0:00:06 > 0:00:08This is the first map of the Milky Way
0:00:08 > 0:00:11made by William Herschel back in 1785
0:00:11 > 0:00:15in a paper he called On The Construction Of The Heavens.
0:00:15 > 0:00:18And it's a wonderful thing, but it's not that accurate.
0:00:18 > 0:00:20It shows the sun at the centre, which is wrong.
0:00:20 > 0:00:21There are no spiral arms.
0:00:21 > 0:00:23But he did get one important thing right.
0:00:23 > 0:00:27He shows the Milky Way as a disc seen almost side on.
0:00:27 > 0:00:31Since then, our knowledge of the Milky Way has greatly improved,
0:00:31 > 0:00:34but these images are actually artists' impressions.
0:00:34 > 0:00:36We think that the Milky Way looks like this,
0:00:36 > 0:00:37but we don't know for certain.
0:00:37 > 0:00:41In fact, we know surprisingly little about our home galaxy.
0:00:41 > 0:00:43But a revolution in our understanding
0:00:43 > 0:00:45of the Milky Way is underway,
0:00:45 > 0:00:47and the more we learn about our galaxy,
0:00:47 > 0:00:50the more surprising and interesting it becomes.
0:00:50 > 0:00:54So tonight, we bring you the bang up-to-date Guide To Our Galaxy
0:00:54 > 0:00:56with the most exciting and unusual discoveries.
0:00:56 > 0:00:58Welcome to The Sky At Night.
0:01:24 > 0:01:27Like all good guides, ours will include
0:01:27 > 0:01:28the best maps available,
0:01:28 > 0:01:30the most interesting destinations
0:01:30 > 0:01:32and, of course, a bit of history.
0:01:32 > 0:01:35We'll be joined by guest presenter
0:01:35 > 0:01:38Neil deGrasse Tyson for a tour
0:01:38 > 0:01:42of his top five strange and spectacular stars.
0:01:42 > 0:01:45This white dwarf is effectively flaying
0:01:45 > 0:01:47the outer layers of this giant,
0:01:47 > 0:01:50consuming it, body and soul.
0:01:51 > 0:01:54Pete will show us how to find some of the highlights of our galaxy
0:01:54 > 0:01:56in the night sky.
0:01:57 > 0:01:59But first, we're going to start
0:01:59 > 0:02:03with the most essential part of any guide, the map.
0:02:03 > 0:02:05We're here at the Institute of Astronomy in Cambridge,
0:02:05 > 0:02:08where the researchers have been making the most detailed
0:02:08 > 0:02:11and the most accurate map of the Milky Way ever.
0:02:11 > 0:02:14To do that, they're using one of the most impressive space telescopes
0:02:14 > 0:02:17ever constructed. If it were here on Earth,
0:02:17 > 0:02:20it would be capable of measuring the thickness of a coin on the moon.
0:02:23 > 0:02:25In December 2013,
0:02:25 > 0:02:29the European Space Agency launched the Gaia space telescope.
0:02:31 > 0:02:33Its mission - to map the stars of the Milky Way
0:02:33 > 0:02:35in unprecedented detail.
0:02:39 > 0:02:41Gaia is actually two telescopes,
0:02:41 > 0:02:45each observing 100,000 stars per hour
0:02:45 > 0:02:49and capturing the data with a one billion pixel camera,
0:02:49 > 0:02:52giving Gaia resolution that is ten times greater
0:02:52 > 0:02:54than most ground-based telescopes.
0:02:56 > 0:02:58Over its five-year mission,
0:02:58 > 0:03:02Gaia will observe each part of the sky 70 times
0:03:02 > 0:03:05in order to measure the exact positions of the stars,
0:03:05 > 0:03:07their distances and their motions.
0:03:09 > 0:03:13This room is where data from Gaia comes pouring in and gets processed.
0:03:13 > 0:03:17It's effectively the nerve centre for understanding our galaxy.
0:03:20 > 0:03:21You get some idea
0:03:21 > 0:03:25of the scale of the data-processing operation in here.
0:03:25 > 0:03:30This supercomputer is made up of 1,296 processing cores...
0:03:32 > 0:03:34..but this is just the tip of the iceberg.
0:03:35 > 0:03:38There are five more centres like this dotted around Europe,
0:03:38 > 0:03:40all handling Gaia's data.
0:03:43 > 0:03:46To convert raw satellite data from Gaia
0:03:46 > 0:03:48into an actual map of the Milky Way
0:03:48 > 0:03:53with precise positions, distances and brightnesses is no mean feat.
0:03:53 > 0:03:54In fact, for the first batch alone,
0:03:54 > 0:03:58they've had to analyse around 120 billion images.
0:04:00 > 0:04:02And to achieve the incredible precision
0:04:02 > 0:04:04required by the Gaia mission,
0:04:04 > 0:04:07the data-processing teams have to try to correct
0:04:07 > 0:04:09for every possible source of error.
