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Hidden within our ever-expanding
cosmos are enigmatic monsters.

Monsters with a huge amount
of matter piled into

their small volume, that bend
spacetime into a bottomless chasm.

These monsters are black holes.

Black holes have long held
the imaginations of many scientists.

Yet it wasn't until recently
that we were able to image in detail

what lies around them.

As a result, they still remain
pretty poorly understood.

But we know they hold a key
to many of our universe's mysteries.

They may even have shaped the cosmos
around us, which is why

understanding them is fundamental.

This month, join us on
The Sky At Night,

as we take a mind-bending journey
into the cosmological mysteries

of black holes.

I'll find out the news
from the largest

gravitational observatory
in the world.

It's a completely new window
on the universe.

I'll be reporting a new discovery -

one of the biggest
supermassive black holes yet.

33 billion times
the mass of the Sun.

Dr George Dransfield visits a lab
where black holes are investigated

right here on Earth.

And our in-house stargazing expert,
Pete Lawrence,

tells us what we can all see in
our night skies this month.

Welcome to The Sky At Night.

In our Milky Way alone, it's thought
that 180 million black holes exist.

And these strange objects push
the limits of our understanding

about matter, space and time.

The first person to think seriously
about black holes

was English scientist John Mitchell
back in 1783,

though he called them dark stars.

Today, Mitchell's discovery
is a crucial part of astrophysics

and cosmology, but there's
still lots to learn

about this cosmic phenomena
and how they're born.

Black holes vary in size and mass -
properties set when they form.

To understand a black hole,
we must first understand

the life and death of stars.

I'm meeting with astrophysicist
Dr Becky Smethurst

from the University of Oxford
to discuss stellar evolution.

Becky, there are lots of different
types of star in the universe.

How do we distinguish them?

So we distinguish them by
their temperature and their mass,

so how big they are.

So let's start with the smallest
of stars, the least massive stars.

They don't actually have to burn
their fuel as quick to actually

resist that crush
of gravity inwards.

And so they live a lot longer.

Stars burn hydrogen into helium,

releasing energy that keeps
the force of gravity at bay.

And then on the other end
of the scale,

you've got the massive stars
that are huge and blue,

because they're so much hotter,
because they're burning their fuel

that much faster to counteract
gravity crushing inwards.

The Sun is sort of in the middle
of those two extremes.

It is burning hydrogen into helium
in the same way that the others are,

but not at the same rate
as a massive star,

but still much faster
than a less massive star.

So a star at this phase of its life
is this balance between gravity

and then energy coming out
from the centre.

But all stars will eventually
run out of fuel at their centres

and gravity will crush them.

Stars similar in size to our sun
will become white dwarfs -

dense, stellar cores.

But more massive stars have a
much more dramatic ending.

So, with more massive stars, let's
say ten times the mass of the Sun,

when that runs out of fuel
in its core in the very centre,

we start to reach
what we call a supernova.

So my favourite analogy to describe
this process is if you take

a tennis ball to represent the core
of the star,

then you take a ping-pong ball
to represent the hydrogen gas

around the outside.

But you put the ping-pong ball
on top of the tennis ball

and drop them together.

As the tennis ball bounces
off the ground,

it then gives all of its energy
to the ping-pong ball,

and the ping-pong ball
will then go absolutely flying.

And essentially the same thing
happens in a supernova.

The hydrogen that's very light
rebounds off that very,

very dense core and gets thrown
out in the supernova process.

A supernova, a catastrophic
explosion in the night sky,

an eruption capable of outshining
an entire galaxy.

And these supernovae are bright.
We see them across the universe.

Yeah, we do. They're so incredibly
bright when this happens.

We actually saw one recently
in M101, and this was thanks to

amateur astronomers
who were observing M101

for a completely different reason.

The images captured
enabled scientists to pin down

the occurrence of the explosion
to within an hour.

This was a core-collapse supernova,

and all that remains
is a dense ball of hot gas.

What happens next, again,
depends on its mass.

But what happens to the core
of a star that's gone supernova?

If you go up to things around about
25 times the mass of the Sun,

that crush of gravity downwards,

we don't know of any process
that is able to resist that crush.

You've crushed it so dense,
it's now dense enough that light

cannot escape from that object.

And that is when you
end up with a black hole.

Black holes are objects
with immense gravitational force.

An invisible line, called
the event horizon,

encircles the black hole.

Anything that passes the

event horizon will never escape,

including light itself.

