0:00:28 > 0:00:31Good evening. It's summer now, at last.
0:00:31 > 0:00:34Now to talk about the summer constellations,
0:00:34 > 0:00:39beginning with the loveliest of all - Cygnus the Swan.
0:00:39 > 0:00:42You can't mistake it, it's right overhead
0:00:42 > 0:00:46and the five main stars make up the form of an "X".
0:00:46 > 0:00:49Some people call it the Northern Cross.
0:00:49 > 0:00:53The Milky Way runs right through it and it's very, very rich.
0:00:53 > 0:00:56So, let's begin by talking about those stars.
0:00:56 > 0:01:00The brightest one, Alpha Cygni Deneb.
0:01:00 > 0:01:04More than 200,000 times brighter than our sun.
0:01:04 > 0:01:11And remember what I call the Summer Triangle - Deneb, Vega, Altair.
0:01:11 > 0:01:15I'm going to bring in a new comment on The Sky At Night, Tom Kerss.
0:01:15 > 0:01:17- Tom, welcome.- Hello, Patrick.
0:01:17 > 0:01:21- You've been looking hard at Cygnus?- As ever.
0:01:21 > 0:01:23We're coming up to summer and Cygnus
0:01:23 > 0:01:25is just a lovely constellation to see in the summer.
0:01:25 > 0:01:29It reminds me of being out on long summer evenings waiting for the sunset
0:01:29 > 0:01:32and then, of course, those five bright stars that you mentioned in Cygnus
0:01:32 > 0:01:34are really among the first to come out.
0:01:34 > 0:01:35Deneb with the summer triangle,
0:01:35 > 0:01:37and some of the other stars which we can talk about.
0:01:37 > 0:01:40Let's say a bit about the various stars.
0:01:40 > 0:01:43Sadr or Gamma, an orange super giant.
0:01:43 > 0:01:46That's right. This is a lovely star to observe in contrast to Deneb.
0:01:46 > 0:01:49It does show quite a nice orange colour,
0:01:49 > 0:01:51particularly if you have a large aperture telescope.
0:01:51 > 0:01:54Now we've moved down from the tail and we're in the swan's back.
0:01:54 > 0:01:57OK, time now for Delta or Rukh.
0:01:57 > 0:02:00Delta is actually an optical double star.
0:02:00 > 0:02:04In fact, it is a triple star, but it's hard to observe all three components.
0:02:04 > 0:02:07The double can be seen in a fairly small telescope of four inches or more.
0:02:07 > 0:02:13Double stars are of two kinds, binaries, some white like this,
0:02:13 > 0:02:14and some very close.
0:02:14 > 0:02:17And optical doubles, where one star's in the background.
0:02:17 > 0:02:19A line of sight effect.
0:02:19 > 0:02:23Now then, come down to my favourite,
0:02:23 > 0:02:27the lower star of the cross, Albireo or Beta Cygni.
0:02:27 > 0:02:28In my view,
0:02:28 > 0:02:31the loveliest coloured double in the entire sky.
0:02:31 > 0:02:36I don't disagree with you, Patrick, but I know that we do disagree on the actual colour.
0:02:36 > 0:02:38Primary is golden yellow.
0:02:38 > 0:02:39Absolutely.
0:02:39 > 0:02:41What about the companion?
0:02:41 > 0:02:44This is a star that you've described to me as being azure blue,
0:02:44 > 0:02:48but I see it a little bit more dull, a little bit closer to Neptune blue.
0:02:48 > 0:02:51I see it as a brilliant blue.
0:02:51 > 0:02:53It is a brilliant blue, don't get me wrong, it's very nice.
0:02:53 > 0:02:58You can see it... A small telescope will do it.
0:02:58 > 0:03:01- In a fairly big telescope, it's a lovely sight.- It is.
0:03:01 > 0:03:04The colours are striking, particularly as they're so different.
0:03:04 > 0:03:07The contrast helps to bring out each member of the star.
0:03:07 > 0:03:11Why are stars different colours? Obviously, they're different temperatures.
0:03:11 > 0:03:14The temperature sets the surface colour of the star.
0:03:14 > 0:03:17In the case of Albireo's main component,
0:03:17 > 0:03:20we've a golden yellow star which is not dissimilar in colour
0:03:20 > 0:03:21from our own sun.
0:03:21 > 0:03:24In the case of the blue component we have a much hotter star.
0:03:24 > 0:03:28In the same way as a blue flame is hotter than a yellow flame.
0:03:28 > 0:03:30There are two other stars we must mention.
0:03:30 > 0:03:34One is 61 Cygni, a faint binary,
0:03:34 > 0:03:38the first star to have its distance measured
0:03:38 > 0:03:40way back in 1838.
0:03:40 > 0:03:43That's right, it was the astronomer Bessel.
0:03:43 > 0:03:45Sometimes we call it Bessel's Star.
0:03:45 > 0:03:47He measured it to be over 10 light years away.
0:03:47 > 0:03:51We now know it's just over 11 light years away. He was very close.
0:03:51 > 0:03:56One of the very closest stars, and on the other side, P Cygni.
0:03:56 > 0:03:59- Yes. - This doesn't look spectacular.
0:03:59 > 0:04:01It looks like an ordinary star.
0:04:02 > 0:04:04It is in fact a Hyper Giant.
