Supernovas ...
gigantic explosions that light up the cosmos.
One of the most spectacular things in the universe
is the death of a giant star.
They live fast, and they die young.
Inside the star's core,
temperatures and pressures are immense.
We're talking about a billion degrees
in the center of one of these stars.
A ticking time bomb
that explodes with indescribable energy.
The last minutes of a giant star's life
are the most cataclysmic events that we see in the universe.
Dramatic finales blazing across space.
That one supernova is brighter
than the hundreds of billions of stars
that constitute the galaxy.
How amazing is that?
But these stellar deaths also hold the key
to life itself.
Understanding supernovas is understanding our story.
We owe our existence to them.
... Captions by vitac ... www.vitac.com
captions paid for by discovery communications
Right now, somewhere in the universe,
a giant star is detonating,
creating a huge cosmic explosion
called a supernova.
supernovas are a big, giant dramatic end to a star's life.
All stars die,
but only the biggest go out with a bang.
For a star to go supernova, we think it has to be
at least eight times more massive than our sun.
It's so easy to think of our sun
as this incredibly gigantic thing,
but our sun is absolutely tiny
compared to some of the giant stars in the sky.
We can see some of these giant stars with the naked eye,
and the 10th brightest in the night sky
is a red supergiant
around 15 times the mass of the sun ...
betelgeuse.
betelgeuse is so big that if you were to place it
in our own solar system,
it would stretch to the orbit of Jupiter.
This is one of the biggest beasts in the galaxy.
It's a star also that is on the verge of death.
Betelgeuse is less than 10 million years old,
but this huge star's days are numbered.
It's ready to blow.
when it does, we will see a region of sky
brighten for 14 days,
until it's nearly as bright as a full moon.
It is going to be one of the most spectacular
shows in history.
and it could happen at any moment.
I mean, this is the thing.
I often stand outside in my yard in the wintertime.
I look up at Orion, and I see betelgeuse.
And I'm like, "explode!"
So what will make betelgeuse go supernova?
To understand a giant star's death,
we need to understand its life.
from the day it's born until the day it dies,
a star's life is a constant battle.
gravity is pulling in,
and energy is pushing out.
The interior of a star
is fusing countless atomic nuclei together.
Atoms are ramming into each other,
getting very, very close.
And if they get close enough,
they'll actually stick and form a larger atom.
Every second, a giant star fuses
7 1/2 billion tons of hydrogen.
That amount of energy is roughly equivalent
to about 100 billion atomic bombs per second.
That's a big-ass explosion.
This explosive energy
threatens to blow the star apart,
but the star's own massive gravity keeps the lid on.
Straughn: Everything in the universe is a fight
between the inward force of gravity
and the outward force of pressure or energy.
Every single star in the sky, even our own sun,
is an incredibly dynamic battleground.
In many ways, stars are an explosion
that are actually too big to explode.
Gravity holds it together.
This battle between these two opposing forces
determines the life and death of the star.
And this is where size matters.
The more massive the star, the more gravity pushes inward,
the harder the star has to push outwards to keep itself alive.
very massive stars are like stars on steroids.
They have a lot of fuel to burn.
They're so powerful
that they use up their fuel at a rapid rate.
Massive stars like betelgeuse are giant factories,
fusing lighter elements into heavier ones.
But the hard work doesn't start until their final years.
For around 90% of their life, they fuse hydrogen into helium,
but eventually, the hydrogen starts running out.
In the core of a supergiant star,
there's a sequence of fusion
that goes from lighter elements to heavier elements,
and it gets faster and faster every step of the way.
The countdown to death begins.
The inward push from gravity takes over,
raising the temperature in the core.
Helium starts fusing to carbon.
There's enough helium to last about a million years,
but it too runs out,
and things start speeding up.
Carbon gets fused into neon.
That takes about 1,000 years.
Neon fusing into silicon?
That takes about one year.
Once it starts fusing silicon into iron,
that takes one day.
It gets more and more frantic.
It's kind of like a cooking-contest show,
where as the clock is running down,
they're trying to do more and more things,
and they get more and more frantic
until, ding, time's up.
