WEBVTT 00:00:07.699 --> 00:00:10.839 Howdy Star Gazers and welcome to Star Trails. 00:00:11.060 --> 00:00:13.880 I'm Drew and I'll be your guide to the night 00:00:13.880 --> 00:00:18.039 sky for the week of August 17th to the 23rd. 00:00:18.539 --> 00:00:21.899 This is a week designed for patient eyes and 00:00:21.899 --> 00:00:25.320 early alarms. We have slim crescents at dawn, 00:00:25.679 --> 00:00:29.559 a lovely planetary trio before sunrise, and the 00:00:29.559 --> 00:00:32.659 darkest weekend of the month for deep sky hunting. 00:00:33.799 --> 00:00:36.539 Then in the second half of the show, we'll widen 00:00:36.539 --> 00:00:39.619 the frame and talk about the Milky Way's hidden 00:00:39.619 --> 00:00:43.060 companions. Dozens of ghostly little galaxies 00:00:43.060 --> 00:00:46.219 that have been circling us all along, almost 00:00:46.219 --> 00:00:49.759 invisibly. and how they're reshaping our sense 00:00:49.759 --> 00:00:53.840 of home in the universe. Let's begin with the 00:00:53.840 --> 00:00:57.240 moon. Right now it's a delicate waning crescent 00:00:57.240 --> 00:01:00.799 hugging the pre -dawn eastern horizon. Each morning 00:01:00.799 --> 00:01:04.359 it slims further and slides closer to the sun's 00:01:04.359 --> 00:01:07.700 glare. By Friday it's a little more than a silver 00:01:07.700 --> 00:01:11.140 thread in brightening twilight. The new moon 00:01:11.140 --> 00:01:14.480 arrives early Saturday morning. This isn't just 00:01:14.480 --> 00:01:17.810 any new moon. It's what astronomers and calendar 00:01:17.810 --> 00:01:21.890 keepers call a seasonal black moon, the rare 00:01:21.890 --> 00:01:25.450 case when one astronomical season contains four 00:01:25.450 --> 00:01:28.969 new moons instead of the usual three, and this 00:01:28.969 --> 00:01:33.150 is the third of the four. It's mostly a calendar 00:01:33.150 --> 00:01:36.310 quirk, but practically speaking, it means Friday 00:01:36.310 --> 00:01:39.209 and Saturday nights bring the darkest skies of 00:01:39.209 --> 00:01:41.680 the month. If you've been waiting for the Milky 00:01:41.680 --> 00:01:44.420 Way to leap out of the sky, or you want to take 00:01:44.420 --> 00:01:47.079 a crack at the Andromeda galaxy with the naked 00:01:47.079 --> 00:01:50.140 eye from a truly dark sight, this is your moment. 00:01:51.180 --> 00:01:53.879 The pre -dawn sky is where the week's best show 00:01:53.879 --> 00:01:58.459 plays out. On Tuesday, August 19th, Mercury reaches 00:01:58.459 --> 00:02:01.980 its greatest western elongation for this apparition. 00:02:02.540 --> 00:02:06.659 About 30 to 40 minutes before sunrise, scan very 00:02:06.659 --> 00:02:10.120 low in the east northeast. Mercury will be bright 00:02:10.120 --> 00:02:13.219 for a small planet, but still close to the horizon. 00:02:13.800 --> 00:02:16.340 A pair of binoculars will punch through the haze 00:02:16.340 --> 00:02:19.860 and help you spot it. Now look higher up that 00:02:19.860 --> 00:02:23.060 same stretch of sky and you'll find Venus and 00:02:23.060 --> 00:02:26.500 Jupiter still sharing the stage after last week's 00:02:26.500 --> 00:02:30.419 conjunction. Venus blazes lower, Jupiter shines 00:02:30.419 --> 00:02:33.939 above it. On Tuesday and Wednesday a paper -thin 00:02:33.939 --> 00:02:36.659 crescent moon joins them in the morning twilight 00:02:36.889 --> 00:02:40.629 turning the scene into a gorgeous quartet. Mercury 00:02:40.629 --> 00:02:44.289 near the horizon, Venus brighter above it, Jupiter 00:02:44.289 --> 00:02:47.990 higher still, and the moon as a thin accent. 