Latest / Space Nuts: Astronomy Insights & Cosmic Discoveries / Stellar Q&A: Unraveling Fusion Mysteries, Martian Caves & Solar Cycles

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Stellar Q&A: Unraveling Fusion Mysteries, Martian Caves & Solar Cycles · Space Nuts: Astronomy Insights & Cosmic Discoveries

0:00
Andrew Dunkley: Hi there. Thanks for joining us. This is a Q
0:02
and A edition of Space Nuts. My name is
0:04
Andrew Dunkley. Thanks for your company. Uh,
0:07
coming up, we're going to answer audience
0:09
questions. Um, one from Ken. Uh,
0:12
I'm going to paraphrase his 500,000 word
0:14
question by saying, why don't the numbers add
0:16
up when turning hydrogen into helium?
0:19
Also, a question from Mark about caves on
0:22
Mars and Casey wants
0:25
to talk about changes in stars.
0:28
That's all coming up on this Q and A edition
0:30
of space nuts. 15 seconds. Guidance is
0:33
internal. 10, 9,
0:36
ignition sequence start. Space nuts.
0:39
Jonti Horner: 5, 4, 3, 2.
0:40
Andrew Dunkley: 1. 2, 3, 4, 5, 5, 4, 3,
0:43
2, 1.
0:44
Jonti Horner: Space nuts.
0:45
Andrew Dunkley: Astronauts report it feels good.
0:48
I just got a text to say my car's been
0:50
serviced, so I'll be back in about 20 minutes
0:52
if you just want to. Hang on. I'm kidding.
0:54
Uh, joining us to answer all those questions
0:56
is Professor Jonty Horner, professor of
0:58
Astrophysics at the University of Southern
1:00
Queensland. Hi, Jonty. Good day.
1:01
Jonti Horner: How are you going to.
1:03
Andrew Dunkley: I am m. All right. Good to see you again.
1:05
Jonti Horner: It's good to be back. I was going to say the
1:06
amount I've been talking too much. You've
1:08
probably got time to go and get the car and I
1:11
could probably
1:11
Andrew Dunkley: ask you a question, bolt down and get the car
1:14
and come back just in time to hear the end
1:17
of the first sentence.
1:18
Jonti Horner: Yes, and I do apologise to listeners if I've
1:20
rambled on too much, but it's when you get to
1:23
talk about your hobby and people have to
1:24
listen, it's, you know, hard not to get
1:26
excited and it is, isn't it?
1:28
Andrew Dunkley: It is.
1:29
Let's get, uh, straight into our first
1:31
question. Hi, Andrew and Jonty. I'm going to
1:34
say it's not what he wrote, but anyway, it
1:35
doesn't matter. Ken, uh, from Maroochydore,
1:38
longtime listener and fan and love the way
1:40
you make complex issues sound simple. I am
1:43
trying to understand the basic fusion
1:45
reaction that both me and my accountant are
1:47
struggling with. Uh, the basic fusion
1:50
reaction in our sun converts hydrogen to
1:52
helium. To summarise the reaction, four
1:55
hydrogen nuclei, I.e. four protons,
1:58
go through two steps to create one
2:01
helium nucleus containing two neutrons
2:04
and two protons. Additionally, gamma rays,
2:06
neutrinos and positrons are released.
2:09
My anatomy textbook tells me that about
2:12
600 million tonnes of hydrogen convert to
2:14
596 million tonnes of helium every
2:17
second. The 4 million tonnes is
2:20
conveyed to energy as per E equals
2:23
MC squared. If neutrons had a lower
2:26
mass than protons it would all make perfect,
2:28
perfect sense, but they don't. They have a
2:31
higher mass. So the mass of the helium
2:34
nucleus is higher than the mass of the four
2:37
protons. All the explanations I've read
2:39
sound pretty dodgy, and my accountant says he
2:42
could never get away with such, such
2:44
explanations with the tax department.
2:47
Could you please explain the devil in the
2:49
detail that I'm missing? I, uh, love it.
2:52
That's a great question. And thanks for the
2:54
research, Ken. Hope all is well in
2:56
Maroochydore. Not far from you, just a bit
2:58
further up the coast.
2:59
Jonti Horner: Yes, out to the coast and up a bit north of
3:02
Brisbane, up on the sunshine course, which is
3:04
kind of lovely area.
3:05
Andrew Dunkley: It's kind of, it's only kind of lovely.
3:08
Jonti Horner: Yeah, only kind of lovely. It's getting,
3:09
getting very aggressively more and more
3:11
touristed. Tracks a slightly different
3:13
tourist demographic to the Gold coast, which
3:15
is south of Brisbane, um, but is still
3:18
a little bit more touristy than you'd like
3:20
it. It's a little bit like hippie central,
3:22
but not to the level of Byron nuts.
3:25
Andrew Dunkley: Yeah, I get you. Yeah, yeah, yeah.
3:27
Byron is the hippie, uh, capital of the, of
3:30
the country, I think. Or to, to
3:33
be more like Nimbin a bit further down.
3:36
That's, that's very, very hippie.
3:39
Anyway, um, so,
3:41
yeah, he doesn't understand the balance. It
3:43
doesn't. It doesn't. To paraphrase, why don't
3:46
the numbers add up when turning hydrogen into
3:48
helium? That's the short version of the
3:50
question.
