Thanks. Can everybody hear me?
Well, welcome everybody and thanks so much for showing up on a Saturday morning.
Today we'll go together through a tour of our universe and explore together the most violent explosions of massive stars.
And the two main things I would like you to get away from this talk is that these explosions are very vital for our lives.
Without them, we wouldn't be here.
But also they have a flip side. They could be very dangerous for our lives.
But as I'll show you, the most recent scientific investigations have shown that they might actually not be very dangerous near our Earth.
So you can relax and go home happily.
So nowadays if you want to know what something is, for example, you just Google it. So what do you find when you Google supernova?
Well, these are the things I found. They're trucks. They're called supernovae.
There's actually a band called supernovae and there was also a music show and they had a big fight about which band or movie show should be called supernovae.
But what are these things really? Well, what is this? Just before we talk about supernovae.
Yes, that's our sun. And our sun is a very normal star. By definition, its mass is one solar mass.
Now our Milky Way, when you look up on mostly summer night, you see a band of stars with a lot of bit of black patches.
So that's dust. And that's our own Milky Way that consists of billions of stars, so 10 to the 9 stars in our own Milky Way.
Now, if you were able to get off from your seat and fly with me above the Earth, above our solar system and keep going,
you will see our own Milky Way from above and it would look like something like this. It would look like a pancake.
You have different things. The astronomers are very inventive. They have different names for different things.
This is the galactic bulge. We have different arms, spiral arms. And the sun is in the outskirts of our own Milky Way.
Of course, that's really hard to do to fly from our seats, but we can look at other Milky Way like galaxies and see what their structures are.
And if we do that in a galaxy far, far away, we see a supernova.
This is a Hubble Space Telescope image of a supernova that occurred in this galaxy 95 million light-years away.
And this is one star that at the end of its life is as luminous as 10 to the 9 stars combined.
So this is quite amazing that one star itself can outshine a whole galaxy and therefore can become a beacon for us,
for our astronomers to see across the universe.
So how do we know that it was a star that exploded?
Well, for some cases in our nearby neighborhood, in this case in the Large Magena Cloud,
we saw the star before it exploded.
So this is an image of the field there and you see this one star.
It doesn't look very different from the rest of it in this very busy field.
But then later on, a week later, bang, the supernova went off and you see something that wasn't there before much, much brighter.
This is a nearby case and this supernova was detected in 1987, 1987,
and therefore the name of the supernova is SN97A because it was the first one of that year.
So these kinds of investigations where you have the image of the star before it exploded
are really powerful to really figure out what star exploded and give rise to this magnificent explosion, a real large firework.
Now, like men and humans, stars live and then they die.
So this is just a cartoon of the life cycle of a massive star.
When I say massive star, I mean 10 times the mass of our own sun.
So stars are formed in what we call nebula going through various stages, protostar phase.
Then they spend most of their lifetime in what we call the main sequence where they happily fuse hydrogen into helium
and that's how they produce the energy we see as light, for example, for our own sun.
So they spend a lot of time on the main sequence, our own star, our own sun spends nine billion years on the main sequence
and then massive stars, 10 times the mass of our sun, become larger, they become giants, super giants
and when they have exhausted the fuel, so various amounts of elements that have been fusing into heavy elements to produce light, they explode.
So most of the stars that we know of that our age or 10 times the mass of the sun explode as what we call type 2 supernovae.
These massive stars live much faster and die much younger than our own sun,
only in astronomical terms 10 billion years, 10 million years for a 20 solar mass star.
So that's actually quite quick compared to the rest of the time scales in astronomy.
So just to give a basic overview, I know people have various backgrounds here.
I just want to make sure we understand when I talk about visible light and other kinds of radiation from other parts of the electromagnetic spectrum.
So light is only a very specific part of the full electromagnetic spectrum.
So this is the visible spectrum where we have the rainbow in terms of the different wavelengths of the light.
But the electromagnetic spectrum is much broader.
The range I will also concentrate today is gamma rays, so those are the highest energies.
Those are photons that have the highest energies in the whole range.
And of course the other ranges, everybody has heard of the radio wavelengths, then there's microwave infrared.
We can't really see the infrared emission, but we feel it more in terms of heat.