0:04:04 > 0:04:07Immensely distant, immensely remote,
0:04:07 > 0:04:10unstable, near the end of its life.
0:04:10 > 0:04:14It will explode as a Super Nova.
0:04:14 > 0:04:16Or possibly even a Hyper Nova.
0:04:16 > 0:04:17It will, just when?
0:04:17 > 0:04:21It might be tomorrow, it might not be for a million years,
0:04:21 > 0:04:23but explode it will.
0:04:23 > 0:04:26And when it does, it will be a real sight.
0:04:28 > 0:04:30Also, as I say, don't forget.
0:04:30 > 0:04:35Take a pair of binoculars and sweep all round the Cygnus.
0:04:35 > 0:04:38Rich star field after rich star field.
0:04:38 > 0:04:40It is always worth doing.
0:04:41 > 0:04:43Tom, thank you very much.
0:04:43 > 0:04:46Well, we talked about things from indoors -
0:04:46 > 0:04:48now it's time to go outdoors,
0:04:48 > 0:04:50right down to Selsey Beach.
0:04:50 > 0:04:56There we find Chris North to say more about the glories of Cygnus.
0:04:56 > 0:05:01With the long, light summer evenings ahead of us, for astronomers
0:05:01 > 0:05:05it's a waiting game before darkness finally washes over us.
0:05:05 > 0:05:09The reward for our long wait is Cygnus.
0:05:09 > 0:05:12There is a rich array of objects to look at,
0:05:12 > 0:05:14such as star clusters and nebulae,
0:05:14 > 0:05:17which come in a variety of shapes and colours.
0:05:17 > 0:05:21The reason why Cygnus is so blessed with such beautiful things to look at
0:05:21 > 0:05:24is its position in our own galaxy - the Milky Way.
0:05:27 > 0:05:31The stars in our galaxy are primarily arranged in a disc.
0:05:31 > 0:05:33If we could look at this from the top,
0:05:33 > 0:05:36we'd see a spiral structure with a bar in the centre.
0:05:36 > 0:05:40Our own sun is located about half to two-thirds of the way out
0:05:40 > 0:05:42in one of the minor spiral arms.
0:05:42 > 0:05:47On this scale, the stars in Cygnus are very close to the sun.
0:05:47 > 0:05:49But as we look through the constellation,
0:05:49 > 0:05:53we're looking along the line of this minor spiral arm.
0:05:53 > 0:05:57That explains the richness of objects we see within Cygnus.
0:05:59 > 0:06:01The galactic arm of the Milky Way
0:06:01 > 0:06:05is filled with stars and also with gas and dust,
0:06:05 > 0:06:06where new stars are forming.
0:06:06 > 0:06:11Some of these interstellar clouds are energised by nearby stars
0:06:11 > 0:06:12and emit their own light
0:06:12 > 0:06:15and we call these emission nebulae.
0:06:15 > 0:06:19These cosmic nurseries make wonderful, complex shapes
0:06:19 > 0:06:21and patterns on the sky,
0:06:21 > 0:06:24which astronomers over the years have given names
0:06:24 > 0:06:27such as the Cocoon, the Butterfly and the Pelican Nebula.
0:06:32 > 0:06:36Pete Lawrence and Paul Abel are set up in Patrick's garden,
0:06:36 > 0:06:38ready to take up the story of Cygnus.
0:06:38 > 0:06:45- Hello, Pete.- Hello, Paul.- It's that time of year again when my favourite summer constellation makes a return.
0:06:45 > 0:06:48Cygnus, what a glorious constellation it is.
0:06:48 > 0:06:52It's fantastic. Cygnus is supposed to represent the swan, of course,
0:06:52 > 0:06:56but the central portion of it is also known as the Northern Cross.
0:06:56 > 0:07:00And quite a large constellation, too. Easily recognised.
0:07:00 > 0:07:03But the height of the cross - if you hold your hand up to the sky,
0:07:03 > 0:07:06at arm's length, it's about the same as the span between
0:07:06 > 0:07:09the tip of your little finger and the end of your thumb.
0:07:09 > 0:07:12- So, really a large constellation. - It is.
0:07:12 > 0:07:14Of course the spiral arm of our galaxy passes through it
0:07:14 > 0:07:18and so the entire region is filled with star formation
0:07:18 > 0:07:20and emission nebulae.
0:07:20 > 0:07:23This is one of the most fascinating things about it -
0:07:23 > 0:07:28- the sheer number of deep sky objects in it.- It's like a chocolate box of the night sky!
0:07:28 > 0:07:32Plenty of time to go through the chocolates until dawn comes up!
0:07:32 > 0:07:36I should start with the largest nebula, the North America Nebula.
0:07:36 > 0:07:42Isn't that fantastic? The North America Nebula is a star-forming region in Cygnus.
0:07:42 > 0:07:46It sits off the upper left corner,
0:07:46 > 0:07:49- if you like, of Cygnus, very close to the star...- Xi Cygni.
0:07:49 > 0:07:52Yes. It's also thought that Deneb illuminates it, too.
0:07:52 > 0:07:54- It is, yes. - It's a very powerful star.
0:07:54 > 0:07:56It is - very, very powerful.
0:07:56 > 0:07:59It's an emission nebulae and this means that it is illuminated
0:07:59 > 0:08:03by the young stars forming within their lights. Photons from them
0:08:03 > 0:08:05travel outwards and illuminate the hot gas.