00:02:49.129 --> 00:02:51.770 Evening planets are quieter but worth a look. 00:02:52.090 --> 00:02:55.090 Mars clings to the low western horizon right 00:02:55.090 --> 00:02:58.849 after sunset. It's faint, small, and sets quickly, 00:02:59.050 --> 00:03:01.729 so you'll need an unobstructed view and a bit 00:03:01.729 --> 00:03:05.250 of luck with atmospheric clarity. By late evening, 00:03:05.689 --> 00:03:08.849 Saturn is the reward, rising in the east -southeast 00:03:08.849 --> 00:03:11.689 and climbing higher night by night as it heads 00:03:11.689 --> 00:03:15.710 towards opposition on September 21st. Even a 00:03:15.710 --> 00:03:18.289 small telescope will show its rings and, with 00:03:18.289 --> 00:03:21.650 patient viewing, the tiny star -like point of 00:03:21.650 --> 00:03:25.090 Titan nearby. If you haven't seen Saturn this 00:03:25.090 --> 00:03:28.189 year, mark it down, late August through September 00:03:28.189 --> 00:03:32.539 is prime time. The Perseid meteor shower peaked 00:03:32.539 --> 00:03:36.099 last week, but it doesn't switch off like a faucet. 00:03:36.539 --> 00:03:39.219 Perseids linger through late August, and a few 00:03:39.219 --> 00:03:42.259 bright stragglers are very possible in the predawn 00:03:42.259 --> 00:03:45.319 hours, especially now that moonlight will not 00:03:45.319 --> 00:03:48.780 be washing out the sky. If you're up early for 00:03:48.780 --> 00:03:51.879 the planetary lineup anyway, give yourself 15 00:03:51.879 --> 00:03:56.060 or 20 minutes just to scan the sky. Face generally 00:03:56.060 --> 00:03:59.879 northeast, but don't fixate on the radiant. Meteors 00:03:59.879 --> 00:04:03.000 can streak across any part of the sky, and you'll 00:04:03.000 --> 00:04:05.479 spot more by letting your peripheral vision do 00:04:05.479 --> 00:04:09.039 the work. If you want targets for the dark evenings 00:04:09.039 --> 00:04:12.039 later this week, let me recommend three summer 00:04:12.039 --> 00:04:15.060 gems that don't get a lot of hype. The first 00:04:15.060 --> 00:04:18.699 is Delphinus, the dolphin, a little diamond and 00:04:18.699 --> 00:04:22.120 tail asterism just east of Altair that resembles 00:04:22.120 --> 00:04:26.600 a leaping dolphin. The second is Scutum, the 00:04:26.600 --> 00:04:30.439 shield. Small and unassuming, but home to M11, 00:04:30.680 --> 00:04:34.259 the wild duck cluster. Through binoculars or 00:04:34.259 --> 00:04:37.139 a small scope, it looks like a burst of stardust 00:04:37.139 --> 00:04:40.180 against the Milky Way, dense, rich, and full 00:04:40.180 --> 00:04:44.120 of texture. The third is Volpecula, the fox, 00:04:44.319 --> 00:04:47.439 which hides the famous coat hanger asterism. 00:04:47.959 --> 00:04:51.160 Six stars and a bar, with four forming the hook. 00:04:51.819 --> 00:04:55.240 Sweep slowly between Altair and Vega, and once 00:04:55.240 --> 00:04:57.639 you see the code hanger, you will never unsee 00:04:57.639 --> 00:05:01.699 it. Under this weekend's moonless sky, all three 00:05:01.699 --> 00:05:06.399 reward a patient scan. In recent episodes, we've 00:05:06.399 --> 00:05:08.680 journeyed out to the darkness at the edge of 00:05:08.680 --> 00:05:11.759 our solar system to explore objects in the Kuiper 00:05:11.759 --> 00:05:15.779 Belt and beyond. But what will we find when we 00:05:15.779 --> 00:05:19.339 venture out to the edge of our galaxy? That's 00:05:19.339 --> 00:05:34.500 coming up after the