At the centre of the black hole

lies the singularity -

a point where matter seems

to become infinitely dense.

The first thing that we always talk
about with a black hole

is the event horizon.

That is that point at which
you'd have to be travelling faster

than the speed of light
to escape the pull of gravity

of the black hole.

What's beyond the event horizon,
though, is anybody's guess, really.

Because light can't escape,

we can't get any information
from it or any data to know

what that looks like. Because
nothing can escape? Exactly that.

Mathematically, we'd describe
it as a singularity.

So where we've taken all the matter
that used to be in the core

of that star and we have crushed
it down into an infinitely small,

infinitely dense point.
And there's this point in the middle

where we know our physics
breaks down? Exactly that.

And it's where our best theory
of gravity,

Einstein's theory
of general relativity,

can't explain what's going on
any more at that exact point.

Light is trapped inside
these deep gravity sinks,

making black holes difficult
to observe and study.

Perhaps because we can't see them,

black holes are often dubbed
as one of the scariest things

in the universe.
They have a bad rap.

But is this true?

Everybody loves a black hole.

But I wouldn't get too close.

Get too close, and nothing -
not even light,

the fastest thing in the universe -
can escape its gravitational pull.

The place where this happens
is called the event horizon.

But in truth, you'd be doomed
long before you got there.

The difference in gravitational pull
between your feet and your head

would likely pull you apart -

a process charmingly known
as "spaghettification".

Some think black holes are
surrounded by intense firewalls,

which would destroy anything
that passes through them.

And we've seen the effect
of black holes ripping stars apart

from halfway across the universe.

In science fiction, black holes
are always spacecraft-menacing

creatures of the night.

They're famous for messing with time
and threatening starships,

like in the 2014 film Interstellar.

The language we use to describe
them reflects this.

We always say they're "lurking"
at the centre of a galaxy.

I think we called them monsters
at the start of the programme.

But actually, black holes
are friendly beasts.

Look at this movie made
by astronomers who go each year

to telescopes on mountaintops
in Chile and Hawaii

to stare at the centre
of our galaxy.

We're watching stars move in orbit
around something that must weigh

as much as a few million suns,
and yet be crammed into a region

smaller than our solar system.

This has to be a black hole
lurking menacingly at the heart

of our galaxy.

But those stars
aren't in any jeopardy.

They're no more going to fall
into the black hole than the Earth

is going to fall into the Sun.

Stay clear, and you can enjoy
a black hole safely.

They can even play a role
in lighting up the universe.

If I want to turn matter into light,

it turns out that creating a star
is a very inefficient way

of going about it.

What I should do instead
is take the matter and throw it

towards a black hole
where the conditions in the disc

that forms around it will make
the matter glow brightly.

Black holes - your friendly
galactic light source,

enlivening the universe -
not a hideous threat to be feared.

Unless you're spaghettified,
of course!

MAGGIE ALDERIN-POCOCK:
The nearest black hole to Earth

was found just a year ago.

It sits a mere 1,600
light years away.

And although it's
on our galactic doorstep,

the strong gravitational pull
at its centre hides it from sight.

The elusive nature of black holes
means that scientists

have had to develop ground-breaking
techniques in order for them

to reveal their hidden secrets.

One of these techniques involves
the detection of an unseen,

yet incredibly fast ripple
in space and time.

Enter LIGO.

LIGO is one of the world's largest
physics experiments,

based in the US,
with its data shared by scientists

around the world.

I've come to meet
one of the scientists,

Dr Tessa Baker from
Queen Mary University of London.

So, Tessa, you're a cosmologist
and you work on data

for an instrument called LIGO.

So what is like LIGO?

So, LIGO is a special
kind of detector

that is unlike any of the other
sort of regular telescopes

we have in astronomy. So your
optical or sort of infrared

or anything like that?
Completely different,

completely different set-up,

and detects a completely different
kind of signal

that comes from the universe,
called gravitational waves.

Gravitational waves are minute
ripples in space-time

caused by violent and energetic
processes in the universe.

This is one of the big discoveries
that Albert Einstein had.

He realised empty space,

you should think of it almost
like a fabric or a fluid.

It's something that can be bent
and stretched and distorted. Yes.

So space and time are kind
of unified into this single thing,

and massive objects
cause that to bend. Yes.

It's only a little bit of a step
further to realise

that if it can bend,
it can also carry waves. Yes.

So a disturbance in space-time
will cause ripples

to travel outwards, like ripples
on the surface of a pond.

