Secrets of the Whirlpool Galaxy The Sky at Night

On this month's Sky at Night,

we'll be taking you to one of the most spectacular

and fascinating places in the whole night sky.

It's officially known as M51,

but most of us know it by its more romantic name -

the Whirlpool Galaxy.

M51 is a stunning sight.

But it's more than just beautiful to look at.

It is also one of the most fascinating places

we have ever discovered.

Its sculptured spiral arms are a maelstrom of star formation...

..lit up by the light of hot young stars.

But it is also a stellar graveyard,

in which we can see neutron stars and black holes

tearing other stars apart.

So why is this galaxy so active,

and how did it get its majestic

and surprisingly prominent spiral shape?

Welcome to The Sky at Night.

We're making this Whirlpool Galaxy Day.

On this one day,

we're going to be pointing as many telescopes as we can at the object.

We've got amateur back-yard telescopes,

professional instruments like the dish behind me,

and on mountaintops around the world, large observatories,

several of which have got better weather than here in Cambridge!

We'll even be going above the weather, because we've got

special permission to use one of Nasa's space telescopes.

We'll be looking M51 at different wavelengths of light and different

magnifications to create a unique portrait of this remarkable galaxy.

We've come to the Mullard Radio Astronomy Observatory

just outside Cambridge.

Founded in 1957,

it helped pioneer a completely new way of observing the sky.

It was here...well, actually, just over that hedge,

that Jocelyn Bell discovered the first pulsars

and, later on, we'll be using some of the modern instruments

that still operate on the site

to take a close look at the Whirlpool Galaxy.

But our exploration of M51 doesn't start in this slightly damp field.

It starts somewhere with much clearer skies.

It's nine o'clock in the morning,

it's a terrible time to be trying to observe a galaxy,

at least from here, but this laptop is connected

to a telescope in Hawaii, where it's still dark.

It's a 0.4-metre telescope on top of the island of Maui,

on a dormant volcano, and you can see from this camera

just outside the observatory that they've got brilliant, clear skies.

See a couple of planets, the nice Milky Way,

and this region of sky, where M51, our target, is located.

Now, the telescope's already slewed to the source

and it's taking a five-minute exposure

and so, in just a few minutes,

we should see our first image of the Whirlpool Galaxy.

Well, there it is, this is our image,

and you can see immediately the spiral arms in the galaxy

which wind round this central, bright nucleus,

and then the second thing you notice

is that there are two galaxies in the frame.

There's this companion which has stretched out this bright material

from the main galaxy, so you get this arm joining the two,

and there's lots of faint gas and dust around that secondary galaxy.

And really that's what the Whirlpool Galaxy gives us -

it's a chance to watch a collision between two galaxies in action.

It's something we can't see this well anywhere else in the sky.

It really is a beautiful image.

Maggie showed this image to galaxy expert Rob Kennicutt

to ask about the Whirlpool's amazing structure

and its relationship with its companion.

It is absolutely wonderful.

So there's two spiral arms. They look almost perfect.

The classic spiral galaxy. So how are they formed?

Almost all disc galaxies like this system do have spiral structure

but, as you say, these are spectacularly prominent arms,

and were quite certain now

that to produce such strong spiral structure,

you need the driving force

of interaction with a companion galaxy.

Now, you see, to me, that seems counterintuitive,

cos when I think of collisions, I think of chaos,

things being thrown everywhere.

I don't think of structure being formed that way.

I agree with you - it's not intuitive,

but this computer simulation is designed to show

how these collisions between galaxies

can excite spiral structure.

So you have two galaxies, disc galaxies, much like the spiral.

It's looking a bit messy now, but... Wow!

You see, actually, the spiral forming due to the collision,

and in both galaxies.

But in this simulation, we had two similar-sized galaxies.

What if you had something that was more akin to the Whirlpool Galaxy?

What we have here is a second computer simulation,

and this one is actually specifically tailored

-to try to reproduce the two galaxies in the Whirlpool system.

-Right.

So this kind of spiral structure you see in the beginning

is actually the sort of spiral structure

that is common

in galaxies like the Milky Way.

So, even before the interaction,

Whirlpool probably was a spiral galaxy,

but now you see the companion making its appearance.

Now, that doesn't look like a companion galaxy.

It looks more like a point mass.

I think that's right, to save on computing time,

I think they modelled the companion galaxy as a point mass,

and now you're beginning to see its effects already -

the spiral pattern is being amplified, and let's keep going.

-And there you are.