There's the alpha violet and then x-rays, and these are the lowest energy photons, packages, so to speak, and these are the highest energy ones.
So I will concentrate today on the visible range, so what our eyes can see,
but also talk about gamma rays that are emitted from these really violent explosions, these gamma ray bursts.
So why are supernova important? Why do we study them? Why do we care about them?
Well, supernova are vital for our own life.
They produce a lot of the elements we consist of.
So this is an image of supernova that exploded in our own Milky Way that was observed in 1604 by Kepler himself, actually.
And the image is color coded by the emission from various kinds of elements.
So this is calcium, important for our bones.
The green stuff is iron that's emitting, so that's really important in our blood.
And then oxygen, of course, really vital for our own breathing life, that's in blue.
And that has led a number of people, including Carl Sagan, to say we are made of stardust.
We're made of elements that were expelled and partly also produced during the explosion of a massive star.
So I think that's a really beautiful image that actually drew me initially to this field of supernova where a very cataclysmic event,
a very morbid event, a death of a star, at the same time gives rise to something vital,
which is all these various elements that make up our own body.
Now supernova are also vital because they trigger the formation of other stars.
So this is an image, again, from the Hubble Space Telescope, and the Hubble Space Telescope is really important,
important telescope for astronomers because it's above the atmosphere.
And so it has very, very good resolution.
It has very good spatial capability to really resolve things on very small spatial case,
where we can really closely look at these amazing astronomical events and objects.
So this is an image from the Hubble Space Telescope showing the Orion Nebula where a lot of stars are forming
and their emission is heating up the nebula around them that is emitting at various wavelengths.
And supernova are really cool.
They leave behind very extreme objects.
I'm sure you've heard about black holes, neutron stars, pulsars and so forth.
So this is an image taken both with Hubble and Chandra and the third telescope that's in outer space called the Spitzer Neonferret Telescope.
And it's a composite of this supernova that exploded in our own Milky Way around 500 years ago.
And sorry, this one is 900 years ago.
And when you zoom into this spot here in the center, you see the actual object that was left behind.
So this is Chandra, an X-ray image of the central object, which is a pulsar.
So it's a star, a neutron star that's very, very dense.
So the mass of it is around one solar mass, so the mass of the sun.
But it's concentrated on a very small volume, the size of New York.
So imagine putting the whole sun into a very small space, the size of New York, and that's what you get.
And it has really strong emission at the poles.
And that's what Chandra saw in the X-rays and also Hubble and the visible.
And if you take a number of images as a function of time, you see that it's changing.
So now I'm going to try to show a movie, and sometimes it works and sometimes it doesn't.
Let's see if it...
Yes, so you see this image is taking over a couple of two weeks, over the term of two weeks.
And you see that a lot of things are happening.
There's this jet coming out.
It's emitting a number of high-energy electrons that are then emitting in the X-rays.
You have a really complex structure.
Again, astronomers like to give names to everything in detail.
So this is the Torah surrounded.
These are wisps and so forth.
And people can really study in detail using those images how exactly the pulsar is moving,
how rapidly it's spinning, and what kind of environment it has around it.
So these amazing explosions leave behind these really extreme objects, black holes that we don't see,
but then other kinds of neutron stars and pulsars we can see.
And supernova tell us about the universe.
So not the kind of supernova that I'm talking about today, which are due to massive stars, but other kinds.
In any case, those are explosions of one specific star.
And as I said, because they're so luminous, compared to 10 to the 9, so billion suns,
they can be seen over large distances throughout the universe.
And they tell us about the universe, how the universe expanded over the last 13 billion years.
And that led to an amazing discovery in 98, which was that of the accelerating universe.
So we always thought the universe was expanding, and perhaps the expansion slowing down
because of the force of gravity, but the surprising result, and here you see this cartoon of Einstein
who's really surprised to see that our universe is actually accelerating.
So this discovery that was made by a number of people, including Alex Flupenko,
who you might have heard earlier this year, a number of other people, his graduate students,
and another group up the hill at LBL, that discovery was voted the best,
the science story of the year of 98 in science.
And they even have really great tools, too, because they're so luminous and can be seen across large distances.
So how does the star look like before it explodes?
