1 00:02:53,887 --> 00:02:57,721 If you wish to make an apple pie from scratch... 2 00:02:58,291 --> 00:03:02,387 ...you must first invent the universe. 3 00:03:02,596 --> 00:03:04,188 Thank you very much. 4 00:03:05,365 --> 00:03:07,799 Suppose I cut a piece... 5 00:03:08,668 --> 00:03:10,602 ...out of this apple pie. 6 00:03:23,283 --> 00:03:25,751 Crumbly, but good. 7 00:03:29,089 --> 00:03:34,026 And now suppose we cut this piece in half, or more or less. 8 00:03:35,629 --> 00:03:38,496 And then cut this piece in half... 9 00:03:39,099 --> 00:03:41,260 ...and keep going. 10 00:03:41,468 --> 00:03:46,132 How many cuts before we get down to an individual atom? 11 00:03:46,339 --> 00:03:51,038 The answer is about 90 successive cuts. 12 00:03:52,145 --> 00:03:55,478 Of course, this knife isn't sharp enough... 13 00:03:55,682 --> 00:03:58,150 ...the pie is too crumbly... 14 00:03:58,351 --> 00:04:01,752 ...and an atom is too small to see in any case. 15 00:04:01,955 --> 00:04:05,391 But there is a way to do it. 16 00:04:08,028 --> 00:04:12,260 It was here at Cambridge University in England... 17 00:04:12,465 --> 00:04:15,764 ...that the nature of the atom was first understood... 18 00:04:15,969 --> 00:04:20,338 ...in part by shooting pieces of atoms at atoms... 19 00:04:20,540 --> 00:04:23,703 ...and seeing how they bounce off. 20 00:04:23,910 --> 00:04:27,437 A typical atom is surrounded... 21 00:04:27,647 --> 00:04:31,413 ...by a kind of cloud of electrons. 22 00:04:31,618 --> 00:04:35,554 The electrons are electrically charged, as the name suggests... 23 00:04:35,755 --> 00:04:39,122 ...and they determine the chemical properties of the atom. 24 00:04:39,326 --> 00:04:43,387 For example, the glitter of gold... 25 00:04:46,366 --> 00:04:48,300 ...or the transparency of the solid... 26 00:04:48,501 --> 00:04:51,527 ...that's made from the atoms silicon and oxygen. 27 00:04:52,806 --> 00:04:55,036 But deep inside the atom... 28 00:04:55,241 --> 00:04:59,302 ...hidden far beneath the outer electron cloud... 29 00:04:59,512 --> 00:05:03,539 ...is the nucleus, composed chiefly of protons and neutrons. 30 00:05:03,750 --> 00:05:06,116 Atoms are very small. 31 00:05:06,319 --> 00:05:10,881 100 million of them, end to end, would be about so big. 32 00:05:11,091 --> 00:05:15,994 And the nucleus is 100,000 times smaller still. 33 00:05:16,496 --> 00:05:20,227 Nevertheless, most of the mass in an atom is in the nucleus. 34 00:05:20,433 --> 00:05:23,493 The electrons are by comparison... 35 00:05:23,703 --> 00:05:26,672 ...just bits of moving fluff. 36 00:05:26,873 --> 00:05:30,036 Atoms are mainly empty space. 37 00:05:30,243 --> 00:05:34,304 Matter is composed chiefly of nothing. 38 00:05:37,217 --> 00:05:40,345 When we consider cutting this apple pie, but... 39 00:05:40,553 --> 00:05:43,989 ...down beyond a single atom... 40 00:05:44,257 --> 00:05:49,092 ...we confront an infinity of the very small. 41 00:05:49,295 --> 00:05:52,264 And when we look up at the night sky... 42 00:05:52,465 --> 00:05:56,458 ...we confront an infinity of the very large. 43 00:05:56,670 --> 00:06:01,369 These infinities are among the most awesome of human ideas. 44 00:06:01,574 --> 00:06:06,011 They represent an unending regress which goes on... 45 00:06:06,413 --> 00:06:10,543 ...not just very far, but forever. 46 00:06:10,750 --> 00:06:14,618 Have you ever stood between two parallel mirrors... 47 00:06:14,821 --> 00:06:16,220 ...in a barbershop, say... 48 00:06:16,423 --> 00:06:19,915 ...and seen a very large number of you? 49 00:06:20,126 --> 00:06:23,061 Or you could use... 50 00:06:24,297 --> 00:06:26,765 ...two flat mirrors... 51 00:06:27,200 --> 00:06:29,600 ...and a candle flame... 52 00:06:29,803 --> 00:06:33,102 ...you would see a large number of images... 53 00:06:33,306 --> 00:06:37,106 ...each the reflection of another image. 54 00:06:39,012 --> 00:06:42,243 You can't really see an infinity of images... 55 00:06:42,449 --> 00:06:45,782 ...because the mirrors aren't perfectly flat and aligned. 56 00:06:45,985 --> 00:06:49,386 And there's a candle or a candle flame in the way... 57 00:06:49,589 --> 00:06:52,319 ...and light doesn't travel infinitely fast. 58 00:06:52,525 --> 00:06:54,959 When we talk of real infinities... 59 00:06:55,161 --> 00:06:59,029 ...we're talking about a quantity larger than any number. 60 00:06:59,232 --> 00:07:03,692 No matter what number you have in mind, infinity is larger. 61 00:07:07,407 --> 00:07:11,707 There's a nice way to write large numbers. 62 00:07:11,911 --> 00:07:13,310 You can... 63 00:07:14,380 --> 00:07:16,814 ...write the number 1000... 64 00:07:17,016 --> 00:07:19,610 ...as 10 to the power three... 65 00:07:19,819 --> 00:07:23,755 ...meaning, a one followed by three zeros. 66 00:07:23,957 --> 00:07:28,894 Or a million is written as 10 to the power six... 67 00:07:29,229 --> 00:07:33,996 ...meaning, a one followed by six zeros. 68 00:07:34,734 --> 00:07:38,864 There's no largest number. If anybody gives you a candidate... 69 00:07:39,072 --> 00:07:41,063 ...you can always add the number one to it. 70 00:07:41,274 --> 00:07:44,209 But there certainly are very big numbers. 71 00:07:44,410 --> 00:07:49,109 The American mathematician Edward Kasner once asked his nephew... 72 00:07:49,315 --> 00:07:52,716 ...to invent a name for an extremely large number: 73 00:07:53,419 --> 00:07:56,513 10 to the power 100... 74 00:07:56,723 --> 00:08:01,660 ...which I can't write out all the zeros because there isn't room. 75 00:08:01,861 --> 00:08:06,628 The boy called it a googol. 76 00:08:08,334 --> 00:08:13,135 If you think a googol is large, consider a googolplex. 77 00:08:13,840 --> 00:08:16,400 It's 10 to the power of a googol. 78 00:08:16,609 --> 00:08:20,067 That is, a one followed, not by 100 zeros... 79 00:08:20,280 --> 00:08:23,443 ...but by a googol zeros. 80 00:08:24,517 --> 00:08:26,280 Now, by comparison... 81 00:08:26,486 --> 00:08:29,148 ...with these enormous numbers... 82 00:08:29,355 --> 00:08:32,552 ...the total number of atoms in that apple pie... 83 00:08:32,759 --> 00:08:35,728 ...is only about 10 to the 26th. 84 00:08:35,929 --> 00:08:38,796 Tiny compared to a googol and... 85 00:08:38,998 --> 00:08:42,559 ...of course, much, much less than a googolplex. 86 00:08:42,769 --> 00:08:44,361 The number of elementary particles... 87 00:08:44,571 --> 00:08:46,539 ...protons, neutrons and electrons... 88 00:08:46,739 --> 00:08:48,434 ...in the accessible universe... 89 00:08:48,641 --> 00:08:50,939 ...is of the order of 10 to the 80th. 90 00:08:51,144 --> 00:08:53,169 A one followed by 80 zeros. 91 00:08:53,379 --> 00:08:55,438 Still much, much less than a googol... 92 00:08:55,648 --> 00:08:59,243 ...and vastly less than a googolplex. 93 00:08:59,452 --> 00:09:04,219 And yet, these numbers, the googol and the googolplex... 94 00:09:04,424 --> 00:09:09,157 ...do not approach, they come nowhere near infinity. 95 00:09:09,362 --> 00:09:14,197 In fact, a googolplex is precisely as far from infinity... 96 00:09:14,400 --> 00:09:17,267 ...as is the number one. 97 00:09:17,470 --> 00:09:20,701 We started to write out a googolplex... 98 00:09:20,907 --> 00:09:22,704 ...but it wasn't easy. 99 00:09:24,544 --> 00:09:27,172 It's a very big number. 100 00:09:46,532 --> 00:09:50,832 Writing out a googolplex is a spectacularly futile exercise. 101 00:09:51,437 --> 00:09:55,464 A piece of paper large enough to contain the zeros in a googolplex... 102 00:09:55,675 --> 00:09:59,873 ...couldn't be stuffed into the known universe. 103 00:10:05,585 --> 00:10:06,552 Fortunately... 104 00:10:08,087 --> 00:10:09,554 ...there's a... 105 00:10:10,857 --> 00:10:13,189 ...much simpler and more concise way... 106 00:10:13,393 --> 00:10:15,486 ...to write a googolplex. 