Your body was forged in the spectacular death of stars

We are all atomically connected.

Fundamentally, universally.

But what does that mean?

I'm an astrophysicist and as such,

it is my responsibility to trace
the cosmic history

of every single one of your atoms.

In fact, I would say

that one of the greatest achievements
of modern astronomy

is the understanding of how our atoms
were actually put together.

While hydrogen and helium were made

during the first two minutes
of the big bang,

the origin of heavy elements,

such as the iron in your blood,

the oxygen we're breathing,

the silicone in your computers,

lies in the life cycle of stars.

Nuclear reactions take lighter elements

and transform them into heavier ones,

and that causes stars to shine

and ultimately explode,

and therefore, enriching the universe
with these heavy elements.

So without stellar death

there would be no oxygen

or other elements
heavier than hydrogen and helium,

and therefore, there would be no life.

There are more atoms in our bodies

than stars in the universe.

And these atoms are extremely durable.

The origins of our atoms

can be traceable to stars
that manufactured them in their interiors

and exploded them
in all across the Milky Way,

billions of years ago.

And I should know this,

because I am indeed a certified
stellar mortician.

(Laughter)

And today, I want to take you on a journey

that starts in a supernova explosion

and ends with the air
that we're breathing right now.

So what is our body made of?

Ninety-six percent consists
of only four elements.

Hydrogen, carbon, oxygen and nitrogen.

Now, the main character
of this cosmic tale is oxygen.

Not only is the vast majority
of our bodies made of oxygen,

but oxygen is the one element
fighting to protect life on earth.

The vast majority of oxygen
in the universe

was indeed produced
over the entire history of the universe,

in this supernova explosions.

These supernova explosions
signal the demise of very massive stars.

And for a brilliant month,

one supernova explosion
can be brighter than an entire galaxy

containing billions of stars.

That is truly remarkable.

That is because the massive
stars burn brighter

and have a spectacular deaths
compared to other stars.

Nuclear fusion is really
the lifeblood of all stars,

including the Sun,

and as a result is the root source
of all the energy on Earth.

You can think of stars
as these fusion factories,

which are powered
by smashing atoms together

in their hot and dense interiors.

Now, stars like our Sun,

which are relatively small,

burn hydrogen into helium,

but heavier stars of about
eight times the mass of the Sun

continue this burning cycle,

even after they exhausted
their helium in their cores.

So at this point,

the massive star if left
with a carbon core,

which, as you know,
is the building block of life.

This carbon core continues to collapse

and as a result, the temperature increases

which allows further
nuclear reactions to take place,

and carbon then burns into oxygen,

into neon, silicon, sulphur

and ultimately iron.

And iron is the end.

Why?

Because iron is the most
bound nuclei in the universe.

Which means that we cannot extract energy

by burning iron.

So when the entire core
of the massive star is made of iron,

it's run out of fuel.

And that's an incredibly
bad day for a star.

(Laughter)

Without fuel it cannot generate heat,

and therefore, gravity has won the battle.

The iron core has no other choice
but to collapse,

reaching incredibly high densities.

Think of 300 million tons

reduced to a space
the size of a sugar cube.

At these extreme high densities
the core actually resists collapse,

and as a result,

all of this infalling material
bounces off the core.

And these dramatic bounds,

which happen in a fraction
of a second or so,

is responsible for ejecting
the rest of the star in all directions,

ultimately forming a supernova explosion.

So, sadly, from the perspective
of an astrophysicist,

the conditions in the centers
of these exploding stars

cannon be recreated in a laboratory.

(Laughter)

Now, thankfully for humanity,
we're not able to do that.

(Laughter)

But what does that mean?

That means that as astrophysicists,

we have to rely on sophisticated
computer simulations

in order to understand
these complex phenomena.

These simulations can be used
to really understand how gas behaves

under such extreme conditions.

And can be used to answer
fundamental questions like,

what ultimately disrupted
the massive star,

how is it that this implosion
can be reversed into an explosion?

There's huge amount
of debate in the field,

but we all agree

that neutrinos,

which are these elusive
elementary particles,

play a crucial role.

I'm about to show you
one of those simulations.

So neutrinos are producing huge numbers

once the core collapses.

And in fact,

they are responsible for transferring
the energy in this core.

Like thermal radiation in a heater

neutrinos pump energy into the core,

increasing the possibility
of disrupting the star.

In fact, for about a fraction of a second,

neutrinos pump so much energy

that the pressure increases high enough

that a schock wave is produced

and the shock wave goes
and disrupts the entire star.

And it is in that shock wave
where elements are produced.

So, thank you neutrinos.

(Laughter)

Supernova shine bright,

and for a brief period of time
they radiate more energy

than the Sun will in its entire lifetime.

