1 00:00:01,440 --> 00:00:06,680 This is our galaxy, the Milky Way, in all its glory. 2 00:00:10,720 --> 00:00:13,160 Or, at least, it might be... 3 00:00:13,160 --> 00:00:17,160 because like every other image that shows the whole of the Milky Way, 4 00:00:17,160 --> 00:00:21,560 this is actually a computer graphic that is based on our best guesswork. 5 00:00:24,040 --> 00:00:27,720 So what DOES the Milky Way really look like? 6 00:00:27,720 --> 00:00:29,920 We're about find out. 7 00:00:58,800 --> 00:01:03,240 Quatre, trois, deux, un... 8 00:01:03,240 --> 00:01:05,080 Decollage. 9 00:01:07,560 --> 00:01:12,640 In December 2013, the European Space Agency launched Gaia - 10 00:01:12,640 --> 00:01:14,840 a scientific instrument that will transform 11 00:01:14,840 --> 00:01:16,560 our understanding of the Milky Way. 12 00:01:16,560 --> 00:01:19,640 It's the most advanced astronomical camera ever made, 13 00:01:19,640 --> 00:01:22,800 and contains the biggest array of sensors ever to leave the Earth. 14 00:01:26,320 --> 00:01:29,280 The camera is packed with a billion pixels - 15 00:01:29,280 --> 00:01:32,560 over 60 times more than the Hubble Space Telescope cameras, 16 00:01:32,560 --> 00:01:36,840 and it will allow Gaia to learn more about our galaxy than ever before. 17 00:01:41,080 --> 00:01:44,480 We've come to the company that built this incredible sensor 18 00:01:44,480 --> 00:01:46,480 to explore the story behind it. 19 00:01:47,640 --> 00:01:51,040 Also, what DOES our galaxy actually look like? 20 00:01:51,040 --> 00:01:55,000 At the moment we think we've got a four-spiral arm structure. 21 00:01:55,000 --> 00:01:56,920 A few years back we thought we only had three. 22 00:01:56,920 --> 00:01:58,680 It's amazing how the picture changes. 23 00:01:59,760 --> 00:02:01,960 And physicist Jim Al-Khalili finds out 24 00:02:01,960 --> 00:02:04,040 how the mission will let us trace 25 00:02:04,040 --> 00:02:07,240 the hidden matter that shapes our galaxy. 26 00:02:07,240 --> 00:02:12,040 Without dark matter, the universe as we see it simply wouldn't exist. 27 00:02:12,040 --> 00:02:16,640 Plus Pete, showing us how to see the Milky Way for ourselves. 28 00:02:22,360 --> 00:02:25,400 Seen from Earth, the Milky Way is a glorious sight 29 00:02:25,400 --> 00:02:27,800 as it stretches across the night sky. 30 00:02:29,200 --> 00:02:32,920 But we only get a very narrow perspective on our galaxy. 31 00:02:32,920 --> 00:02:36,040 And, in fact, we know surprisingly little about it. 32 00:02:38,200 --> 00:02:42,760 We estimate the Milky Way contains at least 100 billion stars, 33 00:02:42,760 --> 00:02:47,080 but so far we've only been able to catalogue a tiny fraction - 34 00:02:47,080 --> 00:02:49,320 around 200 million. 35 00:02:49,320 --> 00:02:52,760 We believe that it's a spiral galaxy, 36 00:02:52,760 --> 00:02:55,680 but its exact structure remains an open question. 37 00:02:57,000 --> 00:03:00,840 As does what lies in the mysterious region around its centre. 38 00:03:02,480 --> 00:03:06,800 But Gaia is hoping to change all this. 39 00:03:06,800 --> 00:03:10,480 Gaia is a wide-field telescope, designed to scan the entire sky. 40 00:03:10,480 --> 00:03:13,480 Instead of staring intently at individual objects, 41 00:03:13,480 --> 00:03:16,360 it's designed to give us a broad perspective. 42 00:03:16,360 --> 00:03:21,120 It's armed with two telescopes that focus light onto a sensor 43 00:03:21,120 --> 00:03:24,520 bristling with a billion photosensitive pixels. 44 00:03:24,520 --> 00:03:28,280 What we have here is a duplicate of one of the imaging sensors 45 00:03:28,280 --> 00:03:30,360 that makes up the Gaia array. 46 00:03:30,360 --> 00:03:33,480 In the full array, we actually have 106 of these, 47 00:03:33,480 --> 00:03:35,640 and so it's pretty impressive. 48 00:03:35,640 --> 00:03:37,520 Each one of these detectors is quite similar 49 00:03:37,520 --> 00:03:39,120 to the sort of thing you'll find 50 00:03:39,120 --> 00:03:42,080 in a consumer digital camera, but with one difference. 51 00:03:42,080 --> 00:03:44,280 The pixels in this are much, much bigger. 52 00:03:44,280 --> 00:03:45,680 The engineers have worked out 53 00:03:45,680 --> 00:03:48,480 that with bigger pixels you can gather a lot more light. 54 00:03:48,480 --> 00:03:51,680 So much so that this sensor is capable of capturing 55 00:03:51,680 --> 00:03:54,520 more than 90% of the light that lands upon it. 56 00:03:54,520 --> 00:03:57,360 Whereas my camera at home would be lucky to get 20%. 57 00:03:59,000 --> 00:04:01,040 Gaia is expected to detect 58 00:04:01,040 --> 00:04:04,360 and measure hundreds of stars every second. 59 00:04:04,360 --> 00:04:08,960 And it will revisit the same patches of sky 70 times, 60 00:04:08,960 --> 00:04:12,480 which will allow it to do something extraordinary. 61 00:04:12,480 --> 00:04:14,800 Not only will we have the most accurate map 62 00:04:14,800 --> 00:04:16,760 of our little corner of the galaxy, 63 00:04:16,760 --> 00:04:21,360 but also, for the first time, we'll record how the stars are moving. 64 00:04:21,360 --> 00:04:23,880 And that is a REALLY powerful tool. 65 00:04:25,280 --> 00:04:28,840 It will allow us to predict what the galaxy will look like in the future, 66 00:04:28,840 --> 00:04:30,600 and how it evolved. 