107 00:10:23,736 --> 00:10:25,135 Like this. 108 00:10:25,338 --> 00:10:27,329 And infinity... 109 00:10:30,510 --> 00:10:32,944 ...can be represented like this. 110 00:10:33,513 --> 00:10:37,415 This is the Cavendish Laboratory at Cambridge University... 111 00:10:37,617 --> 00:10:41,576 ...where the constituents of the atom were first discovered. 112 00:10:41,788 --> 00:10:45,747 The realm of the very small. 113 00:10:47,026 --> 00:10:51,053 From the time of Democritus, in the fifth century B.C... 114 00:10:51,264 --> 00:10:54,995 ...people have speculated about the existence of atoms. 115 00:10:55,201 --> 00:10:58,568 For the last few hundred years, there have been persuasive... 116 00:10:58,771 --> 00:11:02,332 ...but indirect arguments that all matter is made of atoms. 117 00:11:02,575 --> 00:11:07,171 But only in our time, have we actually been able to see them. 118 00:11:07,380 --> 00:11:11,976 Here the red blobs are the random throbbing motions... 119 00:11:12,452 --> 00:11:13,942 ...of uranium atoms... 120 00:11:14,153 --> 00:11:17,145 ...magnified 100 million times. 121 00:11:18,424 --> 00:11:22,952 How Democritus of Abdera would've enjoyed this movie. 122 00:11:27,967 --> 00:11:32,734 We pretty much take atoms for granted. 123 00:11:33,539 --> 00:11:36,531 And yet, there are so many different kinds... 124 00:11:36,743 --> 00:11:41,146 ...lovely and useful at the same time. 125 00:11:41,347 --> 00:11:42,473 Look. 126 00:12:09,776 --> 00:12:13,803 There are some 92 chemically distinct kinds of atoms... 127 00:12:14,280 --> 00:12:16,111 ...naturally found on Earth. 128 00:12:16,315 --> 00:12:20,081 They're called the chemical elements. 129 00:12:35,635 --> 00:12:38,160 Virtually everything we see and know... 130 00:12:38,371 --> 00:12:40,771 ...all the beauty of the natural world... 131 00:12:40,973 --> 00:12:44,465 ...is made of these few kinds of atoms... 132 00:12:44,677 --> 00:12:49,046 ...arranged in harmonious chemical patterns. 133 00:13:05,465 --> 00:13:09,162 Here we've represented all 92 of them. 134 00:13:09,368 --> 00:13:13,702 At room temperature, many of them are solids. 135 00:13:13,906 --> 00:13:15,601 A few are gases. 136 00:13:15,942 --> 00:13:17,273 And two of them... 137 00:13:18,578 --> 00:13:21,604 ...bromine and mercury, are liquids. 138 00:13:23,282 --> 00:13:27,048 They're arranged in order of complexity. 139 00:13:27,253 --> 00:13:31,087 Hydrogen, the simplest element, is element number 1. 140 00:13:31,290 --> 00:13:34,453 And uranium, the most complex... 141 00:13:34,894 --> 00:13:37,294 ...is element 92. 142 00:13:40,633 --> 00:13:43,466 Some elements are very familiar. 143 00:13:44,137 --> 00:13:45,365 For example... 144 00:13:45,571 --> 00:13:49,667 ...silicon, oxygen, magnesium, aluminum, iron... 145 00:13:49,876 --> 00:13:51,400 ...those that make up the Earth. 146 00:13:51,677 --> 00:13:56,171 Or hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur... 147 00:13:56,382 --> 00:13:58,873 ...the elements that are essential for life. 148 00:13:59,085 --> 00:14:03,044 Other elements are spectacularly unfamiliar. 149 00:14:03,256 --> 00:14:06,225 For example, hafnium. 150 00:14:06,826 --> 00:14:08,225 Erbium. 151 00:14:08,427 --> 00:14:10,327 Dysprosium. 152 00:14:10,930 --> 00:14:13,228 Praseodymium. 153 00:14:13,432 --> 00:14:16,333 Elements we don't bump into in everyday life. 154 00:14:17,136 --> 00:14:22,073 By and large, the more familiar an element is, the more abundant it is. 155 00:14:22,375 --> 00:14:24,707 There's a great deal of iron on the Earth. 156 00:14:24,911 --> 00:14:28,506 Not all that much yttrium. 157 00:14:28,714 --> 00:14:29,942 The fact... 158 00:14:30,149 --> 00:14:33,744 ...that atoms are composed of only three kinds of elementary particles... 159 00:14:33,953 --> 00:14:36,421 ...protons, neutrons and electrons... 160 00:14:36,622 --> 00:14:39,022 ...is a comparatively recent finding. 161 00:14:39,225 --> 00:14:41,693 The neutron was not discovered until 1932. 162 00:14:41,894 --> 00:14:46,354 And it, like the electron and the proton, were discovered here... 163 00:14:46,566 --> 00:14:48,534 ...at Cambridge University. 164 00:14:48,734 --> 00:14:52,830 Modern physics and chemistry have reduced the complexity... 165 00:14:53,039 --> 00:14:57,601 ...of the sensible world to an astonishing simplicity. 166 00:14:57,810 --> 00:15:02,042 Three units, put together in different patterns... 167 00:15:02,248 --> 00:15:06,184 ...make, essentially, everything. 168 00:15:11,624 --> 00:15:15,116 A neutron is electrically neutral... 169 00:15:15,328 --> 00:15:17,819 ...as its name suggests. 170 00:15:18,397 --> 00:15:22,231 A proton has a positive electrical charge... 171 00:15:22,802 --> 00:15:27,068 ...and an electron an equal, negative electrical charge. 172 00:15:27,273 --> 00:15:29,798 Since every atom is electrically neutral... 173 00:15:30,009 --> 00:15:32,239 ...the number of protons in the nucleus... 174 00:15:32,511 --> 00:15:36,572 ...must equal the number of electrons far away in the electron cloud. 175 00:15:36,782 --> 00:15:40,843 The protons and neutrons, together, make up the nucleus of the atom. 176 00:15:41,921 --> 00:15:46,858 Now, the chemistry of an atom, the nature of a chemical element... 177 00:15:47,059 --> 00:15:49,289 ...depends only on the number of electrons... 178 00:15:49,495 --> 00:15:53,090 ...which equals the number of protons, which is called the atomic number. 179 00:15:53,299 --> 00:15:55,597 Chemistry is just numbers. 180 00:15:55,801 --> 00:15:58,895 An idea which would have appealed to Pythagoras. 181 00:15:59,105 --> 00:16:00,504 If you're an atom... 182 00:16:00,706 --> 00:16:03,903 ...and you have just one proton... 183 00:16:04,110 --> 00:16:05,577 ...you're hydrogen. 184 00:16:05,778 --> 00:16:07,973 Two protons, helium. 185 00:16:08,180 --> 00:16:10,011 Three, lithium. 186 00:16:10,216 --> 00:16:12,980 Four, beryllium. Five protons, boron. 187 00:16:13,185 --> 00:16:17,747 Six, carbon, and seven, nitrogen. Eight, oxygen, and so on. 188 00:16:17,957 --> 00:16:20,687 All the way to 92 protons... 189 00:16:20,893 --> 00:16:23,885 ...in which case your name is uranium. 190 00:16:24,497 --> 00:16:27,625 Protons have positive electrical charges... 191 00:16:27,833 --> 00:16:31,030 ...but like charges repel each other. 192 00:16:31,237 --> 00:16:33,637 So why does the nucleus hold together? 193 00:16:33,839 --> 00:16:37,070 Why don't the electrical repulsion of the protons... 194 00:16:37,276 --> 00:16:39,210 ...make the nucleus fly to pieces? 195 00:16:39,812 --> 00:16:42,337 Because there's another force in nature. 196 00:16:42,548 --> 00:16:45,108 Not electricity, not gravity... 197 00:16:45,318 --> 00:16:46,751 ...the nuclear force. 198 00:16:46,952 --> 00:16:49,853 We can think of it as short-range... 199 00:16:50,056 --> 00:16:52,456 ...hooks which start working... 200 00:16:52,658 --> 00:16:56,185 ...when protons or neutrons are brought very close together. 201 00:16:56,395 --> 00:16:58,886 The nuclear force can overcome... 202 00:16:59,632 --> 00:17:02,192 ...the electrical repulsion of the protons. 203 00:17:02,401 --> 00:17:05,461 Since the neutrons exert nuclear forces... 204 00:17:05,971 --> 00:17:07,632 ...but not electrical forces... 205 00:17:07,840 --> 00:17:12,334 ...they are a kind of glue which holds the atomic nucleus together. 206 00:17:13,879 --> 00:17:18,407 A lump of two protons and two neutrons... 207 00:17:18,617 --> 00:17:20,517 ...is the nucleus of a helium atom... 208 00:17:20,720 --> 00:17:23,188 ...and is very stable. 209 00:17:23,389 --> 00:17:27,883 Three helium nuclei, stuck together by nuclear forces... 210 00:17:28,094 --> 00:17:29,925 ...makes carbon. 