Well, we think that during the lifetime of the star, it undergoes various degrees of nuclear burning,
where it fuses hydrogen first into helium, and then helium, and it's done with hydrogen,
it spreads helium into oxygen, and therefore goes down the chain of your periodic table.
So this is just the center of your star, so let's say 25 solar masses.
This little symbol here is the solar symbol, and the center of the star looks like an onion.
It has various layers made of different amounts of elements, and in the center it's made of iron.
So now when this massive star reaches its iron core, it no longer can burn iron into something heavier
and still gain energy.
So what happens then is it reaches really the fatal part of its life,
it no longer can extract energy, and the core itself implodes, it starts to contract.
However, when it reaches a very certain density, nuclear densities in the center, it becomes really stiff.
And the outer parts that don't know about the fact that it's become really stiff, they crash on it,
and this crash leads to a shockwave, a shockwave that then travels again outwards through the star
and explodes the star itself.
So it might sound a little bit counterintuitive in the fact that you first have an implosion
somehow that leads to an explosion.
But we think that's what's happening, we have a number of pieces of evidence that support that,
and I had intentions to bring out a very sophisticated model that explains it,
but because as you know, do you see budget cuts and so forth, I couldn't quite do that,
so I used a low budget option.
So I'm going to show you something that's supposed to just again visualize the basic concept of the explosion itself.
Of course, since I'm a scientist, I have to wear these great glasses.
Without them, I'm still missing my lab coat, but then that would be ideal.
So the main point is what I'm going to do is I'm going to have this little gadget hit the floor,
and that's supposed to show you what happens when a certain amount of mass is reaching a certain point where it becomes stiff,
and this little piece is going to be the outer layers of the star.
And we'll see what happens to this red little piece.
Unfortunately, it's quite small, not everybody might see it, but that's the main point.
So let's see.
Okay, that's a good point. I should be in the center here.
Okay, let's see.
I have another one so that people can see, and this time I'm going to try to throw it.
So if anybody is able to catch it, please hold on to it and return it later on.
Again, budget cuts.
So let me find it. Okay, last one.
So what is happening?
Well, what's happening is that these other balls that are much heavier,
much have much more mass than this little red one,
are imparting momentum to this little ball that is much less massive,
and that leads to the fact that this red ball is really shooting off and going into another direction.
It's very similar, basically, or simply speaking, to the outer layers of the star
that is first imploding and then exploding.
So last one, hopefully this time I'm going to be able to do this very straight.
Okay, well this was an asymmetric explosion, then it went this way.
So this is the basic idea how an implosion can lead to the explosion of a star.
A lot of people in astronomy, specific theorists,
have been trying for the last 30 years to simulate this event in their computers.
Unfortunately, and it's actually quite embarrassing,
they haven't been able to do that in detail because the physics is really detailed and very messy.
But that's one of the frontiers in supernova physics and supernova astronomy
to really figure out the details of how it exactly happens.
However, nature, of course, knows how to do it and we see those kinds of explosions many times in various kinds of galaxies.
To give you a sense of it, the occurrence rate of supernova is one supernova per hundred years,
per milky-weight type galaxy.
So those things are quite common, relatively speaking,
hundred years is a very short timescale for astronomy.
And so in our milky-weight, you would expect that one went off every hundred years.
And I will talk about galactic supernova in a second.
Okay, so we astronomers, we have telescopes across the globe.
I'll just give you a little bit of a feel for it.
So our Earth and here we have the US and specifically California has a number of great telescopes.
There are some here in Hawaii, so we have to go to Hawaii, unfortunately, for our work.
Then we have here Chile, there are a number of telescopes in Chile.
And we would like to have telescopes at places that are very high up so you don't have the atmosphere,
much of the atmosphere between you, the telescope, and your celestial object
because the atmosphere leads to twinkling of the celestial object.
The light is smudged out because it really bounces around in the atmosphere,
so ideally we would like to go in space.
But if we can't, we still try to build telescopes on high mountains.
Also we want to be somewhere that's dry because a lot of the scattering happens due to,
an absorption happens due to water in the atmosphere,
so we want to go to places that are very dry, such as Chile, for example,
that has the Atacama Desert, that's one of the driest deserts in the world,
or in the Andes that's high up and a number of telescopes are there.