break. Stay with us. Welcome 00:05:34.500 --> 00:05:38.579 back. Today we're talking about the Milky Way's 00:05:38.579 --> 00:05:42.480 hidden companions. When you stand under a truly 00:05:42.480 --> 00:05:45.459 dark sky this weekend and the Milky Way glows 00:05:45.459 --> 00:05:48.319 like a river of light, it is tempting to think 00:05:48.319 --> 00:05:52.000 we know this city of stars well. We've mapped 00:05:52.000 --> 00:05:55.779 its spiral arms, traced its dusty lanes, and 00:05:55.779 --> 00:05:59.720 even weighed the black hole at its center. But 00:05:59.720 --> 00:06:02.819 there's a surprise waiting just beyond the obvious. 00:06:03.379 --> 00:06:07.259 Our galaxy is surrounded by dozens, maybe hundreds, 00:06:07.519 --> 00:06:11.120 of ghostly little galaxies. quietly orbiting 00:06:11.120 --> 00:06:15.360 the Milky Way like dim satellites. Some are so 00:06:15.360 --> 00:06:17.899 faint that until recent years we didn't know 00:06:17.899 --> 00:06:21.779 they even existed. Astronomers call them dwarf 00:06:21.779 --> 00:06:24.639 galaxies and the faintest among them are known 00:06:24.639 --> 00:06:28.560 as ultra -faint dwarfs. They are small collections 00:06:28.560 --> 00:06:32.339 of stars and dark matter bound together by gravity. 00:06:32.939 --> 00:06:36.259 A typical ultra -faint dwarf may contain only 00:06:36.259 --> 00:06:39.540 a few thousand stars spread over hundreds of 00:06:39.540 --> 00:06:43.079 light years. By comparison, the Milky Way holds 00:06:43.079 --> 00:06:46.860 hundreds of billions of stars. If we think of 00:06:46.860 --> 00:06:49.920 the Milky Way as a bright metropolis, these dwarves 00:06:49.920 --> 00:06:54.100 are outlying towns, sparsely populated, dimly 00:06:54.100 --> 00:06:58.319 lit, and far from the bustle of the core. Why 00:06:58.319 --> 00:07:01.970 did we miss them for so long? The answer is partly 00:07:01.970 --> 00:07:05.110 about surface brightness. It's not just how much 00:07:05.110 --> 00:07:08.529 light an object emits, but how spread out that 00:07:08.529 --> 00:07:11.850 light is in the sky. Many of these dwarves are 00:07:11.850 --> 00:07:14.949 so diffuse that their stars melt into the background. 00:07:15.370 --> 00:07:18.589 They don't pop the way a bright cluster or a 00:07:18.589 --> 00:07:22.310 nebula does. The breakthrough came with enormous 00:07:22.310 --> 00:07:25.689 digital sky surveys and sophisticated search 00:07:25.689 --> 00:07:29.470 techniques. The Sloan Digital Sky Survey was 00:07:29.470 --> 00:07:33.389 a pioneer here, and the Dark Energy Survey dramatically 00:07:33.389 --> 00:07:36.509 expanded the census, revealing a host of ultra 00:07:36.509 --> 00:07:39.910 -faint candidates by looking for tiny over -densities. 00:07:40.689 --> 00:07:43.029 These are little knots where there are just a 00:07:43.029 --> 00:07:45.790 few more stars than you would expect by chance. 00:07:46.529 --> 00:07:49.449 Follow -up spectroscopy then distinguishes true 00:07:49.449 --> 00:07:52.329 galaxies from star clusters by measuring how 00:07:52.329 --> 00:07:55.990 the stars move. Galaxies show the signature of 00:07:55.990 --> 00:07:59.829 dark matter, clusters do not. And the floodgates 00:07:59.829 --> 00:08:03.610 are still opening. The Vera C. Rubin Observatory, 00:08:03.889 --> 00:08:06.930 coming fully online soon, will map the southern 00:08:06.930 --> 00:08:10.769 sky over and over for a decade, essentially making 00:08:10.769 --> 00:08:14.370 a time -lapse movie of the universe. It's