1 00:00:00,012 --> 00:00:05,248 One thing that we noticed about lots of galaxies is that like the Milky Way, 2 00:00:05,248 --> 00:00:10,253 there are Spiral galaxies and so the issue of understanding spi, spiral 3 00:00:10,253 --> 00:00:15,205 galaxies come to be spiral is an old interesting one and I want to explain 4 00:00:15,205 --> 00:00:19,352 like its a problem first. So one could imagine that for some 5 00:00:19,352 --> 00:00:23,052 reasons stars form in a pattern like the one I've. 6 00:00:23,052 --> 00:00:30,017 Right, and therefore you have spirals. That is not the solution to how spirals 7 00:00:30,017 --> 00:00:34,736 form. We understand that because remember that, 8 00:00:34,736 --> 00:00:40,632 within the region where the sun is, rotational velocity. 9 00:00:40,632 --> 00:00:45,745 was roughly constant. If rotational velocity is constant, then 10 00:00:45,745 --> 00:00:51,580 periods of rotation are not constant. Stars on the interior moving at the same 11 00:00:51,580 --> 00:00:57,851 speeds as stars on the exterior, the interior stars will orbit, and with much 12 00:00:57,851 --> 00:01:01,632 shorter periods cause their orbits are shorter. 13 00:01:01,632 --> 00:01:06,362 What this means is if you imagine creating a galaxy with this pattern and 14 00:01:06,362 --> 00:01:11,336 then letting the stars rotate and remember that the Sun has gone through 15 00:01:11,336 --> 00:01:16,237 about 25 orbits of the Milky Way so this is not some theoretical construct. 16 00:01:16,237 --> 00:01:21,284 What happens is the inner stars orbit faster than the outer stars and the arms 17 00:01:21,284 --> 00:01:25,912 get all wound up. And eventually will dissolve after a few 18 00:01:25,912 --> 00:01:30,825 rotations you have no spiral left. You cannot, construct a spiral galaxy as 19 00:01:30,825 --> 00:01:35,688 something you're actually going to find, by having a spiral being a place where 20 00:01:35,688 --> 00:01:38,801 stars are. in general, since the sun takes 230 21 00:01:38,801 --> 00:01:42,008 million years to circumnavigate the galaxy, 22 00:01:42,008 --> 00:01:46,736 And remember that spiral arms are the position where we found O and B, hah, O 23 00:01:46,736 --> 00:01:50,191 and B type stars. And since O and B type stars don't live 24 00:01:50,191 --> 00:01:55,076 230 billion years, by the time the spiral arms continue, complete an orbit out 25 00:01:55,076 --> 00:01:58,506 where the sun is, it's a whole new generation of stars. 26 00:01:58,506 --> 00:02:01,262 Many new generations of stars have come up. 27 00:02:01,262 --> 00:02:06,323 And yet the spiral arms must be stable otherwise you wouldn't see so many spiral 28 00:02:06,323 --> 00:02:11,668 configurations in galaxies.So what is it that allows spiral arms to survive the 29 00:02:11,668 --> 00:02:15,412 model that was proposed by Lenin Shoe in the 1960's. 30 00:02:15,412 --> 00:02:20,240 It's called the density wave model and the idea is that spiral arms are not 31 00:02:20,240 --> 00:02:24,067 material objects. They are propagating density waves 32 00:02:24,067 --> 00:02:28,699 propagating through the disk. The wave propagates as you recall with a 33 00:02:28,699 --> 00:02:32,088 completely velocity that is the wave velocity. 34 00:02:32,088 --> 00:02:36,523 It's completely independent. Of the motion of the actual objects in 35 00:02:36,523 --> 00:02:39,390 the disc, when we had a wave going down the string. 36 00:02:39,390 --> 00:02:43,783 the string wasn't moving at all, and the wave propogated down the string. 37 00:02:43,783 --> 00:02:48,461 So these waves propogate through the disc with their own velocity and what this 38 00:02:48,461 --> 00:02:52,932 implies, is these are like sound waves. There are regions where the density. 39 00:02:52,932 --> 00:02:58,514 In the disk increases by say, 10 or 20%. And how do you generate such a situation? 