Oh, like... Yes, I see!
So, little demo here.

Let's make some ripples!
MAGGIE LAUGHS

So the pond here is
our space-time... Exactly.

..and that was
a gravitational wave. Exactly.

These gravitational waves
are created by binary systems,

which are systems,
in this case black holes,

that orbit each other,
dragging space-time along,

creating ripples that LIGO detects.

The signals that we get
have a wave pattern.

And what we see is that wave
signal gets faster and faster,

and larger and larger as
our black holes get... Ooh!

..closer and closer together,

and then it dies away once they've
merged. Once they've merged, OK.

That's just because they're not
making the ripples any more?

Exactly. Yes. Exactly. And from
looking at the spacing

of those waves, we start to build up
a picture of the

population of black holes.

So, what are their masses?

How much do they weigh?

How fast are they spinning,
for example?

And those properties in turn
are related to how they formed.

So what kinds of environments
do black holes form in?

What kinds of stars
give rise to them?

So all of those are things
we can extract from the data.

The ripples need to travel
light years to reach us,

and by the time they do,

they are so small that LIGO
has to be extremely sensitive

to pick them up.

So how does LIGO detect
these gravitational waves?

So LIGO is made of two
laser beams,

and these two laser beams
are at right angles to one another,

like so. OK, yes. And so there's a
laser shining in here,

it's travelling along both
of these rubber band arms,

it's bouncing off mirrors
at the end,

and the lasers are coming back
together here at the central point.

And the whole thing's set up
so that those two laser beams

cancel each other out.

Now, when a gravitational wave
comes through,

what it's going to do
is it's going to cause the length

of these laser arms to change.
Right.

And that's going to oscillate
backwards and forwards

whilst the gravitational wave moves
through, and there'll be pulses

of light coming out
and reaching our detectors.

This laser interferometer can detect
changes in the length of those arms

that are about one ten
thousandth the size of a proton.

So these laser arms are actually
about 4km long,

and they need to be that long
to be sensitive

to such small changes.

With each observation run,
LIGO has improved its detection,

with 90 merger events
confirmed so far.

And now its fourth
run is under way.

So you're on upgrade four.

So what are you going to get
out of this?

What will these new upgrades
give us?

So every time we upgrade
the detectors,

we make them more sensitive,

and that means we detect more
and more gravitational waves.

And the more we get,
the more we're able to probe,

rather than just seeing
the tip of the iceberg.

How can you detect that
sort of tiny movement,

compared with all the noise
that is around?

These interferometers pick up
the pounding of waves

on the ocean, logging that's
happening hundreds of miles away,

motorway traffic,
even quantum jitter of the atoms

in the detectors themselves. Yes!
MAGGIE LAUGHS

There's a couple of observatories
around the world.

If we get a sort of fluctuation
in one of the detectors,

we can check whether it's real
or not, because a real detection

would be seen by all
the detectors at once.

It's a completely new window
on the universe,

and we're now able to hear
the universe as well as see it,

and that enables us to answer lots
more questions than we could do.

LIGO detects high-frequency
gravitational waves.

But recently, astronomers
around the world detected

lower frequencies
beyond LIGO's range.

These are produced by even bigger
black holes and are helping us

tune into the cosmos even deeper.

Facilities like LIGO
have opened our eyes

to an unseen universe.

I can't wait to see what the
next round of data from LIGO

will reveal to us.

CHRIS LINTOTT: Many black holes
have a mass similar to our sun

and are just a few miles across.

But others are much larger,
containing the same amount

of material as 100 million suns.

These elusive beasts are known
as supermassive black holes,

and new technology is enabling
scientists to observe them

like we never have before.

In 2022, scientists revealed
the first ever image

showing the shadow
of Sagittarius A*,

our Milky Way's
supermassive black hole,

nearly 50 years
after it was identified.

The shape we see is formed
in part due to a phenomenon

known as gravitational lensing.

And earlier this year, the same
effect was used to discover

something even bigger.

I'm meeting Dr James Nightingale
from Durham University,

who discovered one of the largest
supermassive black holes to date.

So, James, what have you found?

What myself and my research team
found is a black hole

at the centre of a galaxy,
which is 33 billion times

the mass of the Sun.

So this is a black hole
that's, you know, over three times

more massive than all
of the stars in our galaxy,

everything we can see with our eyes
in the night skies.

So black holes are elusive,
even big ones.

