1 00:02:46,387 --> 00:02:50,221 If you wish to make an apple pie from scratch... 2 00:02:50,791 --> 00:02:54,887 ...you must first invent the universe. 3 00:02:55,096 --> 00:02:56,688 Thank you very much. 4 00:02:57,865 --> 00:03:00,299 Suppose I cut a piece... 5 00:03:01,168 --> 00:03:03,102 ...out of this apple pie. 6 00:03:15,783 --> 00:03:18,251 Crumbly, but good. 7 00:03:21,589 --> 00:03:26,526 And now suppose we cut this piece in half, or more or less. 8 00:03:28,129 --> 00:03:30,996 And then cut this piece in half... 9 00:03:31,599 --> 00:03:33,760 ...and keep going. 10 00:03:33,968 --> 00:03:38,632 How many cuts before we get down to an individual atom? 11 00:03:38,839 --> 00:03:43,538 The answer is about 90 successive cuts. 12 00:03:44,645 --> 00:03:47,978 Of course, this knife isn't sharp enough... 13 00:03:48,182 --> 00:03:50,650 ...the pie is too crumbly... 14 00:03:50,851 --> 00:03:54,252 ...and an atom is too small to see in any case. 15 00:03:54,455 --> 00:03:57,891 But there is a way to do it. 16 00:04:00,528 --> 00:04:04,760 It was here at Cambridge University in England... 17 00:04:04,965 --> 00:04:08,264 ...that the nature of the atom was first understood... 18 00:04:08,469 --> 00:04:12,838 ...in part by shooting pieces of atoms at atoms... 19 00:04:13,040 --> 00:04:16,203 ...and seeing how they bounce off. 20 00:04:16,410 --> 00:04:19,937 A typical atom is surrounded... 21 00:04:20,147 --> 00:04:23,913 ...by a kind of cloud of electrons. 22 00:04:24,118 --> 00:04:28,054 The electrons are electrically charged, as the name suggests... 23 00:04:28,255 --> 00:04:31,622 ...and they determine the chemical properties of the atom. 24 00:04:31,826 --> 00:04:35,887 For example, the glitter of gold... 25 00:04:38,866 --> 00:04:40,800 ...or the transparency of the solid... 26 00:04:41,001 --> 00:04:44,027 ...that's made from the atoms silicon and oxygen. 27 00:04:45,306 --> 00:04:47,536 But deep inside the atom... 28 00:04:47,741 --> 00:04:51,802 ...hidden far beneath the outer electron cloud... 29 00:04:52,012 --> 00:04:56,039 ...is the nucleus, composed chiefly of protons and neutrons. 30 00:04:56,250 --> 00:04:58,616 Atoms are very small. 31 00:04:58,819 --> 00:05:03,381 100 million of them, end to end, would be about so big. 32 00:05:03,591 --> 00:05:08,494 And the nucleus is 100,000 times smaller still. 33 00:05:08,996 --> 00:05:12,727 Nevertheless, most of the mass in an atom is in the nucleus. 34 00:05:12,933 --> 00:05:15,993 The electrons are by comparison... 35 00:05:16,203 --> 00:05:19,172 ...just bits of moving fluff. 36 00:05:19,373 --> 00:05:22,536 Atoms are mainly empty space. 37 00:05:22,743 --> 00:05:26,804 Matter is composed chiefly of nothing. 38 00:05:29,717 --> 00:05:32,845 When we consider cutting this apple pie, but... 39 00:05:33,053 --> 00:05:36,489 ...down beyond a single atom... 40 00:05:36,757 --> 00:05:41,592 ...we confront an infinity of the very small. 41 00:05:41,795 --> 00:05:44,764 And when we look up at the night sky... 42 00:05:44,965 --> 00:05:48,958 ...we confront an infinity of the very large. 43 00:05:49,170 --> 00:05:53,869 These infinities are among the most awesome of human ideas. 44 00:05:54,074 --> 00:05:58,511 They represent an unending regress which goes on... 45 00:05:58,913 --> 00:06:03,043 ...not just very far, but forever. 46 00:06:03,250 --> 00:06:07,118 Have you ever stood between two parallel mirrors... 47 00:06:07,321 --> 00:06:08,720 ...in a barbershop, say... 48 00:06:08,923 --> 00:06:12,415 ...and seen a very large number of you? 49 00:06:12,626 --> 00:06:15,561 Or you could use... 50 00:06:16,797 --> 00:06:19,265 ...two flat mirrors... 51 00:06:19,700 --> 00:06:22,100 ...and a candle flame... 52 00:06:22,303 --> 00:06:25,602 ...you would see a large number of images... 53 00:06:25,806 --> 00:06:29,606 ...each the reflection of another image. 54 00:06:31,512 --> 00:06:34,743 You can't really see an infinity of images... 55 00:06:34,949 --> 00:06:38,282 ...because the mirrors aren't perfectly flat and aligned. 56 00:06:38,485 --> 00:06:41,886 And there's a candle or a candle flame in the way... 57 00:06:42,089 --> 00:06:44,819 ...and light doesn't travel infinitely fast. 58 00:06:45,025 --> 00:06:47,459 When we talk of real infinities... 59 00:06:47,661 --> 00:06:51,529 ...we're talking about a quantity larger than any number. 60 00:06:51,732 --> 00:06:56,192 No matter what number you have in mind, infinity is larger. 61 00:06:59,907 --> 00:07:04,207 There's a nice way to write large numbers. 62 00:07:04,411 --> 00:07:05,810 You can... 63 00:07:06,880 --> 00:07:09,314 ...write the number 1000... 64 00:07:09,516 --> 00:07:12,110 ...as 10 to the power three... 65 00:07:12,319 --> 00:07:16,255 ...meaning, a one followed by three zeros. 66 00:07:16,457 --> 00:07:21,394 Or a million is written as 10 to the power six... 67 00:07:21,729 --> 00:07:26,496 ...meaning, a one followed by six zeros. 68 00:07:27,234 --> 00:07:31,364 There's no largest number. If anybody gives you a candidate... 69 00:07:31,572 --> 00:07:33,563 ...you can always add the number one to it. 70 00:07:33,774 --> 00:07:36,709 But there certainly are very big numbers. 71 00:07:36,910 --> 00:07:41,609 The American mathematician Edward Kasner once asked his nephew... 72 00:07:41,815 --> 00:07:45,216 ...to invent a name for an extremely large number: 73 00:07:45,919 --> 00:07:49,013 10 to the power 100... 74 00:07:49,223 --> 00:07:54,160 ...which I can't write out all the zeros because there isn't room. 75 00:07:54,361 --> 00:07:59,128 The boy called it a googol. 76 00:08:00,834 --> 00:08:05,635 If you think a googol is large, consider a googolplex. 77 00:08:06,340 --> 00:08:08,900 It's 10 to the power of a googol. 78 00:08:09,109 --> 00:08:12,567 That is, a one followed, not by 100 zeros... 79 00:08:12,780 --> 00:08:15,943 ...but by a googol zeros. 80 00:08:17,017 --> 00:08:18,780 Now, by comparison... 81 00:08:18,986 --> 00:08:21,648 ...with these enormous numbers... 82 00:08:21,855 --> 00:08:25,052 ...the total number of atoms in that apple pie... 83 00:08:25,259 --> 00:08:28,228 ...is only about 10 to the 26th. 84 00:08:28,429 --> 00:08:31,296 Tiny compared to a googol and... 85 00:08:31,498 --> 00:08:35,059 ...of course, much, much less than a googolplex. 86 00:08:35,269 --> 00:08:36,861 The number of elementary particles... 87 00:08:37,071 --> 00:08:39,039 ...protons, neutrons and electrons... 88 00:08:39,239 --> 00:08:40,934 ...in the accessible universe... 89 00:08:41,141 --> 00:08:43,439 ...is of the order of 10 to the 80th. 90 00:08:43,644 --> 00:08:45,669 A one followed by 80 zeros. 91 00:08:45,879 --> 00:08:47,938 Still much, much less than a googol... 92 00:08:48,148 --> 00:08:51,743 ...and vastly less than a googolplex. 93 00:08:51,952 --> 00:08:56,719 And yet, these numbers, the googol and the googolplex... 94 00:08:56,924 --> 00:09:01,657 ...do not approach, they come nowhere near infinity. 95 00:09:01,862 --> 00:09:06,697 In fact, a googolplex is precisely as far from infinity... 96 00:09:06,900 --> 00:09:09,767 ...as is the number one. 97 00:09:09,970 --> 00:09:13,201 We started to write out a googolplex... 98 00:09:13,407 --> 00:09:15,204 ...but it wasn't easy. 99 00:09:17,044 --> 00:09:19,672 SAGAN: It's a very big number. 100 00:09:39,032 --> 00:09:43,332 Writing out a googolplex is a spectacularly futile exercise. 101 00:09:43,937 --> 00:09:47,964 A piece of paper large enough to contain the zeros in a googolplex... 102 00:09:48,175 --> 00:09:52,373 ...couldn't be stuffed into the known universe. 103 00:09:58,085 --> 00:09:59,052 Fortunately... 104 00:10:00,587 --> 00:10:02,054 ...there's a... 105 00:10:03,357 --> 00:10:05,689 ...much simpler and more concise way... 106 00:10:05,893 --> 00:10:07,986 ...to write a googolplex. 