3:51
Jonti Horner: And I totally get that. Because if you look
3:52
at the masses of protons and
3:55
neutrons in isolation and add them together,
3:58
helium is two protons, two neutrons. Add them
4:01
together, taking the mass of a proton and the
4:04
mass of a neutron, and you get a given value
4:06
for the mass of a helium nucleus. And
4:09
then you look at the mass of a helium nucleus
4:10
and it isn't that mass. And that doesn't make
4:13
sense because if you've got four nucleons
4:16
together, surely the mass of the nucleus is
4:19
the four nucleons added together. And that's
4:21
effectively the fundamental of what's being
4:23
said here. Added to which a, uh,
4:26
hydrogen nucleus is a proton, a deuterium
4:29
nucleus is a proton and a neutron, but
4:30
hydrogen nucleus is a proton. Four hydrogen
4:33
nuclei go together to make a helium nucleus,
4:36
which is 2 protons, 2 neutrons and M. In the
4:38
process, you kick a few things out and do a
4:40
few weird things, surely. Therefore,
4:43
four hydrogen nuclei have the mass of
4:46
four protons. One helium nucleus has a mass
4:49
of two Protons plus two neutrons. And when
4:51
you get the numbers off Wikipedia, that would
4:53
suggest that the helium nucleus should be
4:55
more massive than hydrogen and you should
4:57
lose energy rather than create it, because
5:00
you've had to create mass. Fundamentally in
5:02
actuality though, the mass of the helium
5:04
nucleus is lower than the mass of two
5:07
protons plus two neutrons. And that's
5:10
where the misunderstandings coming in, or not
5:12
really misunderstanding, that's where the
5:14
complexity and the confusion comes in
5:17
and takes everybody a little bit to get your
5:19
head around this when you first come across
5:20
it. Helium nucleus, quite rightly is
5:23
made of two protons and two neutrons. But
5:25
those two protons and neutrons are held
5:28
together. They're bound together by the
5:30
nuclear force, held in strongly enough that
5:32
the repulsion from the two positively charged
5:34
things don't blow it apart. So there is
5:37
something going on called the binding energy.
5:39
And the binding energy is the amount of
5:41
energy you would have to throw at a helium
5:43
nucleus to separate the two protons and
5:46
the two neutrons and make them fly through
5:48
space separately. Again with the energy and
5:51
mass equivalence. If you were to do that,
5:54
you would then have 4, 4 nucleons
5:57
independently of each other, which would have
5:58
the mass we've just calculated. But you've
6:01
had to add energy and energy is equivalent to
6:03
mass. So what it's saying is that the mass of
6:06
a helium nucleus is lower than the
6:09
mass you would expect from the four nucleons
6:11
because of the effect of this binding energy
6:13
that's in there. And that binding energy is
6:15
something we can calculate. It's helium is
6:18
remarkably high compared to the things on
6:20
either side of it. Helium 3 hydrogen, 3
6:22
lithium, and that is incredibly tightly
6:25
bound. That binding energy
6:28
is what leads to the little bit of mass
6:30
deficit with the helium atom being lighter
6:32
than the four nucleons that went to make it.
6:35
And it's that energy that's released. Now
6:37
this is why nuclear fusion can work, because
6:40
if you put four nucleons
6:43
together from hydrogen atoms and make ah,
6:45
from hydrogen nuclei to make a helium nuclei
6:48
with the binding energy, it means you get
6:50
more energy out than you get in. You produce
6:53
energy. And that's true if you fuse
6:56
helium. Helium is difficult. You can't fuse
6:58
it to anything until carbon because
7:01
lithium, beryllium, boron have a
7:04
lower binding energy per nucleon than helium
7:06
does. So you actually have to put energy in
7:09
to fuse helium to those things rather than
7:11
getting energy out. So that doesn't happen.
7:13
Helium can fuse to carbon, but you need
7:16
three helium nuclei to collide at once.
7:19
Which is hard. From then on, from carbon
7:22
upwards, you can get a little bit of energy
7:24
from fusing heavier and heavier things
7:25
together until you get to iron. Iron 56
7:28
has the highest amount of binding energy per
7:31
nucleon. So if you try and fuse
7:34
hydrogen, fuse iron atoms to make a heavier
7:36
atom than iron, you have to put more energy
7:39
in than you get out. And that's what causes
7:42
stars fundamentally to go supernova, is that
7:44
they fuse heavier and heavier things to iron
7:46
and then the fuel sources cut off suddenly,
7:49
they then collapse. You get a boom because of
7:51
the shockwave going bouncy, bouncy. Some of
7:54
the energy from that supernova,
7:56
uh, gets taken up in the fusion of iron to
7:59
make heavier elements and gets sunk into
8:01
that, which is where we get all the elements
8:03
heavier than iron, everything up to uranium
8:05
and beyond those things heavier than
8:08
iron, the binding energy per nucleon gets
8:11
lower and lower the further up you go. Which
8:13
is why for things heavier than iron,
8:16
nuclear fusion costs energy, but nuclear
8:19
fission liberates energy because you're going
8:22
back up the slope again. So uranium
8:24
fissioning to be become lighter elements
8:26
gives off energy because of that binding
8:28
energy difference. It's all part of the same
8:30
thing. Now I, I understand that
8:32
intuitively this really isn't a
8:35
satisfying answer because this binding energy
8:37
sounds a bit like your accountant thinking
8:39
you've got a tax dodge. It's a bit like
8:41
capital gains tax or fringe benefits or, or
8:43
uh, what is it? Negative gearing? Binding
8:46
energy may well be the negative gearing of
8:48
the cosmos because it's when you put four
8:50
nucleons together, they're wear less than
8:51
they would do on their own.
8:53
Um, it's ultimate waste loss plan. But it is
8:56
this bind that leads to the mass you
8:58
would measure for a helium nucleus being less
9:01
than the mass you would measure for hydrogen
9:03
nuclei. And it's a mass as you measure, not
9:06
the masses of the individual components. If
9:08
you took them out and put them on their own,
9:10
that is the important thing. This is usually
9:13
skipped in the explanations. And this is
9:15
where the challenges come in. Because in the
9:17
explanations you just say a helium atom is
9:20
less massive than four hydrogen atoms.
