Of like.
Right. I will pick up right where James ended.
No, we're not tried to connect the types of explosions, he explained to you to what we see.
So-called supernova explosions and the later part I will tell you the role supernovae play in the universe, including for ourselves.
Now, first from observation point. What is a supernova?
Now the name says Nova. That's a new star.
Supernova means a bright new star. So the original definition is it's a bright new star that all of a sudden appeared in the sky.
Now, as you know, from James's talk, that's of course, a complete misnomer supernovae not connected with the birth of stars, but the death of stars.
Now they do come in quite a variety of flavours. And let me just show you some.
The first one. Well, is before and after image.
That right, is the before and this the after image, so all of a sudden,
if you look in the sky, you have this new object that looks like a bright star.
This is a particularly important supernova event. It was the last one you could see with the naked eye from our from the Earth.
It occurred in 1987 and therefore called supernova 1987A.
It's a very important event, and it was the first observable supernova for hundreds of years.
But more importantly, you could actually see the neutrinos, as James already alluded to,
that were generated in the collapse of what was a massive star.
Now, this is the before image, and you see this arrow is actually pointed to a particular star.
So in this case, this was actually the first case where we could identify the star that actually exploded.
It was a big surprise because that stuff looked like a middle aged star.
It didn't look like a star at the end of its evolution, and that was a puzzle for a long time.
But not an event like this in our galaxy, because roughly every hundred years or so.
But the hundred billion galaxies in the universe. So there are about seven supernovae occurring somewhere in the universe.
So most of our information on supernovae really comes from supernovae in other galaxies.
And this is an example. You see this galaxy, this fire type galaxy almost edge on and you see this apparent new star that it's a supernova.
While the previous one was connected with the explosion of a massive star, this was connected with one of these nuclear explosions James talked about.
The other types of explosions, and I want to discuss those.
This is a picture of a much more distant event.
So you see a galaxy filled in this blow up version, so you can you see a galaxy and you see this bright spot?
Again, it's a supernova event, but this supernova event was associated with what's called a gamma ray burst.
Another come back to that later, though, some of the most energetic explosions we know in the universe.
So let me now pick up where James essentially ended, right?
He told you really. The evolution of stars is different to different and different channels.
Basically, low mass stars like the Sun later become rejoined and eject burn hydrogen and helium,
but ultimately eject the envelopes and become a white wolf box and white dwarf.
The more massive stars that are more massive than eight times the mass of the Sun
that continue to burn after carbon oxygen all the way burned to form iron cores.
And these are the ones that explode in a supernova. It's the core that collapses two neutron stars.
This is the image of the Crab Nebula, the supernova that occurred in 10:54,
and it leaves a compact remnant either a neutron star 10 kilometres enrages rages or a black hole.
Now, both of these channels can ultimately produce explosive events.
This in the formation of these objects, in the case of the white dwarfs,
if later on, they will excrete and reach the Chinese e-commerce, how that happens?
I get back to later the two basic mechanisms core collapse and some nuclear explosions.
Just a quick recap of what James already told you. The collapse happens at the end of life of a massive star.
When you have an iron core and you've run out for the nuclear fuel, the core collapses to form a neutron star.
And in that phase, it released a lot of gravitational energy. All of the gravitational binding energy that ends up in the neutron star.
And it's about three times 10 to 46 to. Which is much more than what you observe in the ultimate supernova.
And the reason for that is that most of that energy comes out in the form of nutrients.
A small fraction of these neutrinos somehow posited in this in falling star and
reverse this collapse into an explosion because the supernova is an explosion.
This explosion occurs in oaks and white walls when you ignite carbon under general conditions and his James said, that's a powder keg.
It's a it's like a nuclear nuclear bomb. It burns, disrupts the star and you don't expect any remnant.
In this case, the energy source is actually nuclear energy.
The energy that comes out in both cases is comparable, and that has to do with the fact that the structures of these objects,
the iron core in the civil war, are actually quite similar.
Now, someone asked before about the history.
Now the basic idea that neutron stars neutron stars form at the end of the life of massive stars goes back to Oppenheimer.
You already speculated that in the 30s and in fact,
the idea that this could be connected with the supernova was proposed by Fritz Vicki in the thirties.
So only a few years after the discovery of the neutron, this picture had developed.
It took, of course, many decades before we actually started to understand it.