0:04:09 > 0:04:12They even have to account for the gravitational influence
0:04:12 > 0:04:17of the other planets in the solar system on the light from the stars.
0:04:17 > 0:04:19It's a tremendous task,
0:04:19 > 0:04:21so much so that it took a year to process
0:04:21 > 0:04:23the first 14 months' worth of data.
0:04:25 > 0:04:27But it's been worth all the effort,
0:04:27 > 0:04:29because the result is nothing less
0:04:29 > 0:04:33than the first truly accurate map of our home galaxy.
0:04:33 > 0:04:36And Chris went to meet lead scientist Gerry Gilmore
0:04:36 > 0:04:39to see the map for himself.
0:04:39 > 0:04:43So, Chris, welcome to our new map of our Milky Way.
0:04:43 > 0:04:44It's beautiful.
0:04:44 > 0:04:48You can see immediately the sort of big picture structure.
0:04:48 > 0:04:50First thing to notice is that this
0:04:50 > 0:04:51is a map of the entire sky.
0:04:51 > 0:04:53But this stripe across the middle,
0:04:53 > 0:04:55this bright stripe, that's the galaxy.
0:04:55 > 0:04:58That's the Milky Way galaxy, that's right, yes.
0:04:58 > 0:05:01But there is more to the Milky Way than just the main disc.
0:05:01 > 0:05:06There are also millions of stars, both above and below the disc.
0:05:06 > 0:05:09Everything white that you can see in this image is starlight.
0:05:10 > 0:05:12What we need to do is zoom in.
0:05:12 > 0:05:16This is the top-layer map, and now we are zooming in.
0:05:16 > 0:05:18So this is the inner parts of the Milky Way here,
0:05:18 > 0:05:20these big dust clouds and dust lanes,
0:05:20 > 0:05:22the dust lane, these are dust lanes,
0:05:22 > 0:05:25and you start to see that all the white light
0:05:25 > 0:05:27is now breaking up into stars,
0:05:27 > 0:05:29into individual stars.
0:05:29 > 0:05:31So, as you get further in, as we zoom in,
0:05:31 > 0:05:34we see more and more stars individually.
0:05:34 > 0:05:36Look at how they suddenly appear
0:05:36 > 0:05:38as you go that little bit deeper into space and, ping!
0:05:38 > 0:05:42What you thought was just white light is actually starlight.
0:05:42 > 0:05:44And how many stars are there in the galaxy?
0:05:44 > 0:05:48There's maybe 100 billion, 200 billion, 300 billion.
0:05:48 > 0:05:50The number's probably near the high end of that range.
0:05:50 > 0:05:52What we know is that there's maybe twice as many
0:05:52 > 0:05:54as we previously thought.
0:05:54 > 0:05:56There are twice as many stars in the galaxy as we thought?
0:05:56 > 0:05:58That's right, yes. It's just in the bit of the Milky Way
0:05:58 > 0:06:01that Gaia has measured. There are twice as many stars in there.
0:06:01 > 0:06:02How has Gaia doubled...?
0:06:02 > 0:06:04That seems surprising to me.
0:06:04 > 0:06:06I assumed that you didn't know the position before,
0:06:06 > 0:06:08but where have they been hiding?
0:06:08 > 0:06:10It's just because the image quality of Gaia,
0:06:10 > 0:06:13the fact that Gaia has such high spatial resolution.
0:06:13 > 0:06:14Because it can see sharply?
0:06:14 > 0:06:16Exactly, see sharply,
0:06:16 > 0:06:17and so Gaia can tell the difference
0:06:17 > 0:06:20between two or three stars that are very close together,
0:06:20 > 0:06:22but, nevertheless, are separate stars,
0:06:22 > 0:06:25whereas, previously, from our images from the ground,
0:06:25 > 0:06:26blurry sort of things,
0:06:26 > 0:06:28this stuff here would have all been
0:06:28 > 0:06:30merged into what we thought was one star.
0:06:30 > 0:06:33And so, we've been looking at the map as it appears on the sky,
0:06:33 > 0:06:35but one of the exciting things about Gaia is
0:06:35 > 0:06:37that we have three-dimensional information as well.
0:06:37 > 0:06:40We can go beyond this sort of two-dimensional picture.
0:06:40 > 0:06:42Exactly, and that's the unique feature of Gaia.
0:06:42 > 0:06:45Gaia measures distances as well as all these other things.
0:06:45 > 0:06:47And once we get into measuring distances,
0:06:47 > 0:06:50then we can measure the three-dimensional structure
0:06:50 > 0:06:51of our Milky Way.
0:06:51 > 0:06:54So far, we've only just had a taster of that,
0:06:54 > 0:06:58so Gaia's just released two million accurate distances.
0:06:58 > 0:07:01And so, here is the beginnings of a picture in three dimensions.
0:07:01 > 0:07:04This is our sun, and we are about to go and see
0:07:04 > 0:07:06the Hyades and the Pleiades cluster.