67 00:04:32,160 --> 00:04:34,160 It should also help solve a puzzle - 68 00:04:34,160 --> 00:04:36,640 what is the exact shape of our galaxy? 69 00:04:37,920 --> 00:04:41,240 The problem is how do you work it out from the inside? 70 00:04:43,040 --> 00:04:46,160 To find out why this is such an important question, 71 00:04:46,160 --> 00:04:49,240 Chris is speaking to Nicholas Walton. 72 00:04:49,240 --> 00:04:54,320 This is a representation of what our galaxy looks like, top-down. 73 00:04:54,320 --> 00:04:56,240 We can see the spiral arm structures. 74 00:04:56,240 --> 00:04:58,080 We can see a bar structure, 75 00:04:58,080 --> 00:05:01,280 and here we see directly that looking at it from this direction, 76 00:05:01,280 --> 00:05:03,000 looking down on it, 77 00:05:03,000 --> 00:05:05,040 we wouldn't know if that was a flat bar, 78 00:05:05,040 --> 00:05:06,880 we wouldn't know if it was a bulge, 79 00:05:06,880 --> 00:05:09,760 because we haven't got the idea of the 3D volume. 80 00:05:09,760 --> 00:05:12,560 And so this is our best guess as to what the galaxy might look like. 81 00:05:12,560 --> 00:05:14,960 The sun would be here, something like that, 82 00:05:14,960 --> 00:05:17,400 but interesting things happen near the centre. 83 00:05:17,400 --> 00:05:19,520 Yes, towards the centre you can see 84 00:05:19,520 --> 00:05:22,200 that the spirals of our galaxy come together. 85 00:05:22,200 --> 00:05:25,680 At the moment we think we've got a four-spiral arm structure, 86 00:05:25,680 --> 00:05:28,160 but a few years back we thought we only had three. 87 00:05:28,160 --> 00:05:31,520 It's changing all the time. It's amazing how the picture changes, 88 00:05:31,520 --> 00:05:34,520 based on new information and new observations and so forth. 89 00:05:34,520 --> 00:05:37,520 If you go in, you'll see that the star densities actually increase 90 00:05:37,520 --> 00:05:40,360 and you'll see what looks like a bulge. 91 00:05:40,360 --> 00:05:44,000 But now we believe this is a bar. A sort of straight structure? 92 00:05:44,000 --> 00:05:45,560 A straight structure across there, 93 00:05:45,560 --> 00:05:47,600 and you'll notice that the spiral arms actually 94 00:05:47,600 --> 00:05:51,800 start at the ends of the bars, the north and the south bar. 95 00:05:51,800 --> 00:05:53,760 In the centre, the idea is that 96 00:05:53,760 --> 00:05:56,240 actually some of the stars are forming 97 00:05:56,240 --> 00:06:00,440 because of material forming and flowing down the bar into there. 98 00:06:00,440 --> 00:06:03,280 So this material has perhaps flowed along the bar. 99 00:06:03,280 --> 00:06:05,120 And actually there were some recent results 100 00:06:05,120 --> 00:06:06,760 that make it more complicated again. 101 00:06:06,760 --> 00:06:10,600 Yes, indeed. We've had some recent, very exciting survey data, 102 00:06:10,600 --> 00:06:14,120 looking for very bright stars towards the centre, 103 00:06:14,120 --> 00:06:16,920 and now we find that there's evidence of cross structure. 104 00:06:16,920 --> 00:06:20,560 So there's two bars. So, all of a sudden, it's even more complicated 105 00:06:20,560 --> 00:06:24,400 and it's a real challenge to understand how we put this together. 106 00:06:24,400 --> 00:06:26,240 But this is an evolving picture. 107 00:06:26,240 --> 00:06:28,880 We thought we knew what our galaxy was like but actually, 108 00:06:28,880 --> 00:06:31,480 because we're inside this very complicated structure, 109 00:06:31,480 --> 00:06:32,600 it's very difficult. 110 00:06:32,600 --> 00:06:35,280 So when we look ahead a few years to the results from Gaia, 111 00:06:35,280 --> 00:06:38,400 it will give us a definitive picture of what's happening. 112 00:06:38,400 --> 00:06:39,680 So what does it mean? 113 00:06:39,680 --> 00:06:42,960 If our galaxy has these complicated structures at the centre, 114 00:06:42,960 --> 00:06:45,000 and Gaia reveals them to us, 115 00:06:45,000 --> 00:06:47,960 what will that tell us about the formation of the galaxy? 116 00:06:47,960 --> 00:06:49,400 Well, did our galaxy form 117 00:06:49,400 --> 00:06:53,080 because it was the merger of two massive galaxies, for instance, 118 00:06:53,080 --> 00:06:55,720 or was it the merger of a massive galaxy 119 00:06:55,720 --> 00:06:59,680 where we actually then went on and accumulated and accreted lots of... 120 00:06:59,680 --> 00:07:03,080 gobbled up, almost, lots of little, smaller galaxies? 121 00:07:03,080 --> 00:07:06,000 These are the answers that Gaia will enable us to get. 122 00:07:06,000 --> 00:07:08,480 And if we can get them for OUR galaxy, 123 00:07:08,480 --> 00:07:10,280 we can understand other galaxies. 124 00:07:10,280 --> 00:07:11,760 We can understand other galaxies, 125 00:07:11,760 --> 00:07:14,400 we can understand what the universe looked like further back 126 00:07:14,400 --> 00:07:19,000 towards the Big Bang, all the way through to our present-day universe 127 00:07:19,000 --> 00:07:22,520 and get a good understanding of how galaxies were built, 128 00:07:22,520 --> 00:07:26,600 how they were formed, and how stars were formed within those galaxies, 129 00:07:26,600 --> 00:07:29,880 and that links all the way through to how planets are formed. 