- Good evening everybody, and welcome to "The End of Our Sun,"
or at least I should say, welcome to a lecture entitled
"The End of Our Sun." This is in fact lecture two in my series on cosmic conclusions, but it's also the first
of a two lecture series on how stars evolve, what
their life cycles are. And there are two lectures
because the way that stars evolve and the way that they play
out and the way that they come to an end depends very, very strongly on how massive a star is. The subject of tonight's
lecture is going to apply to the lower mass end of
stars, stars much like our sun. And the subject of my next lecture is going to be how massive stars end, much more massive than our sun. And it turns out, they end
in rather different ways. There are some similarities,
both are fairly spectacular, but there are some key differences. So as we think about
the life cycle of stars, I just want to draw your
attention to how it all begins, and to show you this
particularly exciting image which was recently published by NASA, a beautiful result from the new JWST, its newest space telescope. We refer to it as the just
wonderful space telescope. What you can see here
is a very famous nebula in the night sky known as
The Pillars of Creation. I'm sure you've seen this before,
it was an important target imaged by the Hubble Space Telescope, the JWST's predecessor,
at optical wavelengths a few years ago. And while there are a
number of similarities with this image, there are a
number of important differences that I'd like to draw your attention to. So this particular image is
taken at longer wavelengths than the original optical
image from Hubble. Specifically the waveband
that we're talking about is the mid infrared, significantly longer
wavelengths than in the optical. And the importance of observing
at these longer wavelengths in the mid infrared is that it reminds us that there's a lot of
dust in the universe. Clouds of dust, which,
when they're cool enough, can collapse under gravity
and form new stars. Stars themselves are much fainter at these longer wavelengths,
these mid infrared wavelengths, and also dust is much easier to see at these longer wavelengths. So this is the image
of choice if you want, or this is the wave band of
choice, if you want to image a situation where stars are
going to be born in the future. One thing that's really
important about this image at the mid infrared range
is that there has never been such a sharp image at these wavelengths. Until the JWST came along, we had never looked at the universe in such sharp focus at
these long wavelengths. Let me draw a comparison for you with slightly shorter wavelengths, not as short as the optical,
but now the near infrared. So this is exactly the same region of sky, but at a different wavelength. We're seeing a different picture because at these somewhat
shorter wavelengths, the stars are a whole lot brighter so they come booming through. And also, we don't see
the dust quite so readily at these wavelengths and that's
because of the temperature, the relatively cool temperature
that the dust is at. This image too is from the
JWST, that magnificent telescope that was launched on Christmas morning and delayed many of us from
getting to our roast turkey. The launch of this telescope, the deployment of this telescope, the images extracted from this telescope, are a testament to the
dedication and the expertise of a great many scientists and
engineers working in the US, working in Europe, and way beyond. I want to introduce one of
them in particular to you, and that's Gillian Wright,
she's based in Edinburgh. She was the European Principal
Investigator for the camera that works at those mid
infrared wavelengths that I mentioned to
you in my second slide. She's also the Director of the UK Astronomy Technology Center. And the fact that this camera on the JWST works so brilliantly is
a brilliant testament to her and to her teams. So here then are these two
images, and you can see how very many stars there are
at the near infrared images, wavelengths, and how
much there is in the way of dusty clouds at the
mid infrared wavelengths. So beautiful images, we
can spend the whole hour looking at these and talking about these, but we need to get to the
business of the life cycle of stars like our sun and we need to think about how stars form. Now wherever you have gas in the cosmos, if it's cool enough, it will gradually, gradually begin to form stars. As long as that gas cloud
can collapse under gravity, ultimately you will end up with stars. Stars will form wherever
you have the condensation of primordial gas, that's
gas milling around, formed after the big bang,
or indeed interstellar gas, the spewed out remnants of ex stars. More of that a little bit later. But if you have gravitational collapse, we call it jeans collapse
under gravity of gas, ultimately following a competition between gravitational
collapse and thermal energy which is fighting to oppose
the gravitational collapse, ultimately you will end up
with these dense concentrations of gas mixed in with dust, no doubt, which will, once they're cool
enough, properly collapse. But as that gas gets denser and denser, it will get hotter and hotter. And when that happens, once you attain sufficiently
high temperatures, fusion can be ignited. And I just want to play
for you this cartoon movie, which captures rather more eloquently than I can that process. So this is a great big cloud
