I have a slight problem,
but the show's going on.

My blood's red.

Is Viennese blood red?

(Laughter)

I suspect it is.

Why is blood red?

Does anybody know? Can you tell me?

(Audience) It's iron.

It's iron, yes.

It's iron in the hemoglobin,
in our bloodstream,

that makes the blood red.

Iron is one of the chemical elements,

and I'm going to talk
about that in a moment.

But, just first ...

(Laughter)

... tomato ketchup.

We'll hear more about tomatoes later.

(Laughter) (Applause)

Back to the chemical elements and iron.

It is, indeed, 
one of the chemical elements,

and even if you're not a chemist,
you probably know of some others.

An answer given by a student in an exam.

[H2O is hot water and CO2 is cold water.]

(Laughter)

So, you know what H2O is?

(Audience) Water.
Jocelyn Burnell: Water. CO2?

(Audience) Carbon dioxide.
JB: Carbon dioxide.

So, we've got here
another three chemical elements:

hydrogen, oxygen, and carbon.

And while we're dealing
with student exam questions,

here's another one about water:

Water is composed of two gins ...

[Water is composed of two gins,
Oxygin and Hydrogin.]

[Oxygin is pure gin.
Hydrogin is water and gin.]

(Laughter)

These answers come from
the United States of America, but ...

(Laughter) (Applause)

It's a wonderful resource of all sorts
of amazing things that come true.

Maybe some of you
recall seeing a diagram like this

in school chemistry laboratories.

You can see it in other places, too,

even these days on tea towels,
mugs, bags, pens.

It's a tabulation of the 100 plus
chemical elements that we know about.

In Oxford, where I come from,

we have it on taxis and buses, as well -

but that's Oxford.

(Laughter)

Now, in our bodies, there's clearly
iron in the bloodstream,

there's also hydrogen and oxygen
because we're two-thirds water.

There's carbon in our tissues,
calcium in our bones.

I'm going to focus on the iron
because this is a short talk.

Where did that iron, and, indeed,
where did those other things come from?

How did it get into our bodies?

It's not in the air ... much.

It's come through what we've eaten:
plants and animals.

How did the iron
get into the plants and animals?

Well, it came from the earth.

How did it get into the earth?

Where did it come from before that?

What I am going to be telling you about

is how the stars have created
the chemical elements -

the key ingredients of life:
oxygen, carbon, calcium, iron -

with particular emphasis on the iron.

Stars are formed in some
of the dark spots of the galaxy,

the dark patches.

There are particles
of gas and dust milling around,

by chance as a little knot,

it's got extra gravity,
pulls in some more,

puts up the gravity, pulls in more.

And over some millions of years,

this little knot grows into
what's going to be a full-blown star.

When the temperature
in the middle of this lump

reaches about 10 million degrees,

nuclear reactions start,

and, in particular, a nuclear reaction
of hydrogen being converted to helium.

And there's some energy to spare,
and it comes out of starlight.

Our sun's busy doing that:

our sun is burning about 600 million
tons of hydrogen every second.

It's done that for 5 billion years.

It'll do it for about
another 5 billion years.

And shortly after that, it will end,
and it's actually no use for this story.

(Laughter)

We have to focus
on a very small minority of stars,

the extremely massive ones,

10, 20, 30 times the size of our sun.

Examples of these that you might know:

the Pleiades - which is in the winter sky
near the constellation of Orion,

and Betelgeuse -

which is the reddish star,
top left in the constellation of Orion.

These big stars not only convert
hydrogen to helium,

but then the helium to carbon,

and work their way
across the periodic table

till they end up with iron
in the center of the core.

And this is the first place
that we have iron in the universe -

in the cores of some stars.

Not very useful to us
if it's in the cores of stars.

But star death, dramatic star death,
comes to the rescue.

A pair of photographs here:
a "before" and an "after."

We're looking at
a southern hemisphere object

called the Large Magellanic Cloud.

It's a small galaxy,
external to ours, but quite nearby.

We're seeing up top left
a glowing mass of gas,

quite a lot of pink hydrogen gas,

millions of little stars,

and one of them, bottom right,
picked out with an arrow.

For those of you
who are not astrophysicists,

the arrow's added after the photo's taken.

(Laughter)

But, this inconspicuous star
that we had to pick out with an arrow

becomes this,

and you don't need an arrow
to see that thing in the bottom right.

The star has exploded catastrophically.

It was one of these big stars

like the ones in the Pleiades,
or Betelgeuse.

It's gone all the way through
the various chemical elements.

It's got this range of onion shells
with iron in the middle

and the other chemical
elements outside it,

and it has exploded.

The physics of the explosion
is quite complicated,

so I'm not going to go
into the details of that,

but it is a catastrophic explosion.

We used to assume
it was totally catastrophic.

We now know that the pulsars
that Vlad mentioned in his introduction

are formed from the cores
of these exploding stars.

But 95% of the star
is skooshed out into space,

which means that being
fanned out across space

are useful chemical elements
that were inside the star:

oxygen, calcium, carbon, iron,

spread out, made available
by the catastrophic terminal explosion

of this particular star.