130 00:07:34,840 --> 00:07:39,600 The main focus of Gaia's work will be to catalogue millions of stars, 131 00:07:39,600 --> 00:07:42,440 but scientists are hoping that it will do much more, 132 00:07:42,440 --> 00:07:45,120 that it will shed some light on one of the most mysterious 133 00:07:45,120 --> 00:07:47,520 and hard-to-imagine constituents of our galaxy - 134 00:07:47,520 --> 00:07:49,240 dark matter. 135 00:07:50,400 --> 00:07:53,360 Dark matter is thought to be everywhere - 136 00:07:53,360 --> 00:07:56,400 in the depths of space, all around us. 137 00:07:57,760 --> 00:08:01,880 Billions of particles passing through your body every second. 138 00:08:01,880 --> 00:08:04,320 And yet it is also completely invisible. 139 00:08:06,360 --> 00:08:08,400 Physicist Jim Al-Khalili is investigating 140 00:08:08,400 --> 00:08:10,160 what dark matter really is, 141 00:08:10,160 --> 00:08:13,000 and how Gaia might help us find out more about it. 142 00:08:16,120 --> 00:08:17,720 For almost a century, 143 00:08:17,720 --> 00:08:21,920 scientists have been searching for dark matter without success. 144 00:08:21,920 --> 00:08:23,640 But we know something is there. 145 00:08:26,120 --> 00:08:28,360 The key is gravity. 146 00:08:28,360 --> 00:08:31,800 Look up at the night sky, and you expect gravity to explain 147 00:08:31,800 --> 00:08:35,080 the distribution and movement of all the stars. 148 00:08:35,080 --> 00:08:38,960 But the mass of the visible universe only provides 149 00:08:38,960 --> 00:08:44,480 15% of the gravitational pull needed to explain everything we see. 150 00:08:44,480 --> 00:08:48,680 There's no visible evidence of the missing stuff that provides 151 00:08:48,680 --> 00:08:52,000 the other 85% of the required gravity. 152 00:08:53,200 --> 00:08:56,520 Dark matter is simply the name that scientists have given 153 00:08:56,520 --> 00:09:01,840 to whatever it is that's creating the rest of that gravitational pull. 154 00:09:01,840 --> 00:09:06,800 Without dark matter, the universe as we see it simply wouldn't exist. 155 00:09:06,800 --> 00:09:10,800 But if dark matter makes up such a large part of our galaxy, 156 00:09:10,800 --> 00:09:14,280 why haven't we been able to find it yet? 157 00:09:14,280 --> 00:09:17,920 To begin to get your head round just why searching for dark matter 158 00:09:17,920 --> 00:09:22,360 is such a challenge, you first have to understand what it ISN'T. 159 00:09:22,360 --> 00:09:25,680 You might think it would be easy for me to show you dark matter, 160 00:09:25,680 --> 00:09:27,880 but it's much more difficult than that. 161 00:09:29,800 --> 00:09:36,760 Nothing in this wood, no matter how dark, is as dark as dark matter. 162 00:09:36,760 --> 00:09:40,520 That's because all normal matter reacts to light. 163 00:09:40,520 --> 00:09:43,800 Shine a torch, and some of that light will be reflected back. 164 00:09:45,480 --> 00:09:47,560 But dark matter is different. 165 00:09:47,560 --> 00:09:51,240 It's a substance that doesn't respond to light in any way. 166 00:09:51,240 --> 00:09:54,280 It neither emits light nor reflects it. 167 00:09:54,280 --> 00:09:58,480 But even that is only the beginning of just how profoundly different 168 00:09:58,480 --> 00:10:00,600 dark matter is. 169 00:10:00,600 --> 00:10:05,080 You see, dark matter isn't "matter" in the conventional sense at all, 170 00:10:05,080 --> 00:10:09,520 in that it's not made up of the same stuff as normal matter. 171 00:10:09,520 --> 00:10:12,200 Most of the evidence for the existence of dark matter 172 00:10:12,200 --> 00:10:14,160 is therefore indirect. 173 00:10:14,160 --> 00:10:17,840 And this is where the difficulty lies. 174 00:10:29,080 --> 00:10:33,000 The problem is, we just don't know what dark matter is. 175 00:10:34,360 --> 00:10:36,880 And that's why it's so hard to study. 176 00:10:39,040 --> 00:10:41,600 Scientists are trying lots of different ways 177 00:10:41,600 --> 00:10:44,440 to detect and understand dark matter. 178 00:10:44,440 --> 00:10:48,400 For example, there are experiments buried deep underground 179 00:10:48,400 --> 00:10:50,920 trying to capture the particles of dark matter 180 00:10:50,920 --> 00:10:53,040 using specially designed detectors. 181 00:10:53,040 --> 00:10:56,640 There are particle accelerators, like the Large Hadron Collider, 182 00:10:56,640 --> 00:11:00,320 which are trying to create the particles of dark matter. 183 00:11:00,320 --> 00:11:03,160 But Gaia will be trying a different method. 184 00:11:03,160 --> 00:11:05,720 It will allow us to look up at our galaxy 185 00:11:05,720 --> 00:11:08,560 and deduce the properties of dark matter 186 00:11:08,560 --> 00:11:10,320 from what we can actually see. 187 00:11:12,080 --> 00:11:16,680 It will measure, in extreme detail, the effect dark matter has 188 00:11:16,680 --> 00:11:19,760 on the visible matter in our own galaxy. 189 00:11:19,760 --> 00:11:22,240 'Cosmologist Dr Andrew Pontzen 190 00:11:22,240 --> 00:11:25,080 'simulates what this might look like.' 191 00:11:25,080 --> 00:11:28,800 Of course, we think about dark matter being out there today, 192 00:11:28,800 --> 00:11:31,160 but you're interested in the role it played 193 00:11:31,160 --> 00:11:33,760 very early on in the formation of the universe. 