9:23
Ergo some mass has been lost. Therefore
9:25
energy is produced by equals MC squared. And
9:28
it skips all this discussion of the particle
9:30
physics underpinning it on um, this binding
9:32
energy. As always, there is a fairly
9:35
detailed discussion of this in the wikipedia
9:37
page for Helium 4. Talking about the
9:39
stability and that's got the binding energy
9:41
curve in. There is also discussions of
9:43
binding energy and that out there. And
9:45
particle Physics out there. It is basically
9:48
though, that the binding energy causes this
9:50
mass deficit. And it's that mass deficit that
9:52
has been converted to energy that it's
9:55
in liberated in fusion. So I
9:58
appreciate it is not the most satisfying
10:00
answer, but that's our
10:03
understanding of the why behind all of this.
10:06
And the proof is in the pudding. Fusion
10:08
happens. It produces energy at the
10:11
right level that we calculate that all, all
10:13
of our models suggest it should do. So it
10:15
seems that this is a very accurate
10:18
representation of how the world works, even
10:21
if it doesn't immediately feel
10:23
satisfying and commonsensical. And uh,
10:26
part of that is that common sense we've
10:28
developed based on the experience of the
10:31
world around us at our scales, at macroscopic
10:33
scales. And the further you go from the
10:34
conditions in this room, the less accurate
10:37
modern sense, common sense is at predicting
10:39
the outcomes of things. And that's why
10:42
it's hard to work these things out.
10:45
Andrew Dunkley: Yeah, it's like, I mean, listening
10:48
to your explanation would be the same as me
10:50
trying to explain to a kangaroo how to use
10:53
a pedestrian crossing. So, you know,
10:55
it's. I can understand Ken's
10:58
frustration. Um, but
11:02
if I understand your explanation thoroughly,
11:05
um, Ken, what, what Jonty was saying
11:08
was that shift happens.
11:11
That's basically it, I think. But thanks for
11:14
the question. Great to hear from you. This is
11:17
a Q and A edition of Space Nuts with Andrew
11:18
Dunkley and Professor Johnty Horner.
11:25
Jonti Horner: Space Nuts.
11:26
Andrew Dunkley: Okay, Jonty, our next question says,
11:29
uh, Fred Watson was talking about caves on
11:31
Mars and that got me thinking. The caves on
11:34
Earth are, uh, primarily made of limestone,
11:36
which has high concentrations of CO2
11:39
that was locked in by millions of years of
11:42
sea creatures popping into the sea, creating
11:44
a layer on the seabed that could be,
11:47
uh, um, seabed. Could it be the same effect
11:50
on Mars? No. Punctuation caught me out
11:53
there. If so, I need to ask, ah,
11:56
the question. Um, you get,
11:58
uh. If
12:01
so, I don't need to ask the next question.
12:04
Um, you get what I mean. But the next
12:06
question is, if caves are deep enough,
12:09
could it be possible that the atmosphere at
12:12
the base of these caves could be dense enough
12:15
to create a stable atmosphere with higher
12:17
concentrations of oxygen for life? And could
12:19
that life be looking at Earth with
12:22
envious eyes?
12:23
Jonti Horner: Yeah, very funny.
12:24
Andrew Dunkley: Hope not. Uh, Mark? Uh, thank you, Mark.
12:27
So, um, caves on Earth, we. Yeah, not all of
12:30
them, but uh, quite a lot of them are
12:31
limestone. Um, could it be the
12:34
same way as caves were created
12:37
on Mars? I think that's the initial question.
12:40
Jonti Horner: So I'd stress here, I'm Not a geophysicist,
12:43
but the limestone we get on Earth is stuff
12:45
that is now above sea level that was once
12:47
below sea level. You had all these calcium
12:49
shelled creatures die and fall to the bottom
12:52
of the, uh, ocean and then get compacted over
12:54
millions of years to form this rock. And then
12:57
plate tectonics lifted the rock above the
12:59
surface of the ocean. I guess on Mars you'd
13:01
probably argue the oceans went away. There's
13:03
not plate tectonics to lift things up, but
13:05
the places that were ocean now no longer
13:08
are. Ah, this is
13:11
predisposed on the idea that you develop
13:13
things with enough calcium
13:16
to be able to make shells and stuff like
13:18
that. Um, I believe, and I stand to
13:21
be corrected on this, that the things that
13:23
make calcium are typically
13:25
oxygen breathers, not carbon dioxide
13:28
breathers. But I may be wrong on that. Um,
13:30
they die, they precipitate stuff out. Now,
13:32
the first point is whether they could be
13:34
limestone caves on Mars. Ah, now that would
13:36
be predisposed on the appropriate
13:39
evolution of life to get to the point where
13:42
you have things that could leave fossils,
13:43
that could leave shells and stuff. And we
13:45
haven't yet found any fossils of such life on
13:48
Mars. And without such life, you couldn't get
13:51
limestone. I think think will be an open
13:53
question. I suspect if we got Earth, uh,
13:54
scientists and stuff like that in on the
13:56
show, they'd have good reasons why limestone
13:59
will be unlikely to be common on Mars.
14:01
Because, yes, you did have oceans and lakes,
14:04
but was there, ah, enough time for
14:07
enough deposits to be made of shelled
14:09
creatures which we don't even know evolved?