107 00:10:16,236 --> 00:10:17,635 Like this. 108 00:10:17,838 --> 00:10:19,829 And infinity... 109 00:10:23,010 --> 00:10:25,444 ...can be represented like this. 110 00:10:26,013 --> 00:10:29,915 This is the Cavendish Laboratory at Cambridge University... 111 00:10:30,117 --> 00:10:34,076 ...where the constituents of the atom were first discovered. 112 00:10:34,288 --> 00:10:38,247 The realm of the very small. 113 00:10:39,526 --> 00:10:43,553 From the time of Democritus, in the fifth century B.C... 114 00:10:43,764 --> 00:10:47,495 ...people have speculated about the existence of atoms. 115 00:10:47,701 --> 00:10:51,068 For the last few hundred years, there have been persuasive... 116 00:10:51,271 --> 00:10:54,832 ...but indirect arguments that all matter is made of atoms. 117 00:10:55,075 --> 00:10:59,671 But only in our time, have we actually been able to see them. 118 00:10:59,880 --> 00:11:04,476 Here the red blobs are the random throbbing motions... 119 00:11:04,952 --> 00:11:06,442 ...of uranium atoms... 120 00:11:06,653 --> 00:11:09,645 ...magnified 100 million times. 121 00:11:10,924 --> 00:11:15,452 How Democritus of Abdera would've enjoyed this movie. 122 00:11:20,467 --> 00:11:25,234 We pretty much take atoms for granted. 123 00:11:26,039 --> 00:11:29,031 And yet, there are so many different kinds... 124 00:11:29,243 --> 00:11:33,646 ...lovely and useful at the same time. 125 00:11:33,847 --> 00:11:34,973 Look. 126 00:12:02,276 --> 00:12:06,303 There are some 92 chemically distinct kinds of atoms... 127 00:12:06,780 --> 00:12:08,611 ...naturally found on Earth. 128 00:12:08,815 --> 00:12:12,581 They're called the chemical elements. 129 00:12:28,135 --> 00:12:30,660 Virtually everything we see and know... 130 00:12:30,871 --> 00:12:33,271 ...all the beauty of the natural world... 131 00:12:33,473 --> 00:12:36,965 ...is made of these few kinds of atoms... 132 00:12:37,177 --> 00:12:41,546 ...arranged in harmonious chemical patterns. 133 00:12:57,965 --> 00:13:01,662 Here we've represented all 92 of them. 134 00:13:01,868 --> 00:13:06,202 At room temperature, many of them are solids. 135 00:13:06,406 --> 00:13:08,101 A few are gases. 136 00:13:08,442 --> 00:13:09,773 And two of them... 137 00:13:11,078 --> 00:13:14,104 ...bromine and mercury, are liquids. 138 00:13:15,782 --> 00:13:19,548 They're arranged in order of complexity. 139 00:13:19,753 --> 00:13:23,587 Hydrogen, the simplest element, is element number 1. 140 00:13:23,790 --> 00:13:26,953 And uranium, the most complex... 141 00:13:27,394 --> 00:13:29,794 ...is element 92. 142 00:13:33,133 --> 00:13:35,966 Some elements are very familiar. 143 00:13:36,637 --> 00:13:37,865 For example... 144 00:13:38,071 --> 00:13:42,167 ...silicon, oxygen, magnesium, aluminum, iron... 145 00:13:42,376 --> 00:13:43,900 ...those that make up the Earth. 146 00:13:44,177 --> 00:13:48,671 Or hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur... 147 00:13:48,882 --> 00:13:51,373 ...the elements that are essential for life. 148 00:13:51,585 --> 00:13:55,544 Other elements are spectacularly unfamiliar. 149 00:13:55,756 --> 00:13:58,725 For example, hafnium. 150 00:13:59,326 --> 00:14:00,725 Erbium. 151 00:14:00,927 --> 00:14:02,827 Dysprosium. 152 00:14:03,430 --> 00:14:05,728 Praseodymium. 153 00:14:05,932 --> 00:14:08,833 Elements we don't bump into in everyday life. 154 00:14:09,636 --> 00:14:14,573 By and large, the more familiar an element is, the more abundant it is. 155 00:14:14,875 --> 00:14:17,207 There's a great deal of iron on the Earth. 156 00:14:17,411 --> 00:14:21,006 Not all that much yttrium. 157 00:14:21,214 --> 00:14:22,442 The fact... 158 00:14:22,649 --> 00:14:26,244 ...that atoms are composed of only three kinds of elementary particles... 159 00:14:26,453 --> 00:14:28,921 ...protons, neutrons and electrons... 160 00:14:29,122 --> 00:14:31,522 ...is a comparatively recent finding. 161 00:14:31,725 --> 00:14:34,193 The neutron was not discovered until 1932. 162 00:14:34,394 --> 00:14:38,854 And it, like the electron and the proton, were discovered here... 163 00:14:39,066 --> 00:14:41,034 ...at Cambridge University. 164 00:14:41,234 --> 00:14:45,330 Modern physics and chemistry have reduced the complexity... 165 00:14:45,539 --> 00:14:50,101 ...of the sensible world to an astonishing simplicity. 166 00:14:50,310 --> 00:14:54,542 Three units, put together in different patterns... 167 00:14:54,748 --> 00:14:58,684 ...make, essentially, everything. 168 00:15:04,124 --> 00:15:07,616 A neutron is electrically neutral... 169 00:15:07,828 --> 00:15:10,319 ...as its name suggests. 170 00:15:10,897 --> 00:15:14,731 A proton has a positive electrical charge... 171 00:15:15,302 --> 00:15:19,568 ...and an electron an equal, negative electrical charge. 172 00:15:19,773 --> 00:15:22,298 Since every atom is electrically neutral... 173 00:15:22,509 --> 00:15:24,739 ...the number of protons in the nucleus... 174 00:15:25,011 --> 00:15:29,072 ...must equal the number of electrons far away in the electron cloud. 175 00:15:29,282 --> 00:15:33,343 The protons and neutrons, together, make up the nucleus of the atom. 176 00:15:34,421 --> 00:15:39,358 Now, the chemistry of an atom, the nature of a chemical element... 177 00:15:39,559 --> 00:15:41,789 ...depends only on the number of electrons... 178 00:15:41,995 --> 00:15:45,590 ...which equals the number of protons, which is called the atomic number. 179 00:15:45,799 --> 00:15:48,097 Chemistry is just numbers. 180 00:15:48,301 --> 00:15:51,395 An idea which would have appealed to Pythagoras. 181 00:15:51,605 --> 00:15:53,004 If you're an atom... 182 00:15:53,206 --> 00:15:56,403 ...and you have just one proton... 183 00:15:56,610 --> 00:15:58,077 ...you're hydrogen. 184 00:15:58,278 --> 00:16:00,473 Two protons, helium. 185 00:16:00,680 --> 00:16:02,511 Three, lithium. 186 00:16:02,716 --> 00:16:05,480 Four, beryllium. Five protons, boron. 187 00:16:05,685 --> 00:16:10,247 Six, carbon, and seven, nitrogen. Eight, oxygen, and so on. 188 00:16:10,457 --> 00:16:13,187 All the way to 92 protons... 189 00:16:13,393 --> 00:16:16,385 ...in which case your name is uranium. 190 00:16:16,997 --> 00:16:20,125 Protons have positive electrical charges... 191 00:16:20,333 --> 00:16:23,530 ...but like charges repel each other. 192 00:16:23,737 --> 00:16:26,137 So why does the nucleus hold together? 193 00:16:26,339 --> 00:16:29,570 Why don't the electrical repulsion of the protons... 194 00:16:29,776 --> 00:16:31,710 ...make the nucleus fly to pieces? 195 00:16:32,312 --> 00:16:34,837 Because there's another force in nature. 196 00:16:35,048 --> 00:16:37,608 Not electricity, not gravity... 197 00:16:37,818 --> 00:16:39,251 ...the nuclear force. 198 00:16:39,452 --> 00:16:42,353 We can think of it as short-range... 199 00:16:42,556 --> 00:16:44,956 ...hooks which start working... 200 00:16:45,158 --> 00:16:48,685 ...when protons or neutrons are brought very close together. 201 00:16:48,895 --> 00:16:51,386 The nuclear force can overcome... 202 00:16:52,132 --> 00:16:54,692 ...the electrical repulsion of the protons. 203 00:16:54,901 --> 00:16:57,961 Since the neutrons exert nuclear forces... 204 00:16:58,471 --> 00:17:00,132 ...but not electrical forces... 205 00:17:00,340 --> 00:17:04,834 ...they are a kind of glue which holds the atomic nucleus together. 206 00:17:06,379 --> 00:17:10,907 A lump of two protons and two neutrons... 207 00:17:11,117 --> 00:17:13,017 ...is the nucleus of a helium atom... 208 00:17:13,220 --> 00:17:15,688 ...and is very stable. 209 00:17:15,889 --> 00:17:20,383 Three helium nuclei, stuck together by nuclear forces... 210 00:17:20,594 --> 00:17:22,425 ...makes carbon. 