0:07:06 > 0:07:08Both part of the autumn sky.
0:07:08 > 0:07:09Oh, look.
0:07:09 > 0:07:11There's Betelgeuse and the Hyades there
0:07:11 > 0:07:13with Aldebaran in front of them.
0:07:13 > 0:07:14The Pleiades in the corner.
0:07:14 > 0:07:16There goes Aldebaran. It's closer to us.
0:07:16 > 0:07:18And so, here we are, for the first time ever,
0:07:18 > 0:07:21seeing a star cluster in three dimensions.
0:07:21 > 0:07:23So we can tell the difference
0:07:23 > 0:07:27between the front side and the back side of the Hyades.
0:07:27 > 0:07:29So we know exactly in 3-D where all these stars are.
0:07:29 > 0:07:31And this helps understand the cluster.
0:07:31 > 0:07:33We know more about it than we did before we had this map.
0:07:33 > 0:07:35And so, we have already discovered
0:07:35 > 0:07:38not only how deep the Hyades cluster itself is,
0:07:38 > 0:07:39but, actually, it's about twice as big.
0:07:39 > 0:07:43It's amazing. What other features should we look for in this 3-D map?
0:07:43 > 0:07:45So this dramatic Hyades example,
0:07:45 > 0:07:49one very nearby cluster, is just a taster for what's going on.
0:07:49 > 0:07:53We've got good distances now for just two million stars.
0:07:53 > 0:07:56We are going to have one billion at least,
0:07:56 > 0:07:58that will give us a three-dimensional map
0:07:58 > 0:08:00of half of our Milky Way.
0:08:00 > 0:08:02But, even more interestingly,
0:08:02 > 0:08:05because Gaia's continuing to observe over time,
0:08:05 > 0:08:07it's telling us how everything's moving,
0:08:07 > 0:08:09and that combination of where things are
0:08:09 > 0:08:10and how they are moving
0:08:10 > 0:08:14allows us to determine how the galaxy works as a machine,
0:08:14 > 0:08:17how the Milky Way actually functions, what is a spiral arm,
0:08:17 > 0:08:19where is the dark matter and how it's evolving.
0:08:19 > 0:08:21Well, it's going to be very exciting.
0:08:21 > 0:08:23It's going to be fantastic.
0:08:23 > 0:08:27So now we have a map of over a billion stars in our galactic guide,
0:08:27 > 0:08:30but the question is, where do we go and visit?
0:08:30 > 0:08:33What are the most interesting and exciting stars within our galaxy?
0:08:33 > 0:08:36We invited astrophysicist Neil deGrasse Tyson,
0:08:36 > 0:08:39who has written about this very subject
0:08:39 > 0:08:41in his book, Welcome To The Universe,
0:08:41 > 0:08:43to give us a guide
0:08:43 > 0:08:45to the weird and wonderful stars of the Milky Way.
0:08:46 > 0:08:48The sun.
0:08:48 > 0:08:51It's a million times bigger than the Earth.
0:08:52 > 0:08:56It's widely regarded as a yellow orb in the sky, but it's actually white.
0:08:57 > 0:08:59You know, our sun is not the hottest
0:08:59 > 0:09:02or the coolest or the biggest or the smallest.
0:09:02 > 0:09:05There's nothing to distinguish it
0:09:05 > 0:09:09among the hundreds of billions of other stars in the galaxy.
0:09:09 > 0:09:11In fact, if you were an intergalactic alien,
0:09:11 > 0:09:13and you came upon the Milky Way,
0:09:13 > 0:09:15there'd be nothing about our star system
0:09:15 > 0:09:18that would attract your attention.
0:09:18 > 0:09:22But there's plenty of other stars in our galaxy worth a look,
0:09:22 > 0:09:25and I've made a personal list of the top five.
0:09:26 > 0:09:27Coming in at number five
0:09:27 > 0:09:30of our exotic star countdown of the Milky Way
0:09:30 > 0:09:36is SAO 206462, previously thought to just be an ordinary star.
0:09:36 > 0:09:38When you go in and look close,
0:09:38 > 0:09:42and you blot away the light of this relatively new star
0:09:42 > 0:09:43that was just formed,
0:09:43 > 0:09:47you get to see this remarkable spiral structure.
0:09:47 > 0:09:51We are pretty confident that this is a disc of debris
0:09:51 > 0:09:55in the act of forming brand-new planets
0:09:55 > 0:09:58around this relatively newly born star.
0:09:58 > 0:10:03Each of these spiral arms will likely coalesce,
0:10:03 > 0:10:05and, in the not so distant future,
0:10:05 > 0:10:08become single planets, one for each arm,
0:10:08 > 0:10:11in mutual orbit around the host star.
0:10:11 > 0:10:15We've seen discs of material before, around newborn stars,
0:10:15 > 0:10:18hinting that planets are soon to form.
0:10:18 > 0:10:22But this is the first time we've ever seen spiral structure.