14:12
So there's a lot of complexity there. It's
14:14
obviously something, I don't know for
14:16
definite whether you could have limestone
14:19
on Mars. Um, I haven't heard of it being
14:22
detected on Mars, but absence of evidence
14:25
is not evidence of absence. But I don't
14:26
believe it is common, otherwise we'd be
14:28
fairly well aware of it. But there are other
14:30
ways you can get caves on Earth. And I mean,
14:32
we've got lava tubes and lava caves up
14:35
in North Queensland in Uladulla. We've got
14:37
similar things have been found on the Moon
14:39
and Mars. There are skylights and lava tubes
14:42
on Mars that people have even suggested could
14:44
be suitable places for humans to
14:47
go and live. Because if you're underground,
14:49
you're shielded from radiation. And of
14:51
course, if you're in a cave, you can seal the
14:53
entrance and fill it with air, which would be
14:56
good. Now that kind of links to the second
14:58
part of the question here from Mark, which is
15:01
if you have caves deep enough, could you have
15:02
enough atmosphere in those caves to have
15:05
atmospheric pressure? I think that's
15:08
unlikely here because those cave
15:10
systems would probably be connected to the
15:13
surface and air would diffuse out of
15:16
them. So you equilibriate and you don't
15:18
go into the cave in the lava tube on Earth,
15:21
and suddenly it's 3 atmospheres. Because if
15:24
it was 3 atmospheres, the air would be pushed
15:25
out the entrance. So I don't
15:28
think you get to atmospheric pressure in
15:30
these caves unless they were very, very deep
15:33
and were sealed and the air was sealed in
15:35
from when the atmospheric pressure was higher
15:37
and it hadn't escaped. But with the porosity
15:39
of the rocks, I think that would be very
15:40
unlikely. The next thing is about there being
15:43
oxygen in those caves. And I think of all of
15:46
these things, that is the least likely
15:48
because Mars
15:51
doesn't have much, if any free oxygen
15:54
in the atmosphere, because oxygen reacts with
15:56
everything. And on Mars, there are bits of
15:59
methane being produced. We're not quite sure
16:00
what's going on there, but the oxygen, free
16:03
oxygen from Mars has been very
16:06
effectively absorbed into the surface through
16:08
chemistry. Um, big part of why Mars looks
16:11
red, of course, is that you can effectively
16:12
say the surface is rusted iron
16:15
oxide. The oxygen in the air has reacted with
16:18
the iron in the surface and
16:19
Andrew Dunkley: been locked up like a lot of Australia.
16:22
Jonti Horner: Yes. Um, now you can produce some oxygen in
16:25
Mars's atmosphere. If you get water into the
16:28
atmosphere, and there is a very small amount,
16:30
traces of ox of water in Mars's atmosphere,
16:32
we do get water clouds there. Without an
16:35
ozone layer, some of that water, particularly
16:37
the water that gets highest in the
16:38
atmosphere, will get dissociated. It will get
16:41
broken into hydrogen and oxygen by
16:43
ultraviolet radiation and the hydrogen will
16:46
then escape because hydrogen atoms travel so
16:48
quickly that Mars gravity can't hold them,
16:50
which means the hydrogen goes away and the
16:52
oxygen is left behind. So you will be
16:55
producing small trace amounts of oxygen in
16:57
Mars's atmosphere all the time. But then the
16:59
oxygen would then be used up in chemistry and
17:02
removed. So in order to have large amounts of
17:04
oxygen in one of these caves, you'd need a
17:06
source of oxygen, and you'd need that source
17:09
to provide enough oxygen that the oxygen can
17:11
overcome everything that's trying to remove
17:13
it. Now, on Earth, it took a
17:16
huge fraction of Earth's life before you got
17:18
the great oxidation event for life to
17:20
actually get to the point where it could
17:21
produce more oxygen than the Earth, uh,
17:23
system could absorb. So to have abundant
17:26
oxygen on Mars, uh, strikes me as Very
17:28
unlikely. Maybe possible that in the future
17:31
that will change though, because if we went
17:32
to Mars, then one of the ways people are
17:35
thinking the first human habitats will be
17:37
built will be to go to the caves in the lava
17:39
tunnels and live there. And then you can make
17:42
a sealed environment that you can then pump
17:44
with an artificial atmosphere. So it could be
17:46
that if you ask that question in 20 years
17:49
time, Mark, the answer would be yes. There
17:52
are caves with high enough atmospheric
17:53
concentrations for life. We've put them
17:56
there. Would they be looking back at Earth
17:58
with anxious eyes, with envious eyes? I guess
18:00
it depends on the person who's emigrated
18:02
there. There's these fabulous ideas of what
18:05
humanity will look like when we're a multi
18:07
planet species. But the thing that stuck with
18:09
me more than anything else was this amazing
18:11
Talk from a doctor in 2012, 2013,
18:14
2014 at one of our space research conferences
18:17
who basically talked about the difficulty
18:19
humans have reproducing when you go even
18:22
very slightly away from standard temperature
18:24
pressure at sea level. Talked about the
18:26
challenges at invaders had into South
18:29
America when they reached the high Andes.
18:30
They couldn't colonise there because they
18:32
weren't able to reproduce. Talking about very
18:35
slight changes in conditions being enough to
18:37
render our ability to have children
18:40
null and void. On Mars you've got one third
18:42
gravity. Unless you get dystopian science
18:45
fiction future where women enter
18:48
centrifuges for nine months in order to carry
18:50
a child to term, in order to simulate one
18:52
ghost. The perspective is at least in the
18:55
relatively near future, humanity on the
18:58
moon, humanity on Mars will be
19:01
not self sustaining. We won't be able to have
19:03
children there. And so it may well be that
19:05
Mars becomes a interplanetary retirement
19:08
home. People sow their wild oats, live their
19:11
lives and then go to Mars later in life for
19:13
the adventure. And then would they look back
19:15
with envy? Well, they'd probably look back
19:17
with a little bit of longing, but also they'd
19:18
have the excitement of where they are are.