Now would we have been able to predict it without observation? It was another question.
And the answer is no. People are simulating these core collapse calculations.
And until I would say five years ago on the computer, they didn't explode.
All right. They just collapsed.
So if you had to trust computer simulations until a few years ago, we would not have predicted the existence of these core collapse.
So observations really have been key for actually understanding it before guiding the research, the ideas of the thumb, nuclear explosions.
And I think I agree with James this really required first the understanding of nuclear fusion in stars.
It's the first time I would say would be that this was proposed would be the late 60s, early 70s.
But I can't exactly pinpointed anyway, all of this.
All of these explosions occur on a timescale of a second fraction of a second or a few seconds.
So everything James talked about is over after a few seconds.
Now, at that time, you don't see anything, at least not in the actual electromagnetic radiation,
because this happens inside hidden deep inside an envelope.
Either the outer parts of the white wolf or if you have a massive star, it's inside the envelope of the massive star.
But what the energy you you put in there, the 10 to 44 joules,
what they typically do is they drive a shock and that shock passes through the remaining envelope and does two things.
First, it accelerates all the material and makes it explode.
But it also deposits a lot of energy behind the shock. And so the material behind the shock is is quite hot and that that energy in this
shock heated region is actually providing an important source for the energy,
for the light you see later on. Right in the centre.
You also have radioactive heating because in the nuclear in the explosion,
you have nuclear reactions that produce all the radioactive elements and the most important of which is nickel 56,
nickel 56 produced and subsequent decays.
And that's also important energy source for the later observable supernova.
Now, the shock front passes through the embodiment of your half.
And the first time you see any light from this is when the shock front reaches the surface.
Typically produces X Ray Flash and the time between the action.
Cool explosion and the shock outbreak breakout. Depends on the type of star.
If you have a white dwarf or a compact star, it's a few seconds. But if you have a big star like a red supergiant, it actually takes a whole day.
All right, so if you have a supernova, a red supergiant, our galaxy explode, you will see the neutrinos immediately.
But the first light you will see a day later.
But once the shock has passed and now everything's expanding, the ejector start to cool and radiate, and that's what you see.
As material becomes cooler, it becomes neutral and transparent.
And so you can see deeper and deeper into the ejector up to some point.
A photo sphere like photos, we have the Sun where most of the photons escaped from.
Within this, you still have this hot region that tries to pull in right the centre.
You have a radioactive energy source that copiously produces X-rays and gamma rays that then diffuse outwards.
This first process last takes hours, two days. This takes days to a few, perhaps a few years after after that.
Most of the material is transparent and then you go into a different phase going to the supernova remnant phase, which I'll talk about later.
Now, to understand this physics of this is relatively straightforward, not going to get quite that complicated.
What I'm showing you here are two light codes.
This is for type two supernova the core collapse, and this is for type one, a supernova which is similar to explosion.
What you see here is a function of time. Schematically, here is luminosity.
It's a logarithmic luminosity scale and the indicated indicator of the maximum luminosity of a core collapse supernovas about 10 to 36 watts.
Now that doesn't mean much, but that's in solar luminosity.
That's about 10 billion times the luminosity of the Sun. So it's their voluminous.
And that's why, for a brief moment, these supernovae actually can outshine whole galaxies.
And the right hand side, you will see it on this it more or less the same thing, except that this uses a magnitude scale.
You don't need to know what that is. Important thing. It's a logarithmic luminosity scale.
The power output on algorithmic scale, more negative means more luminous.
The peak luminosities in these two types are comparable the type 1A.
The nuclear explosions tend to be a bit more luminous, but they're comparable.
Now, how do you understand? The difference between the two.
Now, the duration of the supernova phase is supernova phase in both cases is ultimately determined by the diffusion time,
how long it takes all the energy, all the photons inside the ejecta to escape at later times.
You see this straight line is this zero growth in scale?
That means it's an exponential.
You also see it here, the beginning of it, and we'll continue at this long exponential tail exponential energy input that suggests radioactivity.
And that's what it is. This is caused by just by the cooling, by the by the decay of 1956, first to Cawood and then to 1856.
We know this very well. You know the half life of the decay for nickel 56 and also for cobalt 56.
And we know how much energy is released in the decays. And so that's very useful because by just measuring the luminosity in this late phase,
we can immediately measure how much mass of this radioactive material was produced.