-Whoa!

You've got the two distinct spiral arms,

and you've got the companion galaxy at one of the ends

-of the spiral arm, so that is a good recreation...

-Indeed.

..of the Whirlpool and its companion.

But notice this is 300 million years after the start of the calculation.

Yes.

-But you think of this as now, in our world, right?

-OK, yes.

So now, the computer's going to predict

what this system will look like in the future.

-Ah, the future of the Whirlpool Galaxy.

-Indeed.

So, as you see, the companion continues to get closer

and closer to the centre of the spiral,

it's still on the way in, and as it goes,

the spiral arms are amplified even more than today,

-if that can be imagined.

-Yes.

-But it's getting a lot messier!

-Indeed.

It's now reached closest approach, and on the way out,

-it's leaving kind of a train wreck behind.

-Yes!

And now, you see, the companion is coming in for a second time,

and the movie stops at this point, but if we were to continue it,

the two galaxies would totally merge together.

But that means that the Whirlpool Galaxy in all its perfection,

as it looks today, is transitory, so it will pass through that phase,

and at some point end up maybe more like this.

Yeah, indeed, and look at the scale, in about 70 million years,

the phase of spiral structure will be long over,

and you'll be left within 100 million years

with one galaxy where there were two before.

So, it just makes me appreciate the Whirlpool Galaxy all the more,

because we can appreciate its beauty right at the moment,

-but it won't last!

-Indeed.

The telescope in Hawaii actually took three pictures of the galaxy,

each at a different wavelength.

When composited together,

they give us a colour view of the Whirlpool in all its glory.

But it's not just professional telescopes

that can capture spectacular images like these.

Pete has been trying to demonstrate

how you can view the galaxy for yourself.

And he's been exploring the history of our observation of this galaxy.

Astronomy in the UK can be a bit frustrating at times,

and the clouds have now come in,

and I really don't think I'm going to get a view of M51 tonight.

So welcome to the great British summer!

It really is a pity that it's cloudy this evening,

because M51 is one of the best deep sky objects up there.

And at the moment, it's really high up in the sky,

which means it's well away from any murk close to the horizon,

and that will give you a good view of it.

And it's pretty simple to find, too.

To locate it, first identify the Plough or Saucepan,

which is part of Ursa Major,

and is one of the most recognisable patterns in the entire night sky.

Identify the star in the middle of the handle

and the one at the end of the handle.

Draw a line between them and turn by 90 degrees,

and move for about half that distance again,

and that'll take you to exactly where M51 is in the sky.

You can't see M51 with the naked eye,

but it is possible to see it with just a pair of binoculars.

And if you've got a telescope like this, then it looks quite amazing,

and that will also allow you to take a picture of it.

And I've got a picture here I took a little while ago,

and you can really start to make out some of the structure of the galaxy.

You've got the spiral arms, they're very evident there.

You've also got the little satellite galaxy, as well.

That's very obvious.

So if you CAN get a view of M51,

it's a really rewarding object to have a look at.

'But not everyone has always thought that M51 was fascinating.'

The Whirlpool Galaxy was discovered by Charles Messier in 1773,

and added to his famous catalogue as the 51st entry, hence the name M51.

But Messier wasn't interested in how fascinating M51 was.

He was a comet hunter,

irritated by wasting his time pointing his telescope

at things that superficially resembled comets,

but which on closer inspection

were revealed to be something else entirely.

So he set about making a catalogue of what were, to him,

frustrating objects,

so he could ignore them in his search for comets.

But the irony was that in doing so,

he had compiled a list of nearly all of the most spectacular

deep sky objects -

nebulae like the Orion Nebula,

open star clusters like the Pleiades,

and galaxies like M51.

'To be fair, Messier probably couldn't resolve the Whirlpool

'to be much more than an indistinct, diffuse cloud -

'a nebula.'

The first time the Whirlpool Galaxy was seen in all its glory

was in 1845, when William Parsons, the third Earl of Rosse,

pointed a 1.8 metre reflecting telescope, based in Birr, Ireland,

which was then the largest telescope in the world, at M51.

And this is a sketch he made of that object,

and it's absolutely incredible.

There's so much structure to see here,

but what's really evident is the spiral nature of the galaxy.

And that was the first time this had ever been recorded

in a celestial object.

The picture was quite a sensation at the time,

and was published all over the world.

It's even suggested that his swirling drawing

was the inspiration for Van Gogh's Starry Night.