19:20
So it could be that in 20 years time, 30
19:22
years time, there will be life in the caves
19:26
with artificially enhanced oxygen levels.
19:29
Looking back at Earth, watching the news and
19:31
all the rest of it, but I don't think at the
19:33
minute that that's the case.
19:35
Andrew Dunkley: Okay, fair enough. Uh, and uh, just one
19:38
more point, Mark. I just did a quick search
19:40
about could there be limestone on Mars? And
19:42
according to an article in Science
19:44
AstroDailyPod which dates back nearly 20
19:46
years now, uh, yes, limestone, specifically
19:49
carbonate minerals likely exist on Mars, but
19:53
probably not in massive thick sedimentary
19:55
beds found On Earth, while early Mars was
19:58
warmer and wetter, supporting the potential
20:00
for carbonate formation, the planet lacked
20:03
the extensive oceans and tectonic plate
20:05
activity required to build large limestone
20:07
deposits. So there you are. They
20:10
think there possibly is limestone
20:13
on Mars, but not uh, enough to do
20:16
what we've seen on Earth. Uh,
20:18
but great question and uh, certainly food for
20:21
thought, uh, and appreciate it. Mark, thanks
20:24
for sending it in.
20:25
This is Space Nuts. Andrew Dunkley here with
20:27
Professor Jonty Horner.
20:33
Jonti Horner: M. Space Nuts.
20:34
Andrew Dunkley: And you're listening to a Q and A edition.
20:37
We've got one more question to tackle. Hello,
20:39
this is Casey from Colorado, she's one of our
20:42
regular contributors. I have some questions
20:45
about stars. Why do the activity levels of
20:47
stars change? What causes solar cycles?
20:50
Does every type of star go through solar
20:53
cycling? Love the show, Hope you're both
20:56
well. Uh, thanks Casey from Colorado.
20:59
Um, it's a good question because we don't
21:01
really talk about this sort of thing much,
21:03
but uh, we're just about to go out of
21:06
uh, the peak of solar activity
21:09
in our own Solar System, the 11 year cycle
21:12
that they talk about. Uh, so that's probably
21:15
where we should start. We know the sun goes
21:17
through an 11 year cycle and um,
21:22
and we witness different things during those
21:24
11 years because it's constantly changing.
21:27
Jonti Horner: Yeah, it's worth stressing Shreya, that
21:29
uh, effectively all stars are uh,
21:32
inherently variable to some degree. And
21:35
our sun is actually incredibly low
21:37
variability compared to many stars.
21:40
The solar cycle we observe is
21:43
arguably a 22ish year cycle with
21:45
two peaks and two minima. And the subtlety
21:48
there is we get solar maximum when there are
21:50
lots of sunspots, lots of activity, more
21:52
aurora and solar minimum when we've got fewer
21:55
sunspots, less activity, fewer aurora.
21:58
And we get those on and about an 11 year
22:00
cycle from one peak to the next, sometimes a
22:02
bit shorter, sometimes a bit longer. But the
22:05
origin of the solar cycles with the
22:08
sun is the Sun's magnetic field. The sun
22:11
has a magnetic field that runs from the North
22:13
Pole to the South Pole. And it
22:15
rotates in such a way that the rotation
22:18
period at the equator and the rotation period
22:20
at the poles are different rotates as a fluid
22:22
body. So the time it takes the Sun's equator
22:25
to rotate is a couple of days quicker than
22:27
the poles. The magnetic field lines running
22:30
from the North Pole to the South Pole get
22:32
hooked up in the material and gradually get
22:34
wound up a bit like an elastic band. And so
22:36
the Sun's magnetic field gets more and more
22:38
complicated through the 11 years, starts to
22:41
get kinks and the kinks break through the
22:42
surface. So you get locations where the
22:44
magnetic field comes out of the surface,
22:46
loops up and goes back down. And in the
22:48
places where it's nearly vertical, it cools
22:50
the surface of the sun because it suppresses
22:52
convection of new energy from, from
22:53
underneath, leading to cooler spots which
22:55
look darker and therefore are sunspots. And
22:58
gradually the sun gets more and more wound
22:59
up. Sunspot activity begins at high latitudes
23:02
and works its way down towards the equator
23:04
and eventually around solar maximum, you get
23:07
the field line starting to snap and break and
23:09
you get a polar reversal happen. The north
23:12
pole becomes a south pole and the south pole
23:14
becomes a north pole and then it all begins
23:16
again. So the reason we talk about a 22 year
23:18
cycle is you get solar maximum
23:21
with north pole to the north, well, north
23:23
pole to the top, and then a minimum, then
23:25
solar maximum with the south pole to the top,
23:27
then a minimum, then you're back to where you
23:29
started from. Roughly. Those
23:32
cycles are driven by the magnetic activity of
23:35
the sun and they vary the brightness of our
23:37
star, um, by a vanishingly small amount. It's
23:39
an incredibly stable star. I think you're
23:42
talking about variability on the level, about
23:44
one part in 2, 10,000, something
23:47
like that. Now that is such a low level of
23:49
variability that if we were observing an
23:51
other star, we probably wouldn't be able to
23:54
pick up the variability in the total
23:56
brightness, but we would be able to pick up
23:58
the magnetic activity. And this is magnetic
24:00
activity. And star spots are uh, one of the
24:03
challenges for people trying to find
24:05
exoplanets because a star spot can mimic as
24:08
an exoplanet and stellar activity like the
24:10
Sun's magnetic cycle and the star. So SAR
24:13
spots that go with it can actually be mimic
24:16
a radial velocity planet. So there's a lot of
24:17
work done in when we think we've got a signal
24:20
confirming that it is actually a planet and
24:22
not a star spot. So you've got that kind of
24:24
stellar activity.