It's a very simple calculation, high school calculation you can do.
And so for. And this tells us really how much of the nickel 56 was actually produced.
You know, core collapse supernova on the left,
the typical mass is about point one solar mass or even less in contrast for the thermonuclear explosions.
It's typically point seven solar masses, so it's much more neatly produced there.
And since the Nicolas Otamendi case to Iron 56,
this is why we believe actually why we know that these thermonuclear explosions are the dominant producers of iron in the universe.
Now to understand the length of the light curve.
When I said it's a diffusion process, and again, it's actually a simple calculation can do.
It's a random walk process, but the photons scatter back and forwards and to escape from the photo sphere.
Now this is just probably false in this simple random walk in one dimension.
If you have a random walk in each step, you move by something else and you want to know where after any steps you are,
if the probability going forward and backwards is equal to the average expectation is zero.
You don't go anywhere. But what changes over time is the envelope of this path.
In fact, if you calculate what's the R squared after in steps, that's end times L script.
So the root mean square distance after in steps is this gridlock end times that.
If at any point you reach the photosphere from which the photons then escaped freely to infinity.
Then this process has ended. And so this tells you how many steps you need.
So if you want to go to a Radius R and each step is L, you know you need R overall squared steps.
It's very common resolve, which I'm sure you've all seen before.
So we can then define the diffusion time just as the number of steps times the time of each step.
I just just l sees the speed of light. And so you get our script over and see.
So all we need to know is what is the length of each step,
which is the mean free path for the photons and that rewrite in terms of one of Chaparro where Kappos called the opacity.
But what it is, it's just a cross-section per unit mass and rose the mass density.
This this absorption per unit mass across the community is something which we know just from basic atomic physics.
Of course, all of this occurs not in a static star. It occurs in an object that's readily expanding.
So we need to take that into account.
But we can estimate the velocity of the ejector bus just taking the energy and calculating the average velocity, which is the scourge of tree over.
And if you put all of that in and it's really very simple physics, very simple mathematics, you can estimate what the diffusion time is.
It ultimately depends on the mass of the ejecta. This absorption coefficient and the energy.
If you put in typical values for massive star, the 10 to 44 jewels and a typical mass of 10 solar masses, that gives you about 150 days.
And that's very typical, very comparable to what you see. If, on the other hand, you have a much more much smaller mass.
Just one solar mass, you get some much shorter time to get more like 20 days again, which is what you see here.
So it's very simple, very simple process.
I mean, I've oversimplified a little bit because the absorption properties in the UK is a bit different, but I think we get the idea.
I want to make this point a bit further to show you here is just a bunch of supernova light curves for assuming four core collapse supernovae,
assuming that in all of these you have the same type of explosion.
You have an energy of 10 to 15, 10 to 44 Joule that's deposited deep inside a massive star.
The top two actually are two observational light curves. This times in a second, but.
Never mind, this is a supernova in 1969.
Because you had this long faced with the light curve is almost flat.
Now this in the later years of this exponential you.
This was the star that exploded here was a big red supergiant, probably a thousand times the radius of the sun.
Also shown here is the light curve of the supernova 1987A, and you see it early time.
It's actually quite a bit fainter.
Now the reason for that is that the star that exploded here was not a big supergiant was a compact star, but 40 times the radius of the Sun.
And a lot of this energy in the early days had to actually go into expanding it.
And that essentially made it a much fainter event at early times, at late times when the heating is just due to radioactivity.
It just depends on how much nickel was produced. To make this point further, in the bottom panel,
you see some theoretical ideas which you generated and what we just did here is we took a massive star of turf solar masses,
a big big red supergiant and took off different amounts of mass.
We didn't take any mask off, then this is the light curve you get.
And you see, again, you have this long.
More or less constant portion that's called a plateau, and observationally you would classify this as a type two p where the P stands for plateau.
But it's basically just a red supergiant with a massive envelope.
If you reduce the mass from eight point eight to five to the plateau, becomes shorter,
ultimately disappears, and then the light curve looks much more like a linear decline.
And observation that's classified as a to L L stands for linear.
As you reduce the even more well, you get a very short peek.
And after a few days, tens of days, you don't see any when you see something different.
These are called type to be and I'm not going to explain why. But if you take of all of the hydrogen, then there are different type.
He would call them a one B. And if you take off also healing, you get one see.
