1 00:00:00,012 --> 00:00:04,370 Okay. We have some understanding what goes on in the disk. Let's look towards 2 00:00:04,370 --> 00:00:07,089 the center. Of course, if you thought it was hard to 3 00:00:07,089 --> 00:00:11,201 figure out the structure of the disk, understanding the structure of the bulge 4 00:00:11,201 --> 00:00:14,547 is more difficult, yet, that is the direction where there is most 5 00:00:14,547 --> 00:00:18,645 obstruction by dust and so on. In fact as I said, we only realized that 6 00:00:18,645 --> 00:00:23,676 the Milky Way has a bar, this was only verified, there were suspicions earlier 7 00:00:23,676 --> 00:00:28,213 in the past decade, so we're still learning very much about the star 8 00:00:28,213 --> 00:00:33,118 structure of the Milky Way galaxy. In the bulge we find, as I said, stars of 9 00:00:33,118 --> 00:00:38,746 varying ages with varying populations an interesting reversal of what we're used 10 00:00:38,746 --> 00:00:43,026 to happens in the bulge. the oldest populations of stars in the 11 00:00:43,026 --> 00:00:48,112 galactic bulge end up having, turned out to have high metallicity, so they have 12 00:00:48,112 --> 00:00:53,543 high, relatively, concentrations of non-hydrogen and helium elements, whereas 13 00:00:53,543 --> 00:00:58,807 the newer stars are more metal-poor. the interpretation that we put on this is 14 00:00:58,807 --> 00:01:03,844 that presumably, early in the galaxy's history, there was this burst of star 15 00:01:03,844 --> 00:01:08,450 formation in the bulge. these, these formed many hot large stars 16 00:01:08,450 --> 00:01:13,492 which exploded as supernovae enriching the bulge, bulge region in metals, and 17 00:01:13,492 --> 00:01:17,547 therefore, subsequent stars that formed shortly thereafter were metal-rich. 18 00:01:17,547 --> 00:01:21,762 Subsequent to all of this, there must have been an infall of fresh unenriched 19 00:01:21,762 --> 00:01:25,832 matter from regions of a galaxy where star formation had not yet started. 20 00:01:25,832 --> 00:01:30,566 So, where there had not been supernova to enrich the environment, this brought in 21 00:01:30,566 --> 00:01:35,465 relatively low metal, metallicity hydrogen, and subsequent generations of 22 00:01:35,465 --> 00:01:40,806 stars formed in the bulge would have formed in a metal-poor environment, and 23 00:01:40,806 --> 00:01:46,391 the bulge is one of the regions in which ongoing star formation seems to be going 24 00:01:46,391 --> 00:01:50,215 on as we speak right now. I think it's hard to see the bulge, 25 00:01:50,215 --> 00:01:54,930 of course, the most interesting action in some sense is that the center, in the 26 00:01:54,930 --> 00:02:00,496 core seeing the core is harder still infrared observations allow us to peer 27 00:02:00,496 --> 00:02:05,484 through the dust to some extent and find that at the center of a galaxy is dense 28 00:02:05,484 --> 00:02:09,258 cluster of stars, including a bunch of stars with you'd 29 00:02:09,258 --> 00:02:14,028 call them type O and B. So, had blue stars, but with luminosities 30 00:02:14,028 --> 00:02:18,644 far exceeding what we expect for main sequence type O and B stars. 31 00:02:18,644 --> 00:02:22,869 luminosities of a million, solar luminosities per star. 32 00:02:22,869 --> 00:02:29,098 one interpretation of this is that these may be Wolf-Rayet stars, so older stars 33 00:02:29,098 --> 00:02:34,944 from a starburst episode about 10 million years ago, which are now very luminous. 34 00:02:34,944 --> 00:02:39,981 Remember, we're measuring their luminosity, the luminosity of an OB star 35 00:02:39,981 --> 00:02:44,002 in the infrared. These may be having extreme infrared 36 00:02:44,002 --> 00:02:47,293 emissions, because they are Wolf-Rayet stars. 37 00:02:47,293 --> 00:02:52,424 there are many puzzles about the on, goings on at the center of the galaxy. 38 00:02:52,424 --> 00:02:58,079 For example, there is evidence for a very dense mollecular cloud but no ongoing 39 00:02:58,079 --> 00:03:00,927 star formations, the conditions are right for star 40 00:03:00,927 --> 00:03:04,241 formation, but star formation does not seem to be going on. 41 00:03:04,241 --> 00:03:08,878 At the center of it all, of course, it's our 3 or 4 million solar mass black hole 42 00:03:08,878 --> 00:03:12,988 in the center of the galaxy. black holes in general, just a black hole 43 00:03:12,988 --> 00:03:17,439 in the middle of nowhere, is hard to detect. Indeed black holes are detectable 44 00:03:17,439 --> 00:03:20,986 when they create matter. the black hole at the center of the 45 00:03:20,986 --> 00:03:23,592 galaxy is creating matter at a modest rate. 46 00:03:23,592 --> 00:03:29,392 there is some x-ray activity there are occasional flares, but there is not large 47 00:03:29,392 --> 00:03:33,409 bursts of activity as we will see occur in some other galaxies. 48 00:03:33,409 --> 00:03:37,532 And to give you a sense for what infrared radiation does for us, 49 00:03:37,532 --> 00:03:44,882 we can watch this video courtesy of ESO, which zooms in on the center of a galaxy, 50 00:03:44,882 --> 00:03:49,207 invisible light. You see we're looking right into the 51 00:03:49,207 --> 00:03:54,339 heart of the constellation the, the tip of the teapot, of the spout of the 52 00:03:54,339 --> 00:03:59,187 teapot, that is Sagittarius. But more importantly, right into the heart of a 53 00:03:59,187 --> 00:04:04,277 dustling where we can see very little, and as we zoom in, the image goes pretty 54 00:04:04,277 --> 00:04:09,296 much greyed and dark. And the only way to see through this dust will be, this is 55 00:04:09,296 --> 00:04:13,752 the visible image, we are going to convert to infrared light 56 00:04:13,752 --> 00:04:17,481 and compare the visible image which is this. Not much. 57 00:04:17,481 --> 00:04:22,469 With, as the slider sweeps over, an infrared image, which allows us to peer 58 00:04:22,469 --> 00:04:27,729 through and realize that these few dim stars that we see in the Mac are in fact 59 00:04:27,729 --> 00:04:32,741 extraordinarily brilliant, of course, the colors to the right are all fake. 60 00:04:32,741 --> 00:04:37,181 This is an infrared image. But this is the way in which infrared 61 00:04:37,181 --> 00:04:41,525 light allows us to peer through the dust and distinguish the goings on at the 62 00:04:41,525 --> 00:04:42,684 center of a galaxy.