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