But the point I want to make here is just by taking a particular star and taking off different amounts of mass,
you can generate a whole series of different light curves and therefore different observations supernova types.
But it's all for the same type of explosion in the centre. In fact, if you have a massive sign, take of all the hydrogen.
It will look very similar to this one, which I said is a separate explosion.
So how can you distinguish that now for that,
you have to do some more forensic analysis and you have to look at what is the what are the ejecta actually made of?
And that leads us to the topic of supernova classification,
what you do is you look at spectra and long before the different types of supernovae were understood.
Observers noticed that some supernovae have hydrogen in the spectrum and others do not.
And that's where the basic classification comes from type 1A supernovae,
those with which don't show hydrogen the spectrum type to show hydrogen in the spectrum.
Now, for a long time, people thought that these two observation classes could actually be connected.
The two explosion mechanisms James talked about the thermal explosions being in white water
because there's no hydrogen perhaps being type one and the core collapse being type two.
We now know that this is no longer. It's not that simple.
You can have probably both types of explosions in both types of envelopes, but you can look further in it.
And what I'm showing you here different spectra of supernovae near maximum light.
So there's just a flux is a function of wavelength.
And if you look at the top one, which has various atomic lines identified, there's a strong line of silicon.
Now, silicon is one of the elements that's copiously produced in a thermonuclear explosion, so it's a telltale sign of some nuclear explosion.
And therefore, this tells us that this was not a core collapse, but a similar explosion.
The next one has this line labelled air elephants a hydrogen line.
It has a funny shape, has an absorption component and emission component, but that's line shape you get when you have spherical gas expanding.
It gives us this profile. So it's a very obvious thing.
So when you see that you immediately know this was the explosion of the hydrogen rich star was the core collapse.
Of course, we then take off the hydrogen. You don't see that, but we also don't see the silicon.
Then you get two objects that somehow lost the hydrogen and the weather.
If they have some helium, they're called one BS. They also don't join helium.
They're called onesies and. Now, why do stars lose their envelopes, and this has a lot to do with.
To binding to actions, most stars in the sky are not single objects, single isolated objects, most massive stars actually have a companion star.
And what that means you have two massive stars orbiting each other, basically replacing the Earth by another star.
Now, if the stars are very far away, that doesn't matter. They will just evolve in like isolating stars.
However. Interestingly, four massive stars.
Up to about 70 percent of all massive stars are so close that there will be a direct interaction between the two stars.
And what that means is that one of the star one of the star grows because it evolves and wants to become bigger.
It can reach a point where the gravitational pull from the other star can pull off material from it.
And in that situation, you then get into a phase where mass is transferred from one star to the other,
so one star loses and one star, it creates metal. Now that will change the structure of the star, but in particular of the envelope.
And as I just showed you earlier, it's the envelope properties that largely determine what a supernova looks like.
Want to go a bit further? What typically happens if you have this in this mess into occurs in a stable way?
The star that loses massive its hydrogen initially it.
This process will only stop when it has actually transfers most of the hydrogen.
So that way you are then just left with the core of the star, which, if it's an evolved star, would just be a helium core.
So then you have helium star. And when it goes further and explodes, it produces one of these core collapse supernovae that have helium.
If you were to lose the helium, then that might happen by mass transfer, you get another type one C.
Now, it's not only the mass losing star, you change is also big treating star.
If the accreting sides are relatively unevolved style early in its evolution, then it will just behave like a more massive star.
We say this to other be rejuvenated. However, if the star is already somewhat evolved and has already finished its hydrogen burning phase,
the further evolution is quite different and it will not become a red supergiant.
The red zone is the normal expected endpoint of a massive star, but this one will just become a blue supergiant like was observed for supernova 1987A.
So this is not the whole story,
because this massive interface may not be stable if the mess to have faced the mess up to rate is so high that the companion star cannot agree it.
Then what happens is that the expanding stack essentially engulfs the companion star.
And you get into a situation where the one star encodes the companion star.
And if the course of both stars orbiting in in a joint envelope.
Different things can happen, but one of the most dramatic outcomes of this is that the two stars merge completely.
So in the end point then, is that you don't have two stars anymore, but the single one.
Now, this might sound like an extreme thing to you, but it actually isn't.
I would estimate that between 10 and 20 percent of all massive stars do this.