211 00:17:22,629 --> 00:17:26,030 Four helium nuclei makes oxygen. 212 00:17:26,233 --> 00:17:29,100 There's no difference between four helium nuclei... 213 00:17:29,336 --> 00:17:31,896 ...stuck together by nuclear forces and the oxygen nucleus. 214 00:17:32,105 --> 00:17:33,595 They're the same thing. 215 00:17:33,974 --> 00:17:37,273 Five helium nuclei makes neon. 216 00:17:37,477 --> 00:17:39,877 Six makes magnesium. 217 00:17:40,080 --> 00:17:43,743 Seven makes silicon. 218 00:17:43,950 --> 00:17:46,510 Eight makes sulfur, and so on. 219 00:17:46,720 --> 00:17:49,120 Increasing the atomic numbers by two... 220 00:17:49,322 --> 00:17:52,189 ...and always making some familiar element. 221 00:17:52,392 --> 00:17:53,324 Every time... 222 00:17:54,294 --> 00:17:56,319 ...we add or subtract one proton... 223 00:17:56,529 --> 00:17:59,225 ...and enough neutrons to keep the nucleus together... 224 00:17:59,432 --> 00:18:01,900 ...we make a new chemical element. 225 00:18:02,102 --> 00:18:04,866 Consider mercury: 226 00:18:05,071 --> 00:18:08,837 If we subtract one proton from mercury... 227 00:18:09,042 --> 00:18:13,240 ...and three neutrons, we convert it into gold. 228 00:18:13,446 --> 00:18:17,542 The dream of the ancient alchemists. 229 00:18:18,551 --> 00:18:22,419 Beyond element 92, beyond uranium... 230 00:18:22,622 --> 00:18:25,090 ...there are other elements. 231 00:18:25,292 --> 00:18:27,453 They don't occur naturally on the Earth. 232 00:18:27,661 --> 00:18:30,095 They're synthesized by human beings and... 233 00:18:30,297 --> 00:18:32,925 ...fall to pieces pretty rapidly. 234 00:18:33,133 --> 00:18:36,762 One of them, element 94, is called plutonium... 235 00:18:36,970 --> 00:18:40,701 ...and is one of the most toxic substances known. 236 00:18:40,907 --> 00:18:45,344 Where do the naturally occurring chemical elements come from? 237 00:18:45,545 --> 00:18:49,982 Perhaps a separate creation for each element? 238 00:18:50,183 --> 00:18:53,050 But all the elements are made of the same elementary particles. 239 00:18:53,253 --> 00:18:56,245 The universe, all of it, everywhere... 240 00:18:56,456 --> 00:18:59,857 ...is 99.9% hydrogen and helium. 241 00:19:00,093 --> 00:19:01,958 The two simplest elements. 242 00:19:02,162 --> 00:19:03,356 In fact, helium... 243 00:19:03,797 --> 00:19:07,699 ...was detected on the sun before it was ever found on the Earth. 244 00:19:07,901 --> 00:19:11,701 Might the other chemical elements have somehow... 245 00:19:11,905 --> 00:19:15,864 ...evolved from hydrogen and helium? 246 00:19:16,076 --> 00:19:18,442 To avoid the electrical repulsion... 247 00:19:19,145 --> 00:19:23,605 ...protons and neutrons must be brought very close together so the hooks... 248 00:19:23,817 --> 00:19:25,580 ...which represent nuclear forces... 249 00:19:25,785 --> 00:19:27,150 ...are engaged. 250 00:19:27,354 --> 00:19:30,482 This happens only at very high temperatures, where particles... 251 00:19:30,690 --> 00:19:35,354 ...move so fast that there's no time for electrical repulsion to act. 252 00:19:35,628 --> 00:19:40,565 Temperatures of tens of millions of degrees. 253 00:19:40,834 --> 00:19:44,235 Such high temperatures are common in nature. 254 00:19:44,437 --> 00:19:45,734 Where? 255 00:19:45,939 --> 00:19:48,874 In the insides of the stars. 256 00:20:04,290 --> 00:20:08,056 Atoms are made in the insides of stars. 257 00:20:08,261 --> 00:20:12,698 In most of the stars we see, hydrogen nuclei are being jammed together... 258 00:20:12,899 --> 00:20:14,457 ...to form helium nuclei. 259 00:20:14,868 --> 00:20:19,396 Every time a nucleus of helium is made, a photon of light is generated. 260 00:20:20,373 --> 00:20:23,831 This is why the stars shine. 261 00:20:31,351 --> 00:20:35,344 Stars are born in great clouds of gas and dust. 262 00:20:35,555 --> 00:20:39,286 Like the Orion Nebula, 1500 light-years away... 263 00:20:39,492 --> 00:20:43,485 ...parts of which are collapsing under gravity. 264 00:20:51,137 --> 00:20:55,540 Collisions among the atoms heat the cloud until, in its interior... 265 00:20:55,742 --> 00:20:58,404 ...hydrogen begins to fuse into helium... 266 00:20:58,611 --> 00:21:01,409 ...and the stars turn on. 267 00:21:07,520 --> 00:21:10,216 Stars are born in batches. 268 00:21:10,423 --> 00:21:12,823 Later, they wander out of their nursery... 269 00:21:13,026 --> 00:21:15,654 ...to pursue their destiny in the Milky Way. 270 00:21:15,862 --> 00:21:18,695 Adolescent stars, like the Pleiades... 271 00:21:18,898 --> 00:21:21,662 ...are still surrounded by gas and dust. 272 00:21:21,868 --> 00:21:26,328 Eventually, they journey far from home. 273 00:21:30,143 --> 00:21:35,080 Somewhere there are stars formed from the same cloud complex as the sun... 274 00:21:35,448 --> 00:21:37,609 ...5 billion years ago. 275 00:21:37,817 --> 00:21:40,377 But we do not know which stars they are. 276 00:21:40,587 --> 00:21:42,487 The siblings of the sun... 277 00:21:42,822 --> 00:21:47,555 ...may, for all we know, be on the other side of the galaxy. 278 00:21:49,162 --> 00:21:54,099 Perhaps they also warm nearby planets as the sun does. 279 00:21:56,703 --> 00:21:59,604 Perhaps they too have presided... 280 00:21:59,806 --> 00:22:03,708 ...over the evolution of life and intelligence. 281 00:22:25,265 --> 00:22:29,998 The sun is the nearest star, a glowing sphere of gas... 282 00:22:30,503 --> 00:22:34,530 ...shining because of its heat, like a red-hot poker. 283 00:22:38,878 --> 00:22:43,815 The surface we see in ordinary visible light is at 6000 degrees centigrade. 284 00:22:44,050 --> 00:22:45,847 But in its hidden interior... 285 00:22:46,586 --> 00:22:50,044 ...in the nuclear furnace where sunlight is ultimately generated... 286 00:22:50,256 --> 00:22:53,851 ...its temperature is 20 million degrees. 287 00:23:02,235 --> 00:23:03,224 In x-rays... 288 00:23:03,436 --> 00:23:07,167 ...we see a part of the sun that is ordinarily invisible... 289 00:23:07,373 --> 00:23:10,001 ...its million-degree halo of gas... 290 00:23:10,210 --> 00:23:12,303 ...the solar corona. 291 00:23:12,745 --> 00:23:15,839 In ordinary visible light, these cooler, darker regions... 292 00:23:16,049 --> 00:23:17,949 ...are the sunspots. 293 00:23:20,687 --> 00:23:25,147 They are associated with great surges of flaming gas... 294 00:23:25,358 --> 00:23:29,727 ...tongues of fire which would engulf the Earth if it were this close. 295 00:23:30,063 --> 00:23:33,829 These prominences are guided into paths determined... 296 00:23:34,033 --> 00:23:36,263 ...by the sun's magnetic field. 297 00:23:48,915 --> 00:23:51,145 The dark regions of the x-ray sun... 298 00:23:51,351 --> 00:23:54,047 ...are holes in the solar corona... 299 00:23:54,254 --> 00:23:58,247 ...through which stream the protons and electrons of the solar wind... 300 00:23:58,458 --> 00:24:02,656 ...on their way past the planets to interstellar space. 301 00:24:05,164 --> 00:24:09,863 All this churning power is driven by the sun's interior... 302 00:24:10,069 --> 00:24:13,971 ...which is converting 400 million tons of hydrogen into helium... 303 00:24:14,173 --> 00:24:15,697 ...every second. 304 00:24:15,908 --> 00:24:19,503 The sun is a great fusion reactor... 305 00:24:19,746 --> 00:24:22,214 ...into which a million Earths would fit. 306 00:24:22,415 --> 00:24:25,543 Luckily for us, it's safely placed... 307 00:24:25,752 --> 00:24:29,279 ...150 million kilometers away. 308 00:24:45,805 --> 00:24:49,707 It is the destiny of stars to collapse. 309 00:24:50,810 --> 00:24:54,211 Of the thousands of stars you see when you look up at the night sky... 310 00:24:54,414 --> 00:24:59,044 ...every one of them is living in an interval between two collapses. 