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Translator: Ivana Korom Reviewer: Krystian Aparta
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,
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 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 these 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 massive stars burn brighter
and have a spectacular death, 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 is 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 this dramatic bounce,
which happens 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
cannot 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 a huge amount of debate in the field,
but we all agree that neutrinos,
which are these elusive elementary particles,
play a crucial role.
Yeah?
I'm about to show you one of those simulations.
So neutrinos are produced in 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 shock 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)
Supernovas 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 5,600 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 do 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 tattered remains of the star,
and are made primarily of hydrogen,
while the blue and red filaments that you see
are the freshly synthesized oxygen.
So studying 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 on 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 the 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.
100,000 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."
(Laughter)
"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 these 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.
(Laughter)
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,
with time spans from billions of years to mere centuries,
all leading to you,
all of you,
witnesses of the universe.
Thank you.
(Applause)
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Your body was forged in the spectacular death of stars | Enrico Ramirez-Ruiz

20 分類 收藏
林宜悉 發佈於 2020 年 3 月 29 日
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