So when you look at a star in the sky, you can't be sure single star in the sky.
You can't be sure that it always was single.
Now, there's a lot to be said about this, but I want to return to supernova and in some way because I believe that this is an example of such a mood.
I already told you that the neutrinos were detected from this event.
This was the event in the satellite galaxy.
18 neutrinos, but it doesn't sound like much, but in actual fact, it was enough to estimate the mass in the explosion.
The total energy estimate in this neutrino flux was about four times 10 to the 46 tool, which is right exactly.
The rest mass energy of a neutron star at the Smithsonian is the binding edge of a neutron star,
so it really confirmed that neutron stuff was formed in this explosion.
Now, that was a big success, but that was the only early success.
I already told you before that the star that exploded was not supposed to, but a few years later some of the anomalies.
But few years later, the Hubble Space Telescope produced this picture.
Probably most of you have seen this. What is it? Well, it shows the nebula around the event.
It shows these three overlapping rings. These rings have nothing to do with the supernova.
The supernova is just what you see at the centre. This little blob there, that's actually the supernova.
These are the supernova ejecta expanding into more expanded.
This is all material that was ejected about 20000 years before the explosion.
You cannot see it because the supernova, the shock breakout, it's ionised meteor.
And now you see the fluorescent meteor in these rings.
Why does the state do that? Actually, before I tell you, can you see the geometry of these rings in three dimensions?
And have been some debates of it, but I'll tell you what it is actually really three rings,
three rings in space the centre ring, the supernova is at the centre of the centre in.
The other rinks are displaced from that centre ring plane above and below.
And when you're observing it from this direction. This is what you see.
Very. OK, now the picture,
which we have developed over the years and quite confident of now is that what happened 20000 years ago was actually the merger of two stars.
And we're not going to in details, but the two stars merged that releases a lot of energy,
which ejects part of the envelope of the star, and the star then wants to become a blue supergiant.
And there's a strong wind that sweeps up any of the structures that ejected in the merger event.
No, this is just the cartoon. These are some calculations, and this is the final product which we predict.
And so this shows the structures you see at the time of the explosion around this time the centre.
This, by the way, is about a light year in size.
And you see the centre ring and you see these bipolar structures, but they're at the end, you have these ring like features.
And when you calculate what that would look like in age alpha, this is what you see.
And, well, we agree this looks very similar to the exhibition.
Of course, we fine tuned it, but. Interesting point.
I mean, this is an old picture. The supernova has now expanded for for more than 30 years.
In fact, the supernova ejecta have now reached the inner ring,
and the inner ring is we now see how the inner ring is being destroyed by the supernova.
And it actually has become bright again and has some interesting structures.
But but ultimately, all of this will be destroyed by the supernova itself.
So this sort of gives you an idea about the importance of binaries for core collapse supernovae, but of course,
for the some nuclear explosions, the even more important this is something James already briefly alluded to.
Now you need to ask why does a lonely CosmoQuest and white dwarf grow to reach the Chinese e-commerce?
For that, it needs at least at least one companion, and the two basic ideas are that you either have a star binary system again,
whether you have a white dwarf accreting from the companion star and once it reaches agenda, Zika comes, it explodes or gets close to it.
It explodes. No, wait, Wolf, systems in binaries are very common, except that white dwarfs don't like to treat.
And so it's not quite clear how that actually works, but it seems like you.
Plausible think the alternative already mentioned by James as well is that rather than having this process,
you have actually two white dwarfs, and if they're close enough, gravitational radiation will bring them closer till they collide.
And this can then, just like a hammer, detonate, detonate and produce its own accord, rather than many variations of this.
These two basic pictures we just organised the conference discussing just that for a whole week and had a rather controversial discussions.
And I can tell you we are not any wiser afterwards. But I mean, these are just want to tell you these are sort of the basic idea.
I mentioned gamma ray bursts. Cameras are really big puzzle for many decades now.
What are they? Well, there are flashes of gamma rays that come from somewhere in the sky.
This is just a random sample of of these events.
You can't read the scale, but I tell you the typical scales varies from milliseconds to thousands of seconds.
So really like a flash?
Now we actually discovered by a US spy satellite that was checking whether the Russians were doing illegal nuclear explosion in the atmosphere.
But rather than finding gamma rays from the Russians, they found these funny gamma rays from all over the sky.