311 00:24:59,252 --> 00:25:00,742 An initial collapse of... 312 00:25:00,953 --> 00:25:04,445 ...a dark interstellar gas cloud to form the star... 313 00:25:04,657 --> 00:25:07,125 ...and a final collapse of the luminous star... 314 00:25:07,327 --> 00:25:09,090 ...on the way to its ultimate fate. 315 00:25:09,562 --> 00:25:14,499 Gravity makes stars contract unless some other force intervenes. 316 00:25:14,901 --> 00:25:17,802 The sun is an immense ball of radiating hydrogen. 317 00:25:18,004 --> 00:25:22,304 The hot gas in its interior tries to make the sun expand. 318 00:25:22,508 --> 00:25:25,909 The gravity tries to make the sun contract. 319 00:25:26,145 --> 00:25:29,376 The present state of the sun is the balance of these two forces... 320 00:25:29,582 --> 00:25:34,144 ...an equilibrium between gravity and nuclear fire. 321 00:25:34,821 --> 00:25:37,483 In this long middle age between collapses... 322 00:25:37,690 --> 00:25:40,386 ...the stars steadily shine. 323 00:25:40,593 --> 00:25:44,427 But when the nuclear fuel is exhausted, the interior cools... 324 00:25:44,630 --> 00:25:47,428 ...the pressure no longer supports its outer layers... 325 00:25:47,633 --> 00:25:49,794 ...and the initial collapse resumes. 326 00:25:50,603 --> 00:25:52,901 There are three ways that stars die. 327 00:25:53,106 --> 00:25:55,540 Their fates are predestined. 328 00:25:55,742 --> 00:25:57,676 Everything depends on their initial mass. 329 00:25:57,877 --> 00:26:00,471 A typical star with a mass like the sun... 330 00:26:00,680 --> 00:26:03,274 ...will one day continue its collapse... 331 00:26:03,483 --> 00:26:06,384 ...until its density becomes very high. 332 00:26:06,586 --> 00:26:08,315 And then the contraction is stopped... 333 00:26:08,521 --> 00:26:10,386 ...by the mutual repulsion of... 334 00:26:10,590 --> 00:26:13,855 ...the overcrowded electrons in its interior. 335 00:26:14,060 --> 00:26:16,824 A collapsing star twice as massive as the sun... 336 00:26:17,029 --> 00:26:19,554 ...isn't stopped by the electron pressure. 337 00:26:19,766 --> 00:26:22,132 It goes on falling in on itself... 338 00:26:22,335 --> 00:26:25,031 ...until nuclear forces come into play... 339 00:26:25,238 --> 00:26:28,332 ...and they hold up the weight of the star. 340 00:26:28,541 --> 00:26:31,704 But a collapsing star three times as massive as the sun isn't... 341 00:26:31,911 --> 00:26:33,970 ...stopped even by nuclear forces. 342 00:26:34,180 --> 00:26:39,117 There's no force known that can withstand this enormous compression. 343 00:26:39,352 --> 00:26:42,412 And such a star has an astonishing destiny. 344 00:26:42,622 --> 00:26:44,351 It continues to collapse... 345 00:26:44,624 --> 00:26:47,354 ...until it vanishes utterly. 346 00:26:48,594 --> 00:26:53,054 Each star is described by the force that holds it up against gravity. 347 00:26:53,266 --> 00:26:56,702 A star that's supported by the gas pressure... 348 00:26:56,903 --> 00:27:00,600 ...is a normal, run-of-the-mill star like the sun. 349 00:27:00,807 --> 00:27:04,504 A collapsed star that's held up by electron forces... 350 00:27:04,710 --> 00:27:06,337 ...is called a white dwarf. 351 00:27:06,546 --> 00:27:10,380 It's a sun shrunk to the size of the Earth. 352 00:27:10,583 --> 00:27:13,950 A collapsed star supported by nuclear forces... 353 00:27:14,153 --> 00:27:16,018 ...is called a neutron star. 354 00:27:16,222 --> 00:27:19,749 It's a sun shrunk to the size of a city. 355 00:27:19,959 --> 00:27:22,894 And a star so massive that in its final collapse... 356 00:27:23,095 --> 00:27:24,790 ...it disappears altogether... 357 00:27:24,997 --> 00:27:26,658 ...is called a black hole. 358 00:27:26,866 --> 00:27:29,858 It's a sun with no size at all. 359 00:27:30,536 --> 00:27:33,232 But on their ways to their separate fates... 360 00:27:33,439 --> 00:27:38,103 ...all stars experience a premonition of death. 361 00:27:38,311 --> 00:27:40,779 Before the final gravitational collapse... 362 00:27:40,980 --> 00:27:45,815 ...the star shudders and briefly swells into some... 363 00:27:46,285 --> 00:27:48,719 ...grotesque parody of itself. 364 00:27:48,921 --> 00:27:53,290 With its last gasp, it becomes a red giant. 365 00:27:57,029 --> 00:27:59,520 Some 5 billion years from now... 366 00:27:59,732 --> 00:28:04,135 ...there will be a last, perfect day on Earth. 367 00:28:07,139 --> 00:28:10,666 Then, the sun will slowly change... 368 00:28:10,877 --> 00:28:13,903 ...and the Earth will die. 369 00:28:17,049 --> 00:28:19,950 There is only so much hydrogen fuel in the sun. 370 00:28:20,152 --> 00:28:22,552 When it's almost all converted to helium... 371 00:28:22,755 --> 00:28:26,418 ...the solar interior will continue its original collapse. 372 00:28:26,626 --> 00:28:31,086 Higher temperatures in its core will make the outside of the sun expand... 373 00:28:31,330 --> 00:28:34,788 ...and the Earth will become slowly warmer. 374 00:28:35,034 --> 00:28:38,299 Eventually, life will be extinguished... 375 00:28:38,671 --> 00:28:41,606 ...the oceans will evaporate and boil... 376 00:28:41,807 --> 00:28:45,834 ...and our atmosphere will gush away to space. 377 00:28:48,648 --> 00:28:52,448 The sun will become a bloated red giant star... 378 00:28:52,652 --> 00:28:54,176 ...filling the sky... 379 00:28:54,420 --> 00:28:58,584 ...enveloping and devouring the planets Mercury and Venus. 380 00:28:58,791 --> 00:29:01,851 And probably the Earth as well. 381 00:29:02,061 --> 00:29:06,998 The inner solar system will reside inside the sun. 382 00:29:10,236 --> 00:29:13,103 But perhaps by then, our descendants... 383 00:29:13,306 --> 00:29:15,866 ...will have ventured somewhere else. 384 00:29:19,979 --> 00:29:24,382 In its final agonies, the sun will slowly pulsate. 385 00:29:24,584 --> 00:29:27,485 By then, its core will have become so hot... 386 00:29:27,687 --> 00:29:31,123 ...that it temporarily converts helium into carbon. 387 00:29:31,324 --> 00:29:35,226 The ash from today's nuclear fusion will become the fuel... 388 00:29:35,528 --> 00:29:40,397 ...to power the sun near the end of its life in its red giant stage. 389 00:29:42,201 --> 00:29:45,034 Then the sun will lose great shells... 390 00:29:45,571 --> 00:29:47,766 ...of its outer atmosphere to space... 391 00:29:47,974 --> 00:29:51,375 ...filling the solar system with eerily glowing gas. 392 00:29:51,577 --> 00:29:54,637 The ghost of a star, outward bound. 393 00:29:54,847 --> 00:29:58,339 Perhaps half the mass of the sun will be lost in this way. 394 00:30:00,286 --> 00:30:03,585 Viewed from elsewhere, our system will then resemble... 395 00:30:03,789 --> 00:30:05,916 ...the Ring Nebula in Lyra... 396 00:30:06,125 --> 00:30:10,755 ...the atmosphere of the sun expanding outward like a soap bubble. 397 00:30:11,631 --> 00:30:14,862 And at the very center will be a white dwarf. 398 00:30:15,101 --> 00:30:17,569 The hot exposed core of the sun... 399 00:30:17,770 --> 00:30:21,331 ...its nuclear fuel now exhausted, slowly cooling... 400 00:30:21,574 --> 00:30:25,010 ...to become a cold, dead star. 401 00:30:28,014 --> 00:30:30,642 Such is the life of an ordinary star. 402 00:30:30,850 --> 00:30:34,047 Born in a gas cloud, maturing as a yellow sun... 403 00:30:34,253 --> 00:30:36,084 ...decaying as a red giant... 404 00:30:36,288 --> 00:30:41,225 ...and dying as a white dwarf enveloped in its shroud of gas. 405 00:30:50,269 --> 00:30:53,363 Suppose, as we traveled through interstellar space... 406 00:30:53,572 --> 00:30:55,403 ...in our ship of the imagination... 407 00:30:55,608 --> 00:30:59,977 ...we could sample the cold, thin gas between the stars. 408 00:31:00,179 --> 00:31:03,171 We would find a great preponderance of hydrogen... 409 00:31:03,382 --> 00:31:05,942 ...an element as old as the universe. 