0:10:24 > 0:10:26This star is just at the beginning of its life,
0:10:26 > 0:10:29and who knows what other strange phenomena
0:10:29 > 0:10:32it will treat us to as it evolves.
0:10:32 > 0:10:36But strange behaviour isn't the preserve of the young.
0:10:36 > 0:10:37Coming in at number four
0:10:37 > 0:10:42among the top five weirdest stars in the Milky Way galaxy
0:10:42 > 0:10:44is Omicron Ceti,
0:10:44 > 0:10:48Ceti referencing the constellation Cetus, the whale.
0:10:48 > 0:10:52Here it is. It happens to be a red giant star.
0:10:52 > 0:10:54Plenty of these in the galaxy,
0:10:54 > 0:10:58not even distinguished for how it looks in visible light.
0:10:58 > 0:11:02But if you, instead, observe this star using ultraviolet...
0:11:03 > 0:11:05..this is what you find.
0:11:05 > 0:11:06The star is up here...
0:11:08 > 0:11:13..and there's this long, 13 light-year plume left behind.
0:11:16 > 0:11:19And right up front, we have a bow shock,
0:11:19 > 0:11:23which is evidence that this thing is ploughing through the galaxy
0:11:23 > 0:11:26between the stars at incredible speeds.
0:11:26 > 0:11:30In fact, we can measure it at 300,000 miles an hour.
0:11:31 > 0:11:33We happen to know that red giants
0:11:33 > 0:11:37only have a tenuous connection to their outer gaseous shells,
0:11:37 > 0:11:39so going that fast,
0:11:39 > 0:11:42it's going to lose some of that outer shell,
0:11:42 > 0:11:46leaving it behind as it punches its way
0:11:46 > 0:11:48through the gas of the interstellar medium.
0:11:48 > 0:11:50We've never seen anything like this.
0:11:52 > 0:11:57But the most spectacular sights to be seen don't just involve one star.
0:11:59 > 0:12:03Coming in at number three is MY Camelopardalis.
0:12:03 > 0:12:05It's not one star, it's two.
0:12:07 > 0:12:12Two supermassive, hot blue stars
0:12:12 > 0:12:14in orbit around one another.
0:12:14 > 0:12:16That alone is not odd.
0:12:16 > 0:12:18More than half the stars you see in the night sky,
0:12:18 > 0:12:20if you pull a telescope out on them,
0:12:20 > 0:12:24what you'll notice is that it's a double or multiple star system.
0:12:24 > 0:12:27So, we're cool with that.
0:12:27 > 0:12:30What's different here is that these two stars
0:12:30 > 0:12:32are in CONTACT with one another.
0:12:32 > 0:12:36We are fairly certain that, given enough time,
0:12:36 > 0:12:39these two stars will completely coalesce,
0:12:39 > 0:12:43joining their fuel supplies into one coherent star.
0:12:44 > 0:12:47What we don't know is will that star be stable...
0:12:48 > 0:12:52..and just join the ranks of other high-mass stars that are out there,
0:12:52 > 0:12:56or will it be unstable and blow itself apart?
0:12:56 > 0:12:58We don't know.
0:12:58 > 0:13:00Just keep watching this space.
0:13:02 > 0:13:06This isn't the only strange binary system to be seen.
0:13:06 > 0:13:08We're down to number two.
0:13:08 > 0:13:13That distinction goes to Gaia 14aae.
0:13:13 > 0:13:17Once again, it's not a single star, it's a binary star system.
0:13:17 > 0:13:20In this case, one of them is a white dwarf,
0:13:20 > 0:13:23the end stage of a medium-mass star such as the sun,
0:13:23 > 0:13:25and a companion star,
0:13:25 > 0:13:29which happens to be going through its giant phase.
0:13:29 > 0:13:32This white dwarf is effectively flaying
0:13:32 > 0:13:34the outer layers of this giant,
0:13:34 > 0:13:37consuming it, body and soul.
0:13:38 > 0:13:40It's basically a stellar cannibal.
0:13:42 > 0:13:46If the growing mass of the white dwarf crosses a certain threshold...
0:13:47 > 0:13:52..the white dwarf will go unstable and explode, become a supernova.
0:13:53 > 0:13:56But this isn't even the weirdest thing that can happen
0:13:56 > 0:13:57when stars collide.
0:13:59 > 0:14:04And now, the number one weirdest star in the Milky Way galaxy.
0:14:04 > 0:14:06It goes by the name of V Hydrae.
0:14:08 > 0:14:10This star, a red giant star,
0:14:10 > 0:14:14would not otherwise call attention to itself,
0:14:14 > 0:14:18were it not for, every eight and a half years,
0:14:18 > 0:14:24blobs of nearly 10,000-degree plasma comes spewing out
0:14:24 > 0:14:27at a half a million miles an hour.
0:14:27 > 0:14:28Then it stops.
0:14:28 > 0:14:31Eight and a half years later, it repeats.