Now they realised they didn't have to keep it secret in 1973. And then this has remained one of the big puzzles for for several decades, actually.
Certainly in the 70s, there were more theories than there were actually gamma ray bursts.
Many more, but the general idea, at least in the early days, was this has probably something to do with neutron stars in our galaxy.
Like having a comet or asteroid falling onto a neutron star and that should produce a flash of cameras.
Now we know now that that's not the case when the 90s was a satellite that that called Betsy that discovered almost 3000 gamma ray bursts.
And what you see here is the distribution in the plane of the sky, and you see it's pretty random.
This coordinate system, the galaxy is along the equator, the galactic centre is right in the centre.
So what you would expect if they are in our galaxy is that you see a strong concentration towards the galaxy.
This didn't solve the problem because they could just be in the outer parts and the galactic halo.
But then in the late 90s.
And was able not just to see the cameras that you can't localise very well, you could also see optical counterparts associated with this.
And once you have an optical counterpart, you can also look at the spectrum, and those spectra showed red shifted absorption lines.
And what that meant is that these gamma ray bursts must have occurred behind some galaxy.
You have a spectrum, you have some galaxy than the light. The the some of the light will be absorbed by the galaxy.
And you can see an absorption line. But because the galaxy is distant, it is red shifted.
And you will see a regifted line so that we solve that puzzle, at least the location.
It means autrement that these explosions were some of the most energetic explosions we know.
You sometimes call them now hypernova.
And because to indicate that a more energetic than normal supernovae, the typical energy of these explosions actually 10 to 45 joules.
So at least a factor of 10 more than this metric tons of 44. So that immediately tells you what's going on here must be something different now.
We don't fully understand it, but I'm giving you here one picture, though to illustrate it.
But we now think is happening is that you have an explosion of a massive star. But but that the core is actually rapidly rotating.
And if you have rapid rotation, then you can just collapse to the centre because the aluminium is conserved.
And so at some point you get in the collapse,
you get centrifugal support and then rather than collapse into the centre, you form some disk like structure.
And then you create from this desk to the centre and somehow in that process and the here for good reasons, somehow you produce a relativistic jet.
And that's probably done by some may need to hide and process.
But these voters, it's important because if you observe the event along these set axes, you get a gamma ray burst.
But the energy that's released in this process is of the order of the rotation energy of this inner part,
and that is of the order of 10 to 45 Joule and that is probably what sets the energy scale.
I'm very vague because if more debates on this than about the to explosions,
but these events are relatively rare only happens to about 1000 core collapse supernovae.
But the camera bills themselves because the cameras actually beamed into very narrow beams can be seen to the very distant universe,
become one of the most distant objects seen so far was a gamma ray burst that was seen in
2011 that occurred at redshift of 9.4 when the universe was only 400 million years old.
In fact, in principle, one can see gamma ray bursts and even larger distances.
Which? Actually, that's not the whole story.
What I would just describe more applies to what's called long duration cannabis.
We know that two types of cannabis that last tens or hundreds of seconds.
Another type that only occurs takes place over a second or less.
These, we think, are caused by having the merger of two neutron stars now.
We actually have fairly good evidence for this now, because as I'm sure you all have heard two years ago,
Legault discovered gravitational waves from the mergers of two neutron stars,
and that merger obsessed merger was associated with one of these short duration benefits.
So we have some confidence that this is probably the right picture, but I will come back to that briefly in a few minutes.
OK, so that gives us some idea of the different types of supernovae, how we analyse them once the supernovae.
Become transparent. The gas expands and you get some beautiful remnants.
But ultimately, all of that material will mix with the interstellar medium.
And that has several consequences.
First, it provides an energy input to the interstellar medium and that can affect star formation can stop far off formation.
Sometimes it may even initiate star formation.
But more importantly, for the last thing, I want to talk about, it mixes chemical elements with the interstellar medium,
all the elements that were produced before in stars and supernovae.
And this is a recurrent cycle that just sort of illustrated here.
So you see if gas, the gas you form, stars, stars produce heavy elements or and supernovae, and then you mix that again with the interstellar medium.
And this process has continued, and it's taken many place many times over the age of the universe.
So supernovae are not only a means of producing heavy elements,
but they are also the main agent of enriching the interstellar medium in producing the next generation of stars.
Now, the core collapse supernovae in the nuclear explosions have slight differences.
