How the Universe Works

There's a killer lurking

in our galaxy, a star ready to explode into a supernova.

These are the most visually stunning

events in the universe.

Seen from Earth, it would have a terrible beauty.

But for us, it could be fatal.

In a few seconds, it can release as much energy

as the sun will over its entire lifetime.

We're trying to hunt it down, but it's lying low.

We haven't seen a supernova in the Milky Way

in over 400 years.

It could be anywhere.

It is nearly impossible to predict where and when

the next supernova will happen.

The hunt is on to find the next supernova

before it finds us.

October 2019, one of the brightest stars in the sky

looks dangerously unstable.

If you look at the constellation of Orion,

one of the shoulders of Orion is a star that is obviously red.

This is Betelgeuse.

I could go into my backyard and see it.

You could clearly see that it was getting dimmer.

Is this a warning?

Is Betelgeuse about to die in a massive cosmic explosion,

a supernova?

We've been studying this star for hundreds of years.

And one thing we're sure about is that it's big, very big.

Betelgeuse is a massive star, maybe 15 or 20 times

the mass of our sun.

And it's near the end of its life.

It is a massive, enormous, luminous star.

And one day, it's going to go boom.

Betelgeuse is on our list of supernova candidates

because of this massive size.

The bigger star they are, actually the

shorter the lifespan.

The lifespan of a star depends

on a delicate balance between two competing forces...

Gravity pulling in and heat and pressure pushing out.

Stars exist because they're held up.

They're not held up by pillars.

They're held up by energy flowing out of the core

toward the surface of the star.

That stops the gravitational contraction.

Stars get their energy

from nuclear fusion reactions right in the core.

And the most basic one is taking two hydrogen atoms

and slamming them together to form a helium atom.

And you might think, OK, the more hydrogen you have,

the more stuff you have, maybe the longer the start will live.

Turns out it's exactly opposite.

The reason... gravity.

The more mass a star has, the stronger

its gravity, gravity that crushes its hydrogen

atoms closer together.

As you crush things more and more,

the temperature gets hotter and hotter and hotter.

And the nuclear fusion reactions burn faster.

So bigger stars burn their fuel very, very quickly

and live short lives.

Smaller stars burn their fuel much more slowly

and live long, protracted lives.

So when you are a big star, you live fast

and you die young.

Betelgeuse burns brighter than 125,000 suns.

But now it's running out of its hydrogen fuel.

So it's burning whatever it has left just to stay alive.

Stars are basically factories for burning

hydrogen into helium.

And then, once the helium is burned,

they start burning heavier and heavier elements, like carbon

and nitrogen and oxygen.

It's a little like, you burn something, you get ash.

But then if you crush the ash enough,

you could burn it again.

And then you crush it some more, and you can burn it yet again.

But this process can't go on forever.

As the size of the atomic nuclei being fused together grows,

the amount of energy released falls.

The fuel the star needs to resist the pull of gravity

is running out.

Unfortunately, the amount of energy

you can extract by putting two nuclei together

gets smaller and smaller the bigger the nuclei are until you

come to making iron, and iron, it turns out,

is the last thing you can make that way.

The problem with iron is, when you fuse it,

it doesn't make energy.

It takes it away.

So when the star builds up that iron core, it's doomed.

It can no longer create energy in its core

to flow out toward the surface strong enough

to keep it from collapsing.

So collapse is what they do.

In a fraction of a second,

the star's core collapses down from the size of a planet

to about the size of a small city.

And when that happens, all hell breaks loose.

A huge amount of energy

is suddenly released, which forces

the collapsing layers back out.

The result... an enormous explosion we call a supernova.

The shockwave from a supernova

rips out at thousands of miles per second.

And for a brief period of time, they're

brighter than an entire galaxy.

A supernova could devastate life on Earth.

And the evidence can be found at the bottom of our oceans.

There are layers and layers

of silt that have built up.

And there seem to be a layer, about 2.6 million years ago,

that was enriched in a very strange chemical element,

something called iron-60.

Iron-60 is a radioactive isotope of iron,

and it doesn't last very long, just a few million years.

And the only place that we know of that can make iron-60

is a supernova in an exploding star.

That means there must have been a supernova close enough

to the Earth within the past couple of million years

to have physically deposited material on our planet.

That freaks me out.

The sign of this shocking assault on our planet

is a thin layer of this very rare type of iron.

We find it in the mud of every ocean floor

and always at the same depth.

This interstellar dust must have drenched

our world in one enormous burst 2.6 million years ago.

It was a terrible time.

A third of large animal species in the sea suddenly died out.

There were some pretty amazing fish.

Probably the most amazing is the megalodon,

a giant shark... teeth the size of dinner plates and so on.

But they went extinct 2.6 million years ago

at the end of the Pliocene.

What happened?

A lot of sea creatures died.

And a lot of them were in shallow waters,

whereas deep-water animals tended to survive.

That sounds kind of like a supernova.

That can do things that would affect our atmosphere,

would affect shallow water, but not deeper water.

Supernovas create huge amounts of cosmic rays.

When they crash into other atoms,

they break up and produce showers of dangerous shrapnel

called muons.

These charged particles are similar to electrons,

only 200 times heavier.

So they penetrate more deeply and cause more damage.

They can pierce through our atmosphere,

pierce through our skin, get into a cell,

and disrupt the DNA.

They'll go right through a mouse but deposit

in the body of a larger animal.

So the impact on an animal the size of a megalodon,

say, could be pretty extreme.

Muons can shatter DNA, causing mutations and cancer.

But their power weakens as they travel

through water, which may be why only

deep sea creatures survived.

The extinction really tells us that we're not

separate and apart from the universe and the goings

on up there, right?

Supernova going off and things like that...

OK, it's a pretty light show.

No.

It is a direct impact to life on Earth and us.

So are we in danger of extinction?

Is Betelgeuse about to explode?

When stars explode as supernovas,

they can devastate planets hundreds of light years away.

Betelgeuse is about 550 light years from Earth.

So, when it dramatically dimmed in 2019,

scientists were concerned.

But Betelgeuse has dimmed before.

Betelgeuse varies

quite a lot over the years.

There are some cycles, and sometimes

these cycles come together, and you get a deep minimum.

So dimming is part of the star's natural cycle

as it nears the end of its life.

But to get a full picture, we took Betelgeuse's temperature.

If the star was dimming, that would mean that the surface

was cooling over time.

We actually made measurements of the temperature of Betelgeuse

and found out that wasn't happening.

It hardly cooled at all.

It cooled, like, 50 or 100 degrees.

You might expect a much, much more

dramatic change in the surface temperature

if it were about to explode.

So, if Betelgeuse wasn't cooling

much, what was making it dim?

To take a closer look, we used a very large telescope

and an exoplanet hunting instrument

called SPHERE and came up with an extraordinary image.

When I first saw this image of Betelgeuse, it blew me away.

I almost gasped.

I may have said a word I can't say on TV.

That was very exciting.