194 00:11:33,760 --> 00:11:36,840 That's right, and we think dark matter had a crucial role in making 195 00:11:36,840 --> 00:11:39,680 the universe like it is today, so a lot of what 196 00:11:39,680 --> 00:11:42,320 I do uses computers to try and work out or model 197 00:11:42,320 --> 00:11:44,360 what dark matter would have done 198 00:11:44,360 --> 00:11:46,600 throughout the history of the universe. 199 00:11:46,600 --> 00:11:50,040 So I can actually show you here simulation of the way that 200 00:11:50,040 --> 00:11:51,520 dark matter behaves. 201 00:11:51,520 --> 00:11:54,360 Because it's in a computer, we can paint the dark matter 202 00:11:54,360 --> 00:11:56,560 any colour we like, we can make it visible. 203 00:11:56,560 --> 00:11:59,600 So I'm going to paint it green and show you what happens over 204 00:11:59,600 --> 00:12:03,480 the first billion-and-a-half years or so. So I can hit go... 205 00:12:05,040 --> 00:12:06,480 And we've just seen the Big Bang. 206 00:12:06,480 --> 00:12:07,720 The whole universe, 207 00:12:07,720 --> 00:12:10,080 or at least this chunk of the universe we're looking at, 208 00:12:10,080 --> 00:12:14,720 is expanding, and as it does so, you've seen what happens 209 00:12:14,720 --> 00:12:18,480 to the dark matter over the first billion-and-a-half years or so. 210 00:12:18,480 --> 00:12:23,080 It started out quite evenly spread out, coming towards us, 211 00:12:23,080 --> 00:12:25,840 but over time it forms into clumps. 212 00:12:25,840 --> 00:12:27,840 So, of course, this is the dark matter 213 00:12:27,840 --> 00:12:29,600 which we wouldn't be able to see. 214 00:12:29,600 --> 00:12:32,240 How does that relate to the visible universe? 215 00:12:32,240 --> 00:12:36,040 We can switch views and show what the computer thinks 216 00:12:36,040 --> 00:12:38,480 the visible universe would look like at this time. 217 00:12:38,480 --> 00:12:42,680 Every dot of light you see here is a forming mini galaxy. 218 00:12:42,680 --> 00:12:45,920 It's got maybe a few million stars in it. 219 00:12:45,920 --> 00:12:49,360 But the key thing is that they wouldn't be there 220 00:12:49,360 --> 00:12:52,640 unless the dark matter that we were seeing just before 221 00:12:52,640 --> 00:12:54,480 is there in the first place. 222 00:12:54,480 --> 00:12:57,600 It's that extra gravitational pull that all that dark matter is 223 00:12:57,600 --> 00:13:01,800 providing that pulls the gas in, and allows it to sit there 224 00:13:01,800 --> 00:13:05,080 and start forming stars and start forming the universe we know today. 225 00:13:05,080 --> 00:13:07,320 This is the important point. They're clumping together 226 00:13:07,320 --> 00:13:10,160 not because of their own gravity, although that must be important, 227 00:13:10,160 --> 00:13:13,360 but because of the gravity of the dark matter, which is much more dominant. 228 00:13:13,360 --> 00:13:16,400 Yeah, there's so much more dark matter than normal matter 229 00:13:16,400 --> 00:13:19,520 that that's what we think the key role of dark matter is, 230 00:13:19,520 --> 00:13:22,520 to pull all this stuff together and actually clump it 231 00:13:22,520 --> 00:13:24,920 into something that can form these stars. 232 00:13:24,920 --> 00:13:27,480 How does that evolve? What does it look like today? 233 00:13:27,480 --> 00:13:30,560 Well, we can use these computer models to work out precisely that. 234 00:13:30,560 --> 00:13:32,960 If I switch to another view, 235 00:13:32,960 --> 00:13:37,360 here we've zoomed in on one of those single points of light that we saw. 236 00:13:37,360 --> 00:13:41,600 This is going to turn into something like our Milky Way galaxy today. 237 00:13:41,600 --> 00:13:46,040 So if I restart time, then what you see happening is 238 00:13:46,040 --> 00:13:50,720 all these different mini galaxies start merging together. 239 00:13:50,720 --> 00:13:54,920 And once again we think dark matter is playing a key role in this. 240 00:13:54,920 --> 00:13:58,320 It's the gravitational pull associated with the dark matter that 241 00:13:58,320 --> 00:14:02,160 actually pulls all the different bits and pieces together and 242 00:14:02,160 --> 00:14:06,000 starts assembling the present-day Milky Way, which grows and grows. 243 00:14:06,000 --> 00:14:09,200 'Andrew's simulation can also predict 244 00:14:09,200 --> 00:14:13,320 'where the dark matter that surrounds our galaxy is today.' 245 00:14:13,320 --> 00:14:17,080 This is the final picture that comes out of the computer model 246 00:14:17,080 --> 00:14:18,640 we've been watching. 247 00:14:18,640 --> 00:14:21,320 What you've got here is that the galaxy as we know it 248 00:14:21,320 --> 00:14:24,600 is some tiny little bit in the centre, which we can't see 249 00:14:24,600 --> 00:14:27,520 cos we're only looking at the dark matter here. 250 00:14:27,520 --> 00:14:32,480 And then you see there are all these other blobs of dark matter around. 251 00:14:32,480 --> 00:14:35,520 Now, a few of these will have some stars in as well, 252 00:14:35,520 --> 00:14:37,840 but most of them are just too small. 