211 00:17:30,129 --> 00:17:33,530 Four helium nuclei makes oxygen. 212 00:17:33,733 --> 00:17:36,600 There's no difference between four helium nuclei... 213 00:17:36,836 --> 00:17:39,396 ...stuck together by nuclear forces and the oxygen nucleus. 214 00:17:39,605 --> 00:17:41,095 They're the same thing. 215 00:17:41,474 --> 00:17:44,773 Five helium nuclei makes neon. 216 00:17:44,977 --> 00:17:47,377 Six makes magnesium. 217 00:17:47,580 --> 00:17:51,243 Seven makes silicon. 218 00:17:51,450 --> 00:17:54,010 Eight makes sulfur, and so on. 219 00:17:54,220 --> 00:17:56,620 Increasing the atomic numbers by two... 220 00:17:56,822 --> 00:17:59,689 ...and always making some familiar element. 221 00:17:59,892 --> 00:18:00,824 Every time... 222 00:18:01,794 --> 00:18:03,819 ...we add or subtract one proton... 223 00:18:04,029 --> 00:18:06,725 ...and enough neutrons to keep the nucleus together... 224 00:18:06,932 --> 00:18:09,400 ...we make a new chemical element. 225 00:18:09,602 --> 00:18:12,366 Consider mercury: 226 00:18:12,571 --> 00:18:16,337 If we subtract one proton from mercury... 227 00:18:16,542 --> 00:18:20,740 ...and three neutrons, we convert it into gold. 228 00:18:20,946 --> 00:18:25,042 The dream of the ancient alchemists. 229 00:18:26,051 --> 00:18:29,919 Beyond element 92, beyond uranium... 230 00:18:30,122 --> 00:18:32,590 ...there are other elements. 231 00:18:32,792 --> 00:18:34,953 They don't occur naturally on the Earth. 232 00:18:35,161 --> 00:18:37,595 They're synthesized by human beings and... 233 00:18:37,797 --> 00:18:40,425 ...fall to pieces pretty rapidly. 234 00:18:40,633 --> 00:18:44,262 One of them, element 94, is called plutonium... 235 00:18:44,470 --> 00:18:48,201 ...and is one of the most toxic substances known. 236 00:18:48,407 --> 00:18:52,844 Where do the naturally occurring chemical elements come from? 237 00:18:53,045 --> 00:18:57,482 Perhaps a separate creation for each element? 238 00:18:57,683 --> 00:19:00,550 But all the elements are made of the same elementary particles. 239 00:19:00,753 --> 00:19:03,745 The universe, all of it, everywhere... 240 00:19:03,956 --> 00:19:07,357 ...is 99.9% hydrogen and helium. 241 00:19:07,593 --> 00:19:09,458 The two simplest elements. 242 00:19:09,662 --> 00:19:10,856 In fact, helium... 243 00:19:11,297 --> 00:19:15,199 ...was detected on the sun before it was ever found on the Earth. 244 00:19:15,401 --> 00:19:19,201 Might the other chemical elements have somehow... 245 00:19:19,405 --> 00:19:23,364 ...evolved from hydrogen and helium? 246 00:19:23,576 --> 00:19:25,942 To avoid the electrical repulsion... 247 00:19:26,645 --> 00:19:31,105 ...protons and neutrons must be brought very close together so the hooks... 248 00:19:31,317 --> 00:19:33,080 ...which represent nuclear forces... 249 00:19:33,285 --> 00:19:34,650 ...are engaged. 250 00:19:34,854 --> 00:19:37,982 This happens only at very high temperatures, where particles... 251 00:19:38,190 --> 00:19:42,854 ...move so fast that there's no time for electrical repulsion to act. 252 00:19:43,128 --> 00:19:48,065 Temperatures of tens of millions of degrees. 253 00:19:48,334 --> 00:19:51,735 Such high temperatures are common in nature. 254 00:19:51,937 --> 00:19:53,234 Where? 255 00:19:53,439 --> 00:19:56,374 In the insides of the stars. 256 00:20:11,790 --> 00:20:15,556 Atoms are made in the insides of stars. 257 00:20:15,761 --> 00:20:20,198 In most of the stars we see, hydrogen nuclei are being jammed together... 258 00:20:20,399 --> 00:20:21,957 ...to form helium nuclei. 259 00:20:22,368 --> 00:20:26,896 Every time a nucleus of helium is made, a photon of light is generated. 260 00:20:27,873 --> 00:20:31,331 This is why the stars shine. 261 00:20:38,851 --> 00:20:42,844 Stars are born in great clouds of gas and dust. 262 00:20:43,055 --> 00:20:46,786 Like the Orion Nebula, 1500 light-years away... 263 00:20:46,992 --> 00:20:50,985 ...parts of which are collapsing under gravity. 264 00:20:58,637 --> 00:21:03,040 Collisions among the atoms heat the cloud until, in its interior... 265 00:21:03,242 --> 00:21:05,904 ...hydrogen begins to fuse into helium... 266 00:21:06,111 --> 00:21:08,909 ...and the stars turn on. 267 00:21:15,020 --> 00:21:17,716 Stars are born in batches. 268 00:21:17,923 --> 00:21:20,323 Later, they wander out of their nursery... 269 00:21:20,526 --> 00:21:23,154 ...to pursue their destiny in the Milky Way. 270 00:21:23,362 --> 00:21:26,195 Adolescent stars, like the Pleiades... 271 00:21:26,398 --> 00:21:29,162 ...are still surrounded by gas and dust. 272 00:21:29,368 --> 00:21:33,828 Eventually, they journey far from home. 273 00:21:37,643 --> 00:21:42,580 Somewhere there are stars formed from the same cloud complex as the sun... 274 00:21:42,948 --> 00:21:45,109 ...5 billion years ago. 275 00:21:45,317 --> 00:21:47,877 But we do not know which stars they are. 276 00:21:48,087 --> 00:21:49,987 The siblings of the sun... 277 00:21:50,322 --> 00:21:55,055 ...may, for all we know, be on the other side of the galaxy. 278 00:21:56,662 --> 00:22:01,599 Perhaps they also warm nearby planets as the sun does. 279 00:22:04,203 --> 00:22:07,104 Perhaps they too have presided... 280 00:22:07,306 --> 00:22:11,208 ...over the evolution of life and intelligence. 281 00:22:32,765 --> 00:22:37,498 The sun is the nearest star, a glowing sphere of gas... 282 00:22:38,003 --> 00:22:42,030 ...shining because of its heat, like a red-hot poker. 283 00:22:46,378 --> 00:22:51,315 The surface we see in ordinary visible light is at 6000 degrees centigrade. 284 00:22:51,550 --> 00:22:53,347 But in its hidden interior... 285 00:22:54,086 --> 00:22:57,544 ...in the nuclear furnace where sunlight is ultimately generated... 286 00:22:57,756 --> 00:23:01,351 ...its temperature is 20 million degrees. 287 00:23:09,735 --> 00:23:10,724 In x-rays... 288 00:23:10,936 --> 00:23:14,667 ...we see a part of the sun that is ordinarily invisible... 289 00:23:14,873 --> 00:23:17,501 ...its million-degree halo of gas... 290 00:23:17,710 --> 00:23:19,803 ...the solar corona. 291 00:23:20,245 --> 00:23:23,339 In ordinary visible light, these cooler, darker regions... 292 00:23:23,549 --> 00:23:25,449 ...are the sunspots. 293 00:23:28,187 --> 00:23:32,647 They are associated with great surges of flaming gas... 294 00:23:32,858 --> 00:23:37,227 ...tongues of fire which would engulf the Earth if it were this close. 295 00:23:37,563 --> 00:23:41,329 These prominences are guided into paths determined... 296 00:23:41,533 --> 00:23:43,763 ...by the sun's magnetic field. 297 00:23:56,415 --> 00:23:58,645 The dark regions of the x-ray sun... 298 00:23:58,851 --> 00:24:01,547 ...are holes in the solar corona... 299 00:24:01,754 --> 00:24:05,747 ...through which stream the protons and electrons of the solar wind... 300 00:24:05,958 --> 00:24:10,156 ...on their way past the planets to interstellar space. 301 00:24:12,664 --> 00:24:17,363 All this churning power is driven by the sun's interior... 302 00:24:17,569 --> 00:24:21,471 ...which is converting 400 million tons of hydrogen into helium... 303 00:24:21,673 --> 00:24:23,197 ...every second. 304 00:24:23,408 --> 00:24:27,003 The sun is a great fusion reactor... 305 00:24:27,246 --> 00:24:29,714 ...into which a million Earths would fit. 306 00:24:29,915 --> 00:24:33,043 Luckily for us, it's safely placed... 307 00:24:33,252 --> 00:24:36,779 ...150 million kilometers away. 308 00:24:53,305 --> 00:24:57,207 It is the destiny of stars to collapse. 309 00:24:58,310 --> 00:25:01,711 Of the thousands of stars you see when you look up at the night sky... 310 00:25:01,914 --> 00:25:06,544 ...every one of them is living in an interval between two collapses. 