The star is now in its death throes.
Sutter: Once iron production has started,
the clock is ticking
towards the cataclysmic end of this star.
A giant ball of incredibly dense iron forms
in the middle of the dying star's core.
This iron sphere is several thousand miles across
and unbelievably hot.
It gets so hot there
that temperature almost becomes meaningless.
I mean, we're talking about a billion degrees
in the center of one of these stars.
This extreme heat is caused by fusion reactions.
More and more reactions create heavier and heavier elements,
and with each step, less and less energy is produced,
until iron is created.
When you try to fuse iron nuclei together,
that takes energy. It doesn't generate energy.
So once the core starts to fuse iron,
it's basically stealing its own energy.
The growing iron core
sucks more and more energy from the star.
Gravity continues pulling in,
overwhelming the outward pressure from inside the star.
everything gets crushed to unimaginable degrees.
All of a sudden, there's no nuclear reaction
to support the star against the crush of gravity.
With nothing left to hold it up,
the star is doomed.
Gravity wins.
The edges of the iron core collapse.
Trillions of tons of dense iron fall inward
at 1/4 the speed of light.
The star now has less than one second left to live.
Things start to fall apart real quickly.
The core collapse is so fast
that the outer layers of the star
don't even have time to react. They're just hanging there.
It's kind of like wile e. Coyote,
when a cliff collapses underneath him,
and he doesn't even fall until he notices.
The rest of the star collapses.
A trillion-trillion-trillion tons of gas hurtles inwards,
following the iron.
Think about the entire mass of a star
that has been held up by nuclear reactions inside.
All of a sudden, those nuclear reactions
go away in a split second.
Everything rushes into the middle.
And that sets off the most dramatic explosion
in the universe.
The spectacular death blow
can outshine all of the stars in a galaxy.
but there's a problem.
We still don't fully understand
how a collapsing ball of iron and tons of falling gas
create a giant fireball.
How this collapsing core triggers a massive explosion
is one of the biggest mysteries in astrophysics.
A supernova ...
one of the most powerful eruptions in the cosmos,
triggered by the collapse of a massive star.
How do you go from a violent collapse
to an incredibly dramatic explosion?
This involves some of the most complex astrophysics
known to humanity, and we don't fully understand
the details of the process.
We're missing something,
because we nearly always spot supernovas too late.
What you're seeing is, you're seeing the star brightening,
and that's really happening after the fact.
So now the magic key is not finding a supernova
but finding the moment that we call the breakout.
The breakout is a giant star's death rattle.
It's the moment after the core has collapsed,
when the star blows apart in a huge flash of visible light.
but in the entire history of astronomy,
this moment has only been caught twice ...
one by NASA's multimillion-dollar
space telescope, kepler,
and once by a very lucky Argentinean amateur.
I love this story.
There's an amateur astronomer named Victor buso.
He has a very nice telescope in an observatory in his yard.
And he was taking photographs repeatedly
of the same galaxy that happened to be overhead.
And he just happened to be looking
at the right region of the sky,
and he luckily caught the shock breakout of a supernova.
The chances of catching this moment
are 1 in 10 million.
What Victor caught was the moment
the shock wave reaches the surface.
Victor noticed this spot
appearing in his photographs.
Realizing he'd captured the first flash of light
from an exploding star,
he alerted professional astronomers across the globe.
when I heard of his discovery, I was like, "no way.
How could this guy, using a camera on his telescope
for the very first time,
pointing at a single random galaxy in the sky,
have found this exploding star
in the first hour of its explosion?
It's almost too good to be true."
Alex filippenko and his team
monitored the brightening light from the star.
What we found when studying the light from buso's supernova
is that the object brightened very quickly for a short time
when a shock wave, a supersonic wave
going through the star burst out through the surface.
And when it gets right to the edge,
that huge amount of energy is released as a tremendous flash.
That is the moment of shock breakout.
The monstrous shock wave travels
at nearly 30,000 miles per hour,
bursting through the surface of the star
and ripping it to pieces.
Fire!
We see shock waves from explosions on earth.