0:08:05 > 0:08:07Quite a big nebula as well, I believe.
0:08:07 > 0:08:09Nearly four lunar diameters across.
0:08:09 > 0:08:14Quite a low surface brightness. In the city, I struggle to see it.
0:08:14 > 0:08:17You do need to go out to dark skies. I go out in the country.
0:08:17 > 0:08:18That's the best way to see it.
0:08:18 > 0:08:21It's one of these strange objects, because it is so large,
0:08:21 > 0:08:24it's not particularly good to look at through a telescope.
0:08:24 > 0:08:27You can see it in detail through a telescope,
0:08:27 > 0:08:30the best way to see all of it is either through the naked eye in a dark sky
0:08:30 > 0:08:33or with a pair of low-power binoculars.
0:08:33 > 0:08:36Then you can really see that it does actually have the shape
0:08:36 > 0:08:38of the North American continent.
0:08:38 > 0:08:40I've seen hundreds of pictures of it!
0:08:40 > 0:08:43It is a very popular thing to photograph and the reason is
0:08:43 > 0:08:44it's quite easy to photograph
0:08:44 > 0:08:47with a standard camera with a normal lens attached.
0:08:47 > 0:08:49Rather like your setup here.
0:08:49 > 0:08:52That's right. This is a telescope on an equatorial mount,
0:08:52 > 0:08:55which is driven so it tracks the stars.
0:08:55 > 0:08:57I've stuck a camera on the back of the telescope,
0:08:57 > 0:09:00set the camera to a reasonably high sensitivity
0:09:00 > 0:09:03and then just take some long exposure photographs of that region.
0:09:03 > 0:09:06That's when you see this beautiful nebula come out
0:09:06 > 0:09:09and its companion, cos there's another nebula right next to it.
0:09:09 > 0:09:11Another favourite - the Pelican Nebula.
0:09:11 > 0:09:14It's separated by a lovely dark lane, isn't it?
0:09:14 > 0:09:17- This comes out well in long-exposure photographs.- Exactly.
0:09:17 > 0:09:22The shape of these nebulae are actually sculpted by dark dust lanes
0:09:22 > 0:09:28around there. The Pelican Nebula and North America Nebula are believed to be the same cloud.
0:09:28 > 0:09:30But moving away from there,
0:09:30 > 0:09:32we also have one of the most fascinating objects
0:09:32 > 0:09:35in this constellation, that's the Crescent Nebula.
0:09:35 > 0:09:39This is one of those nebula that's formed from a Wolf-Rayet star.
0:09:39 > 0:09:43It's really, again, best seen in photographs, long-exposure photographs.
0:09:43 > 0:09:45These pick out the beautiful structure of the crescent.
0:09:45 > 0:09:49You can see it through a telescope. It does look like a crescent.
0:09:49 > 0:09:53With a long-exposure photograph, you can see this amazing structure.
0:09:53 > 0:09:56- It's not immediately obvious what's going on.- It's not.
0:09:56 > 0:10:00It's believed that that star in the centre, about 400,000 years ago,
0:10:00 > 0:10:03it went into a red giant phase
0:10:03 > 0:10:07and it blew off a lot of material as it did this,
0:10:07 > 0:10:10- so that was escaping away from the star.- Yes.
0:10:10 > 0:10:13Another star has generated a very fast stellar wind,
0:10:13 > 0:10:18so it's sending material out much faster than that original ejection.
0:10:18 > 0:10:22- Right.- So that stellar wind has caught up with the red giant stuff,
0:10:22 > 0:10:23which is out there
0:10:23 > 0:10:27and now there's a shell of material where that's getting excited
0:10:27 > 0:10:31- and you have two shockwaves.- Moving in different directions, I gather?
0:10:31 > 0:10:34It's very, very pretty. Very, very interesting.
0:10:34 > 0:10:36Finally, moving on to the other lovely emission nebula,
0:10:36 > 0:10:39the Cocoon Nebula. A lovely thing, isn't it?
0:10:39 > 0:10:40It's a very odd thing.
0:10:40 > 0:10:43You've got the star cluster nearby and then the nebulosity.
0:10:43 > 0:10:45Again, this is an emission nebula.
0:10:45 > 0:10:50What is remarkable about it is that it sits in a very rich star field.
0:10:50 > 0:10:53But there is a finger of darkness that extends over to it.
0:10:53 > 0:10:58- This is a Barnard Cloud, isn't it? - It is. It's known as Barnard-168.
0:10:58 > 0:11:03The actual Cocoon Nebula sits on the very end of that finger.
0:11:03 > 0:11:06It's quite an amazing thing to see.
0:11:06 > 0:11:10Plenty of star formation in this constellation, Pete!
0:11:10 > 0:11:12Oh, very nice!
0:11:12 > 0:11:15CHRIS: When we look at nebulae in different wavelengths,
0:11:15 > 0:11:17we can discover more about them.
0:11:17 > 0:11:19The North America Nebula in the optical
0:11:19 > 0:11:22shows curtains of excited gas and dust,
0:11:22 > 0:11:24which look colourful and full of structure.
0:11:24 > 0:11:27The dark lanes are thick clouds,
0:11:27 > 0:11:30which block out any light from more distant stars.