253 00:14:37,840 --> 00:14:42,080 So we think that our Milky Way galaxy first of all is 254 00:14:42,080 --> 00:14:43,760 shrouded in dark matter, but also has 255 00:14:43,760 --> 00:14:46,360 all these extra blobs of dark matter around it, 256 00:14:46,360 --> 00:14:49,400 which are essentially failed galaxies in their own right. 257 00:14:49,400 --> 00:14:51,200 Of course, this is a simulation. 258 00:14:51,200 --> 00:14:53,640 How do you test that this idea is correct? 259 00:14:53,640 --> 00:14:57,000 That's where something like Gaia comes into the picture 260 00:14:57,000 --> 00:15:00,040 because if we switch to where we think 261 00:15:00,040 --> 00:15:04,640 the stars are in our computer model, it looks something like this. 262 00:15:04,640 --> 00:15:07,600 The main galaxy is still in the centre there, 263 00:15:07,600 --> 00:15:11,680 and as those small bits of dark matter fall into our own Milky Way, 264 00:15:11,680 --> 00:15:13,480 the stars are stripped out 265 00:15:13,480 --> 00:15:17,080 and they are left in these big streams that you can see here. 266 00:15:17,080 --> 00:15:21,160 So, if we could see the streams of stars falling into the centre, 267 00:15:21,160 --> 00:15:24,320 that is evidence that there is dark matter 268 00:15:24,320 --> 00:15:26,480 spread out to large distances. 269 00:15:26,480 --> 00:15:30,400 Exactly, and that's just what we're hoping Gaia will let us confirm. 270 00:15:30,400 --> 00:15:34,680 So Gaia may well give us our best evidence for dark matter 271 00:15:34,680 --> 00:15:36,720 and how it behaves. 272 00:15:38,240 --> 00:15:40,640 But I give you one last thought. 273 00:15:40,640 --> 00:15:43,600 It might be that after we've had time to analyse 274 00:15:43,600 --> 00:15:46,800 the massive amount of information provided by Gaia 275 00:15:46,800 --> 00:15:48,360 over the next five years, 276 00:15:48,360 --> 00:15:51,360 we discover something completely different. 277 00:15:51,360 --> 00:15:54,440 It might be that all our calculations were in fact wrong, 278 00:15:54,440 --> 00:15:57,240 and that dark matter doesn't exist at all. 279 00:15:57,240 --> 00:15:59,680 Now, in my view, this isn't very likely, 280 00:15:59,680 --> 00:16:02,200 but it's what makes astronomy so exciting. 281 00:16:18,160 --> 00:16:20,160 The key to the success of Gaia 282 00:16:20,160 --> 00:16:22,040 are the imaging sensors that make up 283 00:16:22,040 --> 00:16:24,400 the spacecraft's billion-pixel camera. 284 00:16:26,040 --> 00:16:30,200 Producing these sensors requires some very delicate engineering. 285 00:16:30,200 --> 00:16:32,200 We're back here at the e2v laboratories, 286 00:16:32,200 --> 00:16:36,200 and what you can see in front of us is where they assemble the detectors. 287 00:16:36,200 --> 00:16:39,960 It's in a clean room because of contamination, such as dust. 288 00:16:39,960 --> 00:16:42,960 This can play two roles. If it sits on top of the detector, 289 00:16:42,960 --> 00:16:45,200 it stops the light or the photons getting in. 290 00:16:45,200 --> 00:16:47,560 But if it's embedded in the electronics, 291 00:16:47,560 --> 00:16:49,240 it kills the detector dead. 292 00:16:49,240 --> 00:16:52,080 Now, contamination is such an issue that I'm not allowed in. 293 00:16:52,080 --> 00:16:54,280 But we've cleaned up our cameraman, Nick, 294 00:16:54,280 --> 00:16:56,120 and he's showing us the close-up detail. 295 00:16:58,920 --> 00:17:01,560 The battle with dust is extreme. 296 00:17:01,560 --> 00:17:05,240 In this clean room there is 35,000 times fewer dust particles 297 00:17:05,240 --> 00:17:06,680 than in normal air. 298 00:17:07,960 --> 00:17:11,280 They've got over 300 people making sensors 299 00:17:11,280 --> 00:17:13,120 for all sorts of space missions. 300 00:17:15,360 --> 00:17:18,080 As well as Gaia, this company has built sensors 301 00:17:18,080 --> 00:17:20,040 for the Rosetta comet landing mission, 302 00:17:20,040 --> 00:17:24,000 planet-hunter Kepler and the Mars Curiosity rover. 303 00:17:24,000 --> 00:17:26,760 What they're actually doing in there is cleaning 304 00:17:26,760 --> 00:17:29,600 the detectors before they are sent off for testing. 305 00:17:31,240 --> 00:17:34,120 Dust isn't the only challenge in making the sensors. 306 00:17:37,320 --> 00:17:39,600 They also have to be designed to cope 307 00:17:39,600 --> 00:17:42,880 with the violent experience of being launched into space. 308 00:17:44,240 --> 00:17:46,680 'Chief engineer David Morris simulates 309 00:17:46,680 --> 00:17:50,160 'the conditions of a launch with a vibration test.' 310 00:17:50,160 --> 00:17:52,720 So, what sort of force are you putting it under? 311 00:17:52,720 --> 00:17:56,040 It goes up to 50 times the force due to gravity on Earth 312 00:17:56,040 --> 00:17:58,280 simply by this vibration in three axes. 313 00:18:01,800 --> 00:18:05,040 So it seems really odd because you spend so much time making 314 00:18:05,040 --> 00:18:07,840 these wonderful detectors, and then you put them through hell! 315 00:18:07,840 --> 00:18:10,960 They have to go through hell because otherwise we won't be confident 316 00:18:10,960 --> 00:18:13,600 they'll survive when they go through the hell of launch. 