That point of light that you see there,

which was certainly not there before,

burns like a beacon,

clearly indicating the position
where the massive star has died.

In a galaxy like our own Milky Way,

we estimate that about once every 50 years

a massive star dies.

This implies that somewhere
in the universe

there's a supernova explosion
every second or so.

And thankfully for astronomers,

some of them are actually found
relatively close to Earth.

Various civilizations recorded
these supernova explosions

long before the telescope was invented.

The most famous of all of them

is probably the supernova explosion
that gave rise to the Crab Nebula.

Yeah?

Korean and Chinese astronomers
recorded this supernova in 1054.

As did almost certainly Native Americans.

This supernova happened
about 5600 light years away from Earth.

And it was so incredibly bright,

that astronomers could see it
during the day.

And it was visible to the naked eye

for about two years in the night sky.

Fast forward 1,000 years or so later,

and what we see?

We see these filaments

that were blasted by the explosion,

moving at 300 miles per second.

These filaments are essential
for us to understand

how massive stars die.

The image that you see there

was assembled by
the Hubble Space Telescope

over a span of three months.

And it is incredibly important
to astronomers,

because it ultimately carries
the chemical legacy

of the star that exploded.

The orange filaments that you see there

are the [unclear] remainings of the star,

and are made primarily of hydrogen.

While the blue and red
filaments that you see

are the freshly synthesized oxygen.

So stunning supernova remnants,
like the Crab Nebula,

allowed astronomers to firmly conclude

that the vast majority of oxygen on Earth

was produced by supernova explosions

over the history of the universe.

And we can estimate

that in order to assemble
all the atoms of oxygen in our body,

it took of the order
of a 100 million supernova.

So every bit of you,
or at least the majority of it,

came from one of these
supernova explosions.

So now you may be wondering,

how is it that these atoms,

that were generated in such
extreme conditions,

ultimately took residence in our body?

So I want you to follow
the thought experiment.

Imagine that we're in the Milky Way

and a supernova happens.

It blasted tons and tons of oxygen atoms

almost into empty space.

A few of them

were able to be assembled in a cloud.

Now, 4.5 billion years ago,

something unsettled that cloud
and caused it to collapse,

forming the Sun in its center
and the Solar system.

So the Sun, the planets and life on Earth

depend on this beautiful cycle

of stellar birth, stellar death,
and stellar rebirth.

And this continues recycling
of atoms in the universe.

And as a result, astronomy
and chemistry are intimately connected.

We are life forms that have evolved

to inhale the waste products of plants.

But now you know

that we also inhale the waste products
of supernova explosions.

(Laughter)

So take a moment, inhale.

An oxygen atom
has just gone into your body.

It is certain that that oxygen [atom]

remembers that it was
in the interior of a star

and it was probably manufactured
by a supernova explosion.

This atom may have traveled
the entire Solar system

until it splashed on Earth,

long before reaching you.

When we breathe,

we use hundreds of liters
of oxygen every day.

So I'm incredibly lucky to be standing
in front of this beautiful audience.

But I'm actually stealing
your oxygen atoms.

(Laughter)

And because I'm speaking to you,

I'm giving you some of them back,

that once resided in me.

So breathing, yeah,

participates in this
beautiful exchange of atoms.

And you can then ask, "Well,

how many atoms in our body

once belonged to Frida Kahlo?"

(Laughter)

About 100,000 of them.

A hundred thousand more
probably belonged to Marie Curie,

100,000 more to Sally Ride,

or whoever you want to think of.

So breathing is not only filling our lungs

with cosmic history,
but with human history.

I would like to end my talk
by sharing a myth

that is very close to my heart.

A myth from the Chichimeca culture,

which is a very powerful
Mesoamerican culture.

And the Chichimecas believe

that our essence
was assembled in the heavens.

And on its journey towards us

it actually fragmented
into tons of different pieces.

So my abuelo used to say,

"One of the reasons you feel incomplete

is because you are missing your pieces.

But don't be fooled by that.

You've been given an incredible
opportunity of growth.

Why?

Because it's not like those pieces
were scattered on earth

and you have to go and pick them up.

No, those pieces fell into other people.

And only by sharing them
you will become more complete.

Yes, during your life,

there's going to be individuals
that have this huge pieces

that make you feel whole.

But in your quest of being complete,

you have to treasure and share
every single one of those pieces."

Sounds a lot like
the story of oxygen to me.

Which started in the heavens
in a supernova explosion,

and continues today,

within the confines of our humanity.

Our atoms in our body
have embarked on an epic odyssey,

where times pass from billions of years

to mere centuries,

all leading to you,

all of you,

witnesses of the universe.

Thank you.

(Applause)