How did you find this one?
Yeah, so the way we found this one

was actually quite unique.
It's the first black hole found

via this technique,
gravitational lensing.

So people often describe
these as cosmic telescopes.

They are the telescopes
of the universe,

magnifying distant light,

such that it comes
into our telescopes on Earth

with sort of
much brighter emission.

A gravitational lens can occur
when something massive distorts

and magnifies the light
from distant galaxies behind it.

It's like looking through
a giant telescope.

Typically, we observe the light
come straight across the universe

into our telescope.

But if there was a galaxy
between us and that galaxy,

the light of the background galaxy
would actually take kind of

a curved and distorted path
around the foreground galaxy

and into our telescope, and it might
actually take multiple paths.

One way we can sort of represent
that is actually using a wine glass.

When you use this wine glass
to be our sort of foreground

lensing galaxy, causing the path
of light to be distorted,

the light of the candle starts
to stretch, and so we create

these sort of sheared,
stretched, distorted arcs.

Yeah, so I don't see it as a point
of light, but sort of an arc,

as you say.

Think of the candle as a distant
galaxy, and the wine glass

is the lensing galaxy
in front of it.

When we look at the distant galaxy
through the lens,

the light bends through it.

The Hubble Space Telescope captured
an image of this distorted light

and enabled the team to discover
that it was in fact a black hole

that they were looking at.

What we're looking at here is
an image of the gravitational lens

where we believe to have found
this black hole.

What's actually important is this
small sort of sliver of light

that we can see here
next to the lens galaxy.

That's where all of the information
on the black hole

happens to be contained.

So this is what's called the
counter-image, that's happened

to take a really different path
around the lens galaxy

and into our telescope.
So that's the exciting thing here.

Yeah, yeah.
It's this ultra massive thing.

How do you get
the mass of the black hole?

We shoot millions of realisations
of these light rays,

and what we found
was, the counter-image,

we couldn't quite reproduce
something that looked exactly

like this. By including
a black hole, the images begin

to look more and more like the data.

On the other hand, we have obviously
tried models where the black hole

was much larger. The gravitational
lensing due to the black hole

would be too strong. So it was
only when we hit this sweet spot

in and around 33 billion times
the mass of the Sun

that what we produced began to look
like this Hubble Space Telescope

image of the gravitational lens.

In order to measure the masses
of distant black holes,

astronomers have really only
been able to do this

when they're active.

So the fact that we've been able
to measure a black hole mass

in a non-active galaxy that's...is
kind of opening up a new window

onto the universe, we're able
to find and study black holes

in distant galaxies
that were otherwise invisible.

James's hunt for supermassive
black holes doesn't stop here

and will be given a helping hand
by ESA's brand-new mission.

But this is, of course,
one galaxy and one black hole.

What's next?
Are you still black-hole-hunting?

The thing I want to turn
my attention towards is trying

to understand how big are the
largest black holes in the universe.

And so my hope is that by finding
more gravitational lenses

around, you know, the biggest, most
massive galaxies in the universe,

we can use the technique again
to measure the masses

of the most massive
black holes in the universe.

But of course, for gravitational
lensing, you have to be lucky.

You have to have a distant
galaxy and a nearby galaxy

in just the right place.

There are new missions that will
help you find more lenses, right?

So the European Space Agency in July
launched the Euclid Space Mission.

So the expectation is this thing
is going to find potentially

hundreds of thousands
of gravitational lenses.

You know, in the context
of astronomy today,

that's an incredible step forward
over the number of black holes

that we know right now.

We might not be able to see
black holes, but we can see

the stars around them.

Our in-house stargazing expert,
Pete Lawrence, is here to show us

how to spot the next best thing
to a black hole this month.

August is a fantastic month
for spotting the planet Saturn

at its best and brightest,

as well as the constellation Cygnus,
which represents the swan.

Within this swan lurks
a formidable X-ray source.

The first black hole discovered,
7,000 light years away

from Earth - Cygnus X-1.

Even though we can't see Cygnus X-1,

the constellation of Cygnus,

which surrounds it,
is one of the easiest to spot

in the summer and autumn months
from the Northern Hemisphere.

It looks fantastic to the naked eye
and if you've got a telescope,

there's loads of interest
to see there.

If you look directly overhead
as darkness falls,

you should be able to find Cygnus.
It's very distinctive,

thanks to the Northern Cross
asterism at its centre.

Once you've located the cross,
look for the brightest star, Deneb,

and the double star, Albireo,
at either end.