317 00:18:13,600 --> 00:18:16,400 But it is always traumatic, worrying about whether or not 318 00:18:16,400 --> 00:18:18,320 what we've designed and built will survive 319 00:18:18,320 --> 00:18:19,840 this sort of extreme violence. 320 00:18:19,840 --> 00:18:23,800 This process reveals the key to successful space engineering - 321 00:18:23,800 --> 00:18:27,720 extreme precision married with extreme robustness. 322 00:18:32,960 --> 00:18:36,440 Next, how can you view the Milky Way for yourself? 323 00:18:37,920 --> 00:18:41,440 Pete's here with a few tips and a guide to touring the galaxy. 324 00:18:44,840 --> 00:18:47,280 It's a great time of year to spot the Milky Way. 325 00:18:48,720 --> 00:18:52,200 Unlike in the spring and the autumn, the plane of the Milky Way rides 326 00:18:52,200 --> 00:18:55,920 very high in the sky during the winter and the summer. 327 00:18:55,920 --> 00:18:58,800 And each view gives us a completely different perspective 328 00:18:58,800 --> 00:19:00,160 of our own galaxy. 329 00:19:02,280 --> 00:19:04,720 In the summer we look through the Milky Way 330 00:19:04,720 --> 00:19:06,800 towards the galactic centre. 331 00:19:09,240 --> 00:19:12,840 This bright, summer Milky Way snakes across the sky, 332 00:19:12,840 --> 00:19:18,400 revealing a dense path of stars, punctuated by dark dust clouds. 333 00:19:18,400 --> 00:19:21,720 In the winter we get a much more subtle view of the Milky Way, 334 00:19:21,720 --> 00:19:25,400 rather than the bright view we get during the summer months. 335 00:19:25,400 --> 00:19:28,360 And this is because during the winter we are looking out through 336 00:19:28,360 --> 00:19:31,480 a thinner portion of the Milky Way to the universe beyond. 337 00:19:33,720 --> 00:19:36,080 The result is remarkably different, 338 00:19:36,080 --> 00:19:38,760 with a faint band stretching up from the horizon. 339 00:19:40,680 --> 00:19:42,600 However, it is no less beautiful 340 00:19:42,600 --> 00:19:45,440 and is packed with some stunning deep-sky objects. 341 00:19:47,480 --> 00:19:51,080 At this time of year the best place to see the Milky Way 342 00:19:51,080 --> 00:19:53,920 is within what's known as the Winter Triangle, 343 00:19:53,920 --> 00:19:56,280 and this is made up from three bright stars. 344 00:19:58,120 --> 00:20:01,840 The Winter Triangle is formed by drawing an imaginary line 345 00:20:01,840 --> 00:20:05,120 between the stars Betelgeuse in Orion, Sirius in Canis Major, 346 00:20:05,120 --> 00:20:08,120 and Procyon in Canis Minor. 347 00:20:09,960 --> 00:20:12,480 The Winter Triangle is a very clear pattern, 348 00:20:12,480 --> 00:20:15,840 and if you have a dark sky, you can often see the Milky Way 349 00:20:15,840 --> 00:20:17,960 running right the way through the centre of it. 350 00:20:17,960 --> 00:20:20,120 But if you do have trouble making out 351 00:20:20,120 --> 00:20:23,800 that subtle, faint light from our own galaxy, then it's worth 352 00:20:23,800 --> 00:20:26,120 scanning the region with a pair of binoculars 353 00:20:26,120 --> 00:20:29,280 because there are some fantastic clusters to be seen there. 354 00:20:29,280 --> 00:20:35,640 A few favourites are NGC 2244, in the centre of the Rosette Nebula... 355 00:20:37,320 --> 00:20:41,160 ..and M41, located just below the bright star, Sirius. 356 00:20:44,720 --> 00:20:48,520 Of course the Milky Way isn't the only thing on offer this month. 357 00:20:48,520 --> 00:20:50,520 Jupiter is really well positioned 358 00:20:50,520 --> 00:20:53,120 and it's about to offer up a really rare event. 359 00:20:53,120 --> 00:20:54,440 So, with details of this 360 00:20:54,440 --> 00:20:57,720 and other highlights visible in this month's night sky, 361 00:20:57,720 --> 00:20:58,880 here's my star guide. 362 00:21:00,960 --> 00:21:05,960 The open cluster M41 is located just to the south of the Winter Triangle, 363 00:21:05,960 --> 00:21:09,040 below Sirius, and should be easy to find with binoculars. 364 00:21:10,480 --> 00:21:13,800 Southeast of the triangle, two further clusters - 365 00:21:13,800 --> 00:21:16,160 M46 and M47 - can be seen. 366 00:21:19,000 --> 00:21:22,520 The main constellation in this direction is faint Monoceros, 367 00:21:22,520 --> 00:21:23,480 the unicorn. 368 00:21:25,480 --> 00:21:30,440 Within the mythical beast's triangular head sits NGC 2244, 369 00:21:30,440 --> 00:21:33,960 the open cluster at the heart of the wonderful Rosette Nebula. 370 00:21:36,200 --> 00:21:38,120 As the Milky Way heads north, 371 00:21:38,120 --> 00:21:41,720 it passes a distinctive curve of faint stars in Gemini. 372 00:21:41,720 --> 00:21:45,360 And it's here you'll find M35 373 00:21:45,360 --> 00:21:48,320 and the dimmer NGC 2158. 374 00:21:50,280 --> 00:21:52,680 Finally, look out for brilliant Jupiter, 375 00:21:52,680 --> 00:21:55,520 which dominates the view high to the south around midnight. 376 00:21:58,000 --> 00:22:03,200 On the 24th of January, between 0628 and 0653, 377 00:22:03,200 --> 00:22:06,920 three dark moon shadows of Io, Europa and Callisto 378 00:22:06,920 --> 00:22:10,600 will be visible on the planet's disc at the same time. 379 00:22:10,600 --> 00:22:12,320 This doesn't occur very often 380 00:22:12,320 --> 00:22:14,400 so make sure you get outside and have a look. 381 00:22:19,600 --> 00:22:21,360 Now back to Gaia, 382 00:22:21,360 --> 00:22:23,800 and it will be a few years before we get the first 383 00:22:23,800 --> 00:22:28,080 new star catalogues and maps from the data it's sent back to Earth. 