311 00:25:06,752 --> 00:25:08,242 An initial collapse of... 312 00:25:08,453 --> 00:25:11,945 ...a dark interstellar gas cloud to form the star... 313 00:25:12,157 --> 00:25:14,625 ...and a final collapse of the luminous star... 314 00:25:14,827 --> 00:25:16,590 ...on the way to its ultimate fate. 315 00:25:17,062 --> 00:25:21,999 Gravity makes stars contract unless some other force intervenes. 316 00:25:22,401 --> 00:25:25,302 The sun is an immense ball of radiating hydrogen. 317 00:25:25,504 --> 00:25:29,804 The hot gas in its interior tries to make the sun expand. 318 00:25:30,008 --> 00:25:33,409 The gravity tries to make the sun contract. 319 00:25:33,645 --> 00:25:36,876 The present state of the sun is the balance of these two forces... 320 00:25:37,082 --> 00:25:41,644 ...an equilibrium between gravity and nuclear fire. 321 00:25:42,321 --> 00:25:44,983 In this long middle age between collapses... 322 00:25:45,190 --> 00:25:47,886 ...the stars steadily shine. 323 00:25:48,093 --> 00:25:51,927 But when the nuclear fuel is exhausted, the interior cools... 324 00:25:52,130 --> 00:25:54,928 ...the pressure no longer supports its outer layers... 325 00:25:55,133 --> 00:25:57,294 ...and the initial collapse resumes. 326 00:25:58,103 --> 00:26:00,401 There are three ways that stars die. 327 00:26:00,606 --> 00:26:03,040 Their fates are predestined. 328 00:26:03,242 --> 00:26:05,176 Everything depends on their initial mass. 329 00:26:05,377 --> 00:26:07,971 A typical star with a mass like the sun... 330 00:26:08,180 --> 00:26:10,774 ...will one day continue its collapse... 331 00:26:10,983 --> 00:26:13,884 ...until its density becomes very high. 332 00:26:14,086 --> 00:26:15,815 And then the contraction is stopped... 333 00:26:16,021 --> 00:26:17,886 ...by the mutual repulsion of... 334 00:26:18,090 --> 00:26:21,355 ...the overcrowded electrons in its interior. 335 00:26:21,560 --> 00:26:24,324 A collapsing star twice as massive as the sun... 336 00:26:24,529 --> 00:26:27,054 ...isn't stopped by the electron pressure. 337 00:26:27,266 --> 00:26:29,632 It goes on falling in on itself... 338 00:26:29,835 --> 00:26:32,531 ...until nuclear forces come into play... 339 00:26:32,738 --> 00:26:35,832 ...and they hold up the weight of the star. 340 00:26:36,041 --> 00:26:39,204 But a collapsing star three times as massive as the sun isn't... 341 00:26:39,411 --> 00:26:41,470 ...stopped even by nuclear forces. 342 00:26:41,680 --> 00:26:46,617 There's no force known that can withstand this enormous compression. 343 00:26:46,852 --> 00:26:49,912 And such a star has an astonishing destiny. 344 00:26:50,122 --> 00:26:51,851 It continues to collapse... 345 00:26:52,124 --> 00:26:54,854 ...until it vanishes utterly. 346 00:26:56,094 --> 00:27:00,554 Each star is described by the force that holds it up against gravity. 347 00:27:00,766 --> 00:27:04,202 A star that's supported by the gas pressure... 348 00:27:04,403 --> 00:27:08,100 ...is a normal, run-of-the-mill star like the sun. 349 00:27:08,307 --> 00:27:12,004 A collapsed star that's held up by electron forces... 350 00:27:12,210 --> 00:27:13,837 ...is called a white dwarf. 351 00:27:14,046 --> 00:27:17,880 It's a sun shrunk to the size of the Earth. 352 00:27:18,083 --> 00:27:21,450 A collapsed star supported by nuclear forces... 353 00:27:21,653 --> 00:27:23,518 ...is called a neutron star. 354 00:27:23,722 --> 00:27:27,249 It's a sun shrunk to the size of a city. 355 00:27:27,459 --> 00:27:30,394 And a star so massive that in its final collapse... 356 00:27:30,595 --> 00:27:32,290 ...it disappears altogether... 357 00:27:32,497 --> 00:27:34,158 ...is called a black hole. 358 00:27:34,366 --> 00:27:37,358 It's a sun with no size at all. 359 00:27:38,036 --> 00:27:40,732 But on their ways to their separate fates... 360 00:27:40,939 --> 00:27:45,603 ...all stars experience a premonition of death. 361 00:27:45,811 --> 00:27:48,279 Before the final gravitational collapse... 362 00:27:48,480 --> 00:27:53,315 ...the star shudders and briefly swells into some... 363 00:27:53,785 --> 00:27:56,219 ...grotesque parody of itself. 364 00:27:56,421 --> 00:28:00,790 With its last gasp, it becomes a red giant. 365 00:28:04,529 --> 00:28:07,020 Some 5 billion years from now... 366 00:28:07,232 --> 00:28:11,635 ...there will be a last, perfect day on Earth. 367 00:28:14,639 --> 00:28:18,166 Then, the sun will slowly change... 368 00:28:18,377 --> 00:28:21,403 ...and the Earth will die. 369 00:28:24,549 --> 00:28:27,450 There is only so much hydrogen fuel in the sun. 370 00:28:27,652 --> 00:28:30,052 When it's almost all converted to helium... 371 00:28:30,255 --> 00:28:33,918 ...the solar interior will continue its original collapse. 372 00:28:34,126 --> 00:28:38,586 Higher temperatures in its core will make the outside of the sun expand... 373 00:28:38,830 --> 00:28:42,288 ...and the Earth will become slowly warmer. 374 00:28:42,534 --> 00:28:45,799 Eventually, life will be extinguished... 375 00:28:46,171 --> 00:28:49,106 ...the oceans will evaporate and boil... 376 00:28:49,307 --> 00:28:53,334 ...and our atmosphere will gush away to space. 377 00:28:56,148 --> 00:28:59,948 The sun will become a bloated red giant star... 378 00:29:00,152 --> 00:29:01,676 ...filling the sky... 379 00:29:01,920 --> 00:29:06,084 ...enveloping and devouring the planets Mercury and Venus. 380 00:29:06,291 --> 00:29:09,351 And probably the Earth as well. 381 00:29:09,561 --> 00:29:14,498 The inner solar system will reside inside the sun. 382 00:29:17,736 --> 00:29:20,603 But perhaps by then, our descendants... 383 00:29:20,806 --> 00:29:23,366 ...will have ventured somewhere else. 384 00:29:27,479 --> 00:29:31,882 In its final agonies, the sun will slowly pulsate. 385 00:29:32,084 --> 00:29:34,985 By then, its core will have become so hot... 386 00:29:35,187 --> 00:29:38,623 ...that it temporarily converts helium into carbon. 387 00:29:38,824 --> 00:29:42,726 The ash from today's nuclear fusion will become the fuel... 388 00:29:43,028 --> 00:29:47,897 ...to power the sun near the end of its life in its red giant stage. 389 00:29:49,701 --> 00:29:52,534 Then the sun will lose great shells... 390 00:29:53,071 --> 00:29:55,266 ...of its outer atmosphere to space... 391 00:29:55,474 --> 00:29:58,875 ...filling the solar system with eerily glowing gas. 392 00:29:59,077 --> 00:30:02,137 The ghost of a star, outward bound. 393 00:30:02,347 --> 00:30:05,839 Perhaps half the mass of the sun will be lost in this way. 394 00:30:07,786 --> 00:30:11,085 Viewed from elsewhere, our system will then resemble... 395 00:30:11,289 --> 00:30:13,416 ...the Ring Nebula in Lyra... 396 00:30:13,625 --> 00:30:18,255 ...the atmosphere of the sun expanding outward like a soap bubble. 397 00:30:19,131 --> 00:30:22,362 And at the very center will be a white dwarf. 398 00:30:22,601 --> 00:30:25,069 The hot exposed core of the sun... 399 00:30:25,270 --> 00:30:28,831 ...its nuclear fuel now exhausted, slowly cooling... 400 00:30:29,074 --> 00:30:32,510 ...to become a cold, dead star. 401 00:30:35,514 --> 00:30:38,142 Such is the life of an ordinary star. 402 00:30:38,350 --> 00:30:41,547 Born in a gas cloud, maturing as a yellow sun... 403 00:30:41,753 --> 00:30:43,584 ...decaying as a red giant... 404 00:30:43,788 --> 00:30:48,725 ...and dying as a white dwarf enveloped in its shroud of gas. 405 00:30:57,769 --> 00:31:00,863 Suppose, as we traveled through interstellar space... 406 00:31:01,072 --> 00:31:02,903 ...in our ship of the imagination... 407 00:31:03,108 --> 00:31:07,477 ...we could sample the cold, thin gas between the stars. 408 00:31:07,679 --> 00:31:10,671 We would find a great preponderance of hydrogen... 409 00:31:10,882 --> 00:31:13,442 ...an element as old as the universe. 