410 00:31:06,152 --> 00:31:08,848 We would find carbon, oxygen, silicon. 411 00:31:09,055 --> 00:31:12,149 The most abundant atoms in the cosmos, apart from hydrogen... 412 00:31:12,358 --> 00:31:15,794 ...are those most easily made in the stars. 413 00:31:15,995 --> 00:31:19,396 But we would also find a small proportion of rare elements. 414 00:31:19,598 --> 00:31:22,396 Praseodymium, say, or gold. 415 00:31:22,601 --> 00:31:24,762 They're not made in red giants. 416 00:31:24,970 --> 00:31:29,600 Such elements are manufactured in one of the most dramatic gestures... 417 00:31:29,842 --> 00:31:32,333 ...of which a star is capable. 418 00:31:33,479 --> 00:31:36,744 A star more than about one and a half times the mass of the sun... 419 00:31:37,149 --> 00:31:38,776 ...cannot become a white dwarf. 420 00:31:38,984 --> 00:31:41,953 It will end its life by blowing itself up... 421 00:31:42,154 --> 00:31:46,614 ...in a titanic stellar explosion called a supernova. 422 00:31:48,160 --> 00:31:51,823 There has been no supernova explosion in our province of the galaxy... 423 00:31:52,198 --> 00:31:53,927 ...since the telescope's invention... 424 00:31:54,133 --> 00:31:57,034 ...and our sun will not become a supernova. 425 00:31:57,236 --> 00:31:58,863 But in our imagination... 426 00:31:59,405 --> 00:32:02,704 ...we can fulfill the dream of many earthbound astronomers... 427 00:32:03,142 --> 00:32:07,875 ...and safely witness, close-up, a supernova explosion. 428 00:32:12,251 --> 00:32:16,153 Most of stellar evolution takes millions or billions of years. 429 00:32:16,388 --> 00:32:19,915 But the interior collapse that triggers a supernova explosion... 430 00:32:20,159 --> 00:32:21,592 ...takes only seconds. 431 00:32:21,961 --> 00:32:25,954 The star becomes brighter than all the other stars in the galaxy... 432 00:32:26,165 --> 00:32:27,564 ...put together. 433 00:33:03,269 --> 00:33:05,533 If a nearby star became a supernova... 434 00:33:05,738 --> 00:33:10,471 ...it would be calamity enough for the inhabitants of this alien system. 435 00:33:10,676 --> 00:33:13,076 But if their own sun went supernova... 436 00:33:13,279 --> 00:33:16,612 ...it would be an unprecedented catastrophe. 437 00:33:16,816 --> 00:33:19,250 Worlds would be charred and vaporized. 438 00:33:19,451 --> 00:33:23,911 Life, even on the outer planets, would be extinguished. 439 00:33:24,123 --> 00:33:28,321 In our ship of the imagination, we are now backing away from the star. 440 00:33:28,527 --> 00:33:30,188 But the explosion fragments... 441 00:33:30,396 --> 00:33:33,888 ...traveling almost at the speed of light, are overtaking us. 442 00:33:34,099 --> 00:33:38,695 Individual atomic nuclei, accelerated to high speeds in the explosion... 443 00:33:38,904 --> 00:33:40,838 ...become cosmic rays. 444 00:33:41,040 --> 00:33:45,306 This is another way that stars return the atoms they've synthesized... 445 00:33:45,511 --> 00:33:47,376 ...back into space. 446 00:33:48,147 --> 00:33:50,206 The shock wave of expanding gases... 447 00:33:50,416 --> 00:33:52,941 ...heats and compresses the interstellar gas... 448 00:33:53,152 --> 00:33:56,144 ...triggering a later generation of stars to form. 449 00:33:56,355 --> 00:33:57,686 In this sense also... 450 00:33:57,890 --> 00:34:02,293 ...stars are phoenixes rising from their own ashes. 451 00:34:07,132 --> 00:34:10,966 The cosmos was originally all hydrogen and helium. 452 00:34:11,203 --> 00:34:14,570 Heavier elements were made in red giants and in supernovas... 453 00:34:14,773 --> 00:34:16,764 ...and then blown off to space... 454 00:34:16,976 --> 00:34:19,706 ...where they were available for subsequent generations... 455 00:34:19,912 --> 00:34:21,539 ...of stars and planets. 456 00:34:21,747 --> 00:34:25,513 Our sun is probably a third-generation star. 457 00:34:25,718 --> 00:34:27,345 Except for hydrogen and helium... 458 00:34:27,553 --> 00:34:32,490 ...every atom in the sun and the Earth was synthesized in other stars. 459 00:34:32,758 --> 00:34:37,286 The silicon in the rocks, the oxygen in the air, the carbon in our DNA... 460 00:34:37,496 --> 00:34:41,262 ...the gold in our banks, the uranium in our arsenals... 461 00:34:41,467 --> 00:34:45,062 ...were all made thousands of light-years away... 462 00:34:45,271 --> 00:34:47,136 ...and billions of years ago. 463 00:34:47,339 --> 00:34:51,673 Our planet, our society and we ourselves... 464 00:34:51,877 --> 00:34:55,142 ...are built of star stuff. 465 00:35:00,152 --> 00:35:03,121 We're in a lava tube. 466 00:35:03,889 --> 00:35:08,223 A cave carved through the Earth... 467 00:35:08,427 --> 00:35:10,987 ...by a river of molten rock. 468 00:35:11,997 --> 00:35:14,056 To do a little experiment... 469 00:35:15,067 --> 00:35:18,468 ...we've brought a Geiger counter... 470 00:35:21,507 --> 00:35:25,102 ...and a piece of uranium ore. 471 00:35:25,844 --> 00:35:30,781 The Geiger counter is sensitive to high-energy charged particles... 472 00:35:31,150 --> 00:35:34,813 ...protons, helium nuclei, gamma rays. 473 00:35:35,087 --> 00:35:38,488 If we bring it close to the uranium ore... 474 00:35:38,691 --> 00:35:43,253 ...the count rate, the number of clicks, increases dramatically. 475 00:35:45,931 --> 00:35:48,229 We also have a lead canister here. 476 00:35:48,801 --> 00:35:51,361 And if I drop the uranium ore... 477 00:35:51,904 --> 00:35:56,603 ...into the canister, which absorbs the radiation, and cover it up... 478 00:35:58,978 --> 00:36:02,175 ...I then find the count-rate goes down substantially... 479 00:36:02,381 --> 00:36:04,076 ...but it doesn't go down to zero. 480 00:36:05,284 --> 00:36:08,378 What's the source of the remaining counts? 481 00:36:09,488 --> 00:36:13,948 Some of them come from radioactivity in the walls of the cave. 482 00:36:14,159 --> 00:36:16,354 But there's more to it than that. 483 00:36:16,562 --> 00:36:19,998 Some of the counts are due to high-energy charged particles... 484 00:36:20,199 --> 00:36:24,260 ...which are penetrating the roof of the cave. 485 00:36:24,470 --> 00:36:27,598 We are listening to cosmic rays. 486 00:36:28,774 --> 00:36:33,234 Every second they are penetrating my body... 487 00:36:33,445 --> 00:36:34,707 ...and yours. 488 00:36:34,913 --> 00:36:38,178 They don't do much damage. Cosmic rays have bombarded the Earth... 489 00:36:38,417 --> 00:36:41,011 ...for the entire history of life on our planet. 490 00:36:41,220 --> 00:36:43,814 But they do cause some mutations... 491 00:36:44,023 --> 00:36:46,856 ...and they do affect life on the Earth. 492 00:36:48,994 --> 00:36:51,986 The cosmic rays, mainly protons... 493 00:36:52,197 --> 00:36:57,134 ...are penetrating through the meters of rock in the cave above me. 494 00:36:57,336 --> 00:37:00,032 To do this, they have to be very energetic and in fact... 495 00:37:00,406 --> 00:37:03,466 ...they are traveling almost at the speed of light. 496 00:37:04,276 --> 00:37:05,675 Think of it. 497 00:37:06,145 --> 00:37:08,613 A star blows up... 498 00:37:08,947 --> 00:37:11,438 ...thousands of light-years away in space... 499 00:37:11,650 --> 00:37:14,448 ...and produces cosmic rays which... 500 00:37:14,653 --> 00:37:17,918 ...spiral through the Milky Way galaxy for... 501 00:37:18,123 --> 00:37:21,149 ...millions of years until, quite by accident... 502 00:37:21,360 --> 00:37:24,124 ...some of them strike the Earth... 503 00:37:24,329 --> 00:37:28,231 ...penetrate this cave, reach this Geiger counter... 504 00:37:28,500 --> 00:37:29,865 ...and us. 505 00:37:30,235 --> 00:37:35,104 The evolution of life on Earth is driven in part through mutations... 506 00:37:35,307 --> 00:37:38,401 ...by the deaths of distant stars. 507 00:37:38,610 --> 00:37:41,408 We are, in a very deep sense... 