0:11:30 > 0:11:34Switch to the infrared and we can peer through the curtains,
0:11:34 > 0:11:38picking out the stellar nurseries where stars are being born,
0:11:38 > 0:11:41embedded deep in the dense dust lanes.
0:11:41 > 0:11:44Using these different wavelengths
0:11:44 > 0:11:48also helps us see nebulae which are being formed by the death of stars.
0:11:51 > 0:11:54In Cygnus, we find some very massive stars,
0:11:54 > 0:11:57such as the blue-white super giant Deneb.
0:11:57 > 0:12:01Some 200 times the size of the sun, it's consumed all the hydrogen
0:12:01 > 0:12:03and helium in its core,
0:12:03 > 0:12:06and is now powered by the fusion of heavier elements.
0:12:06 > 0:12:10When this fuel is exhausted, it will go supernova.
0:12:10 > 0:12:13This catastrophic explosion will almost completely destroy
0:12:13 > 0:12:17the star, leaving just its remnant hanging in space, like a shroud.
0:12:18 > 0:12:23The Veil Nebula was formed when one such super giant star exploded
0:12:23 > 0:12:25some five to 10,000 years ago,
0:12:25 > 0:12:28and is one of the oldest supernova remnants
0:12:28 > 0:12:29we can easily see in our night sky.
0:12:31 > 0:12:33The supernova would have been so bright,
0:12:33 > 0:12:36it would have been visible by night and day.
0:12:36 > 0:12:40But all we see now is the shell of material hanging in space.
0:12:40 > 0:12:44This shell is continuing to expand, and is now 100 light years across.
0:12:44 > 0:12:48With its massive structure and intricate shapes and colours,
0:12:48 > 0:12:51astronomers have given parts of it fantastical names.
0:12:51 > 0:12:54Pete and Tom know all about the Witch's Broom.
0:12:55 > 0:12:59So, Tom, the Veil Nebula, an absolutely wonderful object.
0:12:59 > 0:13:01Tricky to see with a small telescope,
0:13:01 > 0:13:04but with a larger instrument, it's quite spectacular, isn't it?
0:13:04 > 0:13:07There's a variety of ways. I've seen it through a large telescope,
0:13:07 > 0:13:11but I've also managed to catch it in a modest pair of binoculars
0:13:11 > 0:13:12- under dark skies.- Really? Fantastic.
0:13:12 > 0:13:15You don't need a massive instrument to see it, but you do need
0:13:15 > 0:13:18good conditions, because it is quite difficult to tease out.
0:13:18 > 0:13:21So let's point out where the Veil Nebula is in Cygnus.
0:13:21 > 0:13:23The easiest way to do that is to identify
0:13:23 > 0:13:27that Northern Cross pattern, first of all.
0:13:27 > 0:13:30So, on the horizontal spar arms, if you like, of the cross,
0:13:30 > 0:13:34the star which is on the left-hand side of it is
0:13:34 > 0:13:37- Epsilon Cygni.- Or Gienah.- Gienah.
0:13:37 > 0:13:40And then if you carry along that line, you come to
0:13:40 > 0:13:45the eastern wingtip, which is marked by the star Zeta Cygni.
0:13:46 > 0:13:50Now, below the line joining Epsilon to Zeta,
0:13:50 > 0:13:53there is a fainter star, which is known as 52 Cygni,
0:13:53 > 0:13:54and that's actually the starting point
0:13:54 > 0:13:56- for locating the Veil, isn't it?- Yeah.
0:13:56 > 0:13:58And once you're at 52 Cygni,
0:13:58 > 0:14:02you're actually already in the Veil Nebula, although at first
0:14:02 > 0:14:03you might not see it,
0:14:03 > 0:14:05because the glare of 52 Cygni will compete with it.
0:14:05 > 0:14:08But you're looking at one part of the Veil Nebula.
0:14:08 > 0:14:10- We typically split it into three parts.- Right.
0:14:10 > 0:14:1352 Cygni is neatly embedded in the Western Veil,
0:14:13 > 0:14:15also known as the Witch's Broom,
0:14:15 > 0:14:17because it has quite a distinctive shape.
0:14:17 > 0:14:19For good reason, isn't it?
0:14:19 > 0:14:21- It does look like a witch's broom.- It does, yeah.
0:14:21 > 0:14:25I don't really know what a witch's broom looks like in detail, but yes!
0:14:25 > 0:14:28The first impression I got was that it looked like a broom,
0:14:28 > 0:14:30but it also looks a bit like a bird in flight,
0:14:30 > 0:14:32which is quite apt, for it being in Cygnus.
0:14:32 > 0:14:36But the eastern part of the Veil, that's slightly brighter, isn't it?
0:14:36 > 0:14:38It's easier to see, I think,
0:14:38 > 0:14:41probably because it's not competing with the glare of a bright star.
0:14:41 > 0:14:45Yeah, that's a good point. And then there's the third section, which is
0:14:45 > 0:14:46known as Pickering's Triangle.
0:14:46 > 0:14:48Or Fleming's Triangle. Not to be controversial!
0:14:48 > 0:14:54I like the name, actually, Pickering's Triangular Wisp! Yes!
0:14:54 > 0:14:55Isn't that wonderful?
0:14:55 > 0:14:59It sounds like it should be a snack you have late at night or something!