40 00:02:58,514 --> 00:03:04,377 Well there's this nice image here on the right, what we have here is a bunch of 41 00:03:04,377 --> 00:03:10,140 elliptical or oval orbits and you could imagine that in each of these orbits 42 00:03:10,140 --> 00:03:15,069 there's a whole bunch of stars. Each following that orbit at various 43 00:03:15,069 --> 00:03:20,397 places along the orbit, and these ellipses or ovals are, tilted slightly 44 00:03:20,397 --> 00:03:26,210 with larger and larger radius, and, you see that there, this creates this, this 45 00:03:26,210 --> 00:03:31,232 twisting, creates regions in the galaxy Where the orbits are close together, this 46 00:03:31,232 --> 00:03:34,847 is a region of enhanced density. If you imagine that each oval is equally 47 00:03:34,847 --> 00:03:38,811 populated along it's length by stars. Then, and the regions where the ovals are 48 00:03:38,811 --> 00:03:42,912 close together there will be an increase population of stars and gas, and whatever 49 00:03:42,912 --> 00:03:45,376 it is that is that's orbiting along these orbits. 50 00:03:45,376 --> 00:03:49,070 And the fact that the stars in the inner ellipse orbit faster than the stars on 51 00:03:49,070 --> 00:03:51,502 the outer ellipse, has nothing to do with it. 52 00:03:51,502 --> 00:03:56,192 If these orbits were actually steady, then this pattern would not shift at all. 53 00:03:56,192 --> 00:04:01,012 It would stay completely constant because every star, as it hit the part where the 54 00:04:01,012 --> 00:04:04,382 orbits are bunched up, would experience higher density. 55 00:04:04,382 --> 00:04:07,257 Now in practice, this is not a stable situation. 56 00:04:07,257 --> 00:04:11,652 Stars orbiting along these orbits, anything by the collection of circular 57 00:04:11,652 --> 00:04:15,624 orbits leads to gravitational interaction. 58 00:04:15,624 --> 00:04:20,831 And so, these orbitals precess, if you put stars in these elliptical orbits. 59 00:04:20,831 --> 00:04:26,038 And that precession will lead to, the, the, different tilts of the different 60 00:04:26,038 --> 00:04:31,299 ellipsis changing at different rates. If you have a resonance That between the 61 00:04:31,299 --> 00:04:37,003 precession period and the orbital period, then you can have a situation where, as 62 00:04:37,003 --> 00:04:42,551 the, what the precession sets up is not the complete destruction of the spiral, 63 00:04:42,551 --> 00:04:46,489 but rather a slow rotation of the spiral, of, over time. 64 00:04:46,489 --> 00:04:50,392 And then What you have is a situation in the disk where you start with a 65 00:04:50,392 --> 00:04:54,877 population of stars and dust and gas and whatever but there are regions where the 66 00:04:54,877 --> 00:04:58,077 density is higher. Now stars as they orbit enter this region 67 00:04:58,077 --> 00:05:01,385 and then they exit it. Clouds of gas enter this region and exit 68 00:05:01,385 --> 00:05:03,466 it. When a cloud of gas encounters this 69 00:05:03,466 --> 00:05:06,393 region of higher density That's higher pressure. 70 00:05:06,393 --> 00:05:10,694 The cloud of gases compress. Some clouds may become critical in the 71 00:05:10,694 --> 00:05:13,951 gene sense. They will then begin to collapse and by 72 00:05:13,951 --> 00:05:18,966 the time those clouds are exiting the spiral at the leading edge of the spiral, 73 00:05:18,966 --> 00:05:23,555 you will find brand new O and B stars that have formed while the cloud was 74 00:05:23,555 --> 00:05:27,998 collapsing inside the spiral. Remember, the entire lifetime of an OB 75 00:05:27,998 --> 00:05:30,662 star. Is maybe 10, 20 million years. 76 00:05:30,662 --> 00:05:36,288 Orbital periods near where the sun is is 200 million so the stars that were formed 77 00:05:36,288 --> 00:05:41,758 by entry into the region of increased density will live out their lives, the OB 78 00:05:41,758 --> 00:05:46,604 stars not far from that region. Now even if all stars are formed in the 79 00:05:46,604 --> 00:05:51,166 spiral arms, stars that live longer will have time to orbit away. 80 00:05:51,166 --> 00:05:56,753 And so you'd expect the red stars. To be far more uniformly scattered. 81 00:05:56,753 --> 00:06:02,722 But, the regions of star formation will be at the region of this density wave. 82 00:06:02,722 --> 00:06:09,019 And this we think is how spirals survive. And so simulation of what this looks like 83 00:06:09,019 --> 00:06:15,240 is Here is this, density we've set up and as the stars orbit the arms orbit with a, 84 00:06:15,240 --> 00:06:20,079 angular velocity that has nothing to do with the angular velocity with which the 85 00:06:20,079 --> 00:06:24,867 star is either on the inside or the outside of the, galaxy orbit, and in the 86 00:06:24,867 --> 00:06:29,505 simulation we see that the arms are stable and we've also decorated them with 87 00:06:29,505 --> 00:06:35,244 blue stars, and, Red H2 regions to make it look like this beautiful example on 88 00:06:35,244 --> 00:06:37,936 the right. And maybe one classic what is called 89 00:06:37,936 --> 00:06:41,417 grand design spiral with very coherent spiral arms. 90 00:06:41,417 --> 00:06:45,162 Populated by lots of blue stars and lots of H1 regions. 