Cygnus the Swan The Sky at Night

Good evening. It's summer now, at last.

Now to talk about the summer constellations,

beginning with the loveliest of all - Cygnus the Swan.

You can't mistake it, it's right overhead

and the five main stars make up the form of an "X".

Some people call it the Northern Cross.

The Milky Way runs right through it and it's very, very rich.

So, let's begin by talking about those stars.

The brightest one, Alpha Cygni Deneb.

More than 200,000 times brighter than our sun.

And remember what I call the Summer Triangle - Deneb, Vega, Altair.

I'm going to bring in a new comment on The Sky At Night, Tom Kerss.

-Tom, welcome.

-Hello, Patrick.

-You've been looking hard at Cygnus?

-As ever.

We're coming up to summer and Cygnus

is just a lovely constellation to see in the summer.

It reminds me of being out on long summer evenings waiting for the sunset

and then, of course, those five bright stars that you mentioned in Cygnus

are really among the first to come out.

Deneb with the summer triangle,

and some of the other stars which we can talk about.

Let's say a bit about the various stars.

Sadr or Gamma, an orange super giant.

That's right. This is a lovely star to observe in contrast to Deneb.

It does show quite a nice orange colour,

particularly if you have a large aperture telescope.

Now we've moved down from the tail and we're in the swan's back.

OK, time now for Delta or Rukh.

Delta is actually an optical double star.

In fact, it is a triple star, but it's hard to observe all three components.

The double can be seen in a fairly small telescope of four inches or more.

Double stars are of two kinds, binaries, some white like this,

and some very close.

And optical doubles, where one star's in the background.

A line of sight effect.

Now then, come down to my favourite,

the lower star of the cross, Albireo or Beta Cygni.

In my view,

the loveliest coloured double in the entire sky.

I don't disagree with you, Patrick, but I know that we do disagree on the actual colour.

Primary is golden yellow.

Absolutely.

What about the companion?

This is a star that you've described to me as being azure blue,

but I see it a little bit more dull, a little bit closer to Neptune blue.

I see it as a brilliant blue.

It is a brilliant blue, don't get me wrong, it's very nice.

You can see it... A small telescope will do it.

-In a fairly big telescope, it's a lovely sight.

-It is.

The colours are striking, particularly as they're so different.

The contrast helps to bring out each member of the star.

Why are stars different colours? Obviously, they're different temperatures.

The temperature sets the surface colour of the star.

In the case of Albireo's main component,

we've a golden yellow star which is not dissimilar in colour

from our own sun.

In the case of the blue component we have a much hotter star.

In the same way as a blue flame is hotter than a yellow flame.

There are two other stars we must mention.

One is 61 Cygni, a faint binary,

the first star to have its distance measured

way back in 1838.

That's right, it was the astronomer Bessel.

Sometimes we call it Bessel's Star.

He measured it to be over 10 light years away.

We now know it's just over 11 light years away. He was very close.

One of the very closest stars, and on the other side, P Cygni.

-Yes.

-This doesn't look spectacular.

It looks like an ordinary star.

It is in fact a Hyper Giant.

Immensely distant, immensely remote,

unstable, near the end of its life.

It will explode as a Super Nova.

Or possibly even a Hyper Nova.

It will, just when?

It might be tomorrow, it might not be for a million years,

but explode it will.

And when it does, it will be a real sight.

Also, as I say, don't forget.

Take a pair of binoculars and sweep all round the Cygnus.

Rich star field after rich star field.

It is always worth doing.

Tom, thank you very much.

Well, we talked about things from indoors -

now it's time to go outdoors,

right down to Selsey Beach.

There we find Chris North to say more about the glories of Cygnus.

With the long, light summer evenings ahead of us, for astronomers

it's a waiting game before darkness finally washes over us.

The reward for our long wait is Cygnus.

There is a rich array of objects to look at,

such as star clusters and nebulae,

which come in a variety of shapes and colours.

The reason why Cygnus is so blessed with such beautiful things to look at

is its position in our own galaxy - the Milky Way.

The stars in our galaxy are primarily arranged in a disc.