24:25
Now most stars are significantly, uh, more
24:28
variable than the sun. And there's a lot of
24:29
other ways stars can vary. We think that
24:32
most sun like stars will have sunspot cycles
24:35
like the sun, and it's due to the structure
24:36
of the convective and the radiative zones and
24:39
all the rest of it. The internal structure of
24:41
the sun, a bit like the Earth, has a crust, a
24:42
mantle and a car. The magnetic field in the
24:45
top layer of the sun can get tangled up.
24:49
Stars of different masses have a bit of a
24:50
different structure, but there's A lot of
24:52
other ways that stars can be variable. And so
24:55
we have a very wide variety of different
24:57
types of variable stars. There are flare
24:59
stars like Proxima Centauri, which have
25:02
stellar activity that can occasionally be a
25:04
super flare that can cause the star's
25:06
brightness to change by almost a factor of
25:07
100. And I normally talk about Proxima
25:10
Centauri being 100 times too fancy with the
25:12
naked eye, but in one mega flare it had a few
25:14
years ago, it almost reached the edge of
25:16
naked eye visibility. That flare was that
25:18
intense. You've then got a, ah, large number
25:21
of stars that vary in brightness. Bixa
25:23
pulsate Bixa size changes fundamentally
25:26
and usually these are stars coming towards
25:29
the end of the life entering a bit of
25:30
instability for various reasons. Some of them
25:33
are only just moving off the main sequence or
25:35
are very young. Others are super giant stars
25:38
with different kinds of variability. But
25:41
that variability causes their
25:43
diameter to change, causes them to pulsate.
25:46
And that's because they're just slightly out
25:47
of equilibrium. When they're at their
25:49
smallest they get hotter, they're putting out
25:52
more energy because they're a bit hotter.
25:55
Um, the surface layers therefore are pushed
25:57
outwards with more force than gravity can
26:00
push them in and they start to expand, they
26:02
go through the equilibrium point but because
26:04
they're still expanding they keep going. As
26:06
the stars outer layers get bigger and bigger,
26:09
the star cools. Because when you
26:12
take a gas and you increase its volume,
26:14
you lower the pressure and you lower the
26:16
temperature so that material cools
26:19
and eventually is giving out less energy
26:22
than gravity pulling in would cause and it
26:25
starts to collapse again. So instead of like
26:27
the sun staying at that very fixed radius
26:29
because gravity and radiation are balanced
26:31
perfectly, you can get this oscillating
26:33
behaviour where you overshoot in both
26:34
directions. And um, sometimes that's fairly
26:37
small, sometimes that's fairly large. And it
26:39
happens at different phases of stars lives in
26:41
different ways. One of the most famous types
26:43
of variable stars are known as the Cepheid
26:45
variable stars. And um, these are somewhat
26:48
evolved stars coming towards the end of their
26:50
life where the pulsation period
26:53
is directly linked to how luminous a star is.
26:58
So two similar stars, but one's brighter than
27:00
the other intrinsically, put them at the same
27:03
distance, one's more luminous, looks
27:04
brighter, they will pulsate with different
27:06
periods. And if you can measure the period,
27:08
you can measure how luminous that star is.
27:11
And that makes Cepheid variables an excellent
27:13
step on our distance ladder. Because you see
27:16
a star that's a certain Brightness, you don't
27:17
really know how far away it is. But if you
27:19
can measure the period of the Cepheid
27:21
variable pulsating, that tells you
27:23
intrinsically how luminous that star is,
27:26
which means we can work out its distance. So
27:28
they're really useful.
27:30
Some stars are astonishingly variable.
27:33
Um, among the stars with the biggest
27:35
variation in brightness from brightest to
27:37
faintest, um, are known as the Myra stars.
27:40
Ah, Myra, the archetypal one is known as
27:43
Myra, the wonderful Myra at its brightest
27:45
is easily visible with a naked eye at
27:48
its fantasy, you need a telescope to see it.
27:50
And a few of these stars have
27:53
amplitudes, the difference in brightness
27:54
between the brightest and faintest that are
27:57
such that they vary in brightness by more
27:58
than a factor of 10,000. Um, a couple of
28:01
examples here, Chi Cygni, which at its
28:04
brightness is a magnitude 3.3 star, so
28:06
comfortable with the naked eye, but not that
28:08
bright at, ah, its Faintest is magnitude
28:10
14.2. Um, that is
28:13
a factor of about 25,000 in brightness
28:16
between brightest and faintests. That is
28:19
so long a period that that was discovered,
28:21
that variability back in 1686.
28:25
You've got a wide variety of mirror type
28:27
stars dominating the stars with the biggest
28:29
variability. But there are other type ones in
28:31
there. Arcarona Borealis is a famous one in
28:34
the bowl of the northern crown, normally only
28:36
barely visible with the naked eye. So carbon
28:39
star, where the mirror stars
28:42
have a periodic variation, they're pulsating
28:44
in a broadly periodic way with periods of a
28:47
year or more. Acherona
28:49
Borealis is different. It shines along,
28:51
shines along, shines along and then suddenly
28:52
it's like somebody drops a curtain in front
28:54
of it and its brightness plunges and then it
28:57
gradually brightens up again. And that is not
28:59
exactly periodic. It's known as a carbon
29:01
star. And what's happening is that, ah,
29:03
occasionally it's got a huge amount of carbon
29:05
in its atmosphere. Occasionally the carbon
29:08
condenses into soot, blocking the light from
29:10
underneath. That cools the outer layers,
29:12
which gets this runaway condensation of
29:14
carbon carbon into soot. That traps the
29:16
radiation from inside, so the heat builds up
29:19
inside until eventually the carbon gets
29:20
turned back into a gas again. The clouds
29:22
clear and the star brightens again.