But we still didn't know what the Whirlpool was or where it was,

and it was only in 1924 that Edwin Hubble demonstrated

that nebulous objects like these

were in fact distant galaxies in space.

We now know it's about 30 million light years away,

and although much smaller than the Milky Way, the disc is huge,

measuring 60,000 light years across.

Images like these, taken by Sky At Night viewers,

clearly show why M51 is one of the most exciting places

in the universe.

But they cannot show us

many of the secrets that are hidden deep within those spiral arms.

'To reveal those secrets,

'we need to find different ways to look at the galaxy.'

These dishes are radio telescopes.

They're just a receiver, not too dissimilar to an FM radio,

but they're much, much bigger.

That's because the signals they're trying to pick up from space

are absolutely minuscule.

In fact, it has been calculated

that if you add up every radio signal ever picked up

by all the radio telescopes in the world,

the combined energy of that radiation

would be enough to melt just three snowflakes.

These telescopes are actually lining up right now

to come in line with M51, the Whirlpool Galaxy,

which lies around 30 million light years away in that direction.

These are the radio images

captured by the Mullard Observatory dishes.

Superficially, these pictures are not as impressive,

but they show details that we could never reveal

with conventional telescopes.

The result of intense magnetic fields

and the glow of hot gas around young stars.

And this remarkable image,

from the Very Large Array Radio Telescope in New Mexico,

reveals the distribution of hydrogen throughout the galaxy -

the raw material from which the stars are made

stretching far beyond the main disc.

And as Maggie's been discovering,

radio astronomy is just one of the many alternative ways

we have to observe M51.

For centuries, we only had one way of studying the night sky -

using telescopes that operated in visible light -

that tiny part of the electromagnetic spectrum

that our eyes can detect.

We can only see things if they're actively emitting light

or reflecting light in the visible part of the spectrum,

and if there's no source...

then we can't see anything at all.

But in addition to radio waves,

there's a lot more to the electromagnetic spectrum,

and if we tune into this,

then we can detect a lot more of what's out there,

hidden in the darkness.

Now, this is a camera that is sensitive to infrared light.

That's a wavelength that is slightly longer

than we can detect with our eyes,

and it allows us to see things

that would otherwise appear to be invisible.

We can see things in the infrared because of their temperature.

On Earth, all objects radiate part of their heat as infrared light.

How much and that what frequency depends on how hot they are.

It's just the same in space.

There's a lot of stuff hiding out there

that we just can't detect with visible light,

but if we tune our telescopes to detect infrared wavelengths,

then suddenly, a lot more is revealed.

Most of the infrared radiation from space is absorbed by the atmosphere,

so infrared telescopes have to be situated on top of tall mountains.

This is the Liverpool Telescope,

located at over 2,300 metres on the island of La Palma in the Canaries.

Late on the night of the 31st of May,

it took an infrared image of the Whirlpool Galaxy

especially for The Sky At Night.

This is the infrared image taken for us

by the Liverpool Telescope just last night.

Actually, it's two images,

because the galaxy doesn't fit on a single frame.

What we can see in this image is light from more stars

than we'd otherwise see in the visible.

By using the infrared, we're able to peer through the dust

that would otherwise obscure our view.

But this is an image in the near infrared -

we're only just past the red in the visible,

and we can go further than that,

and the colour here

represents the different wavelengths of infrared light.

You can immediately see there's a difference between the two galaxies.

The small companion galaxy is bright blue,

and in this image, blue light comes from old stars,

so this galaxy has an old stellar population -

there's not much going on there right now.

In contrast, the Whirlpool itself has this brilliant red glow.

That's light from the dust and gas, the fuel of star formation,

which you can see is spread throughout the disc,

and has this wonderful structure, not just the spiral arms,

but these filaments and these spokes in the disc, as well.

But if you look along the spiral arms themselves,

and only on the spiral arms,

you see these bright knots,

these bright blobs that are shining very brightly,

and these are nebulae -

they're places where thousands of stars are being born,

and it's the light from those young stars

that is causing these blobs to glow quite so brightly.

We have similar features in our own galaxy.

Places like the Orion Nebula, where stars are still being born today.

But this is happening on a much grander scale

in the Whirlpool Galaxy.

Each of these bright dots is a stellar nursery

100 times bigger than the Orion Nebula.

These are all signs that the spiral arms in the Whirlpool

are active - very active.

So what this tells us

is that it's not enough to have the raw materials for star formation -

there's dust and there's gas throughout the disc -

but it's only when it gets twisted up into these spiral arms

that it can become dense enough to form stars.