If we could look at this from the top,

we'd see a spiral structure with a bar in the centre.

Our own sun is located about half to two-thirds of the way out

in one of the minor spiral arms.

On this scale, the stars in Cygnus are very close to the sun.

But as we look through the constellation,

we're looking along the line of this minor spiral arm.

That explains the richness of objects we see within Cygnus.

The galactic arm of the Milky Way

is filled with stars and also with gas and dust,

where new stars are forming.

Some of these interstellar clouds are energised by nearby stars

and emit their own light

and we call these emission nebulae.

These cosmic nurseries make wonderful, complex shapes

and patterns on the sky,

which astronomers over the years have given names

such as the Cocoon, the Butterfly and the Pelican Nebula.

Pete Lawrence and Paul Abel are set up in Patrick's garden,

ready to take up the story of Cygnus.

-Hello, Pete.

-Hello, Paul.

-It's that time of year again when my favourite summer constellation makes a return.

Cygnus, what a glorious constellation it is.

It's fantastic. Cygnus is supposed to represent the swan, of course,

but the central portion of it is also known as the Northern Cross.

And quite a large constellation, too. Easily recognised.

But the height of the cross - if you hold your hand up to the sky,

at arm's length, it's about the same as the span between

the tip of your little finger and the end of your thumb.

-So, really a large constellation.

-It is.

Of course the spiral arm of our galaxy passes through it

and so the entire region is filled with star formation

and emission nebulae.

This is one of the most fascinating things about it -

-the sheer number of deep sky objects in it.

-It's like a chocolate box of the night sky!

Plenty of time to go through the chocolates until dawn comes up!

I should start with the largest nebula, the North America Nebula.

Isn't that fantastic? The North America Nebula is a star-forming region in Cygnus.

It sits off the upper left corner,

-if you like, of Cygnus, very close to the star...

-Xi Cygni.

Yes. It's also thought that Deneb illuminates it, too.

-It is, yes.

-It's a very powerful star.

It is - very, very powerful.

It's an emission nebulae and this means that it is illuminated

by the young stars forming within their lights. Photons from them

travel outwards and illuminate the hot gas.

Quite a big nebula as well, I believe.

Nearly four lunar diameters across.

Quite a low surface brightness. In the city, I struggle to see it.

You do need to go out to dark skies. I go out in the country.

That's the best way to see it.

It's one of these strange objects, because it is so large,

it's not particularly good to look at through a telescope.

You can see it in detail through a telescope,

the best way to see all of it is either through the naked eye in a dark sky

or with a pair of low-power binoculars.

Then you can really see that it does actually have the shape

of the North American continent.

I've seen hundreds of pictures of it!

It is a very popular thing to photograph and the reason is

it's quite easy to photograph

with a standard camera with a normal lens attached.

Rather like your setup here.

That's right. This is a telescope on an equatorial mount,

which is driven so it tracks the stars.

I've stuck a camera on the back of the telescope,

set the camera to a reasonably high sensitivity

and then just take some long exposure photographs of that region.

That's when you see this beautiful nebula come out

and its companion, cos there's another nebula right next to it.

Another favourite - the Pelican Nebula.

It's separated by a lovely dark lane, isn't it?

-This comes out well in long-exposure photographs.

-Exactly.

The shape of these nebulae are actually sculpted by dark dust lanes

around there. The Pelican Nebula and North America Nebula are believed to be the same cloud.

But moving away from there,

we also have one of the most fascinating objects

in this constellation, that's the Crescent Nebula.

This is one of those nebula that's formed from a Wolf-Rayet star.

It's really, again, best seen in photographs, long-exposure photographs.

These pick out the beautiful structure of the crescent.

You can see it through a telescope. It does look like a crescent.

With a long-exposure photograph, you can see this amazing structure.

-It's not immediately obvious what's going on.

-It's not.

It's believed that that star in the centre, about 400,000 years ago,

it went into a red giant phase

and it blew off a lot of material as it did this,

-so that was escaping away from the star.

-Yes.

Another star has generated a very fast stellar wind,

so it's sending material out much faster than that original ejection.

-Right.