0:14:59 > 0:15:00Wisp is a good word,
0:15:00 > 0:15:02because it is wispy, and difficult to observe.
0:15:02 > 0:15:05It's really difficult to see, visually, isn't it, that?
0:15:05 > 0:15:08There are ways of improving the view, aren't there?
0:15:08 > 0:15:10You can use a special observing filter,
0:15:10 > 0:15:12known as an O3 filter.
0:15:12 > 0:15:15What it does is attaches to the bottom of the eyepiece.
0:15:15 > 0:15:18If we look into the eyepiece barrel, there's a thread inside.
0:15:18 > 0:15:20You can screw a filter in there.
0:15:20 > 0:15:23That filter will only allow certain types of light to pass.
0:15:23 > 0:15:25That particular light is characteristic
0:15:25 > 0:15:27of nebulae like the Veil.
0:15:27 > 0:15:29With that, even with a small telescope,
0:15:29 > 0:15:31you should be able to pick it out clearer
0:15:31 > 0:15:34than you would if you didn't have that filter.
0:15:34 > 0:15:37The other trick is to keep the magnification quite low.
0:15:37 > 0:15:41The Veil Nebula is a really big object in the sky.
0:15:41 > 0:15:44- About three degrees across.- Several times the size of a full moon.
0:15:44 > 0:15:47Six times the size of the full moon across, which is huge.
0:15:47 > 0:15:50To take it in you need to keep magnification quite low.
0:15:50 > 0:15:54A good way to start is to try scanning the sky with binoculars.
0:15:54 > 0:15:56So, the Veil Nebula, a fantastic object.
0:15:56 > 0:16:00- It's fantastic visually, and a great photographic target.- Yeah.
0:16:00 > 0:16:02A spectacular supernova remnant
0:16:02 > 0:16:04from a star which ended its life
0:16:04 > 0:16:07some thousands of years ago, and now able to be observed by us today.
0:16:07 > 0:16:10I think you'll agree it's worth some time this summer.
0:16:10 > 0:16:11Absolutely.
0:16:11 > 0:16:13When an even bigger star explodes,
0:16:13 > 0:16:15theories are that it can form a black hole,
0:16:15 > 0:16:18an exotic phenomenon which emits no light,
0:16:18 > 0:16:21making it impossible to see.
0:16:21 > 0:16:24Cygnus X-1 is a very bright X-ray source.
0:16:24 > 0:16:26It was discovered 50 years ago
0:16:26 > 0:16:29and became key to confirming whether black holes were just a theory
0:16:29 > 0:16:31or a reality.
0:16:31 > 0:16:36Paul and I went back down to the beach to discuss this object
0:16:36 > 0:16:37at the heart of Cygnus.
0:16:37 > 0:16:41We've seen already how Cygnus has this wonderful supernova remnant.
0:16:41 > 0:16:44That's the Veil Nebula we were looking at earlier.
0:16:44 > 0:16:46A star that's much more massive than the sun.
0:16:46 > 0:16:4825 times the mass of the sun.
0:16:48 > 0:16:52The whole thing has collapsed down onto this singularity
0:16:52 > 0:16:56with an event horizon that nothing can escape from, not even light.
0:16:56 > 0:17:02What we think happens is stars 20-25 solar masses implode inwards.
0:17:02 > 0:17:04When they get to the end of their lives,
0:17:04 > 0:17:08they are so incredibly heavy, they collapse inwards on themselves.
0:17:08 > 0:17:12As they are collapsing in, they form perhaps one of the most exotic,
0:17:12 > 0:17:16unusual, outrageous objects in the universe -
0:17:16 > 0:17:20a singularity, a point of infinite density
0:17:20 > 0:17:23confined to a very small volume, surrounded by event horizon.
0:17:23 > 0:17:25You're talking about a black hole.
0:17:25 > 0:17:29Cygnus X-1 was first discovered in X-rays in the 1960s.
0:17:29 > 0:17:32There's got to be something there that's emitting light,
0:17:32 > 0:17:35just not the black hole itself, if it is indeed a black hole.
0:17:35 > 0:17:38The evidence for that came from the companion object
0:17:38 > 0:17:40that appears to be orbiting it.
0:17:40 > 0:17:43In the 1970s, astronomers using the Isaac Newton telescope
0:17:43 > 0:17:46looked at a blue super giant star that's orbiting Cygnus X-1
0:17:46 > 0:17:49and saw it's orbiting once every five and a half days.
0:17:49 > 0:17:52That star is about 40 times the mass of our sun.
0:17:52 > 0:17:55If you do the maths, the other object has to be about nine times
0:17:55 > 0:17:58the mass of the sun, nine or ten times.
0:17:58 > 0:18:02It can't be a neutron star. They can't go above three solar masses.
0:18:02 > 0:18:04You do start to think, "What else could it be?"
0:18:04 > 0:18:07This is one of those interesting things,
0:18:07 > 0:18:09the black hole is something we almost take for granted,
0:18:09 > 0:18:13that it must exist, and yet there isn't a lot of evidence for it.
0:18:13 > 0:18:16There's only these indirect observations.
0:18:16 > 0:18:18We've never been able to create one in the lab.
0:18:18 > 0:18:22It's quite interesting that we have these mysterious objects
0:18:22 > 0:18:26out there that we can only see their fingerprints, not at work.
0:18:26 > 0:18:28We're down here on the beach, we've got a camp fire.