29:24
Andrew Dunkley: So that's, I was, I was just going to
29:26
stupidly suggest that it was soot.
29:28
Jonti Horner: Yeah, it is, it's a carbon star. And a Corona
29:31
Borealis is the archetypal, most
29:34
famous one. It's described as a low mass
29:36
yellow supergiant. So again, it's a star
29:38
coming towards the end of its life,
29:40
um, every so often can be after just
29:43
a few months, or it can be a few years. It
29:45
can dim by as much as a factor of 10,000
29:48
because it kind of suits up, clouds up.
29:51
And if you look at the light curve of that.
29:53
If you're bored, have a look at the light
29:54
curve on Wikipedia, because it's really
29:57
head scratching. It shows you how random this
29:59
is and how hard it must have been for people
30:01
to understand. Um, there was an incredible
30:04
deep minimum that happened in the, the
30:06
mid-2010s, I think it was, where it
30:09
dimmed and then it stayed dim for ages.
30:12
Normally it's bright and then it dims for a
30:13
bit and then it brightens up fairly quickly.
30:16
But fundamentally, there's a huge variety of
30:19
ways in which stars can vary intrinsically
30:21
themselves. Their brightness can vary. It
30:24
is known from discussions with the
30:27
traditional owners of the land here in
30:28
Australia that the variability of
30:30
Beetlejuice, Nal, Deborah, and two bright red
30:32
giant, giant red supergiant stars in our
30:34
summer sky, Northern Hemisphere winter sky,
30:37
are variable. We saw that with the great
30:39
dimming of Beetlejuice about a decade ago.
30:41
Now, that kind of variability has been known
30:44
for hundreds, if not thousands of years among
30:46
traditional owners around the world who look
30:48
at the night sky so that stars
30:50
varying intrinsically in brightness, the star
30:53
itself varying. And, um, it's a wonderful
30:56
rabbit hole for people to wander down. It's
30:57
another area of astronomy where amateur
30:59
astronomy contribute a lot because there's
31:02
very active variable star observers who will
31:04
go out there and measure the brightness of
31:05
stars repeatedly to track when they vary.
31:08
We get a lot of that knowledge. You've then
31:10
got a second type of stellar variability
31:12
which is not intrinsic, but is extrinsic.
31:15
What I mean by that is an intrinsically
31:18
variable star is a star itself changing. An
31:20
extrinsic variation is something else causing
31:23
the brightness of the star to change. You
31:25
know, put your hand in front of the star. The
31:27
stars dissipate because your hand's
31:28
absorbing. All the lights disappeared. We
31:31
have multiple star systems where we have
31:33
eclipsing binaries. Algol is possibly the
31:36
most famous of these. The winking demon star,
31:38
whose brightness drops by about a factor of
31:40
three every couple of days. And that
31:43
star is bright enough to be easily visible
31:46
with the naked eye. And, um, the
31:48
variability in its brightness is sufficiently
31:50
large that it's easily noticeable with the
31:52
naked eye. So its brightness varies. I
31:55
think it's every 70 hours or so. I just want
31:57
to cheque it out. Um,
32:01
no, it's less often Than that. Algol's
32:03
brightness varies every 2.86 days.
32:07
So every 2.86 days, the brightness of
32:10
the star drops from magnitude 2.1 to 3.4.
32:13
That's a brightness change of about a factor
32:15
of three times, roughly. And, uh, it dims for
32:18
about 10 hours and then brightens up again.
32:20
Became known as a winking demon star that has
32:23
been known to be variable since prehistory.
32:26
In reality, there's allegations
32:29
that perhaps an Egyptian calendar that talked
32:31
about unlucky days may have been linked to
32:32
that. That's questionable. Where it is
32:35
really interesting, though, is the
32:37
variability of Algol was first explained
32:40
by John Goodrich, who is one of
32:43
those heroes of astronomy you don't hear
32:45
about very often, mainly because he lived a
32:47
very short life. He presented findings in May
32:50
1783 to suggest that the
32:53
variability, the periodic variability of
32:55
Algol was caused by a dark body or a dimmer
32:58
body passing in front of it every 2.86 days.
33:01
He was awarded a medal for this, I believe.
33:04
He never got to receive the medal because he
33:06
died, as I say, very, very young. Died at the
33:09
age of 21. Got the Copley Medal in
33:12
1783, I think he was. Uh, passed away
33:14
three years after that. So in just 21 years
33:17
old, he contributed hugely to our modern
33:20
knowledge of variable stars. And, um, you
33:22
know, it's very unfortunate that he passed
33:24
away at such a young age, but he was able to
33:26
explain this variability that had been
33:28
clearly known for a very long time. It's
33:30
obvious to the naked eye that this star gets
33:32
dimmer. It's not a subtle effect, but he
33:35
was the one who was able to explain it.
33:38
Despite his challenges. He was someone with
33:41
certain physical disabilities. He was someone
33:44
who had a very difficult life. But he had
33:46
such a visionary intellect at the time to
33:48
come up with the explanation that this
33:50
periodic variability and the style of it and
33:52
the frequency and the depth of it being so
33:55
repeatable was because this was actually
33:57
two objects going around each other. Ties in
34:00
with the exoplanet chat we had earlier,
34:02
because in a way, this is the indirect
34:04
discovery of the binarity of Algol.