-So that stellar wind has caught up with the red giant stuff,

which is out there

and now there's a shell of material where that's getting excited

-and you have two shockwaves.

-Moving in different directions, I gather?

It's very, very pretty. Very, very interesting.

Finally, moving on to the other lovely emission nebula,

the Cocoon Nebula. A lovely thing, isn't it?

It's a very odd thing.

You've got the star cluster nearby and then the nebulosity.

Again, this is an emission nebula.

What is remarkable about it is that it sits in a very rich star field.

But there is a finger of darkness that extends over to it.

-This is a Barnard Cloud, isn't it?

-It is. It's known as Barnard-168.

The actual Cocoon Nebula sits on the very end of that finger.

It's quite an amazing thing to see.

Plenty of star formation in this constellation, Pete!

Oh, very nice!

CHRIS: When we look at nebulae in different wavelengths,

we can discover more about them.

The North America Nebula in the optical

shows curtains of excited gas and dust,

which look colourful and full of structure.

The dark lanes are thick clouds,

which block out any light from more distant stars.

Switch to the infrared and we can peer through the curtains,

picking out the stellar nurseries where stars are being born,

embedded deep in the dense dust lanes.

Using these different wavelengths

also helps us see nebulae which are being formed by the death of stars.

In Cygnus, we find some very massive stars,

such as the blue-white super giant Deneb.

Some 200 times the size of the sun, it's consumed all the hydrogen

and helium in its core,

and is now powered by the fusion of heavier elements.

When this fuel is exhausted, it will go supernova.

This catastrophic explosion will almost completely destroy

the star, leaving just its remnant hanging in space, like a shroud.

The Veil Nebula was formed when one such super giant star exploded

some five to 10,000 years ago,

and is one of the oldest supernova remnants

we can easily see in our night sky.

The supernova would have been so bright,

it would have been visible by night and day.

But all we see now is the shell of material hanging in space.

This shell is continuing to expand, and is now 100 light years across.

With its massive structure and intricate shapes and colours,

astronomers have given parts of it fantastical names.

Pete and Tom know all about the Witch's Broom.

So, Tom, the Veil Nebula, an absolutely wonderful object.

Tricky to see with a small telescope,

but with a larger instrument, it's quite spectacular, isn't it?

There's a variety of ways. I've seen it through a large telescope,

but I've also managed to catch it in a modest pair of binoculars

-under dark skies.

-Really? Fantastic.

You don't need a massive instrument to see it, but you do need

good conditions, because it is quite difficult to tease out.

So let's point out where the Veil Nebula is in Cygnus.

The easiest way to do that is to identify

that Northern Cross pattern, first of all.

So, on the horizontal spar arms, if you like, of the cross,

the star which is on the left-hand side of it is

-Epsilon Cygni.

-Or Gienah.

-Gienah.

And then if you carry along that line, you come to

the eastern wingtip, which is marked by the star Zeta Cygni.

Now, below the line joining Epsilon to Zeta,

there is a fainter star, which is known as 52 Cygni,

and that's actually the starting point

-for locating the Veil, isn't it?

-Yeah.

And once you're at 52 Cygni,

you're actually already in the Veil Nebula, although at first

you might not see it,

because the glare of 52 Cygni will compete with it.

But you're looking at one part of the Veil Nebula.

-We typically split it into three parts.

-Right.

52 Cygni is neatly embedded in the Western Veil,

also known as the Witch's Broom,

because it has quite a distinctive shape.

For good reason, isn't it?

-It does look like a witch's broom.

-It does, yeah.

I don't really know what a witch's broom looks like in detail, but yes!

The first impression I got was that it looked like a broom,

but it also looks a bit like a bird in flight,

which is quite apt, for it being in Cygnus.

But the eastern part of the Veil, that's slightly brighter, isn't it?

It's easier to see, I think,

probably because it's not competing with the glare of a bright star.

Yeah, that's a good point. And then there's the third section, which is

known as Pickering's Triangle.

Or Fleming's Triangle. Not to be controversial!

I like the name, actually, Pickering's Triangular Wisp! Yes!

Isn't that wonderful?

