Original subtitles

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.