0:14:33 > 0:14:34Who ordered that?
0:14:35 > 0:14:40Our best hypothesis is that this red giant star has a companion...
0:14:42 > 0:14:48..another star in orbit around it that has a highly elliptical orbit,
0:14:48 > 0:14:50a highly elongated orbit.
0:14:50 > 0:14:54Spends a lot of time out far away, but when it comes in close,
0:14:54 > 0:14:57it comes in real close and punches through
0:14:57 > 0:15:00the physical body of the red giant itself,
0:15:00 > 0:15:02pops out the other side.
0:15:02 > 0:15:06We think that interaction of a hot star and the red giant
0:15:06 > 0:15:09is what's responsible for spewing forth
0:15:09 > 0:15:13these blobs of plasma right on schedule...
0:15:14 > 0:15:15..every eight and a half years.
0:15:17 > 0:15:21So, we've never seen this phenomenon before.
0:15:21 > 0:15:25We THINK we have the right explanation for it,
0:15:25 > 0:15:27but it remains a unique object
0:15:27 > 0:15:31in all the star catalogues that we've accumulated.
0:15:31 > 0:15:33And for that reason,
0:15:33 > 0:15:36it is the weirdest star in the Milky Way.
0:15:39 > 0:15:42And if you want to see some of Neil's top five stars for yourself,
0:15:42 > 0:15:45Peter's here to show you how to find them.
0:15:49 > 0:15:50When you look up at the night sky,
0:15:50 > 0:15:52all of the stars you can see individually
0:15:52 > 0:15:55belong to our own Milky Way galaxy.
0:15:55 > 0:15:58Now, in the winter time, the dense part of the Milky Way,
0:15:58 > 0:16:01which is towards the core, isn't visible.
0:16:01 > 0:16:04But, when we look up at the sky in the winter time,
0:16:04 > 0:16:07we're looking outward in the opposite direction.
0:16:07 > 0:16:11This is also a great time to look for some of the five
0:16:11 > 0:16:13must-see stars in the sky.
0:16:13 > 0:16:16Now, I'm going to start with Omicron Ceti,
0:16:16 > 0:16:20which is in the constellation of Cetus, the whale, or sea monster.
0:16:20 > 0:16:22This is the comet-like star,
0:16:22 > 0:16:25which, when viewed in ultraviolet,
0:16:25 > 0:16:27is seen leaving a tail behind it
0:16:27 > 0:16:28as it races through the galaxy.
0:16:30 > 0:16:33So, this is a variable star, and at its brightest,
0:16:33 > 0:16:36it's an easy thing to see with the naked eye, but at its dimmest,
0:16:36 > 0:16:38as it is now, you need a telescope
0:16:38 > 0:16:41or a pair of binoculars to pick it out.
0:16:41 > 0:16:43Now, I'm going to use my camera
0:16:43 > 0:16:47and see if I can take a photograph of the star.
0:16:48 > 0:16:52To find it, first identify the Great Square of Pegasus
0:16:52 > 0:16:54and then, below, and to the left,
0:16:54 > 0:16:58find the faint V-shaped string of stars of Pisces.
0:16:58 > 0:17:02The string comes to a sharp point at the star Alrescha
0:17:02 > 0:17:03and this acts like an arrow
0:17:03 > 0:17:07pointing directly to Omicron Ceti, also called Mira.
0:17:11 > 0:17:15I'm actually finding it quite tricky to locate the field where Mira is
0:17:15 > 0:17:17at the moment, and the reason for that is
0:17:17 > 0:17:19that there is a full moon coming up
0:17:19 > 0:17:21and that's making the sky really bright
0:17:21 > 0:17:23and drowning out all the faint stars.
0:17:23 > 0:17:26But I think I'm more or less in the right area now.
0:17:27 > 0:17:28And I think I've got it.
0:17:28 > 0:17:31There are two stars very close to one another.
0:17:31 > 0:17:34One of them is red, and that is Omicron Ceti,
0:17:34 > 0:17:39or Mira as it's known, and it's red because it's a red giant star.
0:17:39 > 0:17:41Now, I can't see the tail
0:17:41 > 0:17:43because that's visible in UV light and UV light
0:17:43 > 0:17:45is blocked by the atmosphere,
0:17:45 > 0:17:47so that's best left to the space telescopes,
0:17:47 > 0:17:50but I've definitely got it, so I'm happy.
0:17:54 > 0:17:56And in the faint constellation of Camelopardalis,
0:17:56 > 0:17:59which lies to the north of Auriga and Perseus,
0:17:59 > 0:18:04is the merging binary star, MY Camelopardalis.
0:18:04 > 0:18:06Now, this is so far north in the sky
0:18:06 > 0:18:08that from the United Kingdom it never sets,
0:18:08 > 0:18:11but you will need a telescope to see it.
0:18:11 > 0:18:14The best way to find it is to extend a line
0:18:14 > 0:18:18from the famous eclipsing binary star, Algol, through Mirfak,
0:18:18 > 0:18:22the two brightest stars in the constellation of Perseus.