of gas or it's a simulation of a great big cloud of gas in the galaxy. There's a bit of residual
rotational motion, which manifests itself as
spinning faster and faster as it gets closer to the center. You'll notice that this gas
has collapsed into a disc, and that disc is
perpendicular to the direction that the gentle rotation or the spin of the gas is taking place on. As the collapse persists and
progresses further and further, you'll get a star right in the center at the central potential
well at this gas cloud, and you'll get little condensations formed at different radio further out. Those initially are proto planets. And where they're surrounded by discs, those are the progenitors
of rings of material that orbit around planets and subsequently can form satellites, much
like the moons of Jupiter or the moons of Saturn. So that's pretty much how
the life of a star begins, and how the life of
planets around our star begin to take place and
form the solar system. Now with a bit of flagrant disregard for getting everything to scale here, but just for the sake of
getting everything on one slide, this lines up all the planets
that we know and love. This is how the planets look approximately after the elapse of four or
four and a half billion years, after that initial condensation out of the cooling gas cloud. So we're there, the third planet out, and booming away on the left is our sun. The sun itself right now shines brightly, or at least our friends and relations on the other side of
the world will hopefully be assuring us it shines brightly. It's nighttime here in London, of course. So from our vantage point on
Earth or somewhere on Earth, what we see now is a distant
but blindingly bright and I do, of course, mean blindingly
bright light in the sky. That's what we see for
now, we orbit around it, and at some point somewhere on the planet, someone will be seeing that
blindingly bright light in the sky that is our sun. The sun is our faithful companion. In living memory, the sun has been there. It's risen in the East,
it's set in the West. How is this going to change? And how on Earth will this play out? How will it end? How will the sun go away? Is it changing now? Well, one thing I'd like
to just make quite clear is that the end of the
sun is not like a sunset. The sun doesn't just glide
below the horizon of Earth, as it does at the end of the day on a particular location on Earth. Sunsets are a rather
beautiful spectator sport, when we have time and
indeed clear enough skies and clear enough horizons, to watch them. But that's nothing to do with how our sun will end its life as a star. On the contrary, far from
just fading into oblivion, the sun will go through some
rather spectacular processes almost as though it has read
that amazing Dylan Thomas poem, "Rage, rage against the
dying of the light." I learned this poem at school, loved it, and have never forgotten it. "Do not go gentle into that good night," old age, I feel sure
Dylan Thomas was thinking of the main sequence stars
rather than humans at this point. "Old age should burn and
rage at close of day." "Rage, rage against the
dying of the light." That, in poetic form, is
pretty much what our sun does as it approaches the end of its life. So right now the sun is shining brightly, but it is not, as a star, unchanging. We can study its activity from satellites that can fly close to the sun. And we can also study how the sun changes when there's something in the
way to block out the light, and that ideal something is
Earth's own satellite, the moon. By an extraordinary coincidence, our moon is the exact
same size as the sun. The fancy way of saying that
is that the moon subtends the exact same solid angle as viewed from Earth as the sun does. And what that means is that
when you line the two up, such that the moon is in
between the Earth and the sun, if it's the other way around,
you've got an apocalypse, we are talking about an eclipse when the you've got the Earth,
then the moon, then the sun, when they're exactly
lined up and you block out the solar disc, then and only then is it safe to look at the sun with the unaided human eye. And when you do see, when
you do have the eclipse sun and when you do look at
the residual brightness around the outside of the eclipse sun, then you see two rather
important features, which are just about
discernible on this image. One is the corona, that's the white glow. More of that later, but
I want to talk first about these prominences, if
you look at that pink thing at about one o'clock, that's
what is known as a prominence. A prominence is the spewing
out or the spitting out of a filament of hot
plasma by the lively sun. Think of it as being
erupted out or belched out and spat out by an even hotter
and indeed magnetized sun. These prominences are absolutely
huge relative to the Earth. Let's just try and see how huge that is. So here is the Earth, and
now let's scale it down to the size of that prominence. So, that's pretty much how small we are. You can actually fit 109 Earth diameters across the diameter of our sun. So our sun has over one million
times the volume of Earth. Earth is really pretty puny,
but it is a great vantage point with which to study these prominences. It's an okay vantage point for now, but as I'll describe later in my lecture, as the sun evolves, it gets
bigger and more luminous. When that happens, it would
be extraordinarily dangerous to look at the sun, even when the moon comes in front of the solar disc because the kind of anular