expected 00:08:14.370 --> 00:08:17.250 to add dozens, maybe hundreds, more of these 00:08:17.250 --> 00:08:21.129 ghostly satellites to the map. These little galaxies 00:08:21.129 --> 00:08:24.939 matter in big ways. First, they are dark matter 00:08:24.939 --> 00:08:28.600 laboratories. Dwarves are often dominated by 00:08:28.600 --> 00:08:32.059 dark matter, so by measuring the motions of their 00:08:32.059 --> 00:08:34.940 stars, astronomers can probe how dark matter 00:08:34.940 --> 00:08:39.200 clusters on small scales. Second, they are crucial 00:08:39.200 --> 00:08:43.259 clues to galactic evolution. Massive galaxies 00:08:43.259 --> 00:08:46.399 like the Milky Way likely grew by merging with 00:08:46.399 --> 00:08:49.379 smaller ones, and many dwarves are survivors 00:08:49.379 --> 00:08:54.240 or new arrivals in that ongoing story. Some are 00:08:54.240 --> 00:08:57.360 actively being torn apart, leaving long streams 00:08:57.360 --> 00:09:00.679 of stars wrapped around our galaxy like faint 00:09:00.679 --> 00:09:04.759 ribbons. Third, they speak directly to the missing 00:09:04.759 --> 00:09:08.149 satellites problem. Computer simulations predict 00:09:08.149 --> 00:09:11.190 that a Milky Way -sized galaxy should have hundreds 00:09:11.190 --> 00:09:14.529 of tiny companions. We have found a few dozens 00:09:14.529 --> 00:09:18.210 so far. Either most of the rest are simply too 00:09:18.210 --> 00:09:20.990 faint to notice until recently, and we're just 00:09:20.990 --> 00:09:24.590 now finding them, or our models need fine tuning. 00:09:25.149 --> 00:09:29.070 Either way, the science gets sharper. A few examples 00:09:29.070 --> 00:09:33.779 make this concrete. The Sagittarius dwarf Spheroidal 00:09:33.779 --> 00:09:37.240 Galaxy is so close to us that our gravity is 00:09:37.240 --> 00:09:40.240 shredding it. Its stars have been pulled into 00:09:40.240 --> 00:09:43.759 great arcs, tidal streams that circle the sky 00:09:43.759 --> 00:09:47.980 and are visible in deep survey data. The Canis 00:09:47.980 --> 00:09:52.220 Major dwarf galaxy, discovered in 2003, may be 00:09:52.220 --> 00:09:55.259 the closest galaxy to our own, hiding behind 00:09:55.259 --> 00:09:58.240 the thick dust and star fields of the Milky Way's 00:09:58.240 --> 00:10:03.539 disk, only 25 ,000 light -years from us. And 00:10:03.539 --> 00:10:07.059 Segway 1, an ultra -faint dwarf discovered in 00:10:07.059 --> 00:10:10.399 Sloan data, might be the dimmest known galaxy, 00:10:10.779 --> 00:10:14.480 with only a few hundred visible stars. On an 00:10:14.480 --> 00:10:17.220 image, it looks like nothing more than a slightly 00:10:17.220 --> 00:10:21.299 suspicious smudge, yet its star's motions betray 00:10:21.299 --> 00:10:24.990 a deep well of dark matter. These aren't showpieces 00:10:24.990 --> 00:10:28.289 at the eyepiece, but they're gold mines for understanding 00:10:28.289 --> 00:10:32.529 how galaxies, including ours, came to be. If 00:10:32.529 --> 00:10:34.889 you live or travel in the southern hemisphere, 00:10:35.230 --> 00:10:37.909 you can see two of the Milky Way's brighter companions 00:10:37.909 --> 00:10:41.990 with your unaided eyes. The large and small Magellanic 00:10:41.990 --> 00:10:44.850 Clouds, which look like detached pieces of the 00:10:44.850 --> 00:10:47.529 Milky Way itself floating off the side of the 00:10:47.529 --> 00:10:51.100 sky. From northern latitudes, you can make a 00:10:51.100 --> 00:10:53.679 powerful connection by turning binoculars or 00:10:53.679 --> 00:10:57.720 a small scope toward the Andromeda Galaxy, M31. 