Then you have telescopes here in Australia, you have some on the Canary Islands off the coast of Africa,
and again the European astronomers that operate those telescopes always complain
why they have to go to these Canary Islands that have also nice beaches.
So this is a really worldwide effort and specifically studying supernova is a very international effort
because supernova they become luminous but then they fade within a couple of days or weeks
and therefore you have to observe right away with whatever telescopes you have
to really track their light output as a function of time very closely.
So who are the supernova hunters, their number of them?
One very prolific supernova search is done locally here in California
by Professor Alex Filipenko and Dr. Wei Dong Li, and they operate,
it's called the LIC Observatory Supernova Search, near here in San Jose in the mountains on Mount Hamilton,
and they've been incredibly prolific, they find up to 150 supernova per year.
And actually as an undergrad myself when I was here at UC Berkeley I helped his group find supernovae.
So as a freshman I would look through various images and try to find these new objects,
these new exploding stars, and as Alex sometimes would it really build your character
to look through all these details.
But when Wei Dong Li came he automated it and I will tell you in a second how that happened.
And how we do nowadays how we conduct the search for supernovae.
Of course actually a number of amateurs are also really active in contributing to the supernova finding game.
One of the most prolific one is Tim Puckett, shown here in front of his telescope,
and actually 30% of all nearby supernovae are found by amateurs.
And they just have the money and the dedication to get a telescope to buy a camera,
not very unsimilar to your digital camera actually,
and equip those and look at galaxies every night to find these new exploding stars.
And the youngest supernova hunter actually is a 14 year old girl who was featured most recently
and was then invited to go visit the White House when President Obama had a night of astronomy
where they had telescopes out on the lawn and various people,
middle school children were looking through it and she was featured as the youngest supernova hunter in the world.
Then we have the supernova followers, so to speak.
Those people, those astronomers mostly who don't find supernova,
but then follow them up to understand them in detail and the stars before they exploded
and how much energy they're releasing, what kind of elements they're releasing and so forth.
So this is a picture of a telescope in Arizona which I used to go to as a graduate student.
The diameter of this telescope is 6.5 meters,
so the astronomers usually indicate the diameter of the telescope you're using
and the bigger, the better, because it has a larger light gathering power.
And of course when we do start observing we turn off the lights in the dome.
So this is just for press release and various purposes.
And then if you're unlucky like me sometimes it would snow and then the dome would freeze up
and I couldn't observe so you would just get an image of the closed dome.
Of course it's during the day so it makes sense.
Other telescopes as I mentioned we go to and I go sometimes is the Keck Telescope in Hawaii.
So this is an image of the two twin telescopes on the top at 4,000 meters of Hawaii
and it can be cold even in the summer and there can be snow on the ground
while other people at the beach down are enjoying themselves.
So those are some of our telescopes we observe at.
Then you have others in Chile as I mentioned in the Andes.
So these are the Magellan telescopes and that's me a couple of years ago
I was observing at the summit there.
So these are the tools and the toys so to speak we have to observe these exploding stars.
So this is a gallery of various supernova and their host galaxies.
So the galaxies that surround them such are all Milky Way.
So this is one supernova 99em in this galaxy with this very amazing name NGC 1637.
And this is the spiral galaxy.
This supernova actually was found by Kate and this is the supernova here.
Again keep in mind one star that became luminous and really outshone one billion other neighboring stars.
Now this is another image and who can guess which one is the supernova here?
Yes, it's this one where this one is obvious.
Now however on this one where is the supernova here?
Can you guess?
Well these ones are a bit trickier.
In this case it's this one here.
This is the object that wasn't there before an exploding star.
So how do we find these things?
As I said sometimes it's obvious to see in a way but other times it's a bit trickier
because you have a lot of such structure in your galaxy, in the galaxy you're looking at.
So what we do is we subtract images.
Images that have been taking over a large time range or range of different nights.
So you have at one epoch you have one galaxy and then you come back two, three weeks later
and you observe the same fields and you look at it again.
And if you just look at it by eye it doesn't look very different.
However if you do a subtraction where you subtract one from epoch two
then you see something that wasn't there before.