eclipse that you'll get would be disastrous for human eyes. More of that later on. But in terms of these prominences, that tells you that the
sun is a very lively place, spitting out plasma, belching out mass at very high temperatures,
highly electrically charged. Now I took this particular photograph of the eclipsed sun in 2017, in the United States in Idaho, along with some very dear friends. And this particular image,
I took on my camera. The exposure time was
about 1/500th of a second, and I took in the hopes of trying to see some of these pink prominences. But during totality, I also experimented with taking some longer
exposure time images to try and go deeper on the corona, and try and pick that up
because the corona of the sun also tells us about the
activity of the sun. So changing the exposure time
on my camera to half a second brought forth this image. So that dark disc is
still due to the moon. The moon, of course, never
radiates its own light, it only ever reflects the sun if the sun is at a favorable angle. But, of course, when the sun is eclipsed, no light on this side of it. And what you see in that
half second exposure is a considerably extended solar corona, the extremes of which are more
than a solar diameter away from the edge of the
disc of the moon there. You can see it's not at
all circularly symmetric, it's distinctly elongated to about one o'clock and seven o'clock. So I was terribly pleased
to see the solar corona. By the way, I should mention, the sun isn't the only star in this image. One of the many exciting things, and there are many exciting
things about an eclipse, is that stars that are
normally up in the daytime, if they're nearby the sun, when it's eclipsed, you
can see those clearly. So that's Regulus, which you
never normally see in August, which was the time of year when these photographs were taken. So seeing a big corona and
seeing quite a distorted corona is a sign of a very lively sun. It's a sign of magnetized
plasma streaming away from the sun, energized by
all the convective processes going on within it. So if you see a good, big size corona, you know that the sun is
fighting fit and blasting away. Now it was especially
encouraging to see this because at the time of
this eclipse in 2017, we were heading for what's
known as solar minimum. That's a time in the rhythm
of the sun's activity, it's got a 22 year cycle, when it's at its least
active and least explosive. So it was pretty good to
see a couple of prominences during this particular eclipse, as well as to see quite
an extended solar corona. Now the most recent solar minimum was during 2019, and I was
lucky enough to be able to get to another totally eclipse then. This one was in Chile, not in the US. And this was a handheld
exposure during totality where you can see a very extended corona. Let's just zoom into that right now. Again, pretty much a
sun's diameter's worth to the top right and to the bottom left. So even during that solar minimum time, a very active sun. Now the other evidence that you get of a lively and active sun is sunspots. These are much fewer in number during this so-called solar
minimum in the 22 year cycle. Whereas during solar maximum, obviously you see a lot
more in the way of sunspots. Even in that 2017 eclipse, we could see some little
sunspots just appear. We were able to focus in on
them just before totality, and even though it was relatively
close to solar minimum, they were still there. If you'd like to compare the presence, the characteristic presence of sunspots close to solar minimum, as these guys are, with sunspots close to
maximum, then I can show you a photograph imaged by my
friend and colleague, Steve Lee, very close to solar maximum in 2003. And you can see blemishes
all over the sun, the solar disc during this one, a lot more in the way of activity. So there is this cyclic pattern
with the sun every 22 years, and we can zoom in on those
and admire them greatly. I talked a little bit about
the magnetic nature of sunspots in my lecture last year
entitled "Magnetic Universe." So let me now show you something of the cyclic
nature of our changing sun. It's got a rhythm in
the sun every 22 years, as far back as records
began in the 1750's. So these peaks that you see are periodic, with a period that's
around 22 years apart. We are just beginning the 25th cycle of the sun since records began. Were about a third of
the way through that, or not quite a third of
the way through that, but cycle 25 is where we're at. You can see that superimposed
on that 22 year cycle, there are some peaks that
are really quite high, such as cycle number 19,
which was in the 60's, goodness knows what was going on then, in contrast with cycle number
five in the early 1800's when the sun was much more muted. The proxy or the metric for activity here is something referred
to as sunspot number, which is a way of accounting
for the number of sunspots that you've got all over the solar disc and the number of groups of sunspots that you've got over the solar disc. It captures the real-time
activity of the sun. So if you compare cycle
five with cycle 19, you can see there's a certain
amount of variability. It's been suggested that
there are other periodicities besides this very pronounced 22 year cycle that is superimposed on top
of this and giving you beats. But really, we need a few more such cycles to be sure about that. It's also worth commenting that superimposed on