384 00:22:28,080 --> 00:22:30,520 But it's already giving us exciting science 385 00:22:30,520 --> 00:22:34,560 about unusual phenomena that we might call transient signals. 386 00:22:36,920 --> 00:22:39,160 Every now and then our telescopes record 387 00:22:39,160 --> 00:22:40,800 flashes in the night sky. 388 00:22:42,520 --> 00:22:45,720 Many of which are the results of stars exploding 389 00:22:45,720 --> 00:22:48,480 in what we call supernovae. 390 00:22:55,560 --> 00:22:59,320 These are rare events, but they have enormous scientific value. 391 00:22:59,320 --> 00:23:02,760 In these stellar deaths we can reveal how many of our elements 392 00:23:02,760 --> 00:23:06,600 were created, and also how the universe is expanding. 393 00:23:06,600 --> 00:23:11,040 What Gaia will do is help us find more of these things as they happen, 394 00:23:11,040 --> 00:23:15,560 and it might even help us discover a new type of exploding star. 395 00:23:15,560 --> 00:23:18,880 Maggie is talking to Dr Simon Hodgkin, 396 00:23:18,880 --> 00:23:21,880 who'll be releasing information about these signals 397 00:23:21,880 --> 00:23:22,960 as they are found. 398 00:23:22,960 --> 00:23:26,520 Simon, can you describe to me, what is a transient signal? 399 00:23:26,520 --> 00:23:30,880 A transient signal is one that does not last for a terribly long time. 400 00:23:30,880 --> 00:23:34,320 So it's only there for a small amount of time and it's important 401 00:23:34,320 --> 00:23:37,840 to react quickly to learn what it is before the light disappears. 402 00:23:37,840 --> 00:23:40,560 So there is a whole range of transient events. 403 00:23:40,560 --> 00:23:43,240 They can be superflares on stars like our sun, 404 00:23:43,240 --> 00:23:45,960 which can last for hours to days. 405 00:23:45,960 --> 00:23:49,320 But there can be supernova events which last for weeks to months. 406 00:23:49,320 --> 00:23:54,000 So, if you find a supernova, what can it tell us? Supernovae are mystery. 407 00:23:54,000 --> 00:23:55,880 We've known about them for a long time. 408 00:23:55,880 --> 00:23:58,440 We know there are lots of different kinds of supernovae, 409 00:23:58,440 --> 00:24:01,600 but the physics of what kind of star gives rise to which 410 00:24:01,600 --> 00:24:04,960 kind of supernovae is a little bit, I would have to say, flaky. 411 00:24:04,960 --> 00:24:07,920 So we're trying to find as many nearby supernovae. 412 00:24:07,920 --> 00:24:11,240 It's possible to try and understand where they came from 413 00:24:11,240 --> 00:24:13,240 and what kind of supernovae there are. 414 00:24:14,520 --> 00:24:18,280 'And they've already made a number of discoveries. 415 00:24:18,280 --> 00:24:22,680 'One of which caught the scientists totally by surprise.' 416 00:24:22,680 --> 00:24:26,320 We found something really rather rare and really rather exciting. 417 00:24:27,560 --> 00:24:29,800 'They were alerted after Gaia recorded 418 00:24:29,800 --> 00:24:33,200 'an explosion on a star in our galaxy.' 419 00:24:33,200 --> 00:24:36,480 We took an image, and this was taken on a telescope in Italy. 420 00:24:36,480 --> 00:24:39,400 So we decided to take a sequence, a very short-duration image, 421 00:24:39,400 --> 00:24:43,080 so this is... The first 20 or so images look like this, 422 00:24:43,080 --> 00:24:46,040 and this is another one, which looks pretty much the same. 423 00:24:46,040 --> 00:24:48,200 And it's this star here. 424 00:24:48,200 --> 00:24:51,960 That star blew up. And essentially it's still there, it looks good. 425 00:24:51,960 --> 00:24:54,960 The very next image, 30 seconds later, the star vanished. 426 00:24:54,960 --> 00:24:57,560 It's got 1,000 times fainter, essentially. 427 00:24:57,560 --> 00:25:00,160 So suddenly we've gone from a star that's pretty bright to 428 00:25:00,160 --> 00:25:02,640 a star that is not there. We carried on looking at it, 429 00:25:02,640 --> 00:25:06,720 every 30 seconds we took a new exposure, and over the next minute, 430 00:25:06,720 --> 00:25:09,440 it started to reappear and got back to its previous level. 431 00:25:09,440 --> 00:25:11,160 So what's going on there? 432 00:25:11,160 --> 00:25:13,000 So our interpretation is that we have 433 00:25:13,000 --> 00:25:16,120 two degenerate white dwarf stars orbiting each other, 434 00:25:16,120 --> 00:25:19,920 and in fact these disappearances happen periodically. 435 00:25:19,920 --> 00:25:22,680 As one goes in front of the other? As one goes in front of the other. 436 00:25:22,680 --> 00:25:25,880 Every 15 minutes one star is occulted by the other, 437 00:25:25,880 --> 00:25:28,720 it's called an eclipse. But that doesn't make sense to me 438 00:25:28,720 --> 00:25:31,000 because if you've got two bright stars, you have lots 439 00:25:31,000 --> 00:25:33,880 of brightness, half the brightness then lots of brightness again? 440 00:25:33,880 --> 00:25:35,760 So why does it disappear? That's right. 