0:18:28 > 0:18:31You can't have a camp fire without a few stories.
0:18:31 > 0:18:35Let's move on now to the mythology of what we call Cygnus the Swan.
0:18:35 > 0:18:38Most of the great stories come from the Greeks.
0:18:38 > 0:18:41There are a couple of wonderful Greek legends
0:18:41 > 0:18:43as to how the Swan got in the sky.
0:18:43 > 0:18:47One of my favourites is the story of Phaeton and Cygnus.
0:18:47 > 0:18:51One day, they decided they would run a race across the entire sky,
0:18:51 > 0:18:54across the heavens, past the sun and the moon and back down to Earth.
0:18:55 > 0:18:58So they both set off in their chariots and did that.
0:18:58 > 0:19:03Unfortunately, in order to save time, both cut too close to the sun.
0:19:03 > 0:19:07Their chariots burnt up and they both tumbled to Earth.
0:19:07 > 0:19:09Phaeton, unfortunately, died.
0:19:09 > 0:19:12Cygnus survived and he woke up and discovered his friend's body
0:19:12 > 0:19:15was actually at the bottom of the Eridanus river.
0:19:15 > 0:19:19He called out to Zeus, and Zeus, who was listening -
0:19:19 > 0:19:20The King of the Gods!
0:19:20 > 0:19:23The King of the Gods said, "I can turn you into a swan.
0:19:23 > 0:19:26"Then you can go down and collect his body,
0:19:26 > 0:19:29"but if you do that, you must give up your immortality,
0:19:29 > 0:19:32"and you will only live as long as the swan does."
0:19:32 > 0:19:35Cygnus rapidly said, "Yes, I'll do that."
0:19:35 > 0:19:38In honour of this great, unselfish act,
0:19:38 > 0:19:42Zeus placed Cygnus in the sky after he died.
0:19:42 > 0:19:46It's not just the Greeks who had stories, the Arabs had some.
0:19:46 > 0:19:48And the Arabs, sticking with the bird theme,
0:19:48 > 0:19:51the Arabs didn't see a swan, they saw a hen.
0:19:51 > 0:19:55A rather forlorn looking hen, in some of the drawings!
0:19:55 > 0:20:00Less majestic, perhaps. From the Greeks and the Arabs to the Chinese.
0:20:00 > 0:20:05They've got stories which don't revolve around ornithology.
0:20:05 > 0:20:08They don't see birds at all. They have the story of two lovers.
0:20:08 > 0:20:10Zhi Nu and Niu Lang.
0:20:10 > 0:20:13Zhi Nu is the daughter of the goddess.
0:20:13 > 0:20:16She's immortal and lives in the afterlife.
0:20:16 > 0:20:20She grows tired of heaven and immortality and runs away to Earth.
0:20:20 > 0:20:24During this time on earth, she meets Niu Lang, a farmer.
0:20:24 > 0:20:26They fall hopelessly in love together.
0:20:26 > 0:20:30Eventually, the goddess realises her daughter's escaped and summons her.
0:20:30 > 0:20:33Niu Lang doesn't realise what has happened.
0:20:33 > 0:20:36He comes home and finds his wife missing.
0:20:36 > 0:20:41To his even more intense surprise, his ox acquired the power of speech!
0:20:41 > 0:20:45He informs him that if Niu Lang is to kill the ox
0:20:45 > 0:20:47and put the hide on his back,
0:20:47 > 0:20:50he will be able to ascend to heaven and meet his wife, which he does,
0:20:50 > 0:20:52with his two children.
0:20:52 > 0:20:55The goddess who sees this is cross
0:20:55 > 0:20:58and she takes a hairpin out of her hair
0:20:58 > 0:21:01and scratches a scratch in the heavens, which forms the Milky Way,
0:21:01 > 0:21:03so the two lovers can't meet.
0:21:03 > 0:21:07It's said that one night a year, all the magpies in the world -
0:21:07 > 0:21:10- Magpies?- All the magpies - I'm not making it up -
0:21:10 > 0:21:14All the magpies in the world take pity on these two,
0:21:14 > 0:21:16fly up to heaven and build a bridge.
0:21:16 > 0:21:20Their constellation is this bridge where the lovers can meet,
0:21:20 > 0:21:23and then have to go back to leading separate lives.
0:21:23 > 0:21:25That's perhaps the best interpretation.
0:21:25 > 0:21:29With all of these stories from a huge range of cultures,
0:21:29 > 0:21:31one thing you can't help to notice about Cygnus
0:21:31 > 0:21:34is the Milky Way galaxy going through it,
0:21:34 > 0:21:36with the dark lanes of dust,
0:21:36 > 0:21:39which William Herschel called "holes in the heavens."
0:21:39 > 0:21:42We now know they're clouds of dust blocking light from behind,
0:21:42 > 0:21:45sitting in a spiral arm of our own Milky Way galaxy.
0:21:48 > 0:21:50Galaxies were first spotted as smudges in the sky.
0:21:50 > 0:21:53But it was not until the beginning of last century
0:21:53 > 0:21:56that astronomers realised these were island universes,
0:21:56 > 0:21:58some containing trillions of stars.
0:21:59 > 0:22:03With large telescopes, we can now see millions of galaxies,
0:22:03 > 0:22:07out to the reaches of space, and they come in all shapes and sizes.