0:18:22 > 0:18:25Extend the line they make north for the same distance again,
0:18:25 > 0:18:27and you'll be in the right area.
0:18:28 > 0:18:31And if you're willing to stay up until the early hours,
0:18:31 > 0:18:33then you stand a chance of seeing
0:18:33 > 0:18:37the number one strangest star in the galaxy, V Hydrae,
0:18:37 > 0:18:40the star system emitting the plasma balls.
0:18:42 > 0:18:45It lies in the belly of Hydra, the water snake.
0:18:45 > 0:18:50Halfway along the snake, find new Hydrae and 5 degrees below that,
0:18:50 > 0:18:53at the bottom of a Y-shaped pattern of stars, is V Hydrae.
0:18:54 > 0:18:56A telescope is the best way to see it.
0:18:59 > 0:19:02But sadly, the last two of Neil's favourite stars
0:19:02 > 0:19:04are too tricky to see.
0:19:05 > 0:19:10SAO 206462, the star with the spiral arms,
0:19:10 > 0:19:13is too far south to be seen from the UK.
0:19:13 > 0:19:19And Gaia 14aae, the cannibal binary system, is extremely faint,
0:19:19 > 0:19:22so it would be very hard to spot without a large telescope.
0:19:26 > 0:19:28This time of year is also great
0:19:28 > 0:19:30for seeing another of the stellar highlights
0:19:30 > 0:19:34featured earlier, the Hyades star cluster,
0:19:34 > 0:19:36which Gaia has mapped in 3-D
0:19:36 > 0:19:38and which can even be seen with the naked eye.
0:19:40 > 0:19:42But the best way to see them
0:19:42 > 0:19:45is to use a pair of binoculars or a telescope
0:19:45 > 0:19:47and if you use such an instrument,
0:19:47 > 0:19:49you'll see them in far greater detail.
0:19:49 > 0:19:53Now I'm going to take a photograph to see what I can get.
0:19:53 > 0:19:55The easiest way to find the Hyades
0:19:55 > 0:19:58is to extend the line made by Orion's Belt
0:19:58 > 0:20:03up to the north west, or up and right as seen from the UK,
0:20:03 > 0:20:06until you arrive at the bright orange star, Aldebaran,
0:20:06 > 0:20:09which is part of the V-shaped face of Taurus the bull.
0:20:12 > 0:20:15So, the face of Taurus the bull is the Hyades
0:20:15 > 0:20:19and the Hyades are about 625 million years old,
0:20:19 > 0:20:21so they're getting to the end of their life,
0:20:21 > 0:20:25so there are lots of red giants and white dwarfs in this cluster.
0:20:30 > 0:20:33Our understanding of the Milky Way is improving all the time.
0:20:35 > 0:20:38Not only do we now have Gaia's billion-star map,
0:20:39 > 0:20:41but recently, ground-based telescopes
0:20:41 > 0:20:44have created the most comprehensive map ever made
0:20:44 > 0:20:47of the distribution of dust in the galaxy.
0:20:49 > 0:20:52It shows the spiral arms, star-forming regions
0:20:52 > 0:20:53and streams of dust.
0:20:56 > 0:20:57But beyond the stars and dust,
0:20:57 > 0:20:59there are other parts of the galaxy
0:20:59 > 0:21:02that we still know extremely little about.
0:21:04 > 0:21:06The final part of our Guide To The Galaxy
0:21:06 > 0:21:07takes us to one of the Milky Way's
0:21:07 > 0:21:11most mysterious and interesting regions - its outer reaches.
0:21:11 > 0:21:14It's long been thought that the visible disc of the galaxy
0:21:14 > 0:21:17is embedded in a huge corona,
0:21:17 > 0:21:20a vast cloud of hot, ionised gas.
0:21:20 > 0:21:24And if it's there, it might help us solve a problem,
0:21:24 > 0:21:25because when you add up all the dust,
0:21:25 > 0:21:29all the gas and all the stars in the disc, all the normal matter,
0:21:29 > 0:21:33you find there's only about half as much as there should be,
0:21:33 > 0:21:37and it may be that that missing mass is hiding in the corona.
0:21:37 > 0:21:41Well, we know very little about this strange, enigmatic region
0:21:41 > 0:21:44but scientists here have made a new discovery.
0:21:46 > 0:21:48Maggie went to meet Denis Erkal
0:21:48 > 0:21:51to find out what this discovery can tell us
0:21:51 > 0:21:52about the nature of the corona
0:21:52 > 0:21:54and the missing matter in our galaxy.
0:21:56 > 0:21:58So, Denis, thank you for coming up to speak to us
0:21:58 > 0:22:00and this is a glorious picture.
0:22:00 > 0:22:01What are we seeing here?
