1 00:00:00,090 --> 00:00:02,490 The following content is provided under a Creative 2 00:00:02,490 --> 00:00:04,030 Commons license. 3 00:00:04,030 --> 00:00:06,330 Your support will help MIT OpenCourseWare 4 00:00:06,330 --> 00:00:10,720 continue to offer high quality educational resources for free. 5 00:00:10,720 --> 00:00:13,320 To make a donation or view additional materials 6 00:00:13,320 --> 00:00:17,280 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,280 --> 00:00:19,870 at ocw.mit.edu. 8 00:00:19,870 --> 00:00:21,305 MARK HARTMAN: We live-- 9 00:00:21,305 --> 00:00:25,139 this is a cartoon of what our galaxy looks like. 10 00:00:25,139 --> 00:00:25,680 We live here. 11 00:00:25,680 --> 00:00:27,380 The center of our galaxy is there. 12 00:00:27,380 --> 00:00:29,037 Some of these objects-- 13 00:00:29,037 --> 00:00:31,440 4U1822 is right here. 14 00:00:31,440 --> 00:00:32,630 [INAUDIBLE] is over there. 15 00:00:32,630 --> 00:00:33,810 The Crab Nebula's there. 16 00:00:33,810 --> 00:00:36,230 The Orion Nebula's inside there. 17 00:00:36,230 --> 00:00:38,408 And that other neutron star that we looked at 18 00:00:38,408 --> 00:00:39,695 is really close to us. 19 00:00:39,695 --> 00:00:42,350 But now-- and remember we said if our solar system 20 00:00:42,350 --> 00:00:46,414 was the size of a quarter, how large would the galaxy be? 21 00:00:46,414 --> 00:00:47,414 AUDIENCE: North America. 22 00:00:47,414 --> 00:00:48,857 MARK HARTMAN: OK, so [INAUDIBLE],, 23 00:00:48,857 --> 00:00:50,790 can you hold that up? 24 00:00:50,790 --> 00:00:55,040 So our own galaxy would be the size of North America. 25 00:00:55,040 --> 00:00:57,490 What we're going to think about now 26 00:00:57,490 --> 00:01:01,108 is other galaxies, galaxies outside of our own. 27 00:01:01,108 --> 00:01:02,524 And actually, [INAUDIBLE],, if you 28 00:01:02,524 --> 00:01:04,732 could stand up and move to the other end of the room? 29 00:01:09,434 --> 00:01:12,400 I want you guys just very, very briefly 30 00:01:12,400 --> 00:01:14,390 to discuss with your groups if this 31 00:01:14,390 --> 00:01:19,540 was our galaxy, where do you think the next galaxy would be? 32 00:01:19,540 --> 00:01:23,430 If our galaxy was actually this size, right? 33 00:01:23,430 --> 00:01:26,060 So think back to this idea that we had before. 34 00:01:26,060 --> 00:01:27,730 If the solar system was one quarter, 35 00:01:27,730 --> 00:01:29,063 where would the next quarter be? 36 00:01:29,063 --> 00:01:31,254 Where did we say it was? 37 00:01:31,254 --> 00:01:33,246 AUDIENCE: [INAUDIBLE] 38 00:01:33,246 --> 00:01:35,645 MARK HARTMAN: Like a half mile away, on the other side 39 00:01:35,645 --> 00:01:37,860 of campus. 40 00:01:37,860 --> 00:01:41,595 But if our galaxy were this big, where would the next galaxy be? 41 00:01:41,595 --> 00:01:44,236 Where should be tell [INAUDIBLE] to go? 42 00:01:44,236 --> 00:01:47,060 If [INAUDIBLE] was the Milky Way galaxy-- actually, 43 00:01:47,060 --> 00:01:49,455 here, I'm going to give you the [INAUDIBLE] Galaxy. 44 00:01:49,455 --> 00:01:52,810 This is bigger. 45 00:01:52,810 --> 00:01:56,056 So notice how wide this galaxy is. 46 00:01:56,056 --> 00:01:58,070 This galaxy is about the same width. 47 00:01:58,070 --> 00:02:03,312 The next galaxy away from us is about 20 times the width 48 00:02:03,312 --> 00:02:04,423 of one galaxy. 49 00:02:04,423 --> 00:02:05,609 AUDIENCE: [INAUDIBLE]. 50 00:02:05,609 --> 00:02:07,150 MARK HARTMAN: So we were all thinking 51 00:02:07,150 --> 00:02:09,483 they were far away, like on Mars, right? 52 00:02:09,483 --> 00:02:16,540 But if this is one width of a galaxy, one linear width, 2, 3, 53 00:02:16,540 --> 00:02:26,198 4, 5, 6, 7, blah, blah, blah, all the way, like Peter said-- 54 00:02:26,198 --> 00:02:27,160 PETER: Told you! 55 00:02:27,160 --> 00:02:29,368 MARK HARTMAN: To probably the other side of the room. 56 00:02:29,368 --> 00:02:31,690 [LAUGHTER] 57 00:02:31,690 --> 00:02:34,740 So if that is our Milky Way galaxy, 58 00:02:34,740 --> 00:02:37,330 all the way on this side of the room 59 00:02:37,330 --> 00:02:39,325 would be where the Andromeda Galaxy is. 60 00:02:39,325 --> 00:02:42,874 And yes, they are getting closer to each other. 