0:22:07 > 0:22:11Our Milky Way is similar to some of the gorgeous spiral galaxies we see,
0:22:11 > 0:22:17with arms of stars, gas and dust extending out from a central core.
0:22:17 > 0:22:20At the centre of our galaxy is a bar-like feature that is
0:22:20 > 0:22:23thought to be responsible for the spiral structure.
0:22:24 > 0:22:28Gerry Gilmore has been studying this beating heart of the Milky Way.
0:22:28 > 0:22:31So, Gerry, we've been talking about the constellation of Cygnus
0:22:31 > 0:22:34and all the stuff we can see in the plain of our Milky Way galaxy,
0:22:34 > 0:22:38but you're interested in the much larger-scale structure of our galaxy,
0:22:38 > 0:22:42and I have here the handy sketch of the galaxy we had earlier.
0:22:42 > 0:22:45So, here we are, with the spiral arms and the bar,
0:22:45 > 0:22:48and the sun is here at the top of the image.
0:22:48 > 0:22:51So, tell us what we're seeing here in Cygnus.
0:22:51 > 0:22:53Two things here. The first is that, of course,
0:22:53 > 0:22:55the whole Milky Way - it's huge -
0:22:55 > 0:22:58is so well mixed up that we actually see a fair sample of everything
0:22:58 > 0:22:59when we look locally.
0:22:59 > 0:23:02There's only one thing that's unique in the Milky Way,
0:23:02 > 0:23:06as far as we know, and that's this big, massive black hole down here.
0:23:06 > 0:23:09But there's a second thing that's unique, and that's related to
0:23:09 > 0:23:12the shape and the size, and that's why it looks like this.
0:23:12 > 0:23:15Why have we got this bar-like structure in the middle,
0:23:15 > 0:23:17this sort of big hand-shaped thing, with these spiral arms
0:23:17 > 0:23:20coming out of it? And how do we know that? That's hard to see.
0:23:20 > 0:23:23If you can only see round here, then, of course,
0:23:23 > 0:23:24how do you know what's out here?
0:23:24 > 0:23:27Fortunately, we can see in some wavelengths.
0:23:27 > 0:23:30There's a sort of sweet spot for studying the Milky Way.
0:23:30 > 0:23:33If you go too far to the infrared, all you see is the dust,
0:23:33 > 0:23:35you don't see any of the stars.
0:23:35 > 0:23:37And, of course, most of the Milky Way is stars,
0:23:37 > 0:23:40so you miss that picture. There is this sweet spot,
0:23:40 > 0:23:43which happens to be just to the red of the visible, fortunately.
0:23:43 > 0:23:46Then you can see very large distances through this stuff.
0:23:46 > 0:23:48So the whole key to this is what's going on in the centre.
0:23:48 > 0:23:51We have this bar-like structure, a flattened cigar shape,
0:23:51 > 0:23:53tens of thousands of light years long.
0:23:53 > 0:23:56A big structure, and it has these spiral arms coming off it,
0:23:56 > 0:23:57but they're not real things.
0:23:57 > 0:24:01They're not real collections of stars that are moving around together.
0:24:01 > 0:24:02That's correct, yes.
0:24:02 > 0:24:06A spiral arm behaves, as far as the galaxy is concerned,
0:24:06 > 0:24:07like a water wave on the ocean.
0:24:07 > 0:24:10So, a bit of water will go up and down as the wave comes through it,
0:24:10 > 0:24:13- the wave keeps going, but the water stays still.- Right.
0:24:13 > 0:24:15And that's what's driven by this guy in the middle here,
0:24:15 > 0:24:18this big, sort of hand-shaped structure.
0:24:18 > 0:24:20It has stars in it that are actually trapped in it,
0:24:20 > 0:24:22just by the weight of the other stars.
0:24:22 > 0:24:25So, this thing acts like a sort of mini galaxy
0:24:25 > 0:24:26inside the big galaxy.
0:24:26 > 0:24:29It's heavy enough that it traps stuff in it,
0:24:29 > 0:24:31and they just move around in a sort of big bar.
0:24:31 > 0:24:34It's so heavy, what it actually does is
0:24:34 > 0:24:36stir up the rest of the Milky Way.
0:24:36 > 0:24:40It's like a giant egg beater, sitting in the middle, and the waves
0:24:40 > 0:24:43coming off the end of this big egg beater are the spiral arms.
0:24:43 > 0:24:47This magnificent view of our galactic centre towards the bar was
0:24:47 > 0:24:50taken by the great observatories,
0:24:50 > 0:24:54the Hubble, Spitzer and Chandra space telescopes.
0:24:54 > 0:24:58Hubble revealed stars and warm gas, with the centre of our galaxy
0:24:58 > 0:25:00marked by the bright patch in the lower right.
0:25:02 > 0:25:04Spitzer's view in the infrared exposes
0:25:04 > 0:25:08hundreds of thousands of stars, hidden in visible light.
0:25:09 > 0:25:11But it's in the X-ray that we see
0:25:11 > 0:25:15the true, violent nature of the galactic centre.
0:25:15 > 0:25:17This image shows hundreds of small dots,
0:25:17 > 0:25:20which we believe are black holes and other remnants of dead stars.
0:25:21 > 0:25:24Seen towards the lower right of the image,
0:25:24 > 0:25:28a supermassive black hole lurks in the centre of our galaxy.