508 00:37:41,647 --> 00:37:44,377 ...tied to the cosmos. 509 00:37:47,052 --> 00:37:49,987 Our ancestors knew this well. 510 00:37:50,189 --> 00:37:52,851 The movements of the sun, the moon, and the stars... 511 00:37:53,058 --> 00:37:57,654 ...could be used by those skilled in such arts to foretell the seasons. 512 00:37:57,863 --> 00:38:00,798 So the ancient astronomers all over the world... 513 00:38:00,999 --> 00:38:03,092 ...studied the night sky with care... 514 00:38:03,302 --> 00:38:07,363 ...memorizing and recording the position of every visible star. 515 00:38:07,573 --> 00:38:11,976 To them, the appearance of any new star would have been significant. 516 00:38:12,177 --> 00:38:16,614 What would they have made of the apparition of a supernova... 517 00:38:16,815 --> 00:38:20,307 ...brighter than every other star in the sky? 518 00:38:26,692 --> 00:38:31,356 On July 4th, in the year 1054... 519 00:38:31,563 --> 00:38:36,500 ...Chinese astronomers recorded what they called a guest star... 520 00:38:36,702 --> 00:38:38,795 ...in the constellation of Taurus the Bull. 521 00:38:39,004 --> 00:38:43,134 A star never before seen burst into radiance... 522 00:38:43,342 --> 00:38:46,470 ...became almost as bright as the full moon. 523 00:38:46,678 --> 00:38:51,115 Halfway around the world, here in the American Southwest... 524 00:38:51,316 --> 00:38:56,015 ...there was then a high culture, rich in astronomical tradition. 525 00:38:56,221 --> 00:39:00,453 They too must have seen this brilliant new star. 526 00:39:00,659 --> 00:39:04,026 From carbon-14 dating... 527 00:39:04,263 --> 00:39:08,723 ...of the remains of a charcoal fire, we know that in this very spot... 528 00:39:08,934 --> 00:39:12,563 ...there were people living in the 11th century. 529 00:39:13,305 --> 00:39:18,242 The people were the Anasazi, the antecedents of the Hopi of today. 530 00:39:18,744 --> 00:39:21,645 And one of them seems to have drawn... 531 00:39:21,914 --> 00:39:25,247 ...on this overhang, protected from the weather... 532 00:39:25,450 --> 00:39:27,441 ...a picture of the new star. 533 00:39:27,653 --> 00:39:32,090 Its position near the crescent moon would have been just what we see here. 534 00:39:32,291 --> 00:39:35,192 And the handprint is, perhaps... 535 00:39:35,394 --> 00:39:38,124 ...the artist's signature. 536 00:39:38,830 --> 00:39:43,290 This remarkable star is now called the Crab Supernova. 537 00:39:43,502 --> 00:39:46,596 "Nova" from the Latin word for new and "Crab" because... 538 00:39:46,805 --> 00:39:50,002 ...that's what an astronomer centuries later was reminded of... 539 00:39:50,209 --> 00:39:54,009 ...when looking at this explosion or remnant through the telescope. 540 00:39:54,213 --> 00:39:58,206 The Crab is a star that blew itself up. 541 00:39:58,417 --> 00:40:01,181 The explosion was seen for three months. 542 00:40:01,386 --> 00:40:05,254 It was easily visible in broad daylight. 543 00:40:05,457 --> 00:40:08,324 And you could read by it at night. 544 00:40:10,462 --> 00:40:12,589 Imagine the night when that... 545 00:40:12,798 --> 00:40:15,426 ...colossal stellar explosion... 546 00:40:15,634 --> 00:40:18,228 ...first burst forth. 547 00:40:27,412 --> 00:40:28,811 A thousand years ago... 548 00:40:29,047 --> 00:40:32,574 ...people gazed up in amazement at the brilliant new star... 549 00:40:32,784 --> 00:40:35,218 ...and wondered what it was. 550 00:40:39,258 --> 00:40:42,853 We are the first generation privileged to know the answer. 551 00:40:43,061 --> 00:40:46,326 Through the telescope we have seen what lies today... 552 00:40:46,531 --> 00:40:49,762 ...at the spot in the sky noted by the ancient astronomers. 553 00:40:49,968 --> 00:40:53,563 A great luminous cloud, the remains of a star... 554 00:40:53,772 --> 00:40:58,709 ...violently unraveling itself back into interstellar space. 555 00:41:04,049 --> 00:41:07,382 Only the massive red giants become supernovas. 556 00:41:07,586 --> 00:41:10,680 But every supernova was once a red giant. 557 00:41:10,889 --> 00:41:12,151 In the history of the galaxy... 558 00:41:12,357 --> 00:41:16,453 ...hundreds of millions of red giants have become supernovas. 559 00:41:19,131 --> 00:41:22,794 The bit of the star that isn't blown away collapses under gravity... 560 00:41:23,001 --> 00:41:26,266 ...spinning ever faster like a pirouetting ice skater... 561 00:41:26,471 --> 00:41:27,802 ...bringing in her arms. 562 00:41:28,006 --> 00:41:31,407 The star becomes a single, massive atomic nucleus... 563 00:41:31,610 --> 00:41:33,271 ...a neutron star. 564 00:41:33,478 --> 00:41:36,709 The one in the Crab Nebula is spinning 30 times a second. 565 00:41:36,915 --> 00:41:38,974 It emits a beamed pattern of light... 566 00:41:39,184 --> 00:41:43,018 ...and seems to us to be blinking on and off with astonishing regularity. 567 00:41:43,221 --> 00:41:46,952 Such neutron stars are called pulsars. 568 00:41:48,593 --> 00:41:50,618 Neutron star matter... 569 00:41:50,829 --> 00:41:54,731 ...weighs about a mountain per teaspoonful. 570 00:41:54,933 --> 00:41:58,369 So much that if I had a piece of it here and let it go... 571 00:41:58,570 --> 00:42:00,936 ...I could hardly prevent it from falling. 572 00:42:01,139 --> 00:42:04,074 It would effortlessly pass through the Earth like a... 573 00:42:04,276 --> 00:42:06,642 ...a knife through warm butter. 574 00:42:06,845 --> 00:42:10,246 It would carve a hole for itself completely through the Earth... 575 00:42:10,449 --> 00:42:14,476 ...emerging out the other side perhaps in China. 576 00:42:14,686 --> 00:42:18,315 The people there might be walking along when a... 577 00:42:18,523 --> 00:42:23,324 ...tiny lump of neutron star matter comes booming out of the ground... 578 00:42:23,528 --> 00:42:25,155 ...and then falls back again. 579 00:42:25,364 --> 00:42:26,797 The incident might... 580 00:42:27,032 --> 00:42:30,695 ...make an agreeable break in the routine of the day. 581 00:42:30,902 --> 00:42:34,303 The neutron star matter, pulled back by the Earth's gravity... 582 00:42:34,506 --> 00:42:36,974 ...would plunge again through the Earth... 583 00:42:37,175 --> 00:42:41,578 ...eventually punching hundreds of thousands of holes... 584 00:42:41,780 --> 00:42:46,581 ...before friction with the interior of our planet stopped the motion. 585 00:42:46,785 --> 00:42:48,946 By the time it's at rest at the center of the Earth... 586 00:42:49,187 --> 00:42:53,886 ...the inside of our world would look a little bit like Swiss cheese. 587 00:43:09,040 --> 00:43:13,739 There are places in the galaxy where a neutron star and a red giant... 588 00:43:14,045 --> 00:43:17,981 ...are locked in a mutual gravitational embrace. 589 00:43:18,183 --> 00:43:20,549 Tendrils of red giant star stuff... 590 00:43:20,752 --> 00:43:23,585 ...spiral into a disc of accreting matter... 591 00:43:23,789 --> 00:43:27,190 ...centered on the hot neutron star. 592 00:43:37,169 --> 00:43:40,297 Every star exists in a state of tension... 593 00:43:40,505 --> 00:43:42,530 ...between the force that holds it up... 594 00:43:42,741 --> 00:43:45,209 ...and gravity, the force that would pull it down. 595 00:43:45,410 --> 00:43:49,744 If gravity were to prevail, a stellar madness would ensue... 596 00:43:49,948 --> 00:43:54,044 ...more bizarre than anything in wonderland. 597 00:43:55,654 --> 00:43:58,555 Alice and her colleagues feel, more or less... 598 00:43:58,757 --> 00:44:01,317 ...at home in the gravitational pull of the Earth... 599 00:44:01,526 --> 00:44:04,324 ...called one g, "g" for Earth gravity. 600 00:44:04,729 --> 00:44:08,187 What would happen if we made the gravity less, or more? 601 00:44:08,400 --> 00:44:10,459 At lower gravity, things get lighter. 