They can travel through gas, liquid, and solid,
including the layers of a collapsing star.
This observation of the shock wave
reaching the surface of the star was incredibly important,
because Victor managed to catch a star
the moment is actually went supernova.
That is something that is a scientific treasure.
The shock breakout is like cosmic gold dust,
a flash in the pan that lasts 20 minutes ...
just the blink of an eye on astronomical time scales.
but what sets the shock wave off?
Is it just a question of bounce?
A supernova shock wave can be explained
with the help of a basketball.
The thing about an exploding star
is that the nuclear reactions go out in the core,
and then the outer layers fall in
at incredibly high speeds toward the inner core,
and then it rebounds and bounces out.
And what gives it so much energy is the structure of the star.
As the dying star burns through its fuel,
it creates layers of different elements ...
heavy iron at the core,
with layers and layers of lighter elements above.
So, let's say there was only one layer,
and there was a rebound, like dropping this ball.
It doesn't bounce very high.
But let's say it's organized like a star,
where the heavy thing is at the bottom,
the lighter thing is at the top.
And let's see how this rebound goes.
now, that was a rebound.
The tennis ball launches off the basketball
because energy from the basketball's bounce
is transferred upwards.
The same thing happens in a collapsing star,
but with many more layers.
All the different elements collapse inwards.
They heavier layers hit the dense core first,
passing energy to the lighter ones.
And this creates the shock wave.
But this energy isn't enough to propel the shock wave
all the way out of the star.
The problem is, when we looked at this in detail
using computer models, it didn't work.
The shock wave seemed to stall.
We couldn't get the star to explode.
For 50 years, we couldn't figure out what we were missing.
Scientists suspect something else is involved,
something that's almost impossible to detect.
Could there be a ghost in the supernova machine?
When stars as big as betelgeuse die,
their explosive deaths send shock waves
that travel trillions of miles through space.
But how these shock waves are created
has puzzled scientists for decades.
Time and time again,
when we actually went back to our computers and our theories
and looked at how supernovas should work, they just didn't.
They shouldn't actually explode.
In computer models,
the bounce from falling gas on a collapsing core
can't drive the shock wave all the way out of the star.
Something crucial is missing.
What we needed from inside the core of the star
was a completely new source of energy,
something to actually make that final push
to get the star to rip itself apart.
Scientists suspect this energy
comes from an enigmatic particle called a neutrino.
Neutrinos are a type of fundamental physical particle
that are still a little bit mysterious to us.
They're almost like ghost particles.
They travel through us without touching us at all.
Like particles of light, photons,
neutrinos carry no electrical charge.
But unlike photons, they can pass through stars,
planets, and us.
So where do they come from?
Scientists predict the source
is the star itself.
In the middle of the core of the star,
you're producing something called a neutron star ...
an amazing, super-compressed ball of matter
only about 10 miles across.
As the iron core of a star collapses,
the atoms are crushed together.
Protons and electrons are forced to combine to form neutrons.
This process releases vast quantities of neutrinos.
despite being one of the most abundant particles
in the universe,
neutrinos are notoriously difficult to detect.
but in 1987, scientists got lucky.
A massive star went supernova in a nearby galaxy.
In 1987, astronomers got a wonderful gift.
It was the first naked-eye supernova
in about 400 years.
And we had lots and lots of telescopes
with which to study it
throughout the electromagnetic spectrum.
But the 1987a supernova
set off another scientific instrument ...
a neutrino detector hidden deep below a mountain in Japan.
There was a burst of neutrinos associated with the supernova.
This was just a fantastic surprise,
a wonderful added bonus.
When you're trying to capture and measure elusive particles
that you don't even know
if you're gonna get a signal or not,
and you're sitting there waiting at your detector,
and then suddenly, this thing just lights up?
How exciting is that?
This was definitive proof
that supernovas emit neutrinos.
Neutrinos may be ghostly, but they don't gently drift out
from the collapsing core of the star.
They have to burst out.
The amazing thing about the inside of a supernova explosion
is that it's getting dense enough to trap neutrinos.
All of a sudden now, there's pressure.