Everything else that stayed constant in terms of its light output is gone
because you just subtracted it and it's easily gone
but that thing that exploded and is new is still there.
And we can do that now for the last 20 years, 30 years now
since we have these digital cameras that we can use and that have really revolutionized astronomy.
Before that people were observing using plates and then you can't just easily subtract.
However with digital images like the cameras you have with which you take pictures
those are really powerful because you can manipulate those images in detail.
So being a supernova astronomer is really exciting.
You never know what happens.
You can usually not tell in advance if you look at the star that it's going to go supernova
so you have to be really alert.
And that happened in 2008 where a very interesting object was captured
which we now call 2008D, supernova 2008D
and what happened there was it's actually a really really great story
because it shows you how important serendipity is in astronomy discoveries.
So in 2008 two astronomers from Princeton were looking at images
from the NASA satellite called SWIFT that observes in the X-rays
and they were actually looking at a supernova 2007 UI that had gone off in that one galaxy
and they were following it up for their own studies.
Now what happened was on January 9th when they looked at the same frame again
they saw a new much brighter object
this new object that hadn't been there before
and first we weren't sure what it was
the optical images or UV images of the same galaxy
so you see now this host galaxy that is inclined
it showed a little bit of new stuff but we were really excited
and unsure what it exactly was
and I will tell you in a second what it could be
but we had different amounts of gases
we thought it could be some kind of gamma reverse
and I will talk about those in a second
but in order to really tell what kind of object it is
you need spectra
so spectra are tools to split up the light as a function of wavelengths
to see how much flux there is as a function of wavelengths
and that's like a fingerprint of a supernova
because it shows you what kind of elements there are
that give rise to emission or absorption of that light
so these astronomers send out a circular across the globe
saying look we found this really interesting object
we're not sure what it is
it's in this nearby galaxy only 90 million light years away
it would have been if it had been a gamma reverse
really the closest gamma reverse to observed ever
so a lot of astronomers across the world trained their telescopes
onto this object including ourselves
however we also had observed the same galaxy with the supernova before
and we were lucky because we were having these images
taken with a telescope in Arizona in the new infrared called Peritel
we had images of the same galaxy with the supernova
just three hours before the x-ray detection
and then we observed the next night and we saw
well this is a new object this was 2000 AD
and we continued observing the same galaxy
and we could make fancy plots so this is a very technical slide
I apologize for that
but the main point was we could observe this object very early on
we got data in a number of filters
we got spectra so these are the things I mentioned
where we saw that it had certain elements
that gave rise to absorption in the spectrum
specifically helium
and we could study this supernova in detail
and find out that it didn't have hydrogen in its spectrum
therefore the outermost layer of hydrogen was no longer there
but you could see the next inner one of helium
and we took data with Keck four months later
that certain lines had a very specific shape
and from that we could deduce the geometry of the explosion
so we found that it actually wasn't a round explosion
as you might perhaps guess
but it was a spherical
the shape of it was more like a torus like a donut
than a sphere
and that was really exciting
that led to the press release
and a number of people in the audience actually here helped
people women and others contributed their efforts
to this really great story
and when I had a discussion or interview with the NPR
they were really amazed to think of a torus
as a shape for this supernova
and the person who interviewed said
wow it looks like a donut
I guess that's the shape of the donut
so he was quite impressed about that
so now you might wonder
well these are explosions that happen in other galaxies
as I mentioned the one I showed you in 2008
happened in a galaxy that is 90 million light years away
which means that the light took 90 million years to reach us
so the actual event
let's go back to that
happened 90 million years ago
when dinosaurs were roaming the earth
so that's actually quite amazing to think of that
but because of the vast distances in the universe
and the fact that light travels at a constant speed
we have this time delay
where we know about events in the universe
much much later
so for astronomy this was actually quite close
only 90 million light years away
okay so you might wonder
which explosions happened in our own Milky Way
and do we have historic records of those
well we do
therefore we know of
in terms of the recent history
so the explosion supernova 2006
that was detected in 2006
by a number of astronomers across the globe
Chinese, Arab and European astronomers