34:07
You don't know that there are two stars there
34:09
because you t two stars separately. They're
34:10
circles close together. You can't separate
34:13
them with telescopes, modular, hugely massive
34:16
interferometers we have today. But you can
34:18
infer the two stars there by the
34:20
extrinsic variability, the variability of the
34:23
light we receive because one blocks light
34:25
from the other, fundamentally. So that also
34:28
causes cyclical variations in
34:31
brightness. But it's not the star in this
34:33
case varying. It's a result of the
34:36
environment around the star blocking some of
34:38
the light.
34:39
Andrew Dunkley: Yeah. Okay, so the answer to
34:42
Casey's questions are, uh, fundamentally,
34:44
yes, um,
34:46
stars. Most stars probably have some sort of
34:49
variability, some more than others. Uh, the
34:52
solar cycles are caused by the buildup
34:55
of, um, activity
34:59
and um. Yeah. Does every type of
35:01
star go through solar cycling?
35:03
Jonti Horner: Probably. Does it? It's one of. Like I said,
35:05
it's one of the big challenges for us with
35:06
our radial velocity work. Looking for planet
35:08
trans is filtering out the stellar
35:11
cycles and that's particularly a problem for
35:12
finding planets like Jupiter on a Jupiter
35:14
like orbit. Jupiter goes around the sun every
35:17
11.86 years. The solar cycle is about
35:19
11 years. It's m hard to disentangle the two.
35:23
There's also a fascinating branch of science
35:25
and one of my colleagues at Uni SQ is one of
35:27
the world's experts in this. Um, Professor
35:29
Simon Murphy. This is a discipline called
35:32
asteroseismology. We know about the
35:35
Earth's interior because of earthquakes. We
35:37
can figure out the crust, the core, the
35:39
mantle by how different types of seismic
35:41
waves pass through the Earth's interior. So
35:43
we'll listen to the Earth ringing like a bell
35:44
after an earthquake and we can use that
35:46
information to sense what the interior
35:48
structure is and how it varies. The science
35:51
of astroseismology is doing the same kind of
35:53
thing with stars, looking at how they wibble
35:55
and wobble to map out their interior and
35:58
understand it. And that's fundamentally tied
35:59
to the variability. Now Simon's got a couple
36:02
of PhD students working with him and doing
36:04
some fabulous work, um, including
36:07
Guy, um, called Tom Love, who's down in New
36:09
Zealand, who's an amateur astronomer there,
36:10
doing a PhD, just finishing up with us. Where
36:13
they're looking with Simon at these
36:15
asteroseismology, wibbly wobbliness and also
36:18
at the variability of stars. Looking
36:20
at a group of stars called the Delta Scuti
36:23
stars, which are a particular type of
36:25
oscillating, varying vibrating
36:28
star, looking at how their interiors behave,
36:31
looking at how old they are. So we can better
36:33
understanding of the physics going on and a
36:36
better understanding of where these stars sit
36:38
in the storey of stellar lives. So this kind
36:41
of question is one that leads
36:44
to whole, uh, rafts of amazing science that's
36:46
been done. And I think like everything we
36:47
discuss on the show, these questions are all,
36:50
all entryways to rabbit holes that can go as
36:52
deep as you want to.
36:54
Andrew Dunkley: Yes, absolutely. There you are,
36:57
Casey. Thanks for the question, really, uh,
36:59
really interesting. And um, yeah, it's
37:01
fascinating.
37:02
Stars. I, I've been spending a lot of time
37:04
outside of my telescope recently, uh, and
37:07
photographing where I can
37:11
some of the, the big stars that are visible.
37:13
Um, um, I think I did I get serious
37:16
recently. I can't remember. I've got a couple
37:18
of good ones. I got Alpha Centauri the other
37:20
night, which turned out really well. Uh, but
37:23
yeah, thanks for the question, Casey. If you
37:24
have questions for us, please send them in
37:27
via our website spacenutspodcast.com
37:30
and click on the Ask me anything button at
37:32
the top. It's labelled ama. You can leave
37:35
text or audio messages. If you've got a
37:37
device with a microphone, you're all set.
37:39
Such as a, I don't know, cell phone, mobile
37:41
phone, um, tablet, anything like that.
37:44
Or your computer. The got built in mics
37:47
these days and just tell us who you are and
37:49
where you're from and we'd be happy to try
37:51
and solve your riddles. Uh, and have a
37:53
look around while you're there. Cheque out
37:55
the shop. Cheque out. Uh, Astronomy
37:56
AstroDailyPod. Maybe sign up for your daily
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feed of astronomical news and
38:01
click the supporter tab if you'd like to help
38:03
us out. That is totally optional. Uh, and
38:06
thank you Jonty for all your help today.
38:08
Jonti Horner: Absolute pleasure. It's always good to have a
38:09
chat.
38:10
Andrew Dunkley: We'll see you soon when we talk, uh,
38:12
Astrobiology Part two.
38:15
Uh, that is Professor Johnty Horner from the
38:17
University of Southern Queensland. And uh,
38:20
thanks to Huw in the studio, couldn't uh, be
38:23
with us today? Huw? Um, he's an ex radio
38:26
guy so he thinks he's a star,
38:28
uh, which means his equator rotates more than
38:31
his north and south and he's back in hospital
38:33
with a twisted bow. And from me, Andrew
38:36
Dunkley. Terrible. Thanks for your company.
38:38
We'll catch you on the next episode of Space
38:40
Network Nuts. Bye bye.
38:43
Jonti Horner: You've been listening to the Space Nuts
38:45
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38:48
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38:50
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38:53
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38:57
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