441 00:25:35,760 --> 00:25:37,760 So both objects need to be the same size, 442 00:25:37,760 --> 00:25:40,720 but one is essentially bright and the other is very dark. 443 00:25:40,720 --> 00:25:43,680 So when the dark star passes in front of the bright star, 444 00:25:43,680 --> 00:25:44,960 it blocks it out. 445 00:25:44,960 --> 00:25:47,080 When the dark star goes behind the bright star, 446 00:25:47,080 --> 00:25:49,280 you don't really see any change in the brightness. 447 00:25:49,280 --> 00:25:52,000 And because they are so perfectly aligned, we can 448 00:25:52,000 --> 00:25:54,640 do something you can't do with these kind of stars normally. 449 00:25:54,640 --> 00:25:57,760 It is very rare, we're very lucky to have found the explosion 450 00:25:57,760 --> 00:26:01,160 with Gaia and done this follow-up. We can measure their masses, 451 00:26:01,160 --> 00:26:04,120 we can measure how heavy they are and we can measure their radii. 452 00:26:04,120 --> 00:26:06,440 And if they are what we think they are, 453 00:26:06,440 --> 00:26:10,040 and these two stars will merge together as they lose energy through 454 00:26:10,040 --> 00:26:13,760 gravitational radiation, they could form a type 1a supernova. 455 00:26:16,120 --> 00:26:20,520 Type 1a supernovae are a unique form of exploding star 456 00:26:20,520 --> 00:26:22,520 that are important for astronomy. 457 00:26:24,440 --> 00:26:28,560 Cosmologists can use them to measure the size of the universe. 458 00:26:28,560 --> 00:26:31,280 But exactly how they form is a mystery. 459 00:26:32,840 --> 00:26:36,520 So the question is, is this a progenitor of a 1a, 460 00:26:36,520 --> 00:26:39,360 and I hope in my lifetime we'll be able to make the measurement 461 00:26:39,360 --> 00:26:42,000 that will tell us whether or not these are growing together 462 00:26:42,000 --> 00:26:44,640 and could explode, or they're growing apart 463 00:26:44,640 --> 00:26:47,680 and this is not the 1a progenitor we're looking for. 464 00:26:47,680 --> 00:26:49,600 I suppose, if you see them coming together, 465 00:26:49,600 --> 00:26:51,680 then the hope is that they will go supernova. 466 00:26:51,680 --> 00:26:55,000 That is exactly right, but it may take millions of years. 467 00:26:55,000 --> 00:26:56,680 It's a fantastic discovery, 468 00:26:56,680 --> 00:26:59,240 and one of the great things about this mission 469 00:26:59,240 --> 00:27:01,840 is that you could help find others like it. 470 00:27:01,840 --> 00:27:04,680 Because Gaia data isn't just for scientists. 471 00:27:05,960 --> 00:27:08,800 So the idea is, every time we find something dramatic, 472 00:27:08,800 --> 00:27:11,000 a transient event happening in the sky, 473 00:27:11,000 --> 00:27:14,440 we publish it to our website, so we write down the coordinates 474 00:27:14,440 --> 00:27:17,480 and the brightness of the object, and all those data are public, 475 00:27:17,480 --> 00:27:19,280 and anyone can go and look at them. 476 00:27:19,280 --> 00:27:21,600 And if they have access to a telescope they can go and 477 00:27:21,600 --> 00:27:24,960 follow up our objects, and I would love it if they could tell us what 478 00:27:24,960 --> 00:27:27,480 they are, share their data, and help us classify 479 00:27:27,480 --> 00:27:29,320 the kind of things we're finding, 480 00:27:29,320 --> 00:27:31,680 which are potentially very rare, very exciting. 481 00:27:31,680 --> 00:27:34,920 Cos I guess Gaia's going to be inundating you with lots of these, 482 00:27:34,920 --> 00:27:38,160 and you want the amateur community to join in and help solve the problems. 483 00:27:38,160 --> 00:27:40,840 That is absolutely right. I can't keep up. 484 00:27:40,840 --> 00:27:43,000 We are finding three or four or five a day, 485 00:27:43,000 --> 00:27:46,000 we do not have the resources to follow them up, whereas the 486 00:27:46,000 --> 00:27:47,480 amateur community can help 487 00:27:47,480 --> 00:27:49,760 for the ones that are bright enough for them. 488 00:27:49,760 --> 00:27:53,120 And even in schools, there is access to a network of telescopes 489 00:27:53,120 --> 00:27:55,080 that schoolchildren can have access to, 490 00:27:55,080 --> 00:27:57,840 and trigger follow-up observations of objects we have found. 491 00:27:57,840 --> 00:28:00,920 So school kids can get involved and do real science. 492 00:28:00,920 --> 00:28:03,680 That would be fantastic, I would love that to start happening. 493 00:28:03,680 --> 00:28:07,480 Exciting times ahead. It's a busy, exciting time. Yes! 494 00:28:07,480 --> 00:28:09,320 Thank you so much. Thank you. 495 00:28:16,040 --> 00:28:19,240 To find out more about Simon's project, you can visit our website - 496 00:28:23,160 --> 00:28:25,600 It really is a fabulous chance for schools to get involved 497 00:28:25,600 --> 00:28:27,960 in pushing back the frontiers of knowledge. 498 00:28:27,960 --> 00:28:29,720 That's it for this programme. 499 00:28:29,720 --> 00:28:32,480 Next month we'll be scanning the skies for UFOs, 500 00:28:32,480 --> 00:28:34,760 to reveal how the search for little green men 501 00:28:34,760 --> 00:28:37,560 has transformed our knowledge of the universe. 502 00:28:37,560 --> 00:28:40,200 In the meantime, get outside and get looking up. 503 00:28:40,200 --> 00:28:41,520 Good night.