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 --> 00:00:10,690 continue to offer high quality educational resources for free. 5 00:00:10,690 --> 00:00:13,320 To make a donation or view additional materials 6 00:00:13,320 --> 00:00:16,890 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,890 --> 00:00:19,729 at ocw.mit.edu. 8 00:00:19,729 --> 00:00:20,895 PROFESSOR: I would like us-- 9 00:00:25,280 --> 00:00:33,282 we're going to apply Hubble's law again. 10 00:00:33,282 --> 00:00:34,520 AUDIENCE: Oh. 11 00:00:34,520 --> 00:00:37,490 PROFESSOR: Yes, you guys haven't had enough practice with it. 12 00:00:37,490 --> 00:00:41,180 Here we said, here is the hot gas. 13 00:00:41,180 --> 00:00:43,020 Here is different galaxies. 14 00:00:45,920 --> 00:00:47,720 We saw when we looked at Ned that there 15 00:00:47,720 --> 00:00:50,660 was a bunch of those little red circles. 16 00:00:50,660 --> 00:00:53,000 Each one of those red circles was another galaxy, 17 00:00:53,000 --> 00:00:55,490 that it was a part of this galaxy cluster. 18 00:00:55,490 --> 00:00:57,500 We couldn't see them all, because they might 19 00:00:57,500 --> 00:01:00,180 have a small angular size. 20 00:01:00,180 --> 00:01:02,180 I'm sensing there's not a whole lot of attention 21 00:01:02,180 --> 00:01:03,190 being paid in the room. 22 00:01:09,910 --> 00:01:12,435 How did we find the distance to the galaxy cluster? 23 00:01:17,550 --> 00:01:21,180 So the distance to the cluster is that much. 24 00:01:21,180 --> 00:01:22,730 How did we find the distance? 25 00:01:22,730 --> 00:01:23,230 Juan? 26 00:01:26,730 --> 00:01:30,230 AUDIENCE: Velocity-- what is it? 27 00:01:30,230 --> 00:01:32,750 Oh, speed times distance? 28 00:01:32,750 --> 00:01:35,366 I mean, speed equals distance? 29 00:01:35,366 --> 00:01:36,740 PROFESSOR: Speed equals distance? 30 00:01:36,740 --> 00:01:37,430 That's wrong. 31 00:01:37,430 --> 00:01:38,810 Speed does not equal distance. 32 00:01:38,810 --> 00:01:41,235 AUDIENCE: I mean, the speed [INAUDIBLE] times distance 33 00:01:41,235 --> 00:01:43,060 [INAUDIBLE]. 34 00:01:43,060 --> 00:01:45,310 PROFESSOR: How did we get the distance to the cluster? 35 00:01:45,310 --> 00:01:47,500 Look at your solving problems in science sheet. 36 00:01:47,500 --> 00:01:50,302 What process did you go through? 37 00:01:50,302 --> 00:01:52,760 What did we have to do to get the distance to this cluster? 38 00:01:57,220 --> 00:01:57,720 Peter? 39 00:01:57,720 --> 00:02:00,590 AUDIENCE: Divide velocity by [INAUDIBLE].. 40 00:02:00,590 --> 00:02:02,090 PROFESSOR: OK, we used Hubble's law, 41 00:02:02,090 --> 00:02:06,260 and we said, if we can get a measurement for the redshift 42 00:02:06,260 --> 00:02:09,800 of the cluster, how fast it's moving away from us, if we 43 00:02:09,800 --> 00:02:11,270 can get a measurement of that, we 44 00:02:11,270 --> 00:02:15,290 can use Hubble's law to predict what the distance is. 45 00:02:15,290 --> 00:02:20,750 That was for the average for the whole cluster. 46 00:02:20,750 --> 00:02:27,500 What if I wanted the distance to the front of the cluster, which 47 00:02:27,500 --> 00:02:31,970 is here, the distance from us to the front, 48 00:02:31,970 --> 00:02:38,000 and I also wanted the distance to the back of the cluster, 49 00:02:38,000 --> 00:02:39,950 which is over there? 50 00:02:39,950 --> 00:02:41,300 How could I get those values? 51 00:02:51,030 --> 00:02:53,910 Because we figured out how wide the cluster is side to side, 52 00:02:53,910 --> 00:02:55,840 let's figure out how deep it is. 53 00:02:55,840 --> 00:02:59,270 AUDIENCE: I would subtract the linear diameter 54 00:02:59,270 --> 00:03:03,690 from the distance to the cluster. 