0:22:01 > 0:22:03This is a picture that I took in Chile,
0:22:03 > 0:22:05so this is the night sky as seen from Chile,
0:22:05 > 0:22:07so this is the Milky Way right here,
0:22:07 > 0:22:08this is the disc of the Milky Way,
0:22:08 > 0:22:10and these are the objects we are interested in.
0:22:10 > 0:22:12This is the small and large Magellanic Cloud.
0:22:12 > 0:22:14These are two dwarf galaxies
0:22:14 > 0:22:16that, very recently, they had their closest approach to the Milky Way.
0:22:16 > 0:22:19These can only be seen from the southern hemisphere.
0:22:19 > 0:22:21These can only be seen from relatively far south.
0:22:21 > 0:22:23So, what have you discovered so far?
0:22:23 > 0:22:24So, let's just zoom in a little bit.
0:22:24 > 0:22:27So, this is another picture of the large Magellanic Cloud
0:22:27 > 0:22:29and the small Magellanic Cloud.
0:22:29 > 0:22:31This was the optical image, but we can also show what happens
0:22:31 > 0:22:34when we overlay data from a radio telescope.
0:22:34 > 0:22:35So, this shows hydrogen
0:22:35 > 0:22:38around the large and the small Magellanic Cloud.
0:22:38 > 0:22:40What we believe happened was about a few hundred million years ago,
0:22:40 > 0:22:42the small Magellanic Cloud
0:22:42 > 0:22:44passed near the large Magellanic Cloud,
0:22:44 > 0:22:45and as it passed nearby,
0:22:45 > 0:22:48the gravitational force from the large Magellanic Cloud
0:22:48 > 0:22:50shredded part of the small Magellanic Cloud,
0:22:50 > 0:22:52ripping off this gas bridge.
0:22:52 > 0:22:56- Oh, wow.- And so now we can overlay this blue line,
0:22:56 > 0:22:59which just shows us the densest part of the gas.
0:22:59 > 0:23:02So, we've known about this gas bridge for a long time,
0:23:02 > 0:23:04but what we found recently was a stellar bridge.
0:23:04 > 0:23:08And so this shows the data from the Gaia satellite.
0:23:08 > 0:23:11So here, the background image is the one I showed you before,
0:23:11 > 0:23:13and in white, this milky cloud,
0:23:13 > 0:23:15shows the stars from Gaia smoothed over
0:23:15 > 0:23:17to make an artist's impression of the data.
0:23:17 > 0:23:20OK, so Gaia has discovered these stars and it looks like, yeah,
0:23:20 > 0:23:22there's a stellar bridge, a bridge of stars.
0:23:22 > 0:23:25That's right, so this is the stellar bridge coming down here,
0:23:25 > 0:23:28so just like the gas was ripped off from the small Magellanic Cloud,
0:23:28 > 0:23:31we think, at the same time, stars were ripped off.
0:23:32 > 0:23:37So the gas and star bridge were created at the same time and place.
0:23:37 > 0:23:41But there's a mystery, because today, when the images are overlaid,
0:23:41 > 0:23:44you can see that the two bridges are no longer aligned.
0:23:44 > 0:23:50And Denis believes the explanation for this discrepancy is the corona.
0:23:50 > 0:23:51So, the large and small Magellanic Cloud
0:23:51 > 0:23:54are both moving down roughly in this direction,
0:23:54 > 0:23:56so we think, as they move through the corona,
0:23:56 > 0:23:58what happens is the gas gets pushed back
0:23:58 > 0:24:00by the corona from the Milky Way.
0:24:00 > 0:24:02The stars also feel that pressure from the corona,
0:24:02 > 0:24:04but since they're small and dense,
0:24:04 > 0:24:05they basically punch through the corona,
0:24:05 > 0:24:07so they don't get pushed back at all.
0:24:07 > 0:24:09So, this result is really good evidence
0:24:09 > 0:24:11that the corona actually exists.
0:24:11 > 0:24:12That's right, there was some evidence before,
0:24:12 > 0:24:14but we think this adds even stronger evidence,
0:24:14 > 0:24:16so we're very convinced that the corona is there.
0:24:16 > 0:24:18How far does it extend from our galaxy?
0:24:18 > 0:24:20We think it extends out to at least 600,000 light years
0:24:20 > 0:24:22from the centre of our galaxy.
0:24:22 > 0:24:24Wow, but that's sort of well on its way to Andromeda, isn't it?
0:24:24 > 0:24:27That's right, that's about a third of the way to Andromeda,
0:24:27 > 0:24:28so it's really far out.
0:24:28 > 0:24:29- Wow.- It goes at least that far, we think.
0:24:29 > 0:24:32So, with this massive corona around our galaxy,
0:24:32 > 0:24:34can that help account for the missing visible matter
0:24:34 > 0:24:35in our galaxy?
0:24:35 > 0:24:37Yes, we think that this corona can account for
0:24:37 > 0:24:39a significant fraction of
0:24:39 > 0:24:41the missing normal matter around our galaxy.
0:24:41 > 0:24:43Wow. That's quite a result, isn't it?