00:10:58.639 --> 00:11:01.700 It's not a satellite of the Milky Way. We and 00:11:01.700 --> 00:11:04.799 Andromeda are the two big cities in our local 00:11:04.799 --> 00:11:08.399 group. But thinking of us as Andromeda's companion 00:11:08.399 --> 00:11:11.279 gives a helpful sense of scale and relationship. 00:11:11.960 --> 00:11:14.799 There is also a delightful binocular tie -in 00:11:14.799 --> 00:11:17.220 you can do from anywhere in the northern hemisphere. 00:11:17.740 --> 00:11:21.039 Look toward the rich star fields of Sagittarius 00:11:21.039 --> 00:11:25.740 and find the globular cluster M54. This cluster 00:11:25.740 --> 00:11:28.720 is actually embedded in the Sagittarius dwarf 00:11:28.720 --> 00:11:32.779 galaxy. When you gaze at M54, you're peering 00:11:32.779 --> 00:11:36.639 into another galaxy's star system. You can also 00:11:36.639 --> 00:11:39.340 explore these faint satellites from the couch. 00:11:39.629 --> 00:11:43.389 The Sloan Digital Sky Survey's online navigate 00:11:43.389 --> 00:11:47.190 tools let you type in targets like Segway 1 or 00:11:47.190 --> 00:11:51.009 Canis Major Dwarf, and zoom into the exact patches 00:11:51.009 --> 00:11:54.309 of sky where they hide. The Dark Energy Survey 00:11:54.309 --> 00:11:57.029 has public image galleries that show these ultra 00:11:57.029 --> 00:12:00.309 -faint dwarves against deep star fields, and 00:12:00.309 --> 00:12:02.549 they're striking once you know what you're looking 00:12:02.549 --> 00:12:05.669 at. If you want a three -dimensional feel for 00:12:05.669 --> 00:12:09.090 the neighborhood, free software like GaiaSky 00:12:09.279 --> 00:12:11.700 can let you fly around the model of the Milky 00:12:11.700 --> 00:12:14.379 Way and see where the known satellites live in 00:12:14.379 --> 00:12:17.860 our halo. I'll drop links to all of these in 00:12:17.860 --> 00:12:21.259 the show notes. There is a poetic link between 00:12:21.259 --> 00:12:24.299 this week's seasonal black moon and these hidden 00:12:24.299 --> 00:12:28.340 galaxies. The black moon hides in the sun's glare, 00:12:28.879 --> 00:12:31.519 invisible not because it's absent, but because 00:12:31.519 --> 00:12:35.539 of where and when we're looking. The dwarf galaxies 00:12:35.539 --> 00:12:38.809 hide in the Milky Way's glare, invisible to our 00:12:38.809 --> 00:12:42.029 eyes until we gather enough light and apply enough 00:12:42.029 --> 00:12:45.629 patience. In both cases, the sky is reminding 00:12:45.629 --> 00:12:48.529 us that the obvious story is rarely the whole 00:12:48.529 --> 00:12:52.490 story. A darker sky reveals more. A deeper look 00:12:52.490 --> 00:12:56.509 reveals the rest. Our galaxy is not an island 00:12:56.509 --> 00:13:00.230 alone in the dark. It's part of a quiet archipelago 00:13:00.230 --> 00:13:04.149 of companions, faint and fragile, sharing our 00:13:04.149 --> 00:13:11.820 long orbit through the universe. If the stars 00:13:11.820 --> 00:13:14.220 spoke to you this week or if a question's been 00:13:14.220 --> 00:13:17.100 on your mind, I'd love to hear it. Visit our 00:13:17.100 --> 00:13:20.820 website starrtrails .show where you can contact 00:13:20.820 --> 00:13:24.620 me and explore past episodes. Be sure to follow 00:13:24.620 --> 00:13:28.039 us on Blue Sky and YouTube. Links are in the 00:13:28.039 --> 00:13:31.220 show notes. Until we meet again beneath the stars, 00:13:31.799 --> 00:13:32.759 clear skies everyone.