In the case of the core collapse supernovae you eject, in particular, the material was produced in the star before lots of oxygen, lots of magnesium.
Even though we have an iron core, most of the iron cores actually collapse into the neutron star, and it's locked up so they don't freeze much on.
These produce. Or you said the main producer of iron, and since the timescale for these two different processes are different here,
it's determined by the lifetime of a massive star, which is order 10 to seven years.
Yes, depends on the evolution of evolution, which is more 10 to nine years.
And so you can see that you get in by looking at the ratio of the elements that are produced here to here, particularly the Oxendine ratio.
You can actually deduce something about the different timescales where star formation may have occurred.
Now we have a broad understanding of of how most of this works.
But one of the big puzzles for understanding the elements have been the nutrient-rich elements, things like gold and platinum.
And this is the last point I want to make. There has been really a major breakthrough in this in 2017.
This is detection of the merger of two neutron stars.
All right. How you make gold has been the income stream now since those days.
We know quite a bit more. We know how we formed these stars. It's fusion helium, carbon oxygen, ultimately iron, but fusion doesn't go beyond that.
All right. I mean, it's the most stable nucleus.
So by nuclear fusion, you can't build up elements more heavier than iron unless you have some energy source.
But if you look at the abundance pattern, the solar abundance pattern of elements and when it goes to quite high atomic number,
when some of the light elements like helium and some other elements you can produce in the Big Bang.
But most of this here you produce, either in stars or in supernovae.
But where do you how do you produce these very nutrient rich elements, particular things like platinum here or gold?
This has been a long puzzle. What you need for that is a very high nutrient density.
And the density has to be high enough that you start to build up heavy elements.
You've bombarded iron nucleus with a neutron, but before it decays, you want to add another neutron.
That's the only way of how you can build up a neutron rich elements.
For this, you need very high nutrient densities and that the conditions have to be right for a long time, like about 100 seconds.
Now people have looked for this for many decades and the candidates there for many years were supernova explosions,
but they couldn't really identify the conditions that were right. Now, 20 years ago, the idea that this might be merging neutron stars became popular.
And this, I think, is actually something that has been confirmed by this detection of neutrons of.
Of legal. And what a show you want to show you here is actually a simulation of the merger of two neutron stars.
There's a simulation by Stefan Rosberg. See two neutron stars colliding, dissolving and forming a single object.
But the thing I want you to particularly notice is actually the fluffy stuff around it, right?
This is actually material of these two neutron stars that's flung out dynamically by the tilings, or the aluminium has to go somewhere.
And it is ejected in these tidal arms.
Now this is between, let's say, neutron rich. But as a decompress, it now starts to decay, produces protons and all the elements start to fuse again.
Right. Just like James was saying in the last phase of the collapse, you undo or the nuclear reactions and this phase, you do the opposite.
You build it up again. So within a few seconds, you build up all the elements again.
But all of this is in the nutrient rich environment.
And that's why this is the ideal environment to form elements like this neutron rich elements like like gold.
In fact, on the day when this was announced by labour in China and some Chinese told me that on that day, the price of gold rose by 10 percent.
Mm hmm. Right. So this was the event.
The point was the event was could actually be I mean, you were seeing traditional waves,
but they were able to localise it well enough that then anyone who had a telescope in their backyard
used a telescope to look at this part of the sky to see whether they found an optical counterpart.
And about 10 days, 10 hours after the explosion, after the collapse of the military, so it was discovered by this telescope.
You see this top there outside this galaxy and this is a transition because 20 days before it wasn't there.
So this turns into appeared and then everyone looked at it.
It's produced a paper that's a short paper, but it has about 3600 authors and said everyone who had a telescope pointed and.
I think you do get to come to an end. You could observe the spectrum of this event and how it evolves.
And I'm not going to explain this, but if you analyse it, you can estimate how much mass was ejected.
It was about two or three 2.5 solar masses and that this material must have been extremely neutron rich.
So if you combine that with the fact that you roughly know the rate of these events.
These neutron star mergers are actually more than enough to produce all the gold and or the platinum in the universe.
So I think that was a major breakthrough.
But to end, I actually want to leave you with another thought since I told you all the heavy elements produced even in stars and supernovae.
And so all the material you are made of is really stardust.
So we have been in this tiny supernova at some point.
OK, thank you.