an explosion that was visible with the naked eye actually
and it was visible with the naked eye for three years
and we have historical records of those
then there's another one SN54
then more recent ones Taiko
supernova in 1572
and then Kepler supernova in 1604
and we can still see the so to speak remnants
of those explosions because the matter that was expelled
is moving outwards and interacting
with the medium that is between the star
and other stars giving rise to a lot of emission
so we can train our telescopes in this case
the Chandra X-ray telescope
and outer space to observe these
what we call supernova remnants
and still see a lot of interesting structure
of the material that is moving outwards
and interacting with the circumstantial material
so now of course the next question is
when is the next supernova going to happen in our own Milky Way
as I said the usual rate is 1 per 100 years per Milky Way
we haven't had one in 400 years
so we really overdue for a supernova
so what are the kinds of candidates we have
well who can recognize this constellation
Orion exactly since it's actually up
also now at around 9, 10 and so forth
PM if you look up you see the distinctive feature
the Orion's belt and a number of other stars
and actually this star here Betelgeuse in Orion
is actually our next candidate
because it is a star that is undergoing
the last phases of its life
and as far as we know could explode any second
so Betelgeuse is actually a really large star
it's 1,000 times the size of our Sun
so if you were to put Betelgeuse in your own solar system
it would swallow the inner planets including Earth
and just be out close to our Saturn's orbit
so it's actually a really, really big star
a red giant
another candidate we have is Eta Coraina
and if you watched Nathan Smith's presentation
a couple of months ago he works on this specific star itself
and again this is a very, very massive star
I think around 100 solar masses that could explode any second
so we astronomers actually do have proposals
of what to do exactly if the next galactic supernova happens
and if it's close by it would be really, really bright
so bright that you might be even seeing it during the day
and perhaps so bright that even our own telescopes
which are used to very faint objects would be saturated
completely if this galactic supernova happens
however we have a lot of dust in our own Milky Way
so a lot of the light might get absorbed
so it might not be as bright as we think or it could be
but if you hear next time the news, the galactic supernova
you know what it is
so now I will switch gears and talk about gamma reverse
really the monster explosions of the universe
and the story again is a very interesting story
of how these were discovered in the first place
so in 63 American President there
signed a partial nuclear test ban treaty
to make sure that there are no nuclear tests being conducted
specifically in Russia
and they started to have various kinds of satellites
gamma ray satellites that were orbiting the Earth
to check that no tests, nuclear tests are being conducted
and those kinds of nuclear tests would emit gamma rays
and in the 70s those satellites, the Vella satellites
were up in space and they detected gamma ray emission
however that gamma ray emission didn't come from Earth
it came from all other directions except Earth
and so that led to the discovery of these gamma ray bursts
and as I mentioned gamma rays are the most energetic photons
and so the objects that produce it
emit a lot of these high energy radiation
and that includes a lot of high energies
so as I said it was discovered by Vella in the late 1960s
the big mystery though was well what is their distance
where do they come from, do they come from our own Milky Way
are they local or are they cosmological
do they come from very far away galaxies
and if they came from very far away galaxies
it meant that the intrinsic energy that they emitted
must have been really really large
so to test for that one of the satellites called BATSI
well the instruments called BATSI, the Compton Telescope
observed a number of 2,704 gamma ray bursts
and saw that they were actually across the whole sky
so this is actually a projection of the whole sky in all directions
if you were able to look through the Earth and so forth
and look around you this is the view you would have
in certain deprojection maps
and the point is though that all these points
were distributed across the whole sky
however if you were to be confined to just the Milky Way
you would expect they would be in the plane of the Milky Way
where we see a lot of the other objects in our own Milky Way
so this was very strong clue that these were cosmological events
that they came from other galaxies that they distributed around us
and not from our own Milky Way
and then in 97 people got what we call the first redshift
to really pinpoint the distance to that gamma ray burst
and it showed that it was a cosmological burst
and so forth we've had a number of other bursts where we know the distance
and they're not in our own Milky Way
so these gamma ray bursts are amazing
they outshine the whole universe in a few seconds in gamma rays
so they're very powerful
and we think now that they're due to deaths of very massive stars
and so this is a cartoon and animation done with NASA
that shows how, or again this illustrates
how a star is emitting what we call jets
