1 00:00:00,000 --> 00:00:04,000 Bright pink nebulae almost completely encircle 2 00:00:04,000 --> 00:00:09,000 a spiral galaxy in this NASA/ESA Hubble Space Telescope 3 00:00:09,000 --> 00:00:13,000 image of NGC 922 4 00:00:15,000 --> 00:00:17,000 The ring structure and the galaxy's distorted shape 5 00:00:17,000 --> 00:00:21,000 results from a smaller galaxy scoring a cosmic bullseye 6 00:00:21,000 --> 00:00:28,000 hitting the centre of NGC 922 some 330 million years ago 7 00:00:45,000 --> 00:00:48,000 Episode 60: Galaxy hits the bullseye 8 00:00:50,000 --> 00:00:57,000 Hubble's image of NGC 922 shows that the galaxy is far from being an ordinary spiral 9 00:00:57,000 --> 00:01:03,000 Its arms are disrupted; a stream of stars extends out into space 10 00:01:03,000 --> 00:01:06,000 and a bright ring of nebulae encircles the core 11 00:01:06,000 --> 00:01:13,000 Observing NGC 922 with NASA's Chandra satellite reveals more chaos 12 00:01:13,000 --> 00:01:18,000 in the form of ultraluminous X-ray sources dotted around the galaxy 13 00:01:23,000 --> 00:01:30,000 NGC 922's unusual form today has its origins hundreds of millions of years ago 14 00:01:30,000 --> 00:01:37,000 At that time, a smaller galaxy plunged right through the heart of NGC 922 15 00:01:40,000 --> 00:01:44,000 Although it's outside Hubble's field of view now 16 00:01:44,000 --> 00:01:45,000 images from a survey telescope on the ground 17 00:01:45,000 --> 00:01:49,000 show the interloper shooting away from the scene of the crash 18 00:01:52,000 --> 00:01:56,000 As the small galaxy passed through the middle of NGC 922 19 00:01:56,000 --> 00:02:00,000 it sent out ripples that disrupted the clouds of gas 20 00:02:00,000 --> 00:02:03,000 and triggered the formation of new stars 21 00:02:03,000 --> 00:02:08,000 The radiation from these new stars in turn lit up the gas 22 00:02:08,000 --> 00:02:11,000 The bright pink colour of the resulting nebulae 23 00:02:11,000 --> 00:02:18,000 is a characteristic sign of this process, and it is caused by excited hydrogen gas 24 00:02:18,000 --> 00:02:21,000 the dominant element in interstellar gas clouds 25 00:02:21,000 --> 00:02:27,000 It's a similar process to that used in neon signs here on Earth 26 00:02:28,000 --> 00:02:32,000 In theory, if two galaxies are aligned in just the right way 27 00:02:32,000 --> 00:02:36,000 with the smaller galaxy passing right through the centre of the larger one 28 00:02:36,000 --> 00:02:40,000 the ring of nebulae will form a perfect circle 29 00:02:40,000 --> 00:02:44,000 But more often, the two galaxies are slightly offset 30 00:02:44,000 --> 00:02:50,000 forming a circle like this one, which is noticeably brighter on one side than on the other 31 00:02:53,000 --> 00:02:59,000 These objects, called collisional ring galaxies, are relatively rare in our cosmic neighbourhood 32 00:02:59,000 --> 00:03:03,000 Although mergers and collisions are quite common 33 00:03:03,000 --> 00:03:06,000 The precise alignment and ratio of sizes 34 00:03:06,000 --> 00:03:09,000 necessary to form a ring is not 35 00:03:09,000 --> 00:03:15,000 Only a few are known in our cosmic neighbourhood and NGC 922 36 00:03:15,000 --> 00:03:21,000 and the Cartwheel Galaxy, also snapped by Hubble, are the best known examples 37 00:03:25,000 --> 00:03:29,000 Hubblecast is produced by ESA/Hubble at the European Southern Observatory in Germany 38 00:03:29,000 --> 00:03:32,000 The Hubble mission is a project of international cooperation between NASA and the European Space Agency 39 00:03:37,000 --> 00:03:39,000 www.spacetelescope.org 40 00:03:39,000 --> 00:03:42,000 Transcribed by ESA/Hubble. Translation: ----