When scientists add neutrino pressure
to the computer models,
the shock wave gets farther away from the core,
but the supernova still doesn't explode.
One more ingredient is needed ... disorder.
Because stars are round, it's tempting to think
that a supernova explosion too will be round.
But supernova aren't perfectly symmetric.
Energy from the shock wave
and the neutrinos heats up the gas
in chaotic, unpredictable ways.
They cause hot bubbles to rise and then come back down
and rise and come back down.
It's sort of a boiling motion.
This imparts a lot of turbulence into the gas.
Researchers add all the ingredients
to a supercomputer and let it run.
this simulation is the result.
When the shock wave stalls on its way out of the core,
it creates tiny ripples in the falling elements above.
The ripples become giant sloshing waves.
Neutrinos bursting out from the neutron star
heat the layers of elements above it,
causing them to bubble and rise.
eventually, the intense heat combines
with the pressures of these violent motions,
driving the shock wave out like an interstellar Tsunami,
smashing the star to pieces.
it turns out, stars do explode.
Nature knows what it's doing.
It was the computer models. They were too simple.
Once the models became more complex,
starting taking into account all the dimensions of a star,
the supernova models started to explode.
We think of supernova as effectively simple events ...
very violent events, but simple.
And this is just a beautiful illustration of the fact
that when you dig deep down,
these are really exquisitely complex
and elegant fluid-dynamics problems.
The shock wave travels through
all the layers of the elements that make up the massive star.
It takes hours for it to reach the outer edge
and trigger the first flash of light,
but this flash is just the start of the supernova.
The spectacular light show is just beginning,
a light show that will create elements essential for life.
We see the light from supernovas
all the way across the cosmos,
but what we're seeing isn't the explosive first flash.
That's just the opening act before the main event.
Supernova are some of the most energetic events
in the universe.
The galaxy has hundreds of billions of stars in it,
and yet the death of this one star
can outshine those hundred billions of stars.
One of the interesting things about supernovas
is that when the star explodes,
it's not at its maximum brightness immediately.
It takes days and weeks.
The first flash is the explosive part of a supernova,
blasting tons of matter into space around the dying star.
But it's this ejected debris that makes supernovas shine,
often glowing brighter than the explosion itself.
heavy elements are formed inside the cores of massive stars,
and even heavier elements are formed
during the explosion event itself.
As the star rips apart,
temperatures and pressures are immense.
The elements that once made up the layers of the star
fuse together, creating heavier elements.
And some of these are radioactive.
The decay of these radioactive elements
actually produces light.
That gives it more brightness over a longer period of time
than it otherwise would have.
This cloud of brightly shining matter
can last for months and sometimes years.
these supernova remnants light up the universe
like cosmic fireworks.
these are oftentimes beautiful,
beautiful things in the night sky,
because they are ... you see remnants
of everything that the supernova has generated in its explosion.
But these aren't just pretty light show.
They are crucial for the evolution of galaxies
and solar systems.
Necessary ingredients ...
things like sulfur, things like phosphorous,
things like carbon and oxygen.
And even the elements necessary to build a rocky planet
like the earth itself
can only be formed inside of massive stars
and can only be spread through supernova explosions.
NASA's chandra space telescope studies
one of the most famous objects in the milky way...
...supernova remnant cassiopeia "a."
cassiopeia "a" is a relatively young supernova remnant,
not even 400 years old.
Ever since its star exploded,
cassiopeia "a" has been expanding.
It is now 29 light years across.
Using x-rays, the chandra space telescope
has looked inside this massive cloud.
New observations of cassiopeia "a" have shown us
that the ejecta from this event
has created tens of thousands of times
the earth mass of really important materials.
Filippenko: 70,000 earth masses worth of iron,
and a whopping 1 million earth masses worth of oxygen.
Now, these are elements that are important to life,
to earth, to us.
The iron in your blood, the calcium in your bones,
these were forged in supernova explosions
billions of years ago.
The new study reveals
something even more extraordinary.
Cassiopeia "a" also holds
the building blocks of life.
We see every single atom necessary for DNA
in that one supernova remnant.
One of the really cool things about supernovas
is that our very existence depends on them.