It sounds like it should be a snack you have late at night or something!

Wisp is a good word,

because it is wispy, and difficult to observe.

It's really difficult to see, visually, isn't it, that?

There are ways of improving the view, aren't there?

You can use a special observing filter,

known as an O3 filter.

What it does is attaches to the bottom of the eyepiece.

If we look into the eyepiece barrel, there's a thread inside.

You can screw a filter in there.

That filter will only allow certain types of light to pass.

That particular light is characteristic

of nebulae like the Veil.

With that, even with a small telescope,

you should be able to pick it out clearer

than you would if you didn't have that filter.

The other trick is to keep the magnification quite low.

The Veil Nebula is a really big object in the sky.

-About three degrees across.

-Several times the size of a full moon.

Six times the size of the full moon across, which is huge.

To take it in you need to keep magnification quite low.

A good way to start is to try scanning the sky with binoculars.

So, the Veil Nebula, a fantastic object.

-It's fantastic visually, and a great photographic target.

-Yeah.

A spectacular supernova remnant

from a star which ended its life

some thousands of years ago, and now able to be observed by us today.

I think you'll agree it's worth some time this summer.

Absolutely.

When an even bigger star explodes,

theories are that it can form a black hole,

an exotic phenomenon which emits no light,

making it impossible to see.

Cygnus X-1 is a very bright X-ray source.

It was discovered 50 years ago

and became key to confirming whether black holes were just a theory

or a reality.

Paul and I went back down to the beach to discuss this object

at the heart of Cygnus.

We've seen already how Cygnus has this wonderful supernova remnant.

That's the Veil Nebula we were looking at earlier.

A star that's much more massive than the sun.

25 times the mass of the sun.

The whole thing has collapsed down onto this singularity

with an event horizon that nothing can escape from, not even light.

What we think happens is stars 20-25 solar masses implode inwards.

When they get to the end of their lives,

they are so incredibly heavy, they collapse inwards on themselves.

As they are collapsing in, they form perhaps one of the most exotic,

unusual, outrageous objects in the universe -

a singularity, a point of infinite density

confined to a very small volume, surrounded by event horizon.

You're talking about a black hole.

Cygnus X-1 was first discovered in X-rays in the 1960s.

There's got to be something there that's emitting light,

just not the black hole itself, if it is indeed a black hole.

The evidence for that came from the companion object

that appears to be orbiting it.

In the 1970s, astronomers using the Isaac Newton telescope

looked at a blue super giant star that's orbiting Cygnus X-1

and saw it's orbiting once every five and a half days.

That star is about 40 times the mass of our sun.

If you do the maths, the other object has to be about nine times

the mass of the sun, nine or ten times.

It can't be a neutron star. They can't go above three solar masses.

You do start to think, "What else could it be?"

This is one of those interesting things,

the black hole is something we almost take for granted,

that it must exist, and yet there isn't a lot of evidence for it.

There's only these indirect observations.

We've never been able to create one in the lab.

It's quite interesting that we have these mysterious objects

out there that we can only see their fingerprints, not at work.

We're down here on the beach, we've got a camp fire.

You can't have a camp fire without a few stories.

Let's move on now to the mythology of what we call Cygnus the Swan.

Most of the great stories come from the Greeks.

There are a couple of wonderful Greek legends

as to how the Swan got in the sky.

One of my favourites is the story of Phaeton and Cygnus.

One day, they decided they would run a race across the entire sky,

across the heavens, past the sun and the moon and back down to Earth.

So they both set off in their chariots and did that.

Unfortunately, in order to save time, both cut too close to the sun.

Their chariots burnt up and they both tumbled to Earth.

Phaeton, unfortunately, died.

Cygnus survived and he woke up and discovered his friend's body

was actually at the bottom of the Eridanus river.

He called out to Zeus, and Zeus, who was listening -

The King of the Gods!

The King of the Gods said, "I can turn you into a swan.

"Then you can go down and collect his body,

"but if you do that, you must give up your immortality,

"and you will only live as long as the swan does."

Cygnus rapidly said, "Yes, I'll do that."