0:25:29 > 0:25:31High-energy X-rays are screaming out at us,
0:25:31 > 0:25:35as the black hole destroys anything that gets too close.
0:25:35 > 0:25:39Gas is heated to millions of degrees, solar systems are ripped apart
0:25:39 > 0:25:41and stars are pulverised -
0:25:41 > 0:25:45a cosmic catastrophe which is normally shielded from view.
0:25:45 > 0:25:50The centre of our galaxy is a crowded, vibrant and violent place.
0:25:50 > 0:25:53But amateurs can safely enjoy the pleasures of the Milky Way
0:25:53 > 0:25:57by taking long exposures of the Cygnus Rift.
0:25:57 > 0:26:01This lane of dark, dense dust blocks the light from background stars,
0:26:01 > 0:26:03and for the astronomer is a particular pleasure
0:26:03 > 0:26:05in the gentle summer nights.
0:26:06 > 0:26:09Paul, Tom and Pete have some final delights
0:26:09 > 0:26:12the chocolate box of Cygnus has to offer.
0:26:12 > 0:26:14Well, chaps, end of the evening,
0:26:14 > 0:26:17and we've seen some lovely things in Cygnus.
0:26:17 > 0:26:20I thought just before we close the programme we'd just give
0:26:20 > 0:26:21some of our favourite objects.
0:26:21 > 0:26:24So let's just go with Tom first. You're big into star clusters.
0:26:24 > 0:26:26You love open clusters, don't you?
0:26:26 > 0:26:28Open clusters, galactic clusters, absolutely love them.
0:26:28 > 0:26:31And there are two of note in the constellation of Cygnus.
0:26:31 > 0:26:35The first of the two is M39, which is quite a sparse,
0:26:35 > 0:26:37- not very dense cluster.- OK.
0:26:37 > 0:26:41The other cluster is somewhat less large, and perhaps somewhat
0:26:41 > 0:26:44less magnificent, but it is a favourite of mine, and that's M29,
0:26:44 > 0:26:46which is just below the star Gamma,
0:26:46 > 0:26:48nestled in the centre of the constellation.
0:26:48 > 0:26:54- To me it looks like a small analogue to the Pleiades.- I agree.
0:26:54 > 0:26:56It's known as the Cooling Tower cluster.
0:26:56 > 0:26:59There's a sort of squarish shape to the stars in there.
0:26:59 > 0:27:03I've never seen that. I'll presume you're not just making it up
0:27:03 > 0:27:06- for the programme! - It sounds terribly derogatory!
0:27:06 > 0:27:08Peter, what's yours? What's your favourite object?
0:27:08 > 0:27:11I'm going to go for the obvious one. The Hen's Beak.
0:27:11 > 0:27:12Yes, Albireo.
0:27:12 > 0:27:15Albireo, which is a beautiful double star.
0:27:15 > 0:27:18That's one of the first double stars I ever looked at
0:27:18 > 0:27:21when I was a small boy getting into astronomy.
0:27:21 > 0:27:24- And it takes your breath away... - It does, the colours.
0:27:24 > 0:27:27With very low power, you can see that bright, yellowish primary,
0:27:27 > 0:27:31and that lovely bluish secondary next to it.
0:27:31 > 0:27:33- Absolutely stunning.- It's timeless.
0:27:33 > 0:27:37Imagine planets on there, what sunrise and sunset would be like! Stunning.
0:27:37 > 0:27:38Go on, then, what's yours?
0:27:38 > 0:27:41My favourite one is the little planetary nebula,
0:27:41 > 0:27:43the blinking nebula. Have you heard of this?
0:27:43 > 0:27:46- I have.- This is quite an interesting planetary nebula,
0:27:46 > 0:27:49because the central star is quite bright - magnitude 10.4.
0:27:49 > 0:27:51Now, it does this very peculiar thing.
0:27:51 > 0:27:53If you look at the nebula straight on,
0:27:53 > 0:27:55a four-inch telescope will show it quite well.
0:27:55 > 0:27:58If you look away but use averted vision...
0:27:58 > 0:28:01- So you're looking out the side of your eye.- That's right.
0:28:01 > 0:28:03Then, the nebula appears to blink back on.
0:28:03 > 0:28:07And because natural eye movement, that's going on all the time,
0:28:07 > 0:28:10this blinking effect, and I think that's marvellous.
0:28:10 > 0:28:12A great object - when you look at it, you can't see it!
0:28:12 > 0:28:16That's right! Visual astronomy, Peter!
0:28:16 > 0:28:20Some good stuff in Cygnus, and it was a real pleasure to have
0:28:20 > 0:28:22a constellation like this to look at in the summer, I think.
0:28:22 > 0:28:24- It is.- Absolutely.
0:28:26 > 0:28:30Well, there's so much to see in the summer sky,
0:28:30 > 0:28:32so do go and have a look.
0:28:32 > 0:28:34When we come back next month,
0:28:34 > 0:28:38I'll be talking about the procession of the equinoxes,
0:28:38 > 0:28:43and we'll be paying a visit to the South Downs Planetarium.
0:28:43 > 0:28:46Until then, good night.
0:29:10 > 0:29:12Subtitles by Red Bee Media Ltd
0:29:12 > 0:29:14E-mail subtitling@bbc.co.uk