that periodic behavior is a lot of stochastic,
more random behavior. And so if I now just zoom
in on the end of this plot, just showing the last couple of cycles and now swapping the proxy for activity with the kind of radio flux,
the intensity of radiation that we get at radio wavelengths
from the sun over cycle 24, just at the beginning of cycle 25, you can see that there's a lot of stochastic activity
superimposed on that. So for a start, you can
see even though the model of the sun's behavior
given by this rather smooth red sine wave is a very clean prediction, reality is not like that. In particular, for cycle 24,
it came through a minimum just after 2010 and then rose up. And it was as though we had two peaks in the solar activity then, before it decayed back to
its normal minimum levels around the time of the
2017 total solar eclipse and the 2019 total solar eclipse. Since the early 2020's, the
solar activity has risen again, ahead of what the prediction
of the model says. So the black line there
is the observation, and the blue line is the sort
of smoothed approximation to that black data. And as you can see, the
sun is much more active than the model is predicting. So the sun is not following
anybody's rules in detail, it's following the basic rhythm that we've seen in the past few centuries. But right now, the sun is very active. One beautiful consequence, by
the way, of the sun's activity is when some of the ejected
mass from the surface of the sun, the so-called
coronal mass ejections, actually end up all the way
around Earth's magnetosphere and then propel in towards
the poles of our planet, the magnetic poles, and
give rise to the beautiful Northern Lights in the Northern Hemisphere and Southern Lights in
the Southern Hemisphere. These so-called aurorae
are absolutely spectacular. And with the sun being
so active at present, it's a terrific time to go up to Scotland, and on the assumption that
you get a non-cloudy night in Scotland, there are high chances that you'll see some
really beautiful aurorae. Some spectacular examples have
already been seen this year. But there are times, in contrast with now, when the sun has been
really quite inactive. So let's now turn our attention to that. So there was something referred
to as the Maunder Minimum, which was a period of extreme
inactivity by our sun. This was pretty much between,
it began around 1645, and it ended around 1715. So that's a span of about five Gresham professors of astronomy. UK prime ministers hadn't
been invented back then. That didn't happen until
1721, with Sir Robert Walpole. And, of course, I wouldn't
dream of suggesting that the increase in
hot air after that event was in any way linked, that
would be quite inappropriate. Well, detailed records of the activity of the sun hadn't begun at the time of the Maunder Minimum, in the sense of the quantitative records, counting numbers of sunspots
and all that kind of thing is the case these days
and has been the case for the last 24 and a bit 22 year cycles. But the fact there were no proper quantitative measurements
available doesn't mean that we have zero evidence of the sun's activity during that time. There's a particular heroin in our story at this point, thanks to the German artist and observer, Maria Clara Eimmart. During an eclipse that took place within the Maunder Minimum, she made a beautiful and
accurate, we believe, sketch of what the
solar corona looked like at totality during an eclipse
in, I think it was, 1706. So that's her drawing, and it's available in a library in Germany even today. Now before we get sniffy about the fact that this is the
representation by an artist, not by NASA or some satellite, let me draw your attention
to some important details. The solar corona as depicted in this image is this blue concentric annulus centered on the dark gray eclipsed sun, the dark gray, of course,
being the surface of the moon that's facing closest to us here on Earth. It's circularly symmetric,
no hint of those elongated streaks in the solar corona. Now before you dismiss
this as some inaccurate representation of someone
using artistic license, it turns out, and there's
a paper by Hayakawa et al., that if you look at the
representations by other artists of this exact same eclipse, then they all show the corona, when you scale the eclipsed solar disc, to be the same size as Maria Clara's, then you find that the corona has pretty much the same size as well. So I think for this particular
eclipse which took place in 1706, which was in the
middle of the Maunder Minimum, the idea that there was no
expansive asymmetric corona I think holds water. In another eclipse that took place during the Maunder Minimum, John Wybard who was able
to view the 1652 eclipse in Carrickfergus in Northern
Ireland said the following, "The ring around the sun
had a uniform breadth of half a digit." I think that's old fashioned
language for a finger, "Half a digit or a third
of a digit at least that it emitted a bright
and radiating light, and that it appeared concentric
with the sun and moon when the two bodies were in conjunction," i.e. when they were in eclipse. So very small circularly symmetric corona goes together with inactive
sun, which is what was the case we think during the Maunder Minimum. Well the sun is much,
much livelier these days, and indeed it's a very
important source of energy for those of us who live and
eat here on planet Earth. So the luminosity of