0:24:43 > 0:24:44Yeah, it's pretty exciting.
0:24:44 > 0:24:48There's another layer of significance to this finding,
0:24:48 > 0:24:51because there's evidence of a similar corona of hot gas
0:24:51 > 0:24:54around our neighbouring galaxy, Andromeda.
0:24:55 > 0:24:58Andromeda is about 2.5 million light years away,
0:24:58 > 0:25:02but it's on a collision course with the Milky Way.
0:25:02 > 0:25:04We've measured that Andromeda has a corona,
0:25:04 > 0:25:06and it extends halfway to our galaxy,
0:25:06 > 0:25:08and now we're getting this strong evidence
0:25:08 > 0:25:10that we've got a corona that extends a long way.
0:25:10 > 0:25:12Is there a possibility they could be touching?
0:25:12 > 0:25:14That's right, it is very possible.
0:25:14 > 0:25:16From our models, we think it's very likely
0:25:16 > 0:25:18that the very outer extents of our galaxy
0:25:18 > 0:25:21and Andromeda are touching right now.
0:25:21 > 0:25:23So, I've often heard it quoted that in, you know, four,
0:25:23 > 0:25:25five billion years' time,
0:25:25 > 0:25:27the Milky Way and Andromeda will collide,
0:25:27 > 0:25:30but it sounds as if the collision's already begun.
0:25:30 > 0:25:32In a way it has, yes, but these are just the very outer,
0:25:32 > 0:25:35tenuous edges of the coronas of the Milky Way and Andromeda, but, yes,
0:25:35 > 0:25:37the collision is starting to start now.
0:25:37 > 0:25:39Well, I think that's pretty exciting.
0:25:39 > 0:25:41Thank you so much for coming in and talking to us.
0:25:41 > 0:25:43- Thanks, it's been a pleasure. Thank you.- Thank you.
0:25:46 > 0:25:48The collision of the Milky Way and Andromeda
0:25:48 > 0:25:51will eventually create a new, giant galaxy.
0:25:52 > 0:25:55It's extraordinary to think that the first contact
0:25:55 > 0:25:58between the two galaxies may have already begun,
0:25:58 > 0:26:01but it'll be another four billion years or so
0:26:01 > 0:26:02before the merger is complete.
0:26:04 > 0:26:07It turns out that this intergalactic collision
0:26:07 > 0:26:09will not be a new experience for the Milky Way...
0:26:10 > 0:26:12..because Gaia has revealed
0:26:12 > 0:26:14that our galaxy is the product of many previous
0:26:14 > 0:26:17smaller galactic mergers.
0:26:17 > 0:26:19The main disc of the galaxy
0:26:19 > 0:26:22is surrounded by a halo of stars and clusters,
0:26:22 > 0:26:25which live both above and below the disc, like this.
0:26:25 > 0:26:28But what's new is that, thanks to Gaia,
0:26:28 > 0:26:31we can work out how this whole system is moving.
0:26:31 > 0:26:34Now, the disc is rotating, we've known that for a long while,
0:26:34 > 0:26:37so it's spinning in this sort of direction.
0:26:37 > 0:26:40But what Gaia has found is that some of these clusters
0:26:40 > 0:26:41are moving through the disc
0:26:41 > 0:26:45and they're doing so in the opposite direction
0:26:45 > 0:26:48to the rotation of the disc itself, and so what that means is
0:26:48 > 0:26:51that these clusters are the remnants of galaxies,
0:26:51 > 0:26:53which our Milky Way has accreted.
0:26:53 > 0:26:57Our galaxy is growing through galactic cannibalism.
0:26:57 > 0:26:59And this process is still going on.
0:26:59 > 0:27:02The Magellanic Clouds Denis was talking to Maggie about earlier
0:27:02 > 0:27:05will almost certainly be eaten in the next few hundred million years.
0:27:07 > 0:27:09That's it for now from our Guide To The Galaxy.
0:27:09 > 0:27:12But don't forget, our understanding of the Milky Way
0:27:12 > 0:27:14is undergoing a revolution,
0:27:14 > 0:27:16so tune in in a year or so's time,
0:27:16 > 0:27:19when we get the next batch of data from Gaia.
0:27:19 > 0:27:22Next month we have a special programme.
0:27:22 > 0:27:24We'll be taking over some of the telescopes
0:27:24 > 0:27:27under the clear skies of La Palma in the Canary Islands.
0:27:27 > 0:27:32And we want you to tell us what we should point the telescopes at.
0:27:32 > 0:27:34Find out more on our website
0:27:34 > 0:27:36and then e-mail or tweet your suggestions
0:27:36 > 0:27:38to the usual addresses on the screen now.
0:27:41 > 0:27:43And don't forget you can already check out
0:27:43 > 0:27:45our star guide on the website.
0:27:45 > 0:27:46In the meantime, get outside
0:27:46 > 0:27:49- and get looking up.- Good night.