In honour of this great, unselfish act,

Zeus placed Cygnus in the sky after he died.

It's not just the Greeks who had stories, the Arabs had some.

And the Arabs, sticking with the bird theme,

the Arabs didn't see a swan, they saw a hen.

A rather forlorn looking hen, in some of the drawings!

Less majestic, perhaps. From the Greeks and the Arabs to the Chinese.

They've got stories which don't revolve around ornithology.

They don't see birds at all. They have the story of two lovers.

Zhi Nu and Niu Lang.

Zhi Nu is the daughter of the goddess.

She's immortal and lives in the afterlife.

She grows tired of heaven and immortality and runs away to Earth.

During this time on earth, she meets Niu Lang, a farmer.

They fall hopelessly in love together.

Eventually, the goddess realises her daughter's escaped and summons her.

Niu Lang doesn't realise what has happened.

He comes home and finds his wife missing.

To his even more intense surprise, his ox acquired the power of speech!

He informs him that if Niu Lang is to kill the ox

and put the hide on his back,

he will be able to ascend to heaven and meet his wife, which he does,

with his two children.

The goddess who sees this is cross

and she takes a hairpin out of her hair

and scratches a scratch in the heavens, which forms the Milky Way,

so the two lovers can't meet.

It's said that one night a year, all the magpies in the world -

-Magpies?

-All the magpies - I'm not making it up -

All the magpies in the world take pity on these two,

fly up to heaven and build a bridge.

Their constellation is this bridge where the lovers can meet,

and then have to go back to leading separate lives.

That's perhaps the best interpretation.

With all of these stories from a huge range of cultures,

one thing you can't help to notice about Cygnus

is the Milky Way galaxy going through it,

with the dark lanes of dust,

which William Herschel called "holes in the heavens."

We now know they're clouds of dust blocking light from behind,

sitting in a spiral arm of our own Milky Way galaxy.

Galaxies were first spotted as smudges in the sky.

But it was not until the beginning of last century

that astronomers realised these were island universes,

some containing trillions of stars.

With large telescopes, we can now see millions of galaxies,

out to the reaches of space, and they come in all shapes and sizes.

Our Milky Way is similar to some of the gorgeous spiral galaxies we see,

with arms of stars, gas and dust extending out from a central core.

At the centre of our galaxy is a bar-like feature that is

thought to be responsible for the spiral structure.

Gerry Gilmore has been studying this beating heart of the Milky Way.

So, Gerry, we've been talking about the constellation of Cygnus

and all the stuff we can see in the plain of our Milky Way galaxy,

but you're interested in the much larger-scale structure of our galaxy,

and I have here the handy sketch of the galaxy we had earlier.

So, here we are, with the spiral arms and the bar,

and the sun is here at the top of the image.

So, tell us what we're seeing here in Cygnus.

Two things here. The first is that, of course,

the whole Milky Way - it's huge -

is so well mixed up that we actually see a fair sample of everything

when we look locally.

There's only one thing that's unique in the Milky Way,

as far as we know, and that's this big, massive black hole down here.

But there's a second thing that's unique, and that's related to

the shape and the size, and that's why it looks like this.

Why have we got this bar-like structure in the middle,

this sort of big hand-shaped thing, with these spiral arms

coming out of it? And how do we know that? That's hard to see.

If you can only see round here, then, of course,

how do you know what's out here?

Fortunately, we can see in some wavelengths.

There's a sort of sweet spot for studying the Milky Way.

If you go too far to the infrared, all you see is the dust,

you don't see any of the stars.

And, of course, most of the Milky Way is stars,

so you miss that picture. There is this sweet spot,

which happens to be just to the red of the visible, fortunately.

Then you can see very large distances through this stuff.

So the whole key to this is what's going on in the centre.

We have this bar-like structure, a flattened cigar shape,

tens of thousands of light years long.

A big structure, and it has these spiral arms coming off it,

but they're not real things.

They're not real collections of stars that are moving around together.

That's correct, yes.

A spiral arm behaves, as far as the galaxy is concerned,

like a water wave on the ocean.

So, a bit of water will go up and down as the wave comes through it,

-the wave keeps going, but the water stays still.