our sun, that is to say, the power radiated by our
sun is absolutely tremendous. It's something like 10
to the power 30 watts, 10 to the power 30 joules every second. Now planet Earth is
sufficiently close to the sun that we can receive on average, for every square meter of Earth, an average of 1,000 watts for each of those square meters from it. So that's quite a lot. And if we can collect it and harvest it, it is enough, more than enough, to sustain life on the planet,
allowing plants to grow and be eaten by animals
who eat other animals. The sun is the engine behind
all of that good stuff. And the sun's energy,
accumulated over millennia, metamorphoses into chemical energy, which is stored under the
ground in coal and oil and gas, that we burn in a flash in our
internal combustion engines. There are other ways that we
can do this, but nonetheless, my main point is our
nearest star is the source of all our energy for living,
eating, and having our being. So what does the sun do? Well, I find it amazing to think that it's the source of our energy, as well as the source of
gravitational attraction, that means that we orbit around it. And I find a real resonance
with what Galileo said some years ago, "The sun,
with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else
in the universe to do." I'd like to update this
very slightly by saying the sun, with all those planets revolving around it and dependent on it, can still charge your photovoltaic
panels and your batteries as if it had nothing else
in the universe to do. The sun is the source of
energy is particularly close to my heart because one
of my observatories, my observatory located at a
school in rural southern India, could not function where it
not for directly collected, solar energy collected,
in photovoltaic panels, 12 square meters of
solar panels shown here with some Indian school girls for scale. My observatory simply couldn't function without that as the source of energy. And it's super important for communicating to these impressionable teenagers that once you've got the
infrastructure in place of panels, a few regulators, and some batteries, thereafter for free, you
get energy enduringly out of the sky. As it happens, on Friday,
I'm going to be heading to my school observatory in South Africa to start building the exact same thing because the power situation in
South Africa is a nightmare, not just for astronomy at
night but for daily life. In that part of the world, they're undergoing all
sorts of power outages. And when you get a power outage
four, five, six times a day, then all kinds of things stuff up. Freezers don't freeze,
traffic lights don't stop the traffic or they do,
but it all gets snarled up, it doesn't regulate traffic. The internet goes down,
communications go afoot, business and growth comes to a halt. And so, solar energy has
an important role to play in both astronomy and daily life. So hooray for energy from the sun, but where does that energy come from? What's the origin of all that energy? Well, it's nuclear fusion. It's the fusing together of nuclei where you have atoms or
irons, ionized atoms, with sufficient energy
that they can overcome the strong repulsive electrostatic forces that would normally keep
them apart and enable them to fuse together to become heavier nuclei. This happens in the center
of stars like our sun, where the temperature
exceeds 10 million degrees. And it really doesn't matter
whether the degree scale here is Celsius or Kelvin when you're talking
about that many degrees. But when you are at those
sorts of temperatures, a proton and a proton can fuse together with neutrons and other
particles and ultimately form deuterium, heavy
hydrogen, and then helium, and other so-called light elements that are reasonably high
up in the periodic table, like beryllium, from which we ultimately get helium, as I'll show you in a second. The exact same fusion processes
with different pathways can give us the heavier elements as well, such as carbon and nitrogen and oxygen. But nucleosynthesis within
stars, within our own sun, is what's going on here. It's been happening since surely after the beginning of time, it happens throughout the universe. Wherever you have a star shining, it is shining because of fusion within. This is definitely the case in
our nearest star at present. And despite the gloomy nature
of the title of my talk, it will continue to go on in the center, in the core of our nearest star, for another probably 5 billion years. So, let me reassure you about that. Some naive calculations
said that surely the sun would only last for about a million years. Well, with a proper understanding of both nuclear physics
and quantum mechanics, then the evidence is that the sun would last much, much longer. And I would say that we've
got at least 5 billion years before we use up all our nuclear fuel. And I'm thinking in the
first instance of hydrogen, a hydrogen nucleus is just a simple proton and that fuses together to
form heavy hydrogen, deuterium, and beryllium, as I said, in a pathway that ultimately
leads us to helium. Helium can fuse with helium. And what's particularly significant
about the element helium is that it, despite being the
second most abundant element in the universe, wasn't
discovered on our planet at all, it was discovered in the sun. It's relatively rare on Earth because if it's in gaseous
form, it's whizzing around at the typical temperatures
in our atmosphere so fast that it exceeds the