410 00:31:13,652 --> 00:31:16,348 We would find carbon, oxygen, silicon. 411 00:31:16,555 --> 00:31:19,649 The most abundant atoms in the cosmos, apart from hydrogen... 412 00:31:19,858 --> 00:31:23,294 ...are those most easily made in the stars. 413 00:31:23,495 --> 00:31:26,896 But we would also find a small proportion of rare elements. 414 00:31:27,098 --> 00:31:29,896 Praseodymium, say, or gold. 415 00:31:30,101 --> 00:31:32,262 They're not made in red giants. 416 00:31:32,470 --> 00:31:37,100 Such elements are manufactured in one of the most dramatic gestures... 417 00:31:37,342 --> 00:31:39,833 ...of which a star is capable. 418 00:31:40,979 --> 00:31:44,244 A star more than about one and a half times the mass of the sun... 419 00:31:44,649 --> 00:31:46,276 ...cannot become a white dwarf. 420 00:31:46,484 --> 00:31:49,453 It will end its life by blowing itself up... 421 00:31:49,654 --> 00:31:54,114 ...in a titanic stellar explosion called a supernova. 422 00:31:55,660 --> 00:31:59,323 There has been no supernova explosion in our province of the galaxy... 423 00:31:59,698 --> 00:32:01,427 ...since the telescope's invention... 424 00:32:01,633 --> 00:32:04,534 ...and our sun will not become a supernova. 425 00:32:04,736 --> 00:32:06,363 But in our imagination... 426 00:32:06,905 --> 00:32:10,204 ...we can fulfill the dream of many earthbound astronomers... 427 00:32:10,642 --> 00:32:15,375 ...and safely witness, close-up, a supernova explosion. 428 00:32:19,751 --> 00:32:23,653 Most of stellar evolution takes millions or billions of years. 429 00:32:23,888 --> 00:32:27,415 But the interior collapse that triggers a supernova explosion... 430 00:32:27,659 --> 00:32:29,092 ...takes only seconds. 431 00:32:29,461 --> 00:32:33,454 The star becomes brighter than all the other stars in the galaxy... 432 00:32:33,665 --> 00:32:35,064 ...put together. 433 00:33:10,769 --> 00:33:13,033 If a nearby star became a supernova... 434 00:33:13,238 --> 00:33:17,971 ...it would be calamity enough for the inhabitants of this alien system. 435 00:33:18,176 --> 00:33:20,576 But if their own sun went supernova... 436 00:33:20,779 --> 00:33:24,112 ...it would be an unprecedented catastrophe. 437 00:33:24,316 --> 00:33:26,750 Worlds would be charred and vaporized. 438 00:33:26,951 --> 00:33:31,411 Life, even on the outer planets, would be extinguished. 439 00:33:31,623 --> 00:33:35,821 In our ship of the imagination, we are now backing away from the star. 440 00:33:36,027 --> 00:33:37,688 But the explosion fragments... 441 00:33:37,896 --> 00:33:41,388 ...traveling almost at the speed of light, are overtaking us. 442 00:33:41,599 --> 00:33:46,195 Individual atomic nuclei, accelerated to high speeds in the explosion... 443 00:33:46,404 --> 00:33:48,338 ...become cosmic rays. 444 00:33:48,540 --> 00:33:52,806 This is another way that stars return the atoms they've synthesized... 445 00:33:53,011 --> 00:33:54,876 ...back into space. 446 00:33:55,647 --> 00:33:57,706 The shock wave of expanding gases... 447 00:33:57,916 --> 00:34:00,441 ...heats and compresses the interstellar gas... 448 00:34:00,652 --> 00:34:03,644 ...triggering a later generation of stars to form. 449 00:34:03,855 --> 00:34:05,186 In this sense also... 450 00:34:05,390 --> 00:34:09,793 ...stars are phoenixes rising from their own ashes. 451 00:34:14,632 --> 00:34:18,466 The cosmos was originally all hydrogen and helium. 452 00:34:18,703 --> 00:34:22,070 Heavier elements were made in red giants and in supernovas... 453 00:34:22,273 --> 00:34:24,264 ...and then blown off to space... 454 00:34:24,476 --> 00:34:27,206 ...where they were available for subsequent generations... 455 00:34:27,412 --> 00:34:29,039 ...of stars and planets. 456 00:34:29,247 --> 00:34:33,013 Our sun is probably a third-generation star. 457 00:34:33,218 --> 00:34:34,845 Except for hydrogen and helium... 458 00:34:35,053 --> 00:34:39,990 ...every atom in the sun and the Earth was synthesized in other stars. 459 00:34:40,258 --> 00:34:44,786 The silicon in the rocks, the oxygen in the air, the carbon in our DNA... 460 00:34:44,996 --> 00:34:48,762 ...the gold in our banks, the uranium in our arsenals... 461 00:34:48,967 --> 00:34:52,562 ...were all made thousands of light-years away... 462 00:34:52,771 --> 00:34:54,636 ...and billions of years ago. 463 00:34:54,839 --> 00:34:59,173 Our planet, our society and we ourselves... 464 00:34:59,377 --> 00:35:02,642 ...are built of star stuff. 465 00:35:07,652 --> 00:35:10,621 We're in a lava tube. 466 00:35:11,389 --> 00:35:15,723 A cave carved through the Earth... 467 00:35:15,927 --> 00:35:18,487 ...by a river of molten rock. 468 00:35:19,497 --> 00:35:21,556 To do a little experiment... 469 00:35:22,567 --> 00:35:25,968 ...we've brought a Geiger counter... 470 00:35:29,007 --> 00:35:32,602 ...and a piece of uranium ore. 471 00:35:33,344 --> 00:35:38,281 The Geiger counter is sensitive to high-energy charged particles... 472 00:35:38,650 --> 00:35:42,313 ...protons, helium nuclei, gamma rays. 473 00:35:42,587 --> 00:35:45,988 If we bring it close to the uranium ore... 474 00:35:46,191 --> 00:35:50,753 ...the count rate, the number of clicks, increases dramatically. 475 00:35:53,431 --> 00:35:55,729 We also have a lead canister here. 476 00:35:56,301 --> 00:35:58,861 And if I drop the uranium ore... 477 00:35:59,404 --> 00:36:04,103 ...into the canister, which absorbs the radiation, and cover it up... 478 00:36:06,478 --> 00:36:09,675 ...I then find the count-rate goes down substantially... 479 00:36:09,881 --> 00:36:11,576 ...but it doesn't go down to zero. 480 00:36:12,784 --> 00:36:15,878 What's the source of the remaining counts? 481 00:36:16,988 --> 00:36:21,448 Some of them come from radioactivity in the walls of the cave. 482 00:36:21,659 --> 00:36:23,854 But there's more to it than that. 483 00:36:24,062 --> 00:36:27,498 Some of the counts are due to high-energy charged particles... 484 00:36:27,699 --> 00:36:31,760 ...which are penetrating the roof of the cave. 485 00:36:31,970 --> 00:36:35,098 We are listening to cosmic rays. 486 00:36:36,274 --> 00:36:40,734 Every second they are penetrating my body... 487 00:36:40,945 --> 00:36:42,207 ...and yours. 488 00:36:42,413 --> 00:36:45,678 They don't do much damage. Cosmic rays have bombarded the Earth... 489 00:36:45,917 --> 00:36:48,511 ...for the entire history of life on our planet. 490 00:36:48,720 --> 00:36:51,314 But they do cause some mutations... 491 00:36:51,523 --> 00:36:54,356 ...and they do affect life on the Earth. 492 00:36:56,494 --> 00:36:59,486 The cosmic rays, mainly protons... 493 00:36:59,697 --> 00:37:04,634 ...are penetrating through the meters of rock in the cave above me. 494 00:37:04,836 --> 00:37:07,532 To do this, they have to be very energetic and in fact... 495 00:37:07,906 --> 00:37:10,966 ...they are traveling almost at the speed of light. 496 00:37:11,776 --> 00:37:13,175 Think of it. 497 00:37:13,645 --> 00:37:16,113 A star blows up... 498 00:37:16,447 --> 00:37:18,938 ...thousands of light-years away in space... 499 00:37:19,150 --> 00:37:21,948 ...and produces cosmic rays which... 500 00:37:22,153 --> 00:37:25,418 ...spiral through the Milky Way galaxy for... 501 00:37:25,623 --> 00:37:28,649 ...millions of years until, quite by accident... 502 00:37:28,860 --> 00:37:31,624 ...some of them strike the Earth... 503 00:37:31,829 --> 00:37:35,731 ...penetrate this cave, reach this Geiger counter... 504 00:37:36,000 --> 00:37:37,365 ...and us. 505 00:37:37,735 --> 00:37:42,604 The evolution of life on Earth is driven in part through mutations... 506 00:37:42,807 --> 00:37:45,901 ...by the deaths of distant stars. 507 00:37:46,110 --> 00:37:48,908 We are, in a very deep sense... 