That's the tail and the beak.

About one third of the way
along from Deneb to Albireo

are the wings, joining the
Deneb-Albireo line

at the star Sadr.

If you travel further
towards its beak at Albireo,

you'll get to the star Eta Cygni,

with the supergiant star
HD226868 nearby,

which is in mutual orbit
with the black hole.

Another impressive sight this
August is the planet Saturn

reaching opposition, an annual event
where the Earth is positioned

between Saturn and the Sun,
making Saturn look at its biggest

and brightest.

Through binoculars,
Saturn looks like a bright,

slightly oval disk,
but a small telescope

will easily reveal its rings.

If you miss the 27th of August
opposition, don't worry.

Saturn will remain visible
in the evening sky,

offering excellent
viewing opportunities

for many weeks to come.

Don't forget to show us
your photographs

of this year's opposition and upload
them to our Sky At Night Flickr.

We'll share our favourites
on the programme.

Since last month, you've been
sending us your stunning images

of the supermoon
and Perseid meteor shower.

MAGGIE ALDERIN-POCOCK: Observing
black holes out in the cosmos

is proving a success,

but testing theories in space
comes with its restrictions.

Exoplaneteer Dr George Dransfield
is in Nottingham

to discover a solution.

The universe can't always
be understood through observation,

as there are times and places
that are simply out of reach

to our instruments.

Now, we may never be able
to peer inside a black hole,

but we can use our knowledge
of the cosmos,

combined with emerging
theories of physics,

to do some rather
exciting science.

I'm meeting
Professor Silke Weinfurtner

at the University of Nottingham,

who is taking the theoretical
physics of black holes

and putting them to the test.

So, behind you is what looks to me
like a giant fish tank.

How is this testing the theoretical
physics of black holes?

What we are looking at is a system
that could mimic how black hole

interacts with its environment.

Very strange physics unique to
black holes is actually something

that occurs really naturally
in many, many different systems.

Yeah. So we use fluids as simulators
for black hole physics.

Black holes and their effect
on the surrounding environment

are hard to observe directly.
So instead of relying on theory,

Silke experiments with
a fluid system that behaves

in a similar way.

How are these fluid systems here

creating a black hole analogue
that kind of mimics

what a black hole is doing?

And so what we have here
is a vortex flow.

It's the same thing you would get
when you're sitting in your bathtub,

when you pull the plug,
you see the vortex forming. Yeah.

And what we're interested in
is the interactions

between the wave and the vortex.

And what you can show
mathematically, that the equation

that describes this interaction

is the same as you get for waves

around a black hole.

And if there's the same mass,
then you should expect

the same physics to occur.

Silke uses water as a testing ground
for hypothesised characteristics

of black holes,
such as superradiance -

a phenomenon where intense
spinning amplifies the energy

of light waves.

If we study light waves
around black holes,

light interacts with the black hole
and it comes back

and it has increased
its amplitude,

meaning it is brighter. OK.

So it's a really bizarre
and intriguing process.

It hadn't been observed
in nature ever before. OK.

So through this analogue system,
you can turn these abstract ideas

into reality.

So we are not using light waves,
but we're using water waves,

and this wave machine
creates wave fronds. Yeah.

These straight lines that go,
propagate towards that vortex.

The waves get taller
as they're deflected away

from the rotating vortex,
a result identical to light waves

after interacting with a certain
area around a rotating black hole.

I think I did actually see
the waves on the other side

get higher in amplitude.

That's wicked cool!

Well, it was quite exciting that
after many, many months of work

that we could see superradiance
as predicted around black holes,

to observe it in an analogue
gravity system for the first time.

Observing phenomena experimentally
in the lab can open windows

to new possibilities
of space experimentation.

If we learn how to detect
these effects

in these analogue systems,

perhaps we can also get some clues
of how to start looking

for them in outer space.

CHRIS LINTOTT: Black holes were
thought of as theoretical

for so long,
and then as curiosities.

However, researchers now reveal
just how fundamental they are

to how our universe has evolved.

With new results from JWST,
LIGO, and analogue experiments

here on Earth, the mysteries
of these cosmological anomalies

are being revealed.

That's all we've got time
for tonight.

But do join us next month
when I'll be out in Chile

getting a sneak preview
of the ELT -

the Extremely Large Telescope.

And the month after that,
we'll be doing

our Question Time special
from Exeter.

So do send in your questions
to the email link below.

Goodnight.