escape velocity of Earth. And so it flies off,
utterly unbound by Earth. So we never tend to see too much of it in Earth's atmosphere,
although there are cases where it's trapped underground,
just as well for us. Helium arises on Earth as the
result of radioactive decay from much heavier
elements such as uranium. But it was, as I say, discovered on our nearest star, the sun. It was discovered during a solar eclipse. The eclipse where it was
discovered was the 1868 one, where Pierre Janssen discovered
that there were lines in the solar corona of the eclipsed sun that couldn't readily be
accounted for in terms of all the elements that were known
in the optical spectrum of elements that were
already known on Earth. It was independently discovered
by the British astronomer Norman Lockyer later in the
same year, but also in the sun. Initially there was a
suggestion it was a new element, but that was ridiculed. But, of course, that was the right answer. Helium had never been
seen before on Earth. So, of course, it was
startling and extraordinary to suggest that there
should be a new element. But a few years later,
when Palmieri was examining larva from the volcano Mount Vesuvius, the exact same spectral line was seen corresponding to helium here on Earth. The element helium is named
after the name for the sun, the Greek name for the sun, helios, so that's where that name comes from. And if you'd like to see an image of the solar corona during the eclipse when helium was first
discovered, this is it. Forget any concentric ring
of a very inactive sun here, the corona was splurging out
and so it was relatively easy to examine the light from this
corona during that eclipse. So we've said that fusion gives us stars when we've had the collapse
of those gas clouds, that I talked about at
the start of my talk, into sufficiently dense coagulates, that the temperature
would become high enough to give you fusion. Fusion is what gives us sunshine. Fusion in normal stars, like our sun, is when you can transform
hydrogen into helium and, importantly, heat and light. You will get, in the course
of the sun's lifetime and in the lifetimes of other
much more massive stars, other heavier elements,
but that's the subject of my next lecture. But for this lecture, I
just want to introduce you to the ideas that at different
radii within the ball of hot plasma that is our
sun, you have different zones. And it's the very central
zone, the central core, where fusion of hydrogen into helium, via a pathway I'll discuss in
just a moment, takes place. That's happening now,
that's going to persist for a few more billion years. It's way further outside
that we get sunspots and prominences and indeed,
the solar corona itself. Let me tell you a little
bit about the fusion pathway that takes place right in
that central core of the sun. Each pair of hydrogen nuclei, the central nuclei of hydrogen atoms, also known as protons, fuse
together releasing a neutrino and a neutron to give us heavy hydrogen. These can then form with another proton, a normal hydrogen nucleus, releasing a gamma ray, forming
what's called three helium, two protons and one neutron. So a charge of plus two,
but a mass of three. When two of those lightweight
helium nuclei get together, they're not that stable,
but when they get together, hydrogen protons are again released, but we form stable for helium, two protons and two neutrons. But the heat and the light, the gamma there symbolizes
a ray of light, a photon. That heat and light is the side product of the fusion process, which gives us this much heavier nuclei. So that process is ongoing and ongoing and will continue for the
next few billion years, roughly maintaining the temperatures. And according to all the
other physical processes that are going on in the sun, the conservation of angular momentum, the laws which govern the dynamo processes of the magnetized plasma
that I discussed a little bit in my lecture on the magnetized universe, those are all ongoing for the time being. The heat and light that's given off is, of course, what we receive at Earth. And this will totally continue until we run out of the
fuel that's on the left. When all the hydrogen is used up, big changes are going to happen. But what govern the fusion
processes for the time being is, again, that usual
competition between gravity, that is mass, the attraction due to mass, pulling everything in on the
one hand and gas pressure and radiation pressure
tending to oppose that. Gravity squashes in and gas
pressure and radiation pressure tend to hold up the star
against gravitational collapse. The temperatures inside
the stars are describable and characterizable in terms
of T is the temperature and V is the speed at which
these nuclei whizz around within the hot plasma within the sun. The mass is just the mass of whichever particle you're talking about,
and kB is just a constant of thermodynamics known
as the Boltzmann constant. But roughly speaking,
that's the relationship between the temperature
that the sun is at, and how rapidly stuff is
whizzing around within it. The temperature of the sun will change if there isn't any more
hydrogen to fuse together into helium, it will
start to collapse down. But when it starts to collapse down, the density will increase and
the pressure will increase. And so, things will
start to whizz up again and the temperature will increase again. And so this whole cycle,
while being a bit modified on small time scales for
