1 00:00:00,012 --> 00:00:04,725 So, armed with all these observational tools, we have a pretty good idea of what 2 00:00:04,725 --> 00:00:08,428 the Milky Way is made of. In this first clip, let's sort of lay out 3 00:00:08,428 --> 00:00:12,757 the pieces, and later we'll study each of them in term and see what we can say 4 00:00:12,757 --> 00:00:15,642 about them. So, what is the Milky Way made of? Well, 5 00:00:15,642 --> 00:00:20,188 the Milky Way is mostly a disk of stars that are orbiting, and they have a sense 6 00:00:20,188 --> 00:00:23,645 in which they orbit much as in the solar system. 7 00:00:23,645 --> 00:00:28,361 You imagine some primordial angular momentum being conserved, so the Milky 8 00:00:28,361 --> 00:00:33,133 Way has a north pole and a south pole typically drawn to the top and the bottom 9 00:00:33,133 --> 00:00:36,745 of our images. Sorry, southern hemisphere dwellers. 10 00:00:36,745 --> 00:00:41,611 and in the disk, in fact, the disk is broken up into two different components, 11 00:00:41,611 --> 00:00:47,148 this is a relatively recent find. There is a thin disk only about a third 12 00:00:47,148 --> 00:00:52,392 of a kiloparsec, it's only about 300 parsecs thick, a 1000 light years and, 13 00:00:52,392 --> 00:00:58,056 over the radius of about 25 kiloparsecs. And remember, the sun that way being 8 14 00:00:58,056 --> 00:01:03,720 kiloparsecs from the center is about a third of the way to the edge of the disk. 15 00:01:03,720 --> 00:01:10,399 the thin disk has the bulk of the mass the, the stars in the galaxy the stellar 16 00:01:10,399 --> 00:01:16,015 mass in a thin disk is about 60 billion solar masses, and the stars in the thin 17 00:01:16,015 --> 00:01:21,244 disk are, by and large, young. Their ages are less than 8 billion years, 18 00:01:21,244 --> 00:01:26,423 so the sun is certainly a young star. And the thin disk is surrounded by a 19 00:01:26,423 --> 00:01:31,203 thicker disk with about the same radius but 3 times the thickness. 20 00:01:31,203 --> 00:01:34,902 So, 3,000 light years, a kiloparsec in thickness. 21 00:01:34,902 --> 00:01:40,797 And, the total mass in the thick disk is neglible if you want, only 3 billion 22 00:01:40,797 --> 00:01:46,264 solar masses. But its charaterized this population of this thick disk and it's 23 00:01:46,264 --> 00:01:52,135 not that it's surrounded, of course. the thickness permeates the thin disk as 24 00:01:52,135 --> 00:01:57,070 well but the stars are scattered more diffusely and the thing that 25 00:01:57,070 --> 00:02:02,563 characterizes the thick disk population [LAUGH] is that the ages of the stars are 26 00:02:02,563 --> 00:02:08,782 older typically 8 to 11 billion years are the, or 10 to 11 depending on exactly 27 00:02:08,782 --> 00:02:13,433 where you draw the line, are the ages of the stars in the thick disk. So, two 28 00:02:13,433 --> 00:02:17,760 distinct stellar population but the bulk is in the thin disk. 29 00:02:17,760 --> 00:02:22,523 at the center is a bulge where the galaxy bulges to a greater thickness. 30 00:02:22,523 --> 00:02:26,784 Of course, the disk thickens out as it approaches the bulge, but there is a 31 00:02:26,784 --> 00:02:31,962 distinct structure called the bulge and as it has been discovered in the past 32 00:02:31,962 --> 00:02:36,399 decade, it's actually not a sphere as drawn in this image, but it has a bar 33 00:02:36,399 --> 00:02:39,530 shaped. and it's and therefore while its height 34 00:02:39,530 --> 00:02:45,290 is about 2 kilo parsecs, it's length along the long axis of the bulges 5 kilo 35 00:02:45,290 --> 00:02:48,954 parsecs, and this contains about 10 billion solar masses. 36 00:02:48,954 --> 00:02:53,600 the stars in the bulge, have all kinds of ages, there are different populations, 37 00:02:53,600 --> 00:02:57,869 perhaps 3 different populations, including very young stars, 200 million 38 00:02:57,869 --> 00:03:00,452 years or younger, and some very old stars. 39 00:03:00,452 --> 00:03:02,884 And we'll talk about the bulge population. 40 00:03:02,884 --> 00:03:07,836 And, this is the disk with the bulge and then surrounding all of this is the halo 41 00:03:07,836 --> 00:03:11,397 in which, for example, one finds the globular clusters orbiting. 42 00:03:11,397 --> 00:03:15,702 These are stars, remember they don't sit there, they're orbiting. 43 00:03:15,702 --> 00:03:20,236 So, these are objects that are orbiting in, in orbits with high inclination to 44 00:03:20,236 --> 00:03:24,707 the galactic plane. And so some of them might at any given time be inside the 45 00:03:24,707 --> 00:03:29,379 disk, but they would be distinguished because their velocity would then be 46 00:03:29,379 --> 00:03:34,195 perpendicular roughly, to the direction of motion of the stars in the disk. 47 00:03:34,195 --> 00:03:39,532 And the math of the halo stars is again very small, only 3 billion solar masses. 48 00:03:39,532 --> 00:03:43,012 And the halo certainly contains the oldest stars in the galaxy. 49 00:03:43,012 --> 00:03:47,202 Their ages range between 11 and 13 billion years, that's essentially the age 50 00:03:47,202 --> 00:03:50,022 of the galaxy. Remember, it's almost the age of the 51 00:03:50,022 --> 00:03:52,442 universe. The asterisk is because there's a 52 00:03:52,442 --> 00:03:56,622 component of the halo that I have not discussed here, it's been recently 53 00:03:56,622 --> 00:03:59,500 discovered. And we'll discuss it when the time comes.