55 00:03:03,690 --> 00:03:04,530 PROFESSOR: OK. 56 00:03:04,530 --> 00:03:06,840 So the linear diameter on our model 57 00:03:06,840 --> 00:03:08,850 here is measuring this way. 58 00:03:08,850 --> 00:03:10,140 AUDIENCE: The radius. 59 00:03:10,140 --> 00:03:11,650 PROFESSOR: The radius, right. 60 00:03:11,650 --> 00:03:13,920 But how do we know what the cluster looks like? 61 00:03:13,920 --> 00:03:16,920 Maybe the cluster is football shaped. 62 00:03:16,920 --> 00:03:20,940 When we look at it on the sky, it's just going to be a blob. 63 00:03:20,940 --> 00:03:24,570 We want to know, what is the shape of this cluster? 64 00:03:24,570 --> 00:03:26,640 Is it as wide as it is deep? 65 00:03:29,620 --> 00:03:32,109 Because if it was round, if it was spherical, yeah, 66 00:03:32,109 --> 00:03:34,150 we could say, all right, the distance to the back 67 00:03:34,150 --> 00:03:35,774 is just we'll take the radius and we'll 68 00:03:35,774 --> 00:03:38,430 add the radius on for the back, and we'll take the radius 69 00:03:38,430 --> 00:03:41,200 and we'll subtract the radius off for the front. 70 00:03:45,490 --> 00:03:46,600 How could we do this? 71 00:03:50,760 --> 00:03:55,090 What's at the front of this galaxy cluster? 72 00:03:55,090 --> 00:03:56,810 AUDIENCE: Stars. 73 00:03:56,810 --> 00:03:58,768 PROFESSOR: Are there stars? 74 00:03:58,768 --> 00:04:02,377 AUDIENCE: [INAUDIBLE]. 75 00:04:02,377 --> 00:04:03,460 PROFESSOR: Say that again. 76 00:04:03,460 --> 00:04:04,376 AUDIENCE: [INAUDIBLE]. 77 00:04:04,376 --> 00:04:08,410 PROFESSOR: OK, yeah, there's dust in the way here. 78 00:04:08,410 --> 00:04:12,100 If we could look at the redshift of this galaxy 79 00:04:12,100 --> 00:04:14,920 and the redshift of this galaxy, could we 80 00:04:14,920 --> 00:04:17,140 use those to predict the distance 81 00:04:17,140 --> 00:04:18,350 to the front and the back? 82 00:04:25,182 --> 00:04:26,402 AUDIENCE: Maybe. 83 00:04:26,402 --> 00:04:27,110 PROFESSOR: Maybe? 84 00:04:27,110 --> 00:04:28,579 Yeah, maybe. 85 00:04:28,579 --> 00:04:30,620 What I want you to do is this-- go ahead, Bianca. 86 00:04:30,620 --> 00:04:32,540 AUDIENCE: If it's in front of the other one, 87 00:04:32,540 --> 00:04:35,910 won't you be able to not see the one behind it? 88 00:04:35,910 --> 00:04:38,426 PROFESSOR: Oh, right? 89 00:04:38,426 --> 00:04:40,050 So Bianca's saying, if this one's here, 90 00:04:40,050 --> 00:04:42,180 wouldn't that block out the one behind? 91 00:04:42,180 --> 00:04:43,140 Well, I don't know. 92 00:04:43,140 --> 00:04:45,240 I just drew a diagram here. 93 00:04:45,240 --> 00:04:49,200 What we need to do is, let's go into the cluster 94 00:04:49,200 --> 00:04:55,170 and let's find the galaxy that is moving away the slowest. 95 00:04:55,170 --> 00:04:59,820 And then let's find the galaxy that's moving away the fastest. 96 00:04:59,820 --> 00:05:03,150 If we find the galaxy that's moving away from us slowest, 97 00:05:03,150 --> 00:05:06,120 couldn't we get the distance to the front of the galaxy? 98 00:05:06,120 --> 00:05:09,510 And if we find the galaxy that's moving away fastest, 99 00:05:09,510 --> 00:05:14,380 wouldn't that be the galaxy that's far back in the cluster? 100 00:05:14,380 --> 00:05:18,990 So what I want you guys to do-- here's your task-- 101 00:05:18,990 --> 00:05:22,740 I want you to predict the distance 102 00:05:22,740 --> 00:05:25,620 to the front of the cluster and the distance 103 00:05:25,620 --> 00:05:28,010 to the back of the cluster.