-Right.

And that's what's driven by this guy in the middle here,

this big, sort of hand-shaped structure.

It has stars in it that are actually trapped in it,

just by the weight of the other stars.

So, this thing acts like a sort of mini galaxy

inside the big galaxy.

It's heavy enough that it traps stuff in it,

and they just move around in a sort of big bar.

It's so heavy, what it actually does is

stir up the rest of the Milky Way.

It's like a giant egg beater, sitting in the middle, and the waves

coming off the end of this big egg beater are the spiral arms.

This magnificent view of our galactic centre towards the bar was

taken by the great observatories,

the Hubble, Spitzer and Chandra space telescopes.

Hubble revealed stars and warm gas, with the centre of our galaxy

marked by the bright patch in the lower right.

Spitzer's view in the infrared exposes

hundreds of thousands of stars, hidden in visible light.

But it's in the X-ray that we see

the true, violent nature of the galactic centre.

This image shows hundreds of small dots,

which we believe are black holes and other remnants of dead stars.

Seen towards the lower right of the image,

a supermassive black hole lurks in the centre of our galaxy.

High-energy X-rays are screaming out at us,

as the black hole destroys anything that gets too close.

Gas is heated to millions of degrees, solar systems are ripped apart

and stars are pulverised -

a cosmic catastrophe which is normally shielded from view.

The centre of our galaxy is a crowded, vibrant and violent place.

But amateurs can safely enjoy the pleasures of the Milky Way

by taking long exposures of the Cygnus Rift.

This lane of dark, dense dust blocks the light from background stars,

and for the astronomer is a particular pleasure

in the gentle summer nights.

Paul, Tom and Pete have some final delights

the chocolate box of Cygnus has to offer.

Well, chaps, end of the evening,

and we've seen some lovely things in Cygnus.

I thought just before we close the programme we'd just give

some of our favourite objects.

So let's just go with Tom first. You're big into star clusters.

You love open clusters, don't you?

Open clusters, galactic clusters, absolutely love them.

And there are two of note in the constellation of Cygnus.

The first of the two is M39, which is quite a sparse,

-not very dense cluster.

-OK.

The other cluster is somewhat less large, and perhaps somewhat

less magnificent, but it is a favourite of mine, and that's M29,

which is just below the star Gamma,

nestled in the centre of the constellation.

-To me it looks like a small analogue to the Pleiades.

-I agree.

It's known as the Cooling Tower cluster.

There's a sort of squarish shape to the stars in there.

I've never seen that. I'll presume you're not just making it up

-for the programme!

-It sounds terribly derogatory!

Peter, what's yours? What's your favourite object?

I'm going to go for the obvious one. The Hen's Beak.

Yes, Albireo.

Albireo, which is a beautiful double star.

That's one of the first double stars I ever looked at

when I was a small boy getting into astronomy.

-And it takes your breath away...

-It does, the colours.

With very low power, you can see that bright, yellowish primary,

and that lovely bluish secondary next to it.

-Absolutely stunning.

-It's timeless.

Imagine planets on there, what sunrise and sunset would be like! Stunning.

Go on, then, what's yours?

My favourite one is the little planetary nebula,

the blinking nebula. Have you heard of this?

-I have.

-This is quite an interesting planetary nebula,

because the central star is quite bright - magnitude 10.4.

Now, it does this very peculiar thing.

If you look at the nebula straight on,

a four-inch telescope will show it quite well.

If you look away but use averted vision...

-So you're looking out the side of your eye.

-That's right.

Then, the nebula appears to blink back on.

And because natural eye movement, that's going on all the time,

this blinking effect, and I think that's marvellous.

A great object - when you look at it, you can't see it!

That's right! Visual astronomy, Peter!

Some good stuff in Cygnus, and it was a real pleasure to have

a constellation like this to look at in the summer, I think.

-It is.

-Absolutely.

Well, there's so much to see in the summer sky,

so do go and have a look.

When we come back next month,

I'll be talking about the procession of the equinoxes,

and we'll be paying a visit to the South Downs Planetarium.

Until then, good night.

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