WEBVTT FILE ﻿1 00:00:00.000 --> 00:00:03.704 We all know what a galaxy looks like, right? It’s a huge 2 00:00:03.704 --> 00:00:06.106 collection of stars and other matter that’s shaped like a 3 00:00:06.106 --> 00:00:09.376 spiral or an ellipse, and if you’re an astronomy fan you 4 00:00:09.376 --> 00:00:11.945 probably know that most of the mass is from invisible, 5 00:00:11.945 --> 00:00:16.250 mysterious material called dark matter. Well, NASA’s Hubble 6 00:00:16.250 --> 00:00:20.020 Space Telescope just took an image of a galaxy that is none 7 00:00:20.020 --> 00:00:23.757 of those things. For the first time, astronomers have strong 8 00:00:23.757 --> 00:00:27.527 evidence for a galaxy not having a significant amount of dark 9 00:00:27.527 --> 00:00:31.031 matter. Most astronomers currently believe that dark 10 00:00:31.031 --> 00:00:34.368 matter plays a fundamental role in our universe and the 11 00:00:34.368 --> 00:00:37.204 formation of galaxies. This is because galaxies seem to have a 12 00:00:37.204 --> 00:00:40.340 lot more mass than what we can account for based on just the 13 00:00:40.340 --> 00:00:43.644 stars we see. How much mass is in a system is determined by 14 00:00:43.644 --> 00:00:46.313 measuring the speed at which galaxies rotate or individual 15 00:00:46.313 --> 00:00:50.484 stars in a galaxy move. Without the gravity from that mass, a 16 00:00:50.484 --> 00:00:53.186 galaxy would fly apart if everything in it is moving as 17 00:00:53.186 --> 00:00:57.157 quickly as we observe it moving. Decades of research have led 18 00:00:57.157 --> 00:01:01.495 astronomers to the extraordinary conclusion that 85% of the mass 19 00:01:01.495 --> 00:01:05.666 in our universe is invisible in all wavelengths of radiation, 20 00:01:05.666 --> 00:01:08.602 and that it’s composed of matter that does not contain protons or 21 00:01:08.602 --> 00:01:12.406 neutrons or any type of particle we’ve detected before, AND that 22 00:01:12.406 --> 00:01:16.043 this invisible material is all around us, passing through us 23 00:01:16.043 --> 00:01:20.247 without interacting with regular matter except by gravity. For 24 00:01:20.247 --> 00:01:23.050 some, that’s a tough pill to swallow, and a minority of 25 00:01:23.050 --> 00:01:25.919 astronomers wonder if maybe we just don’t completely understand 26 00:01:25.919 --> 00:01:30.190 how gravity works. If that were the case, and it was an inherent 27 00:01:30.190 --> 00:01:32.926 property of gravity that causes galaxies to move the way they 28 00:01:32.926 --> 00:01:36.997 do, then we could expect all galaxies to behave the same way. 29 00:01:36.997 --> 00:01:39.599 In other words, they would all seem to have about the same 30 00:01:39.599 --> 00:01:43.403 portion of “dark matter.” But with the galaxy in this Hubble 31 00:01:43.403 --> 00:01:46.540 image, astronomers looked at the velocities of ten globular 32 00:01:46.540 --> 00:01:49.676 clusters in the galaxy, each a spherical collection of hundreds 33 00:01:49.676 --> 00:01:52.679 of thousands of stars, and calculated that their movements 34 00:01:52.679 --> 00:01:56.183 can be accounted for entirely by the mass of the visible material 35 00:01:56.183 --> 00:02:00.153 in this system. That means this galaxy has little to no dark 36 00:02:00.153 --> 00:02:04.257 matter. Strangely, this absence of dark matter actually provides 37 00:02:04.257 --> 00:02:07.394 evidence that dark matter is real. It shows that dark matter 38 00:02:07.394 --> 00:02:09.663 isn’t always coupled with regular matter – that it’s 39 00:02:09.663 --> 00:02:13.500 something separate. You can have regular matter without dark 40 00:02:13.500 --> 00:02:16.803 matter. This galaxy is really weird even beyond the dark 41 00:02:16.803 --> 00:02:19.473 matter thing. You may have noticed you can see straight 42 00:02:19.473 --> 00:02:22.609 through it. That’s because this galaxy is what’s called an 43 00:02:22.609 --> 00:02:26.113 “ultra diffuse galaxy,” which as the name implies, is extremely 44 00:02:26.113 --> 00:02:29.549 low density. This galaxy is about the same volume as our own 45 00:02:29.549 --> 00:02:34.921 Milky Way galaxy, but only has about 0.5% the amount of stars. 46 00:02:34.921 --> 00:02:37.190 Though astronomers have known about ultra diffuse galaxies 47 00:02:37.190 --> 00:02:40.227 since the early 1980s, they can be difficult to find since 48 00:02:40.227 --> 00:02:43.630 they’re so faint. A team of astronomers is using an array of 49 00:02:43.630 --> 00:02:46.633 telephoto lenses called Dragonfly to seek out these 50 00:02:46.633 --> 00:02:49.770 ghostly-looking objects. They obtained observations from 51 00:02:49.770 --> 00:02:53.640 Dragonfly, the Sloan Digital Sky Survey, the Gemini Observatory, 52 00:02:53.640 --> 00:02:56.610 and the Keck Observatory, then requested time on the Hubble 53 00:02:56.610 --> 00:03:00.514 Space Telescope to take a closer look at this unusual galaxy. 54 00:03:00.514 --> 00:03:03.116 Having images and data from multiple sources allowed the 55 00:03:03.116 --> 00:03:05.686 team to determine that this galaxy does not have a 56 00:03:05.686 --> 00:03:08.922 significant amount of dark matter. This was definitely 57 00:03:08.922 --> 00:03:12.492 surprising to find. No other galaxies so far have appeared to 58 00:03:12.492 --> 00:03:16.396 be so lacking in dark matter. In fact, other ultra diffuse 59 00:03:16.396 --> 00:03:19.332 galaxies seem to have an overabundance of dark matter. 60 00:03:19.332 --> 00:03:22.536 The same team who studied this galaxy discovered a different 61 00:03:22.536 --> 00:03:28.075 ultra diffuse galaxy in 2016 that they calculated was 99.9% 62 00:03:28.075 --> 00:03:32.245 dark matter. Yet another weird thing about this galaxy – the 63 00:03:32.245 --> 00:03:35.048 globular clusters used to measure the galaxy’s rotation 64 00:03:35.048 --> 00:03:38.518 are way brighter than normal globular clusters. The 65 00:03:38.518 --> 00:03:40.787 researchers have written a different paper that focuses on 66 00:03:40.787 --> 00:03:44.925 just these oddball collections of stars. So, this is a very 67 00:03:44.925 --> 00:03:48.395 strange galaxy in several ways. Astronomers will be looking at 68 00:03:48.395 --> 00:03:51.565 Hubble observations of other ultra diffuse galaxies to see if 69 00:03:51.565 --> 00:03:54.534 there are any other examples of galaxies with unusually low or 70 00:03:54.534 --> 00:03:57.838 high amounts of dark matter. With more samples, astronomers 71 00:03:57.838 --> 00:03:59.906 will be able to better understand the nature of dark 72 00:03:59.906 --> 00:04:03.076 matter, the formation and evolution of galaxies, and the 73 00:04:03.076 --> 00:04:05.712 overall structure of our universe. 74 00:04:05.712 --> 00:00:00.000 www.nasa.gov/hubble @NASAHubble