508 00:37:49,147 --> 00:37:51,877 ...tied to the cosmos. 509 00:37:54,552 --> 00:37:57,487 Our ancestors knew this well. 510 00:37:57,689 --> 00:38:00,351 The movements of the sun, the moon, and the stars... 511 00:38:00,558 --> 00:38:05,154 ...could be used by those skilled in such arts to foretell the seasons. 512 00:38:05,363 --> 00:38:08,298 So the ancient astronomers all over the world... 513 00:38:08,499 --> 00:38:10,592 ...studied the night sky with care... 514 00:38:10,802 --> 00:38:14,863 ...memorizing and recording the position of every visible star. 515 00:38:15,073 --> 00:38:19,476 To them, the appearance of any new star would have been significant. 516 00:38:19,677 --> 00:38:24,114 What would they have made of the apparition of a supernova... 517 00:38:24,315 --> 00:38:27,807 ...brighter than every other star in the sky? 518 00:38:34,192 --> 00:38:38,856 On July 4th, in the year 1054... 519 00:38:39,063 --> 00:38:44,000 ...Chinese astronomers recorded what they called a guest star... 520 00:38:44,202 --> 00:38:46,295 ...in the constellation of Taurus the Bull. 521 00:38:46,504 --> 00:38:50,634 A star never before seen burst into radiance... 522 00:38:50,842 --> 00:38:53,970 ...became almost as bright as the full moon. 523 00:38:54,178 --> 00:38:58,615 Halfway around the world, here in the American Southwest... 524 00:38:58,816 --> 00:39:03,515 ...there was then a high culture, rich in astronomical tradition. 525 00:39:03,721 --> 00:39:07,953 They too must have seen this brilliant new star. 526 00:39:08,159 --> 00:39:11,526 From carbon-14 dating... 527 00:39:11,763 --> 00:39:16,223 ...of the remains of a charcoal fire, we know that in this very spot... 528 00:39:16,434 --> 00:39:20,063 ...there were people living in the 11th century. 529 00:39:20,805 --> 00:39:25,742 The people were the Anasazi, the antecedents of the Hopi of today. 530 00:39:26,244 --> 00:39:29,145 And one of them seems to have drawn... 531 00:39:29,414 --> 00:39:32,747 ...on this overhang, protected from the weather... 532 00:39:32,950 --> 00:39:34,941 ...a picture of the new star. 533 00:39:35,153 --> 00:39:39,590 Its position near the crescent moon would have been just what we see here. 534 00:39:39,791 --> 00:39:42,692 And the handprint is, perhaps... 535 00:39:42,894 --> 00:39:45,624 ...the artist's signature. 536 00:39:46,330 --> 00:39:50,790 This remarkable star is now called the Crab Supernova. 537 00:39:51,002 --> 00:39:54,096 "Nova" from the Latin word for new and "Crab" because... 538 00:39:54,305 --> 00:39:57,502 ...that's what an astronomer centuries later was reminded of... 539 00:39:57,709 --> 00:40:01,509 ...when looking at this explosion or remnant through the telescope. 540 00:40:01,713 --> 00:40:05,706 The Crab is a star that blew itself up. 541 00:40:05,917 --> 00:40:08,681 The explosion was seen for three months. 542 00:40:08,886 --> 00:40:12,754 It was easily visible in broad daylight. 543 00:40:12,957 --> 00:40:15,824 And you could read by it at night. 544 00:40:17,962 --> 00:40:20,089 Imagine the night when that... 545 00:40:20,298 --> 00:40:22,926 ...colossal stellar explosion... 546 00:40:23,134 --> 00:40:25,728 ...first burst forth. 547 00:40:34,912 --> 00:40:36,311 A thousand years ago... 548 00:40:36,547 --> 00:40:40,074 ...people gazed up in amazement at the brilliant new star... 549 00:40:40,284 --> 00:40:42,718 ...and wondered what it was. 550 00:40:46,758 --> 00:40:50,353 We are the first generation privileged to know the answer. 551 00:40:50,561 --> 00:40:53,826 Through the telescope we have seen what lies today... 552 00:40:54,031 --> 00:40:57,262 ...at the spot in the sky noted by the ancient astronomers. 553 00:40:57,468 --> 00:41:01,063 A great luminous cloud, the remains of a star... 554 00:41:01,272 --> 00:41:06,209 ...violently unraveling itself back into interstellar space. 555 00:41:11,549 --> 00:41:14,882 Only the massive red giants become supernovas. 556 00:41:15,086 --> 00:41:18,180 But every supernova was once a red giant. 557 00:41:18,389 --> 00:41:19,651 In the history of the galaxy... 558 00:41:19,857 --> 00:41:23,953 ...hundreds of millions of red giants have become supernovas. 559 00:41:26,631 --> 00:41:30,294 The bit of the star that isn't blown away collapses under gravity... 560 00:41:30,501 --> 00:41:33,766 ...spinning ever faster like a pirouetting ice skater... 561 00:41:33,971 --> 00:41:35,302 ...bringing in her arms. 562 00:41:35,506 --> 00:41:38,907 The star becomes a single, massive atomic nucleus... 563 00:41:39,110 --> 00:41:40,771 ...a neutron star. 564 00:41:40,978 --> 00:41:44,209 The one in the Crab Nebula is spinning 30 times a second. 565 00:41:44,415 --> 00:41:46,474 It emits a beamed pattern of light... 566 00:41:46,684 --> 00:41:50,518 ...and seems to us to be blinking on and off with astonishing regularity. 567 00:41:50,721 --> 00:41:54,452 Such neutron stars are called pulsars. 568 00:41:56,093 --> 00:41:58,118 Neutron star matter... 569 00:41:58,329 --> 00:42:02,231 ...weighs about a mountain per teaspoonful. 570 00:42:02,433 --> 00:42:05,869 So much that if I had a piece of it here and let it go... 571 00:42:06,070 --> 00:42:08,436 ...I could hardly prevent it from falling. 572 00:42:08,639 --> 00:42:11,574 It would effortlessly pass through the Earth like a... 573 00:42:11,776 --> 00:42:14,142 ...a knife through warm butter. 574 00:42:14,345 --> 00:42:17,746 It would carve a hole for itself completely through the Earth... 575 00:42:17,949 --> 00:42:21,976 ...emerging out the other side perhaps in China. 576 00:42:22,186 --> 00:42:25,815 The people there might be walking along when a... 577 00:42:26,023 --> 00:42:30,824 ...tiny lump of neutron star matter comes booming out of the ground... 578 00:42:31,028 --> 00:42:32,655 ...and then falls back again. 579 00:42:32,864 --> 00:42:34,297 The incident might... 580 00:42:34,532 --> 00:42:38,195 ...make an agreeable break in the routine of the day. 581 00:42:38,402 --> 00:42:41,803 The neutron star matter, pulled back by the Earth's gravity... 582 00:42:42,006 --> 00:42:44,474 ...would plunge again through the Earth... 583 00:42:44,675 --> 00:42:49,078 ...eventually punching hundreds of thousands of holes... 584 00:42:49,280 --> 00:42:54,081 ...before friction with the interior of our planet stopped the motion. 585 00:42:54,285 --> 00:42:56,446 By the time it's at rest at the center of the Earth... 586 00:42:56,687 --> 00:43:01,386 ...the inside of our world would look a little bit like Swiss cheese. 587 00:43:16,540 --> 00:43:21,239 There are places in the galaxy where a neutron star and a red giant... 588 00:43:21,545 --> 00:43:25,481 ...are locked in a mutual gravitational embrace. 589 00:43:25,683 --> 00:43:28,049 Tendrils of red giant star stuff... 590 00:43:28,252 --> 00:43:31,085 ...spiral into a disc of accreting matter... 591 00:43:31,289 --> 00:43:34,690 ...centered on the hot neutron star. 592 00:43:44,669 --> 00:43:47,797 Every star exists in a state of tension... 593 00:43:48,005 --> 00:43:50,030 ...between the force that holds it up... 594 00:43:50,241 --> 00:43:52,709 ...and gravity, the force that would pull it down. 595 00:43:52,910 --> 00:43:57,244 If gravity were to prevail, a stellar madness would ensue... 596 00:43:57,448 --> 00:44:01,544 ...more bizarre than anything in wonderland. 597 00:44:03,154 --> 00:44:06,055 Alice and her colleagues feel, more or less... 598 00:44:06,257 --> 00:44:08,817 ...at home in the gravitational pull of the Earth... 599 00:44:09,026 --> 00:44:11,824 ...called one g, "g" for Earth gravity. 600 00:44:12,229 --> 00:44:15,687 What would happen if we made the gravity less, or more? 601 00:44:15,900 --> 00:44:17,959 At lower gravity, things get lighter. 