Our DNA molecules are made up of material
that was once in the core of a massive star.
So somewhere out there,
some unnamed supernova eons ago,
led to you watching me talking about supernovas.
That's awesome.
Supernovas create all the elements needed
to build everything from planets to humans.
Dying stars give us life.
It's a cosmic recycling process.
But what if some stars are faking their own deaths?
For thousands of years,
humans have wondered about bright, new stars
appearing in the sky,
and supernovas continue to surprise us.
Our fascination with supernova has grown
with each discovery of a new event.
The study of supernovas
is really going through a revolution.
We're learning more and more.
We're better able to find them and observe them.
And it turns out not all supernovas are the same.
Some are the result of white dwarf stars
stealing matter from a twin
and growing so big, they explode.
all other supernovas are massive stars
collapsing under their own gravity.
but just to confuse things further,
scientists also categorize supernovas
based on whether hydrogen is present.
Type I are missing hydrogen.
Type ii are not.
So, astronomers have these categories for supernova,
and that might make you think
that we've got them all figured out,
but here's a spoiler ... we don't.
September 2014.
A supernova appears in the great bear constellation
and glows brightly for 600 days.
When scientists check the records,
they discover a supernova was sighted
at the exact same spot 60 years before.
A star seemed to be dying
over and over again.
This particular star was something
we had never seen before,
and it seemed so strange, it was almost impossible.
It actually brightened and faded about five times
over a several-year time span.
And each of these brightenings would have qualified
as a supernova in terms of its total energy.
It's the supernova that would never die.
So how could it happen with the same star again
and again and again?
This really did seem to be a zombie star.
How can a star have multiple deaths?
The answer lies in its sheer size.
We're talking about a very massive star here,
about 100 or more times the mass of the sun,
really the upper limit of what a star can be
without tearing itself apart.
This star is so big
that reactions in the core are off the charts.
And these energetic reactions
produce more than just elements.
It can actually get so hot in the interior
that you produce gamma rays.
This is the most energetic form of light imaginable.
The gamma rays' extreme energy
supports the dying star
against the crushing forces of gravity pushing in,
but it also affects the gamma rays themselves.
Gamma rays above a certain energy can do something weird.
They can transform themselves into matter.
This transformation affects the delicate balance
between gravity and energy in the star's core.
The core starts to collapse.
When it collapses, it generates more energy.
This energy leaks out of the outer layers of the star,
and we sudden brightening of the star, a pulse.
filippenko: And it brightens and fades a bunch of times,
each time releasing some material
but not quite exploding.
It's almost supernova levels of energy.
That's what fooled the astronomers at first.
Eventually, the pulsations stop.
The star calms down, ready to live another day.
astronomers still don't know if this "zombie" supernova
has finally died.
Filippenko: We think that we've seen this final explosion
of the zombie supernova,
but honestly, we're not sure yet.
Maybe it's currently fading, but next year,
it'll surprise us and brighten once again.
But this isn't the only mysterious supernova
that has scientists scratching their heads.
Meet supernova sn 2014c.
Supernova 2014c was a bit of a strange one.
It was initially classified as a type I.
Astronomers classify supernovas as type I or type ii,
depending on whether they contain hydrogen.
If you break the light up coming in from a supernova
into its individual colors, you take its spectrum.
If there's the signature of hydrogen in that spectrum,
that's a type ii supernova.
If the hydrogen is missing, that's type I.
When sn 2014c was first discovered,
hydrogen was missing.
But then later on, hydrogen suddenly appeared,
and we realized, no, this is actually a type ii.
It's sort of a chameleon supernova.
It went from being type I, free of hydrogen,
to type ii, full of hydrogen.
How can a supernova change
from not having hydrogen to having hydrogen?
The chameleon supernova baffled scientists,
until they looked around it with the nustar X-ray telescope.
it revealed that the star had spewed out
a huge amount of hydrogen.
But this wasn't during the supernova event.
This was many decades before.
This star is very massive and relatively unstable.
And it underwent an explosive event about a century ago ...
not big enough to be a supernova,
but it expelled all the hydrogen in that star,
so it was a type I.