the next few billion years, will undergo dramatic change at the point all the hydrogen is gone. And this is a very significant milestone in the life cycle of the sun, when the sun begins to turn into something known as a red giant star. What happens is that that innermost core which used to contain hydrogen doesn't contain hydrogen
anymore, it only contains helium. It scrunches down, it's collapsed down, and so that increases the temperature. The surrounding shell of that
helium core now heats up, and now fusion can take
place further out in the star because that gravitational
potential energy that's released when the helium collapses
heats up its surroundings. And now you get hydrogen
fusion taking place outside of the core of our sun. You no longer get fusion
taking place in the core of the sun, that's all
helium for the time being, but you do get fusion
taking place in the hydrogen that's outside of the helium core. So this is the beginnings of the star turning into a red giant. Now that we've got a shell burning, burning meaning fusing,
outside the helium core, the extra heat from that
collapsed helium core will enhance the fusion of
hydrogen in that outer shell. Because that process
increases, a lot more energy is given off, and the star
will expand and get bigger. It'll actually get a little
bit cooler in the process as it puffs up, but the
radius will increase by something like a factor of 100. When this happens to our sun, it is goodbye to Mercury
and it is goodbye to Venus. Now our sun is by no
means the first red giant in the history of the universe. On the contrary, there are some
very well known stars nearby that you can see in the winter sky, when you haven't got clouds in the way, that you might like to look at. If you find Orion, which
is probably the best known constellation in the Northern sky, and you go up from his belt
through the top of his bow, you end up at a bright
star called Aldebran. It's on route to the Pleiades, which I've discussed previously. But Aldebran is a very
famous red giant star, so too is Betelgeuse in the top shoulder there of Orion. There are a great many red
giant stars in the sky, many of the named stars,
or quite a number of them, are red giants because they're big and they're very, very luminous. Because the stars are very
expanded, the opacity is lower, so more photons get out more quickly than in the sun when it's in
its current evolutionary stage. Aldebran, its name is
derived or believed to derive from the Arabic (foreign language), which means the follower
because it follows the Pleiades through the sky. It's the 14th brightest star in the sky, and its radius is 44
times that of our sun, but it's got about the same mass. But it's vastly more
luminous than the sun, it's something like 450 times
the luminosity of our sun. It's also redder, so that's
why it's called a red giant. It's a giant because it's bigger, and it's red because the
spectrum is more red. I've shown you this spectral
view of the Orion constellation in my lecture on unraveling
rainbows, the year before last. And if you look at
Betelgeuse, the red giant or red super giant even,
and you resolve the light from it into its spectrum, you can see that there's a
big chunk of red emission if you disperse its light into a rainbow. If you compare that
with a rather blue star at the opposite end of
the Orion constellation, then you see, relatively speaking, more cyan purple light
and a bit less red light. There's not zero red light in a blue star anymore than there's zero
blue light in a red star. But what the relative redness or blueness of a star's appearance does communicate is the different temperature within. So in fact, out of the stars in Orion, Rigel is way hotter than
Betelgeuse and than our sun. It's got a temperature
of about 1,200 degrees. Now you can approximate the temperature of the fusing ball of gas that is a star by a thermodynamic function
called a black body function, which has this sort of
shape of intensity of light that it radiates against
frequency of light increasing this way or, equivalently, wavelength of light going that way. So Rigel, being much hotter,
has a much flatter spectrum in the visible part of the spectrum that our eyes are sensitive to. Red is on the left here
and blue is on the right. So the flat spectrum means that
Rigel, relatively speaking, has a bit more blue light, whereas Betelgeuse has a distinct slope. It's got distinctly less blue light relative to its red light because it's at a lower temperature, because it's so expanded, a temperature of only about 3,600 degrees. So that's why the color
of a star that we see with our human eyes is
a very direct indicator of its underlying temperature, and its temperature is a
strong indicator of its size, but it's not quite that simple. In fact, the governing
parameter of a star's evolution and luminosity and behavior and all that sort of thing is its mass, and that's going to be the
subject of my next lecture. But for now, when you
see different colors, that's telling you about
different temperatures of a star. So the way that stellar evolution unfolds is primarily governed by its mass. But, in turn, it's governed
by how much nuclear fuel you've got left at the center. When you've used up all
the hydrogen in your core, you will collapse whatever's
left, in this case, the end products of hydrogen
fusion into a helium core. You will then heat up
an outer hydrogen shell, that will fuse, that will