602 00:44:18,169 --> 00:44:20,933 Near zero g, the slightest motion sends our friends... 603 00:44:21,138 --> 00:44:23,129 ...floating and tumbling in the air. 604 00:44:23,341 --> 00:44:26,139 Little blobs of liquid tea are everywhere. 605 00:44:26,577 --> 00:44:28,101 Curious. 606 00:44:28,646 --> 00:44:31,114 If we now return the gravity to one g... 607 00:44:31,649 --> 00:44:35,710 ...it's raining tea, and our friends fall back to Earth. 608 00:44:37,088 --> 00:44:40,580 I've been to a couple of parties like that myself. 609 00:44:42,593 --> 00:44:46,051 At higher gravities, two or three g's, say, things get... 610 00:44:46,263 --> 00:44:48,891 ...really laid back. 611 00:44:49,100 --> 00:44:53,093 Everyone feels heavy and leaden. 612 00:44:55,773 --> 00:44:59,209 Except by special dispensation... 613 00:44:59,510 --> 00:45:01,273 ...the Cheshire cat. 614 00:45:02,079 --> 00:45:05,071 As a kindness, we remove them. 615 00:45:05,282 --> 00:45:08,774 At thousands of g's, trees become squashed. 616 00:45:08,986 --> 00:45:13,423 At 100,000 g's, rocks become crushed by their own weight. 617 00:45:13,624 --> 00:45:17,253 At all these gravities, a beam of light remains unaffected... 618 00:45:17,461 --> 00:45:19,429 ...continuing up in a straight line. 619 00:45:19,630 --> 00:45:21,325 But at billions of g's... 620 00:45:21,532 --> 00:45:26,060 ...a beam of light feels the gravity and begins to bend back on itself. 621 00:45:26,270 --> 00:45:29,728 Curiouser and curiouser. 622 00:45:29,940 --> 00:45:34,001 Such a place, where the gravity is so large that even light can't get out... 623 00:45:34,211 --> 00:45:36,702 ...is called a black hole. 624 00:45:36,914 --> 00:45:40,179 It's a star in which light itself is imprisoned. 625 00:45:40,384 --> 00:45:42,682 Black holes were theoretical constructs... 626 00:45:42,887 --> 00:45:46,186 ...speculated about since 1783. 627 00:45:46,390 --> 00:45:49,985 But in our time, we've verified the invisible. 628 00:45:50,194 --> 00:45:53,823 This bright star has a massive, unseen companion. 629 00:45:54,031 --> 00:45:58,866 Satellite observatories find the companion to be an x-ray source... 630 00:45:59,070 --> 00:46:01,595 ...called Cygnus X-1. 631 00:46:01,806 --> 00:46:04,366 These x-rays are like the footprints... 632 00:46:04,575 --> 00:46:09,308 ...of an invisible man walking in the snow. 633 00:46:14,618 --> 00:46:17,678 The x-rays are thought to be generated by friction... 634 00:46:17,888 --> 00:46:21,380 ...in the accretion disc surrounding the black hole. 635 00:46:21,592 --> 00:46:24,789 The matter in the disc slowly disappears... 636 00:46:24,995 --> 00:46:26,963 ...down the black hole. 637 00:46:29,467 --> 00:46:33,563 Massive black holes, produced by the collapse of a billion suns... 638 00:46:33,771 --> 00:46:36,365 ...may be sitting at the centers of other galaxies... 639 00:46:36,574 --> 00:46:40,567 ...curiously producing great jets of radiation... 640 00:46:40,778 --> 00:46:42,871 ...pouring out into space. 641 00:46:45,783 --> 00:46:49,480 At high enough density, the star winks out... 642 00:46:49,687 --> 00:46:53,555 ...and vanishes from our universe leaving only its gravity behind. 643 00:46:53,757 --> 00:46:58,592 It slips through a self-generated crack in the space-time continuum. 644 00:46:58,796 --> 00:47:03,096 A black hole is a place where a star once was. 645 00:47:03,801 --> 00:47:06,668 Here we have a flat two- dimensional surface... 646 00:47:06,871 --> 00:47:11,638 ...with grid lines on it, something like a piece of graph paper. 647 00:47:12,543 --> 00:47:14,704 Suppose we take a small mass... 648 00:47:14,912 --> 00:47:19,645 ...drop it on the surface and watch how the surface distorts... 649 00:47:19,850 --> 00:47:23,718 ...or puckers into the third physical dimension. 650 00:47:27,591 --> 00:47:32,028 Gravity can be understood as a curvature of space. 651 00:47:33,764 --> 00:47:36,790 If our moving ball approaches a stationary distortion... 652 00:47:37,001 --> 00:47:40,368 ...it rolls around it like a planet orbiting the sun. 653 00:47:40,571 --> 00:47:44,974 In this interpretation, due to Einstein, gravity is only a pucker... 654 00:47:45,176 --> 00:47:48,737 ...in the fabric of space which moving objects encounter. 655 00:47:48,946 --> 00:47:53,883 Space is warped by mass into an additional physical dimension. 656 00:47:56,387 --> 00:48:00,983 The larger the local mass, the greater is the local gravity... 657 00:48:01,192 --> 00:48:04,650 ...and the more intense is the distortion... 658 00:48:04,862 --> 00:48:08,059 ...or pucker, or warp of space. 659 00:48:08,265 --> 00:48:10,358 So, by this analogy... 660 00:48:10,568 --> 00:48:15,198 ...a black hole is a kind of bottomless pit. 661 00:48:15,406 --> 00:48:17,271 What would happen if you fell in? 662 00:48:17,474 --> 00:48:20,671 Assuming you could survive the gravitational tides... 663 00:48:20,878 --> 00:48:25,542 ...and the intense radiation flux, it is just barely possible... 664 00:48:25,749 --> 00:48:28,843 ...that by plunging into a black hole... 665 00:48:29,053 --> 00:48:32,819 ...you might emerge in another part of space-time. 666 00:48:33,023 --> 00:48:34,718 Somewhere else in space... 667 00:48:34,925 --> 00:48:38,520 ...some-when else in time. 668 00:48:38,729 --> 00:48:40,663 In this view, space is... 669 00:48:40,864 --> 00:48:43,856 ...filled with a network of wormholes... 670 00:48:44,068 --> 00:48:45,933 ...something like the wormholes in an apple. 671 00:48:46,136 --> 00:48:49,230 Although by no means is this point demonstrated... 672 00:48:49,440 --> 00:48:52,637 ...it is merely an exciting suggestion. 673 00:48:52,843 --> 00:48:54,242 If it is true... 674 00:48:54,445 --> 00:48:58,438 ...then perhaps there exist gravity tunnels... 675 00:48:58,649 --> 00:49:02,050 ...a kind of interstellar or intergalactic subway... 676 00:49:02,253 --> 00:49:03,982 ...which would permit you to get from... 677 00:49:04,188 --> 00:49:06,679 ...here to there in much less than the usual time. 678 00:49:06,890 --> 00:49:10,849 A kind of cosmic rapid transit system. 679 00:49:13,564 --> 00:49:15,964 We cannot generate black holes... 680 00:49:16,166 --> 00:49:18,896 ...our technology is far too feeble to... 681 00:49:19,103 --> 00:49:21,799 ...move such massive amounts of matter around. 682 00:49:22,006 --> 00:49:24,998 But perhaps someday, it will be possible to voyage hundreds or... 683 00:49:25,209 --> 00:49:29,077 ...thousands of light-years to a black hole like Cygnus X-1. 684 00:49:29,280 --> 00:49:31,441 We would plunge down to emerge... 685 00:49:31,649 --> 00:49:34,709 ...in some unimaginably exotic time and place. 686 00:49:34,918 --> 00:49:39,582 Our common-sense notions of reality severely challenged. 687 00:49:41,825 --> 00:49:44,487 Perhaps the cosmos is infested with wormholes... 688 00:49:44,695 --> 00:49:47,027 ...every one of them a tunnel to somewhere. 689 00:49:47,231 --> 00:49:51,463 Perhaps other civilizations, with vastly more advanced technologies... 690 00:49:51,669 --> 00:49:56,072 ...are today riding the gravity express. 691 00:49:58,375 --> 00:50:02,072 It's even possible that a black hole is a gate... 692 00:50:02,279 --> 00:50:05,646 ...to another, and quite different, universe. 693 00:50:43,687 --> 00:50:46,178 The lives and deaths of the stars... 694 00:50:46,390 --> 00:50:49,791 ...seem impossibly remote from human experience... 695 00:50:49,993 --> 00:50:53,895 ...and yet we're related in the most intimate way to their life cycles. 696 00:50:54,098 --> 00:50:56,066 The very matter that makes us up... 697 00:50:56,266 --> 00:51:01,203 ...was generated long ago and far away in red giant stars. 698 00:51:02,306 --> 00:51:05,969 A blade of grass, as Walt Whitman said... 699 00:51:06,176 --> 00:51:08,406 "...is the journey work of the stars." 