous work, um, including
00:36:07 --> 00:36:09 Guy, um, called Tom Love, who's down in New
00:36:09 --> 00:36:10 Zealand, who's an amateur astronomer there,
00:36:10 --> 00:36:13 doing a PhD, just finishing up with us. Where
00:36:13 --> 00:36:15 they're looking with Simon at these
00:36:15 --> 00:36:18 asteroseismology, wibbly wobbliness and also
00:36:18 --> 00:36:20 at the variability of stars. Looking
00:36:20 --> 00:36:23 at a group of stars called the Delta Scuti
00:36:23 --> 00:36:25 stars, which are a particular type of
00:36:25 --> 00:36:28 oscillating, varying vibrating
00:36:28 --> 00:36:31 star, looking at how their interiors behave,
00:36:31 --> 00:36:33 looking at how old they are. So we can better
00:36:33 --> 00:36:36 understanding of the physics going on and a
00:36:36 --> 00:36:38 better understanding of where these stars sit
00:36:38 --> 00:36:41 in the storey of stellar lives. So this kind
00:36:41 --> 00:36:44 of question is one that leads
00:36:44 --> 00:36:46 to whole, uh, rafts of amazing science that's
00:36:46 --> 00:36:47 been done. And I think like everything we
00:36:47 --> 00:36:50 discuss on the show, these questions are all,
00:36:50 --> 00:36:52 all entryways to rabbit holes that can go as
00:36:52 --> 00:36:53 deep as you want to.
00:36:54 --> 00:36:57 Andrew Dunkley: Yes, absolutely. There you are,
00:36:57 --> 00:36:59 Casey. Thanks for the question, really, uh,
00:36:59 --> 00:37:01 really interesting. And um, yeah, it's
00:37:01 --> 00:37:02 fascinating.
00:37:02 --> 00:37:04 Stars. I, I've been spending a lot of time
00:37:04 --> 00:37:07 outside of my telescope recently, uh, and
00:37:07 --> 00:37:09 photographing where I can
00:37:11 --> 00:37:13 some of the, the big stars that are visible.
00:37:13 --> 00:37:16 Um, um, I think I did I get serious
00:37:16 --> 00:37:18 recently. I can't remember. I've got a couple
00:37:18 --> 00:37:20 of good ones. I got Alpha Centauri the other
00:37:20 --> 00:37:23 night, which turned out really well. Uh, but
00:37:23 --> 00:37:24 yeah, thanks for the question, Casey. If you
00:37:24 --> 00:37:27 have questions for us, please send them in
00:37:27 --> 00:37:29 via our website spacenutspodcast.com
00:37:30 --> 00:37:32 and click on the Ask me anything button at
00:37:32 --> 00:37:35 the top. It's labelled ama. You can leave
00:37:35 --> 00:37:37 text or audio messages. If you've got a
00:37:37 --> 00:37:39 device with a microphone, you're all set.
00:37:39 --> 00:37:41 Such as a, I don't know, cell phone, mobile
00:37:41 --> 00:37:44 phone, um, tablet, anything like that.
00:37:44 --> 00:37:47 Or your computer. The got built in mics
00:37:47 --> 00:37:49 these days and just tell us who you are and
00:37:49 --> 00:37:51 where you're from and we'd be happy to try
00:37:51 --> 00:37:53 and solve your riddles. Uh, and have a
00:37:53 --> 00:37:55 look around while you're there. Cheque out
00:37:55 --> 00:37:56 the shop. Cheque out. Uh, Astronomy
00:37:56 --> 00:37:58 AstroDailyPod. Maybe sign up for your daily
00:37:58 --> 00:38:01 feed of astronomical news and
00:38:01 --> 00:38:03 click the supporter tab if you'd like to help
00:38:03 --> 00:38:06 us out. That is totally optional. Uh, and
00:38:06 --> 00:38:08 thank you Jonty for all your help today.
00:38:08 --> 00:38:09 Jonti Horner: Absolute pleasure. It's always good to have a
00:38:09 --> 00:38:10 chat.
00:38:10 --> 00:38:12 Andrew Dunkley: We'll see you soon when we talk, uh,
00:38:12 --> 00:38:15 Astrobiology Part two.
00:38:15 --> 00:38:17 Uh, that is Professor Johnty Horner from the
00:38:17 --> 00:38:20 University of Southern Queensland. And uh,
00:38:20 --> 00:38:23 thanks to Huw in the studio, couldn't uh, be
00:38:23 --> 00:38:26 with us today? Huw? Um, he's an ex radio
00:38:26 --> 00:38:28 guy so he thinks he's a star,
00:38:28 --> 00:38:31 uh, which means his equator rotates more than
00:38:31 --> 00:38:33 his north and south and he's back in hospital
00:38:33 --> 00:38:36 with a twisted bow. And from me, Andrew
00:38:36 --> 00:38:38 Dunkley. Terrible. Thanks for your company.
00:38:38 --> 00:38:40 We'll catch you on the next episode of Space
00:38:40 --> 00:38:41 Network Nuts. Bye bye.
00:38:43 --> 00:38:45 Jonti Horner: You've been listening to the Space Nuts
00:38:45 --> 00:38:48 podcast available at
00:38:48 --> 00:38:50 Apple Podcasts, Spotify,
00:38:50 --> 00:38:53 iHeartRadio or your favourite podcast
00:38:53 --> 00:38:55 player. You can also stream on
00:38:55 --> 00:38:56 demand@bytes.com.
00:38:57 --> 00:38:59 Andrew Dunkley: this has been another quality podcast
00:38:59 --> 00:39:01 production from bytes.com.