The spiral arms are where the action is.

But to find out what's actually going on in there,

we're going to need yet another way of looking at the galaxy,

and to use a very, very special piece of kit.

RADIO CRACKLE

MUSIC PLAYS ON RADIO

'Radio waves and infrared

'are both from the lower energy end of the electromagnetic spectrum.'

SHE TURNS RADIO OFF

'But we can also pick up higher energy radiation.

'Radiation in the ultraviolet and X-ray bands of the spectrum.'

They're both familiar to us.

X-rays can penetrate our skin and flesh, but not the bone,

and that turns out to be really useful medically.

UV rays from the sun are powerful enough to damage our skin -

that's what causes sunburn.

In space, this high-energy radiation is only generated

in really extreme conditions,

where the temperature is impossibly high - millions of degrees.

Now, UV and X-ray emissions coming from something as far away

as the M51 galaxy is so weak that it gets absorbed by our atmosphere,

so the only way to observe it is to get up above our atmosphere.

That's why we're incredibly lucky

to have been given time on one of Nasa's space telescopes, Swift.

And right now,

it's slewing its way round to set its sights on the Whirlpool Galaxy.

The Swift satellite sits in an orbit

almost 600km above the Earth's surface.

It is armed with telescopes designed to detect gamma rays,

ultraviolet, X-rays, and visible light.

And these are the images that the Swift telescope captured for us.

They show the galaxy in both ultraviolet light,

revealing the familiar spiral again,

and in X-rays that transform the galaxy

into a patchwork of bright points of light.

I asked astronomer Karen Masters what these images tell us

about star formation in M51.

When you, somebody who studies galaxies, look at this,

what do you see?

Well, obviously you can see the main body of the Whirlpool Galaxy here.

The spiral arms are really, really emphasised in the UV.

When we see a galaxy that's bright in the UV,

we know that it's forming stars vigorously,

and that's because it's only the very hottest, most massive stars

that glow so brightly in the UV,

and those massive, hot stars have very short lifetimes -

just tens of millions of years.

And that's pretty quick for anything astronomical.

That's hugely quick for astronomical timescales.

And so, when you see a galaxy that's very bright in UV,

you know that it's been forming stars vigorously.

Basically right now on astronomical timescales,

ten million years is nothing.

And how would this compare to our Milky Way?

Well, if you do the sums, you can sort of estimate

the star formation rate of the galaxy from that UV image.

You have it forming about five solar masses' worth of stars every year.

That's two, three, four, five times the rate of the Milky Way.

That's right, yeah, and the Whirlpool Galaxy is not as big

as the Milky Way, either,

so it's a galaxy smaller than the Milky Way,

forming stars at a faster rate than the Milky Way.

Well, that's the ultraviolet, but we can have a look at a different view.

So Swift is also an X-ray telescope, and here, slightly smaller,

is the X-ray view, and it looks like the spiral arms have disappeared.

What are we seeing here?

So, there's a nice contrast here with X-ray and UV.

The UV is picking out the birth of stars,

whereas the X-ray here is really going to be picking out

mostly the deaths of stars.

To create X-rays,

the very short wavelength, very energetic emission,

we need some of the most energetic processes

that happen in the universe,

and we need material that's falling onto massive objects,

and what we've got going on in the centre of these galaxies

is a supermassive black hole.

There's a supermassive black hole in the centre of pretty much

every galaxy, but again, what you're seeing glowing in X-ray there

is the material falling down onto that black hole,

and the gravitational energy it picks up

as it falls onto that black hole.

And the friction as it moves against the rest of the material

falling onto the black hole and orbiting it in this accretion disc

makes that material so hot that it starts glowing in X-rays.

The black hole at the centre of M51 is affecting its surroundings.

We can see that in this remarkable picture of the galactic core,

showing two doughnut-shaped rings of dust surrounding the black hole.

But the Swift image also shows many other sources of X-ray emission -

places where we must find equally extreme conditions.

Mostly, these are going to be binary stars in the galaxy,

but very special binary stars -

one of the pair will have come to the end of its main sequence life,

gone supernovae, and either turned into a neutron star or black hole,

and managed to do that without completely disrupting its companion.

But the companion then gets close enough

that it starts losing material onto that black hole or neutron star,

which spirals around it in an accretion disc,

and all of the energy that it picks up makes it incredibly hot,

and so starts glowing in the X-ray,

and so, you see stellar death, really, in this image.