602 00:44:10,669 --> 00:44:13,433 Near zero g, the slightest motion sends our friends... 603 00:44:13,638 --> 00:44:15,629 ...floating and tumbling in the air. 604 00:44:15,841 --> 00:44:18,639 Little blobs of liquid tea are everywhere. 605 00:44:19,077 --> 00:44:20,601 Curious. 606 00:44:21,146 --> 00:44:23,614 If we now return the gravity to one g... 607 00:44:24,149 --> 00:44:28,210 ...it's raining tea, and our friends fall back to Earth. 608 00:44:29,588 --> 00:44:33,080 I've been to a couple of parties like that myself. 609 00:44:35,093 --> 00:44:38,551 At higher gravities, two or three g's, say, things get... 610 00:44:38,763 --> 00:44:41,391 ...really laid back. 611 00:44:41,600 --> 00:44:45,593 Everyone feels heavy and leaden. 612 00:44:48,273 --> 00:44:51,709 Except by special dispensation... 613 00:44:52,010 --> 00:44:53,773 ...the Cheshire cat. 614 00:44:54,579 --> 00:44:57,571 As a kindness, we remove them. 615 00:44:57,782 --> 00:45:01,274 At thousands of g's, trees become squashed. 616 00:45:01,486 --> 00:45:05,923 At 100,000 g's, rocks become crushed by their own weight. 617 00:45:06,124 --> 00:45:09,753 At all these gravities, a beam of light remains unaffected... 618 00:45:09,961 --> 00:45:11,929 ...continuing up in a straight line. 619 00:45:12,130 --> 00:45:13,825 But at billions of g's... 620 00:45:14,032 --> 00:45:18,560 ...a beam of light feels the gravity and begins to bend back on itself. 621 00:45:18,770 --> 00:45:22,228 Curiouser and curiouser. 622 00:45:22,440 --> 00:45:26,501 Such a place, where the gravity is so large that even light can't get out... 623 00:45:26,711 --> 00:45:29,202 ...is called a black hole. 624 00:45:29,414 --> 00:45:32,679 It's a star in which light itself is imprisoned. 625 00:45:32,884 --> 00:45:35,182 Black holes were theoretical constructs... 626 00:45:35,387 --> 00:45:38,686 ...speculated about since 1783. 627 00:45:38,890 --> 00:45:42,485 But in our time, we've verified the invisible. 628 00:45:42,694 --> 00:45:46,323 This bright star has a massive, unseen companion. 629 00:45:46,531 --> 00:45:51,366 Satellite observatories find the companion to be an x-ray source... 630 00:45:51,570 --> 00:45:54,095 ...called Cygnus X-1. 631 00:45:54,306 --> 00:45:56,866 These x-rays are like the footprints... 632 00:45:57,075 --> 00:46:01,808 ...of an invisible man walking in the snow. 633 00:46:07,118 --> 00:46:10,178 The x-rays are thought to be generated by friction... 634 00:46:10,388 --> 00:46:13,880 ...in the accretion disc surrounding the black hole. 635 00:46:14,092 --> 00:46:17,289 The matter in the disc slowly disappears... 636 00:46:17,495 --> 00:46:19,463 ...down the black hole. 637 00:46:21,967 --> 00:46:26,063 Massive black holes, produced by the collapse of a billion suns... 638 00:46:26,271 --> 00:46:28,865 ...may be sitting at the centers of other galaxies... 639 00:46:29,074 --> 00:46:33,067 ...curiously producing great jets of radiation... 640 00:46:33,278 --> 00:46:35,371 ...pouring out into space. 641 00:46:38,283 --> 00:46:41,980 At high enough density, the star winks out... 642 00:46:42,187 --> 00:46:46,055 ...and vanishes from our universe leaving only its gravity behind. 643 00:46:46,257 --> 00:46:51,092 It slips through a self-generated crack in the space-time continuum. 644 00:46:51,296 --> 00:46:55,596 A black hole is a place where a star once was. 645 00:46:56,301 --> 00:46:59,168 Here we have a flat two- dimensional surface... 646 00:46:59,371 --> 00:47:04,138 ...with grid lines on it, something like a piece of graph paper. 647 00:47:05,043 --> 00:47:07,204 Suppose we take a small mass... 648 00:47:07,412 --> 00:47:12,145 ...drop it on the surface and watch how the surface distorts... 649 00:47:12,350 --> 00:47:16,218 ...or puckers into the third physical dimension. 650 00:47:20,091 --> 00:47:24,528 Gravity can be understood as a curvature of space. 651 00:47:26,264 --> 00:47:29,290 If our moving ball approaches a stationary distortion... 652 00:47:29,501 --> 00:47:32,868 ...it rolls around it like a planet orbiting the sun. 653 00:47:33,071 --> 00:47:37,474 In this interpretation, due to Einstein, gravity is only a pucker... 654 00:47:37,676 --> 00:47:41,237 ...in the fabric of space which moving objects encounter. 655 00:47:41,446 --> 00:47:46,383 Space is warped by mass into an additional physical dimension. 656 00:47:48,887 --> 00:47:53,483 The larger the local mass, the greater is the local gravity... 657 00:47:53,692 --> 00:47:57,150 ...and the more intense is the distortion... 658 00:47:57,362 --> 00:48:00,559 ...or pucker, or warp of space. 659 00:48:00,765 --> 00:48:02,858 So, by this analogy... 660 00:48:03,068 --> 00:48:07,698 ...a black hole is a kind of bottomless pit. 661 00:48:07,906 --> 00:48:09,771 What would happen if you fell in? 662 00:48:09,974 --> 00:48:13,171 Assuming you could survive the gravitational tides... 663 00:48:13,378 --> 00:48:18,042 ...and the intense radiation flux, it is just barely possible... 664 00:48:18,249 --> 00:48:21,343 ...that by plunging into a black hole... 665 00:48:21,553 --> 00:48:25,319 ...you might emerge in another part of space-time. 666 00:48:25,523 --> 00:48:27,218 Somewhere else in space... 667 00:48:27,425 --> 00:48:31,020 ...some-when else in time. 668 00:48:31,229 --> 00:48:33,163 In this view, space is... 669 00:48:33,364 --> 00:48:36,356 ...filled with a network of wormholes... 670 00:48:36,568 --> 00:48:38,433 ...something like the wormholes in an apple. 671 00:48:38,636 --> 00:48:41,730 Although by no means is this point demonstrated... 672 00:48:41,940 --> 00:48:45,137 ...it is merely an exciting suggestion. 673 00:48:45,343 --> 00:48:46,742 If it is true... 674 00:48:46,945 --> 00:48:50,938 ...then perhaps there exist gravity tunnels... 675 00:48:51,149 --> 00:48:54,550 ...a kind of interstellar or intergalactic subway... 676 00:48:54,753 --> 00:48:56,482 ...which would permit you to get from... 677 00:48:56,688 --> 00:48:59,179 ...here to there in much less than the usual time. 678 00:48:59,390 --> 00:49:03,349 A kind of cosmic rapid transit system. 679 00:49:06,064 --> 00:49:08,464 We cannot generate black holes... 680 00:49:08,666 --> 00:49:11,396 ...our technology is far too feeble to... 681 00:49:11,603 --> 00:49:14,299 ...move such massive amounts of matter around. 682 00:49:14,506 --> 00:49:17,498 But perhaps someday, it will be possible to voyage hundreds or... 683 00:49:17,709 --> 00:49:21,577 ...thousands of light-years to a black hole like Cygnus X-1. 684 00:49:21,780 --> 00:49:23,941 We would plunge down to emerge... 685 00:49:24,149 --> 00:49:27,209 ...in some unimaginably exotic time and place. 686 00:49:27,418 --> 00:49:32,082 Our common-sense notions of reality severely challenged. 687 00:49:34,325 --> 00:49:36,987 Perhaps the cosmos is infested with wormholes... 688 00:49:37,195 --> 00:49:39,527 ...every one of them a tunnel to somewhere. 689 00:49:39,731 --> 00:49:43,963 Perhaps other civilizations, with vastly more advanced technologies... 690 00:49:44,169 --> 00:49:48,572 ...are today riding the gravity express. 691 00:49:50,875 --> 00:49:54,572 It's even possible that a black hole is a gate... 692 00:49:54,779 --> 00:49:58,146 ...to another, and quite different, universe. 693 00:50:36,187 --> 00:50:38,678 The lives and deaths of the stars... 694 00:50:38,890 --> 00:50:42,291 ...seem impossibly remote from human experience... 695 00:50:42,493 --> 00:50:46,395 ...and yet we're related in the most intimate way to their life cycles. 696 00:50:46,598 --> 00:50:48,566 The very matter that makes us up... 697 00:50:48,766 --> 00:50:53,703 ...was generated long ago and far away in red giant stars. 698 00:50:54,806 --> 00:50:58,469 A blade of grass, as Walt Whitman said... 699 00:50:58,676 --> 00:51:00,906 "...is the journey work of the stars." 