91 00:06:45,162 --> 00:06:50,440 it's H2 regions, I mean, ionized regions where hydrogen has been ionized by the 92 00:06:50,440 --> 00:06:56,265 ultraviolet radiation from, these new hot stars, and so, the density weight model 93 00:06:56,265 --> 00:07:01,788 is very successful model for describing these Its a little more difficult when 94 00:07:01,788 --> 00:07:07,355 you come to describe a what is called a flocculent spiral like as you see MGC 95 00:07:07,355 --> 00:07:12,294 4414 over here. When a spiral structure is. 96 00:07:12,294 --> 00:07:15,737 Lets go here. The model that may play part in 97 00:07:15,737 --> 00:07:19,950 describing. The spiral structure which is nonetheless 98 00:07:19,950 --> 00:07:25,728 present here may be a model called SSPSF for stochastic self-propagating star 99 00:07:25,728 --> 00:07:29,381 formation, proposed by Miller and Arnet in 1976. 100 00:07:29,381 --> 00:07:34,354 And the idea is that star formation begins in a roughly uniform disc 101 00:07:34,354 --> 00:07:38,147 randomly, at some point. Stars begin to form, some cloud 102 00:07:38,147 --> 00:07:40,802 collapses. The first stars to form as the cloud 103 00:07:40,802 --> 00:07:45,427 collapses, of course, are the biggest ones because they form faster and so you 104 00:07:45,427 --> 00:07:48,457 get O and B stars. The O and B stars live their life and end 105 00:07:48,457 --> 00:07:53,357 their main sequence existence before many of the smaller stars have even passed the 106 00:07:53,357 --> 00:07:56,727 proto star stage. And then the supernova explosions from 107 00:07:56,727 --> 00:08:00,134 these stars. create shock waves that propagate through 108 00:08:00,134 --> 00:08:04,906 the clouds and basically blow it apart inhibiting the growth of further stars, 109 00:08:04,906 --> 00:08:09,502 so whatever young stars that past the proto- star stage are going to get onto 110 00:08:09,502 --> 00:08:14,165 the main sequence and stay on the main sequence cause stars are not blown away 111 00:08:14,165 --> 00:08:18,811 by the shock wave but the gas and dust is going to be blown apart and therefore 112 00:08:18,811 --> 00:08:22,597 star formation ceases. So if you start with star formation in 113 00:08:22,597 --> 00:08:27,067 one place, it basically blows itself apart, like a forest fire burns itself 114 00:08:27,067 --> 00:08:29,617 out. But like a forest fire, the same shock 115 00:08:29,617 --> 00:08:34,307 wave can be like a spark because farther out when it is no longer as concentrated 116 00:08:34,307 --> 00:08:38,142 as when it blew this cloud to smitherines, farther out after it's 117 00:08:38,142 --> 00:08:41,842 diluted a bit, this shock wave can be precisely the trigger. 118 00:08:41,842 --> 00:08:45,694 Remember, we need a trigger to cause a cloud to suddenly become critical. 119 00:08:45,694 --> 00:08:49,514 It could be the density wave, it could be the shockwave from a supernova. 120 00:08:49,514 --> 00:08:53,145 Causing a nearby cloud, to now become critical and start collapsing. 121 00:08:53,145 --> 00:08:57,250 And so like a forest fire, this process of star formation will propagate in all 122 00:08:57,250 --> 00:09:01,462 directions through the disc. when it propogates, through the disc, the 123 00:09:01,462 --> 00:09:06,752 differential rotation will wind Whatever shapes these regions propagating star 124 00:09:06,752 --> 00:09:11,957 formation initially had the differential rotation, the fact that stars on the 125 00:09:11,957 --> 00:09:15,897 interior orbit faster, will wind it into this spirally shape. 126 00:09:15,897 --> 00:09:18,812 And this might be the model that describes 127 00:09:18,812 --> 00:09:24,019 Flocculent Spirals and maybe there is some combination of the 2 that is in 128 00:09:24,019 --> 00:09:28,867 action at all times because uh,we see that both of them seem to be reasonable 129 00:09:28,867 --> 00:09:32,028 mechanisms. But the question of spiral arms is at 130 00:09:32,028 --> 00:09:33,131 least intriguing.