heat up its surroundings, you'll have a red giant. When you've used up all the hydrogen, so fusion is no longer happening, there's another big hiccup, there's another bit of a collapse. And then when all the
helium scrunches together, you can sometimes have a helium flash. And, to some extent, the
exact same process repeated, but with helium being the star of the show where you get a helium and
a helium fusing together to give carbon and nitrogen and oxygen. It's plausible that this could play out in the case of our sun. How long does it take for a
star to become a red giant? Well, it's definitely a few
hundred millions of years and that won't take place, as I say, until another five or so
billion years from now. But like I say, how stars
evolve depends on their mass. When low mass stars,
and by low mass stars, I mean stars with a mass
comparable with our sun, that one solar mass, when they've used up
all their hydrogen fuel, it will expand to give
you a big red giant. All stars like our sun
will do the same thing, the radius will expand and
engulf Mercury and Venus, I should say that the jury is out on whether the sun will expand as far as Earth's orbit around
the sun, that isn't clear. It's widely agreed that Mercury and Venus will certainly be engulfed. It seems to be widely agreed
that Mars will not be engulfed. We're in the danger zone. We are likely to get buffeted and blow-torched by prominences and wind from the expanded red giant
characteristic of our sun. So you are here, for now. Earth's orbit won't
change too much probably. I say probably because the mass of the sun isn't going to change a whole lot, and that's what governs
the orbit of the Earth around the sun according to Kepler's laws, which are built on Isaac Newton's laws. But there will be some subtle
effects caused by the very expanded and distributed
nature of the sun's mass. There'll be tidal effect
that start to cause the Earth to probably spiral in a little bit. There won't be life on
Earth at that point. The smart money is on getting over to Neptune, I would suggest. But let's ignore that
suggestion for the time being. What happens to a red giant? Well, as I've said, when low
mass stars us up all their fuel and expand into a red
giant, when they use up all their fuel, collapse
inevitably follows. You might get a new burst of life when helium is the star
of the fusion show, but ultimately collapse under gravity when all those fusion fuels are spent and used up will inevitably follow. And you'll be left with
something that is a white dwarf, a type of compact object that's very well known in the universe. I've previously illustrated
in a past lecture, or tried to illustrate, the
density of white dwarf material by illustrating that a tablespoon
of white dwarf material weighs the same as a family of elephants. But I want to give you a
new way of thinking about the density of a white dwarf,
the relic of a star, today. So a white dwarf is a compact object having the mass of our sun,
but the volume of the Earth, that's how dense a white dwarf is. A white dwarf undergoes
no fusion, it's compact, it is simply supported by
electron degeneracy pressure. If it were any greater, it
would form a different kind of compact object, either a
neutron star or a black hole, which are the end points
of much more massive stars when they approach the
ends of their lives. But that is what I'll be talking
about in my next lecture. As a white dwarf forms, it
blows off a shell of gas, all that gas that's on
the outmost periphery of the solar surface as
the core collapses inwards. But that shell of gas
can be very beautiful and it's at this moment
as the white dwarf, that compact object,
forms in the very center that something else beautiful is born. And that's something else is
known as a planetary nebula. A planetary nebula is not at all a planet, but it was so named we
believe by William Herschel because it was a bit extended
and it was not star-like, it was planet-like. But, of course, as telescope
technology improved, it's possible to image them,
as shown here in the case of the really beautiful Helix nebula, probably the closest
planetary nebula to Earth. If you image it just in oxygen light, you can see a very strong
central concentration. If you image it in hydrogen
light, this with some of my Global Jet Watch
observatory instrumentation, if you image it in hydrogen light, you see much more of a shell
structure being blown off. And if you image it in nitrogen, it looks different, very
different to oxygen, a little bit more similar to the hydrogen. Hydrogen is rather more
pervasive in the center, and there's a bit less
nitrogen in the middle as well. You can combine these
into a multicolor image, and that's when you see this beautiful structure
that's shown here. I'm showing it with a
different transfer function just to show you the
richness of the beauty of a planetary nebula that
gets formed around the outside. Spat out and thrown off
and gradually expanding, ejected from the collapse
of that central white dwarf, really beautiful shock
structures can be seen. So I'd like to end this
lecture with the thought that the end of our sun is not imminent, but it will be spectacular and it will be beautiful, thank you. (audience applause) - Thank you Prof Blundell for a very beautiful lecture, as you said. Unfortunately we won't have
time for questions this evening, but please join me in
thanking Professor Blundell for another fascinating lecture. (audience applause)