700 00:51:01,112 --> 00:51:02,636 The formation of the solar system... 701 00:51:02,847 --> 00:51:05,975 ...may have been triggered by a nearby supernova explosion. 702 00:51:06,184 --> 00:51:07,674 After the sun turned on... 703 00:51:07,886 --> 00:51:10,719 ...its ultraviolet light poured into our atmosphere. 704 00:51:10,922 --> 00:51:12,913 Its warmth generated lightning. 705 00:51:13,124 --> 00:51:16,025 And these energy sources sparked the origin of life. 706 00:51:17,061 --> 00:51:19,393 Plants harvest sunlight... 707 00:51:19,597 --> 00:51:22,498 ...converting solar into chemical energy. 708 00:51:23,301 --> 00:51:26,395 We and the other animals are parasites on the plants. 709 00:51:26,604 --> 00:51:30,540 So we are, all of us, solar-powered. 710 00:51:30,942 --> 00:51:33,433 The evolution of life is driven by mutations. 711 00:51:33,645 --> 00:51:37,775 They are caused partly by natural radioactivity and cosmic rays. 712 00:51:37,982 --> 00:51:42,282 But they are both generated in the spectacular deaths of massive stars... 713 00:51:42,487 --> 00:51:44,478 ...thousands of light-years distant. 714 00:51:46,024 --> 00:51:49,653 Think of the sun's heat on your upturned face... 715 00:51:49,861 --> 00:51:52,125 ...on a cloudless summer's day. 716 00:51:52,330 --> 00:51:55,458 From 150 million kilometers away... 717 00:51:55,667 --> 00:51:58,135 ...we recognize its power. 718 00:51:58,336 --> 00:52:02,170 What would we feel on its seething, self-luminous surface... 719 00:52:02,373 --> 00:52:05,831 ...or immersed in its heart of nuclear fire? 720 00:52:06,044 --> 00:52:10,981 And yet, the sun is an ordinary, even a mediocre star. 721 00:52:11,215 --> 00:52:15,049 Our ancestors worshiped the sun and they were far from foolish. 722 00:52:15,253 --> 00:52:18,051 It makes good sense to revere the sun and the stars. 723 00:52:18,256 --> 00:52:22,215 Because we are their children. 724 00:52:25,196 --> 00:52:29,292 We have witnessed the life cycles of the stars. 725 00:52:29,500 --> 00:52:33,937 They are born, they mature and then they die. 726 00:52:34,138 --> 00:52:36,402 As time goes on, there are more white dwarfs... 727 00:52:36,607 --> 00:52:39,098 ...more neutron stars, more black holes. 728 00:52:39,310 --> 00:52:42,143 The remains of the stars accumulate... 729 00:52:42,347 --> 00:52:44,440 ...as the eons pass. 730 00:52:44,649 --> 00:52:48,415 But interstellar space also becomes enriched in heavy elements... 731 00:52:48,619 --> 00:52:52,578 ...out of which form new generations of stars and planets... 732 00:52:52,790 --> 00:52:54,849 ...life and intelligence. 733 00:52:55,059 --> 00:52:59,257 The events in one star can influence a world halfway across the galaxy... 734 00:52:59,464 --> 00:53:01,694 ...and a billion years in the future. 735 00:53:09,107 --> 00:53:12,634 The vast interstellar clouds of gas and dust... 736 00:53:12,844 --> 00:53:14,471 ...are stellar nurseries. 737 00:53:14,679 --> 00:53:18,740 Here first begins the inexorable gravitational collapse... 738 00:53:18,950 --> 00:53:22,010 ...which dominates the lives of the stars. 739 00:53:22,220 --> 00:53:26,623 Massive suns may evolve through the red giant stage in only millions of years. 740 00:53:26,824 --> 00:53:31,090 Dying young, never leaving the cloud in which they were born. 741 00:53:33,131 --> 00:53:37,693 Other suns, longer-lived, wander out of the nursery. 742 00:53:37,902 --> 00:53:40,029 Our sun is such a star... 743 00:53:40,238 --> 00:53:43,298 ...as are most of the stars in the sky. 744 00:53:54,485 --> 00:53:59,047 Most stars are members of double or multiple star systems... 745 00:53:59,257 --> 00:54:03,557 ...and live to process their nuclear fuel over billions of years. 746 00:54:03,761 --> 00:54:06,821 The galaxy is 10 billion years old. 747 00:54:07,031 --> 00:54:08,623 Old enough to have spawned... 748 00:54:08,833 --> 00:54:12,599 ...only a few generations of ordinary stars. 749 00:54:17,408 --> 00:54:20,741 The objects we encounter in a voyage through the Milky Way... 750 00:54:20,945 --> 00:54:25,075 ...are stages in the life cycle of the stars. 751 00:54:25,283 --> 00:54:27,513 Some are bright and new... 752 00:54:27,718 --> 00:54:31,711 ...and others are as ancient as the galaxy itself. 753 00:54:43,568 --> 00:54:47,095 Surrounding the Milky Way is a halo of matter... 754 00:54:47,305 --> 00:54:50,297 ...which includes the globular clusters... 755 00:54:50,508 --> 00:54:53,944 ...each containing up to a million elderly stars. 756 00:54:54,145 --> 00:54:58,514 At the centers of globular clusters and at the core of the galaxy... 757 00:54:58,716 --> 00:55:03,380 ...there may be massive black holes ticking and purring... 758 00:55:03,588 --> 00:55:06,113 ...the subject of future exploration. 759 00:55:24,008 --> 00:55:26,943 We on Earth marvel, and rightly so... 760 00:55:27,145 --> 00:55:30,171 ...at the daily return of our single sun. 761 00:55:30,381 --> 00:55:34,147 But from a planet orbiting a star in a distant globular cluster... 762 00:55:34,352 --> 00:55:37,185 ...a still more glorious dawn awaits. 763 00:55:37,388 --> 00:55:40,653 Not a sunrise, but a galaxy-rise. 764 00:55:40,858 --> 00:55:44,589 A morning filled with 400 billion suns... 765 00:55:44,795 --> 00:55:47,059 ...the rising of the Milky Way. 766 00:55:47,265 --> 00:55:51,167 An enormous spiral form with collapsing gas clouds... 767 00:55:51,369 --> 00:55:55,100 ...condensing planetary systems, luminous supergiants... 768 00:55:55,306 --> 00:55:57,274 ...stable middle-aged stars... 769 00:55:57,475 --> 00:56:01,844 ...red giants, white dwarfs, planetary nebulas, supernovas... 770 00:56:02,046 --> 00:56:06,483 ...neutron stars, pulsars, black holes and... 771 00:56:06,717 --> 00:56:09,686 ...there is every reason to think, other exotic objects... 772 00:56:09,921 --> 00:56:12,116 ...that we have not yet discovered. 773 00:56:15,126 --> 00:56:18,994 From such a world, high above the disc of the Milky Way... 774 00:56:19,197 --> 00:56:23,224 ...it would be clear as it is beginning to be clear on our world... 775 00:56:23,434 --> 00:56:26,494 ...that we are made by the atoms in the stars... 776 00:56:26,704 --> 00:56:30,765 ...that our matter and our form are determined... 777 00:56:30,975 --> 00:56:34,570 ...by the cosmos of which we are a part. 778 00:56:42,920 --> 00:56:47,186 I only have a moment, but I wanted you to see a picture of Betelgeuse... 779 00:56:47,391 --> 00:56:49,325 ...in the constellation Orion. 780 00:56:49,527 --> 00:56:52,621 The first image of the surface of another star. 781 00:56:52,897 --> 00:56:55,559 But the most exciting recent stellar discovery... 782 00:56:55,766 --> 00:56:57,597 ...has been of a nearby supernova... 783 00:56:57,802 --> 00:56:59,997 ...in a companion galaxy to the Milky Way. 784 00:57:00,204 --> 00:57:04,504 We are here seeing chemical elements in the process of synthesis... 785 00:57:04,709 --> 00:57:08,270 ...and have had our first glimpse of the supernova... 786 00:57:08,479 --> 00:57:11,642 ...through a brand-new field: neutrino astronomy. 787 00:57:11,849 --> 00:57:15,046 And we're now seeing, around neighboring stars... 788 00:57:15,620 --> 00:57:19,317 ...discs of gas and dust just like those needed to explain... 789 00:57:19,523 --> 00:57:22,253 ...the origin of the planets in our solar system. 790 00:57:22,460 --> 00:57:25,190 Worlds may be forming here. 791 00:57:25,396 --> 00:57:28,661 It's like a snapshot of our solar system's past. 792 00:57:29,100 --> 00:57:32,228 And there are so many such discs being found these days... 793 00:57:32,436 --> 00:57:36,964 ...that planets may be very common among the stars of the Milky Way.