700 00:51:08,612 --> 00:51:10,136 The formation of the solar system... 701 00:51:10,347 --> 00:51:13,475 ...may have been triggered by a nearby supernova explosion. 702 00:51:13,684 --> 00:51:15,174 After the sun turned on... 703 00:51:15,386 --> 00:51:18,219 ...its ultraviolet light poured into our atmosphere. 704 00:51:18,422 --> 00:51:20,413 Its warmth generated lightning. 705 00:51:20,624 --> 00:51:23,525 And these energy sources sparked the origin of life. 706 00:51:24,561 --> 00:51:26,893 Plants harvest sunlight... 707 00:51:27,097 --> 00:51:29,998 ...converting solar into chemical energy. 708 00:51:30,801 --> 00:51:33,895 We and the other animals are parasites on the plants. 709 00:51:34,104 --> 00:51:38,040 So we are, all of us, solar-powered. 710 00:51:38,442 --> 00:51:40,933 The evolution of life is driven by mutations. 711 00:51:41,145 --> 00:51:45,275 They are caused partly by natural radioactivity and cosmic rays. 712 00:51:45,482 --> 00:51:49,782 But they are both generated in the spectacular deaths of massive stars... 713 00:51:49,987 --> 00:51:51,978 ...thousands of light-years distant. 714 00:51:53,524 --> 00:51:57,153 Think of the sun's heat on your upturned face... 715 00:51:57,361 --> 00:51:59,625 ...on a cloudless summer's day. 716 00:51:59,830 --> 00:52:02,958 From 150 million kilometers away... 717 00:52:03,167 --> 00:52:05,635 ...we recognize its power. 718 00:52:05,836 --> 00:52:09,670 What would we feel on its seething, self-luminous surface... 719 00:52:09,873 --> 00:52:13,331 ...or immersed in its heart of nuclear fire? 720 00:52:13,544 --> 00:52:18,481 And yet, the sun is an ordinary, even a mediocre star. 721 00:52:18,715 --> 00:52:22,549 Our ancestors worshiped the sun and they were far from foolish. 722 00:52:22,753 --> 00:52:25,551 It makes good sense to revere the sun and the stars. 723 00:52:25,756 --> 00:52:29,715 Because we are their children. 724 00:52:32,696 --> 00:52:36,792 We have witnessed the life cycles of the stars. 725 00:52:37,000 --> 00:52:41,437 They are born, they mature and then they die. 726 00:52:41,638 --> 00:52:43,902 As time goes on, there are more white dwarfs... 727 00:52:44,107 --> 00:52:46,598 ...more neutron stars, more black holes. 728 00:52:46,810 --> 00:52:49,643 The remains of the stars accumulate... 729 00:52:49,847 --> 00:52:51,940 ...as the eons pass. 730 00:52:52,149 --> 00:52:55,915 But interstellar space also becomes enriched in heavy elements... 731 00:52:56,119 --> 00:53:00,078 ...out of which form new generations of stars and planets... 732 00:53:00,290 --> 00:53:02,349 ...life and intelligence. 733 00:53:02,559 --> 00:53:06,757 The events in one star can influence a world halfway across the galaxy... 734 00:53:06,964 --> 00:53:09,194 ...and a billion years in the future. 735 00:53:16,607 --> 00:53:20,134 The vast interstellar clouds of gas and dust... 736 00:53:20,344 --> 00:53:21,971 ...are stellar nurseries. 737 00:53:22,179 --> 00:53:26,240 Here first begins the inexorable gravitational collapse... 738 00:53:26,450 --> 00:53:29,510 ...which dominates the lives of the stars. 739 00:53:29,720 --> 00:53:34,123 Massive suns may evolve through the red giant stage in only millions of years. 740 00:53:34,324 --> 00:53:38,590 Dying young, never leaving the cloud in which they were born. 741 00:53:40,631 --> 00:53:45,193 Other suns, longer-lived, wander out of the nursery. 742 00:53:45,402 --> 00:53:47,529 Our sun is such a star... 743 00:53:47,738 --> 00:53:50,798 ...as are most of the stars in the sky. 744 00:54:01,985 --> 00:54:06,547 Most stars are members of double or multiple star systems... 745 00:54:06,757 --> 00:54:11,057 ...and live to process their nuclear fuel over billions of years. 746 00:54:11,261 --> 00:54:14,321 The galaxy is 10 billion years old. 747 00:54:14,531 --> 00:54:16,123 Old enough to have spawned... 748 00:54:16,333 --> 00:54:20,099 ...only a few generations of ordinary stars. 749 00:54:24,908 --> 00:54:28,241 The objects we encounter in a voyage through the Milky Way... 750 00:54:28,445 --> 00:54:32,575 ...are stages in the life cycle of the stars. 751 00:54:32,783 --> 00:54:35,013 Some are bright and new... 752 00:54:35,218 --> 00:54:39,211 ...and others are as ancient as the galaxy itself. 753 00:54:51,068 --> 00:54:54,595 Surrounding the Milky Way is a halo of matter... 754 00:54:54,805 --> 00:54:57,797 ...which includes the globular clusters... 755 00:54:58,008 --> 00:55:01,444 ...each containing up to a million elderly stars. 756 00:55:01,645 --> 00:55:06,014 At the centers of globular clusters and at the core of the galaxy... 757 00:55:06,216 --> 00:55:10,880 ...there may be massive black holes ticking and purring... 758 00:55:11,088 --> 00:55:13,613 ...the subject of future exploration. 759 00:55:31,508 --> 00:55:34,443 We on Earth marvel, and rightly so... 760 00:55:34,645 --> 00:55:37,671 ...at the daily return of our single sun. 761 00:55:37,881 --> 00:55:41,647 But from a planet orbiting a star in a distant globular cluster... 762 00:55:41,852 --> 00:55:44,685 ...a still more glorious dawn awaits. 763 00:55:44,888 --> 00:55:48,153 Not a sunrise, but a galaxy-rise. 764 00:55:48,358 --> 00:55:52,089 A morning filled with 400 billion suns... 765 00:55:52,295 --> 00:55:54,559 ...the rising of the Milky Way. 766 00:55:54,765 --> 00:55:58,667 An enormous spiral form with collapsing gas clouds... 767 00:55:58,869 --> 00:56:02,600 ...condensing planetary systems, luminous supergiants... 768 00:56:02,806 --> 00:56:04,774 ...stable middle-aged stars... 769 00:56:04,975 --> 00:56:09,344 ...red giants, white dwarfs, planetary nebulas, supernovas... 770 00:56:09,546 --> 00:56:13,983 ...neutron stars, pulsars, black holes and... 771 00:56:14,217 --> 00:56:17,186 ...there is every reason to think, other exotic objects... 772 00:56:17,421 --> 00:56:19,616 ...that we have not yet discovered. 773 00:56:22,626 --> 00:56:26,494 From such a world, high above the disc of the Milky Way... 774 00:56:26,697 --> 00:56:30,724 ...it would be clear as it is beginning to be clear on our world... 775 00:56:30,934 --> 00:56:33,994 ...that we are made by the atoms in the stars... 776 00:56:34,204 --> 00:56:38,265 ...that our matter and our form are determined... 777 00:56:38,475 --> 00:56:42,070 ...by the cosmos of which we are a part. 778 00:56:50,420 --> 00:56:54,686 I only have a moment, but I wanted you to see a picture of Betelgeuse... 779 00:56:54,891 --> 00:56:56,825 ...in the constellation Orion. 780 00:56:57,027 --> 00:57:00,121 The first image of the surface of another star. 781 00:57:00,397 --> 00:57:03,059 But the most exciting recent stellar discovery... 782 00:57:03,266 --> 00:57:05,097 ...has been of a nearby supernova... 783 00:57:05,302 --> 00:57:07,497 ...in a companion galaxy to the Milky Way. 784 00:57:07,704 --> 00:57:12,004 We are here seeing chemical elements in the process of synthesis... 785 00:57:12,209 --> 00:57:15,770 ...and have had our first glimpse of the supernova... 786 00:57:15,979 --> 00:57:19,142 ...through a brand-new field: neutrino astronomy. 787 00:57:19,349 --> 00:57:22,546 And we're now seeing, around neighboring stars... 788 00:57:23,120 --> 00:57:26,817 ...discs of gas and dust just like those needed to explain... 789 00:57:27,023 --> 00:57:29,753 ...the origin of the planets in our solar system. 790 00:57:29,960 --> 00:57:32,690 Worlds may be forming here. 791 00:57:32,896 --> 00:57:36,161 It's like a snapshot of our solar system's past. 792 00:57:36,600 --> 00:57:39,728 And there are so many such discs being found these days... 793 00:57:39,936 --> 00:57:44,464 ...that planets may be very common among the stars of the Milky Way.