And so, from Swift,

you might be forgiven for thinking there are just these few

interesting sources, but I've got another X-ray image,

this one from Chandra, which is a larger,

more sensitive X-ray telescope, and here,

you see not just the individual points,

the X-ray stars that we were looking at,

but now there's this purple glow, as well.

What are we seeing there?

This is hot gas, but not tied to individual stars.

This is now hot shocked gas in the spiral arms.

It's actually revealing the physics

which is starting the star formation in those spiral arms -

the compression of gas being driven very quickly from supernovae,

or winds from hot stars shocking it, and causing star formation to start.

Really, just seeing this glowing

is evidence that this is a dynamic place.

What's driving those changes?

The whole structure is driven by this companion

and the interaction with this companion,

and the timescales there are hundreds of millions of years.

So it did its last pass about 100 million years ago,

and a few hundred million years from now,

it's going to have merged completely with the Whirlpool Galaxy.

It's a strange thought, isn't it,

that it's going to change quite so utterly,

and this familiar shape will disappear.

I suppose we're just lucky to be able to see it

while all of this is going on.

This is the beauty of modern astronomy.

By using all the tools available

to look at M51 in different wavelengths,

we can reveal the secrets of the Whirlpool Galaxy.

We can understand its past and its future.

The optical wavelengths show us light

from just some of the stars in the galaxy,

radio reveals the distribution of gas,

and infrared, the clouds of dust from which the stars are made.

UV light detects the hot young stars formed from that gas and dust...

..and X-ray emissions pinpoint

what happens to those stars when they die.

And all these processes are concentrated in the spiral arms -

dense waves of material created by the collision

between the Whirlpool and its companion galaxy.

All this information allows us to build a portrait of the galaxy -

one that reveals it in all of its beauty and complexity.

And isn't it amazing to think what else might be going on in M51?

Around those stars, there must be planets.

But is there life?

Or even other civilisations looking back at us?

Thanks to everybody who contributed images for tonight's programme.

But if you want to do some more astronomy,

even if you don't have a telescope,

we're letting you participate in some live research,

all part of the BBC's Do Something Great season,

because we want you to become comet hunters.

We need you to take part in an exciting project,

searching for a new population of comets hidden in the asteroid belt.

'It's run by Meg Schwamb.'

-Hi, Meg.

-Hi.

Now, you're one of the lead scientists on this project,

but can you tell me, why are you hunting comets?

Well, we're looking for a unique type of comet,

and these are really wolves in sheep's clothing,

so these are asteroids that start looking like comets.

So when you think of a traditional comet,

you think of an icy body coming in from the outer solar system

and heating up, and sublimating its ices off,

but these are very different. So these are typical, plain,

ordinary asteroids sitting between Jupiter and Mars,

and when we occasionally look at these objects,

we start to see that they have tails.

Like in this image here, where you see this dust tail.

Very similar to what you see with a comet, but that's still an asteroid.

So what do you think is causing this?

We don't know what the dominant mechanism is,

but one source is water ice, which shouldn't be there,

but there could be buried water ice

underneath the surfaces of some of these asteroids

that somehow gets exposed.

So why is this of scientific interest,

the fact that we have these comets,

and they could potentially contain water?

Well, water is important for life,

and one of the interesting things is to wonder,

where did Earth's water come from?

And we don't fully know.

And potentially, there's now a new source of water ice

that could have been deposited on the Earth early on,

and so understanding where these

main-belt comets originally came from,

how did they get into the asteroid belt,

and how many of them are there,

might tell us about whether or not this could be a source

for Earth's water.

So the point of the project is to find many, many more examples,

hopefully. How can our viewers help?

Well, you can go to comethunters.org and get started right away.

All you need is a web browser and your eyes.

So what does the website look like?

So, what we're showing is two images of the asteroid

taken at two different times during the night,

and what we want you to do is use your eyes to spot the tail.

Human beings are really good at spotting patterns

and things that don't match and so we need the eyeballs of anyone

who wants to look, and for the viewers of The Sky At Night

to help us because you are better than a machine at finding these.

This is your chance to be able to help discover a comet.

Well, you've convinced me. I want to go and find a comet,

so thank you very much.

If you want to get involved, the website is on the screen

right now, so good luck, and happy hunting.

That's all for this month, but when we come back next month,

we'll be reporting from Nasa's Jet Propulsion Laboratory

as the Juno probe arrives at Jupiter.

-Until then, get outside, get looking up!

-Good night.