Now, getting from there to us
is quite a long story,

and I'm going to do this bit by mime.

You've probably got some sense

that physics professors
have a slightly dubious reputation.

(Laughter)

The female ones are utterly nuts!

And I'm just about to prove it!

(Applause)

So, this stage is the Milky Way -

our galaxy,

and this is a story
that involves all the Milky Way.

Over here in the Milky Way
is one of these dark clouds

where stars sometimes form,

particles of gas, molecules,
dust milling around.

By chance, there's a little knot,

it has extra gravity, it pulls in
some more bits of dust and gas,

puts up the mass, puts up the gravity,

pulls in some more bits.

To save time, folks, this is going to be
one of these very massive stars,

otherwise we're here for a long time.

So, this gradually grows, gradually grows.

And at the point when it's grown so much

that the temperature in the middle
has reached about 10 million degrees,

it starts its sequence
of nuclear reactions,

and it burns, converts hydrogen to helium.

Brrrr!

Then it starts to run out
of hydrogen in its core.

So it starts converting helium to carbon.

Brrrr!

That doesn't last as long.

Then it runs out of helium in its core,

so it converts carbon
to oxygen, oxygen ...

Brrr! Brr! Brr! Brr!

Boom!

(Laughter)

And millions and millions
and millions of tons of stuff, gas,

fan out from this explosion site

in one part of our Milky Way.

It percolates, slowly,

but we've got eons, there's no rush.

(Laughter)

It can travel,

and it does travel,
gradually, in all directions,

but we're interested in this bit.

And some comes over here

to where there is another
of these dark clouds

with particles of gas
and dust milling around.

And some of the material
from that distant explosion

finds its way over here,

and that material is rich
in carbon and calcium

and iron and oxygen, and so on.

So it joins this cloud,

and by chance a little knot forms,
it's got extra gravity,

pulls in some more particles,
puts up the mass, puts up the gravity,

pulls in some more particles,

puts up the mass, puts up the gravity,

and over a million years,
10 million years,

it grows and grows and grows.

And once again,
I have to crave your indulgence,

could this also be
one of these big stars?--

otherwise we're here all night.

So this big star grows,

and the nuclear reactions start,
and it burns, converts hydrogen to helium.

Brrrr!

Runs out of hydrogen, burns helium. Brrrr!

Runs out of helium, burns carbon.

Brrr! Brr! Brr! Brr! Boom!

(Laughter)

Now, you know the next bit of the story.

Millions and millions and millions
of tons of stuff fan out across space,

and some of it makes its way over here

to another dark part
of the galaxy - the Milky Way -

where there's a star beginning to form.

The material that comes from there

is doubly enriched in carbon
and calcium and iron, and so on,

because of the stuff
that that star generated,

plus the stuff it got
from that star, as well.

So, what's arriving here
is double dose carbon,

calcium, iron, and so on.

And here in this cloud,
a star called the Sun is forming,

and it's made from the stuff

that happens to be
in this patch of the galaxy,

plus the stuff that's come from that star,

plus the stuff
that's come direct from there,

and maybe from some other
exploding stars, as well.

Our sun is a third-generation star.

Our star is a late-forming star,

and it has to be

or we wouldn't be here.

We can only exist close to a young star

that's been enriched
by previous solar cycles.

So, the sun forms,

some of the material is left over.

You've perhaps seen pictures 
of the planet Saturn

with its rings around it.

This is a giant version of that.

So you've got a sun

and some of the debris
in a giant ring around it.

Let's focus on the debris -

little bits going around.

Little bits occasionally collide
with other bits,

and they go around
and collide with other bits

and go around.

And ultimately, you end up with planets

and the ring, the rest
of the ring has disappeared.

The planets are made
of the same stuff as the sun,

which, you remember,
is made of the stuff that was here,

plus stuff from there,
plus stuff from there.

It makes eight planets,

not Pluto.

(Laughter)

Pluto was grabbed later.

You can think of Pluto
as an adopted child, if you wish.

The rest are birth children.

So, these planets are basically
made of the same stuff as the sun,

which is made up of stuff that was here,

plus debris from exploding stars
actually all over our galaxy.

It's not that we can say,
"It was that one and that one."

It's that one and that one
and that one and that one

and that one and that one
and that one and that one -

doubly enriched with all
these useful chemical elements.

And the planets, likewise,
are of the same stuff.

There has been some change

in that the planets
closest to the sun got hot,

and the material that most easily
evaporates has boiled off.

Further out, you can see
the original composition rather better.

But that's broadly what has happened.

So we who eat the plants and the animals
that absorb elements from the earth,

we are made
of the same stuff as the earth,

and the earth is made
of the same stuff as the sun,

which is made up
of the rest of the galaxy.

So, iron was created
in those very massive stars,

ones that went through
lots of nuclear reactions.

And that iron was made available

by the catastrophic death
of those big stars.

So there's life and death already.

If it wasn't for those stars,
particularly the ones that had died,

we would not be here.

And we're intimately and ultimately

children of the stars,

to such an extent that, actually,

we are stars.

Thank you.

(Applause)