Then the star exploded again,
but this event was massive.
Filippenko: The ejected gases from the supernova
smashed into the hydrogen
that had been previously expelled
by the star before exploding.
And once the ejected gases crashed in,
well, that caused that hydrogen gas to glow.
And then we saw hydrogen in the spectrum,
and it became a type ii.
The more scientists learn about supernovas,
the more complicated they become.
So, now it seems that we've seen
every type of supernova that must be possible.
And we've seen some very, very strange ones,
things that are zombies or chameleons.
But there has to be something out there that's stranger still.
There may be a whole zoo
of undiscovered supernovas out there ...
exciting, perplexing, deadly.
And they may have been shaping the solar system,
and earth, since the beginning of time.
The death of a giant star ...
it's more than just an epic explosion.
It unleashes a storm of elements
that form the universe around us.
There's a wonderful cycle of death and life in the universe.
Individual stars are born, they live their lives,
and they die.
When they die, they enrich the universe
with new atoms and new chemicals.
Those go on to form new stars and new planets.
Dust blows out from the explosion,
forming spectacular interstellar clouds ... nebulas,
the nursery of stars, including our solar system.
One of the biggest pieces of evidence we have
is that supernova themselves
produce some very rare radioactive elements,
radioactive elements that we can still see
embedded in the solar system today.
It's sprinkled like radioactive salt.
These radioactive elements,
found right across our planet, are only produced in supernovas,
proof that earth and the solar system
were created from exploding stars
4.6 billion years ago.
But supernovas may have affected earth
much more recently.
We do have some evidence
that there was a particular supernova explosion
that rained down on the earth about 2 1/2 million years ago
and deposited a specific kind of iron.
Iron-60 is a radioactive element
made during supernova.
It's found in fossils from around this time.
We see it embedded in the crust of the earth itself.
We see pieces of evidence.
2 1/2 million years ago,
life on earth changed dramatically.
Africa lost much of its forests to grasslands,
various plants and animals went extinct,
and many new species appeared.
But how could a supernova change life on earth
so dramatically without destroying it completely?
when a supernova explodes,
it produces a tremendous amount of gamma rays.
And if that supernova is close enough to the earth,
you could imagine it really doing damage to our atmosphere.
Some of the incredible amounts of energy
found in a supernova
leave the star in gamma-ray beams.
If that beam were to be pointing at earth,
then the ozone layer could be harmed.
it affects our ozone layer,
which affects the amount of U.V. radiation
that can hit the surface, which can trigger mutations,
which can trigger different forms of vegetation,
which can kill off algae in the in the oceans.
There's a lot of potential effects.
Mutations drive evolution in all forms of life,
from the simplest to the most complex.
So it's conceivable that, as a result
of a relatively nearby supernova,
the mutations led to early hominids
and then homo sapiens.
That actually affected the evolution of life on earth,
and humans in particular.
Is it just coincidence that ancient humans
started to appear at around this time?
Or was our humanity sparked by a supernova?
Supernovas seem to be an example of violent death.
But there were so many steps in the formation
of our solar system, the formation of you,
that are intimately related to supernova.
They created the chemical elements
and maybe even drove our evolution.
We very likely would not exist
if it were not for exploding stars.
From the elements in our DNA to the solar system
and the world we live in, supernovas have made us.
The reason we study astronomy at all
is to actually answer the question as to who we are,
where we came from, and we're going.
And with supernovas, that's all wrapped up
into this amazing story.
Literally, you are the death of a star.
These epic explosions
are unlocking the biggest mysteries of our existence.
The story of supernova have become more interesting
and more complex with every discovery.
So as we learn more, we discover
what it is that we don't understand yet.
The cosmos is something that can seem so distant
and so unreachable, but stars are the things,
the brilliant light to the cosmos,
with which we have the most strong connection.
There are so many things to love about exploding stars.
They are what give rise to the elements of life.
from the most intimate to the most gigantic scales imaginable,
supernovas are the key to all of that.
So, thank you, supernova.
Hats off to you.
Now, please, stay very, very far away.