that those are again the objects that then emit gamma rays
and again this is a movie
so you'll see here these are really narrow jets
that are able to drill themselves through the star
so this is supposed to be a 30 solar mass star or so
and over time is emitting very quickly
so this whole thing happens just within a couple of seconds actually
so the animation takes longer than the real thing
and then at the end what you left behind is this central dense object
that we think is a black hole itself and that's then left behind
and the characteristic feature of these gamma ray bursts
are these really narrow jets that emit a lot of radiation
so what do we think are the stars that explode as gamma ray bursts
well if you look at a star that is more massive than our sun
perhaps ten times or so
then as I mentioned it has this onion structure
where you have the hydrogen layer at the very outermost layer
and the next inner one is helium and so forth
you go down the elements of the periodic table
and then you have at the very end this iron core
and if this star explodes
it will give rise to a normal, a regular supernova
however if you have stars whose outer envelopes have been stripped
so who are so to speak more naked in that sense
don't have the hydrogen or the helium layer at the outermost layers
and those then explode
those are the progenitor stars of these gamma ray bursts
and we still don't fully understand how exactly they make the jets
during the actual core collapse itself
and a lot of people are working on that
but from the supernova spectrum we see in these gamma ray bursts
we see that they have no trace of hydrogen nor helium
they have to be these really highly stripped stars before they explode
and again these explosions are beacons
so the highest redshift or most distant object was detected in 2009
in April 23rd of April
so this is why it's called GRBO90423
and it made worldwide news that it was the most distant object
after people were able to get an estimate of its redshift, of its distance
and this gamma ray burst is at a distance of 12.2 billion light years
so that's only 640 million years after the Big Bang
so very, very early on in the infant universe
and this is really the record for the highest, largest distance explosion
so these are powerful, powerful beacons
then in 2008 you had another gamma ray burst
that was observed with the NASA satellite SWIFT
so this is the X-Rays optical which was actually a naked eye gamma ray burst
so gamma ray bursts don't give rise to only gamma ray emission
but they also give rise to a lot of other emission
including that that can be seen with their own eyes
it's an invisible light and X-Rays
and the emission for a few seconds was so bright
that if you had been able to look at it, you would have seen it with your own eyes
but that only lasted for a couple of seconds
and then what we call the afterglow faded very quickly
and this is an artist's conception of what you saw there
where you saw again these jets
but in this case you might have had two kinds of jets
one that had a wider angle
so that was shown here in yellow
and then you had a much narrower jet in the center here
that had much higher energies which we then saw
because we were in the beam of this jet
so the next question you might ask is
well what happens if GRB goes off in our own Milky Way
and we have to be in again exactly the cone of these very energetic jets
so people astronomers and biologists who looked at that
and the impact of high energy radiation on our own Earth
found that actually it could be quite detrimental
a GRB near Earth could destroy one half of our ozone layer
it could trigger climate change
of course the details are still debated
and could even cause mass extinction
so those could be really really dangerous indeed
if they were to go off in our own Milky Way
now that's a very important question to consider
and look where GRBs go off
and which kind of environments do GRBs explode
so I led a project to look into detail
at some of these GRBs that happened in the local universe
and I'm sorry this is again a very technical plot
but the main point is where I plotted supine without GRBs
and supine with GRBs
and found that there are a specific part of this diagram
in comparison though the Milky Way is up here
so it is in this range for what we call
metallic or oxygen abundance, one of the intrinsic characteristic features
but the Milky Way is much further out
where all the other GRBs supinova are down here
so our conclusion was that actually the Milky Way is very unlikely
to host a GRB because it does not contain
such low metallicities as these other galaxies and environments contain
and so it's very unlikely
again in here for comparison exactly is where the Sun is
and the Milky Way has a spread of these features
the metallicities themselves
and so we concluded
and a number of other people standing at all
in 2006 that actually Earth is very safe from GRBs
the chances that one happens in our own Milky Way
is very small, less than 0.1% or so
so you can relax, you can go home today
and say well probably no supinova might happen
in our own Milky Way but GRB probably not
so this is the concluding slide
I hope I've given you a tour of our amazing universe
including these cosmic fireworks that are both very vital for our life
but also could be dangerous but again we can relax today
thank you very much for your attention