61 00:02:42,874 --> 00:02:44,915 But we'll have to wait to learn a little bit more 62 00:02:44,915 --> 00:02:47,280 about how fast galaxies move before we can figure out 63 00:02:47,280 --> 00:02:49,113 how long it's going to take them to collide. 64 00:02:49,113 --> 00:02:50,070 Good, Peter. 65 00:02:50,070 --> 00:02:50,800 Nice, well done. 66 00:02:50,800 --> 00:02:52,263 [APPLAUSE] 67 00:02:52,263 --> 00:02:53,030 Nice guess. 68 00:02:53,030 --> 00:02:56,322 [INTERPOSING VOICES] 69 00:02:58,810 --> 00:03:00,488 So why was that surprising? 70 00:03:00,488 --> 00:03:03,624 Why did everybody guess these big huge distances? 71 00:03:03,624 --> 00:03:04,800 AUDIENCE: What's that? 72 00:03:04,800 --> 00:03:05,664 MARK HARTMAN: Why did everybody else 73 00:03:05,664 --> 00:03:06,780 guess these big huge distances? 74 00:03:06,780 --> 00:03:07,594 AUDIENCE: Because-- 75 00:03:07,594 --> 00:03:09,380 [INTERPOSING VOICES] 76 00:03:09,380 --> 00:03:11,630 AUDIENCE: [INAUDIBLE] 77 00:03:11,630 --> 00:03:12,530 [LAUGHTER] 78 00:03:12,530 --> 00:03:13,930 MARK HARTMAN: Same thing happened with the quarter, 79 00:03:13,930 --> 00:03:14,638 what do you mean? 80 00:03:14,638 --> 00:03:17,980 AUDIENCE: Because anyone would guess, like, I'd say 2 inches, 81 00:03:17,980 --> 00:03:19,570 $0.50. 82 00:03:19,570 --> 00:03:23,732 And all the people were like, oh no, you're to Mars. 83 00:03:23,732 --> 00:03:26,177 And somebody say it's over there. 84 00:03:26,177 --> 00:03:28,841 MARK HARTMAN: There's a stock answer for everything, I guess. 85 00:03:28,841 --> 00:03:29,340 [LAUGHTER] 86 00:03:29,340 --> 00:03:33,378 AUDIENCE: If you come all the way, I was like, half a mile? 87 00:03:33,378 --> 00:03:34,210 MARK HARTMAN: Yeah. 88 00:03:34,210 --> 00:03:36,842 AUDIENCE: And I was like-- 89 00:03:36,842 --> 00:03:37,550 MARK HARTMAN: OK. 90 00:03:37,550 --> 00:03:42,258 Stars in a galaxy are relatively far apart, 91 00:03:42,258 --> 00:03:44,250 because they're so small compared 92 00:03:44,250 --> 00:03:45,990 to how far apart they are. 93 00:03:45,990 --> 00:03:48,550 Galaxies, on the other hand, are large. 94 00:03:48,550 --> 00:03:50,552 They're big huge things. 95 00:03:50,552 --> 00:03:52,050 Relative to the size of a galaxy, 96 00:03:52,050 --> 00:03:55,554 the space in between galaxies is not so much. 97 00:03:55,554 --> 00:03:57,640 Now David told us this morning that we're part 98 00:03:57,640 --> 00:03:59,640 of a local cluster of galaxies. 99 00:03:59,640 --> 00:04:02,310 So yeah, the Andromeda Galaxy is fairly close to us. 100 00:04:02,310 --> 00:04:04,230 If you're in a cluster of galaxies, 101 00:04:04,230 --> 00:04:06,870 the galaxies are going to be fairly close. 102 00:04:06,870 --> 00:04:13,200 If we're talking about the distances to further galaxies, 103 00:04:13,200 --> 00:04:14,950 it may not be quite as close. 104 00:04:14,950 --> 00:04:17,819 But most galaxies have at least a couple 105 00:04:17,819 --> 00:04:19,620 of other galaxies around them. 106 00:04:19,620 --> 00:04:22,800 In our local group, there's a decent number of galaxies. 107 00:04:22,800 --> 00:04:26,540 It's not huge, it's not more than six to 10, 108 00:04:26,540 --> 00:04:28,140 is that about right, Peter? 109 00:04:28,140 --> 00:04:30,640 The number of galaxies in our kind of local group? 110 00:04:30,640 --> 00:04:32,100 PETER: That sounds right. 111 00:04:32,100 --> 00:04:33,600 MARK HARTMAN: It sounds about right. 112 00:04:33,600 --> 00:04:35,683 So there's about six galaxies that are hanging out 113 00:04:35,683 --> 00:04:36,560 with us together. 114 00:04:36,560 --> 00:04:37,175 So-- 115 00:04:37,175 --> 00:04:37,470 AUDIENCE: Bless you. 116 00:04:37,470 --> 00:04:39,678 MARK HARTMAN: If the galaxy was the size of a poster, 117 00:04:39,678 --> 00:04:41,060 where's the next galaxy? 118 00:04:41,060 --> 00:04:46,810 20 galaxy diameters away. 119 00:04:46,810 --> 00:04:57,819 20 galaxy diameters away, all right? 120 00:04:57,819 --> 00:04:58,360 AUDIENCE: 35. 121 00:04:58,360 --> 00:04:59,470 MARK HARTMAN: Is it 35? 122 00:04:59,470 --> 00:05:01,227 AUDIENCE: There's 35 in the local group. 123 00:05:01,227 --> 00:05:02,810 MARK HARTMAN: Oh, there's 35 galaxies? 124 00:05:02,810 --> 00:05:03,390 OK. 125 00:05:03,390 --> 00:05:11,310 And there are 35 galaxies in our local group of galaxies. 126 00:05:19,410 --> 00:05:21,260 All right?