1 00:00:00,000 --> 00:00:06,000 For astronomers, Omega Centauri has been an outcast amongst globular clusters for a long time. 2 00:00:06,000 --> 00:00:11,000 A new result obtained by the NASA/ESA Hubble Space Telescope and the Gemini Observatory 3 00:00:11,000 --> 00:00:16,000 provides a surprising explanation for Omega Centauri’s peculiarities. 4 00:00:35,000 --> 00:00:37,000 This is the Hubblecast! 5 00:00:37,000 --> 00:00:41,000 News and Images from the NASA/ESA Hubble Space Telescope. 6 00:00:41,000 --> 00:00:46,000 Travelling through time and space with our host Doctor J a.k.a. Dr Joe Liske. 7 00:00:47,000 --> 00:00:49,000 Welcome to the Hubblecast. 8 00:00:49,000 --> 00:00:53,000 Today’s cosmic guest star is a very special object. 9 00:00:53,000 --> 00:00:59,000 Omega Centauri has long been known to be the largest and brightest globular cluster visible in the night sky. 10 00:00:59,000 --> 00:01:08,000 A globular cluster is a nearly spherical group of tens to hundreds of thousands of old stars tightly bound together by gravity, 11 00:01:08,000 --> 00:01:12,000 and found on the outskirts of many galaxies including our own Milky Way. 12 00:01:12,000 --> 00:01:19,000 Beautiful, but enigmatic, Omega Centauri has always been a bit of a puzzle to astronomers. 13 00:01:23,000 --> 00:01:29,000 Omega Centauri lies in the constellation of Centaurus and is visible from Earth with the naked eye. 14 00:01:29,000 --> 00:01:33,000 It is one of the favourite celestial objects for southern hemisphere stargazers, 15 00:01:33,000 --> 00:01:38,000 appearing almost as large as the full Moon when seen from a dark site. 16 00:01:38,000 --> 00:01:43,000 Exactly what type of object Omega Centauri is, has long been a contentious topic. 17 00:01:43,000 --> 00:01:49,000 It was first listed in Ptolemy’s catalogue as a single star nearly two thousand years ago. 18 00:01:49,000 --> 00:01:54,000 In 1677, Edmond Halley reported it as a nebula. 19 00:01:54,000 --> 00:02:04,000 In the 1830s the English astronomer John Herschel was the first to recognize it as a globular cluster, a classification that it has kept ever since. 20 00:02:13,000 --> 00:02:18,000 Omega Centauri has several characteristics that separate it from other globular clusters: 21 00:02:18,000 --> 00:02:29,000 compared to a run-of-the-mill globular, Omega Centauri has a highly flattened shape, it rotates faster, and it includes several generations of stars – 22 00:02:29,000 --> 00:02:35,000 which is an unusual feature for globulars, which normally contain only a single generation of old stars. 23 00:02:35,000 --> 00:02:44,000 Moreover, Omega Centauri is almost 10 times more massive as other globular clusters – almost as massive as a small galaxy. 24 00:02:44,000 --> 00:02:50,000 Now, new images obtained with the Advanced Camera for Surveys onboard the NASA/ESA Hubble Space Telescope 25 00:02:50,000 --> 00:02:54,000 and data obtained by the GMOS spectrograph at the Gemini Observatory 26 00:02:54,000 --> 00:03:01,000 show that Omega Centauri appears to be harbouring an elusive intermediate-mass black hole at its centre. 27 00:03:06,000 --> 00:03:13,000 The black hole was discovered after astronomers measured the motions and brightnesses of stars at the centre of Omega Centauri. 28 00:03:13,000 --> 00:03:19,000 They found that these stars were moving much faster than expected given their total number and brightness. 29 00:03:19,000 --> 00:03:26,000 Such behaviour clearly indicates the existence of something extraordinarily massive at the centre of the cluster. 30 00:03:26,000 --> 00:03:32,000 The intense gravitational field of a black hole with a mass of 40,000 solar masses 31 00:03:32,000 --> 00:03:36,000 provides just the kick necessary to explain the measurements. 32 00:03:38,000 --> 00:03:44,000 This is Dr Eva Noyola of the Max Planck Institute for Extraterrestrial Physics in Germany. 33 00:03:44,000 --> 00:03:49,000 So Eva, you were the lead author on this study. How did you find this black hole? 34 00:03:49,000 --> 00:03:56,000 The way we found this black hole was by looking at the Hubble image and measuring how many stars are there at the very centre of this cluster. 35 00:03:56,000 --> 00:04:00,000 And then we take the GMOS spectrograph and measured velocities, the velocities of these stars. 36 00:04:00,000 --> 00:04:04,000 What we find is that the velocities are much higher than what we would expect just from the stars, 37 00:04:04,000 --> 00:04:09,000 and that implies that there is an extra amount of matter, which means that we found a black hole. 38 00:04:09,000 --> 00:04:11,000 And what’s so special about this black hole? 39 00:04:11,000 --> 00:04:16,000 The very special thing about this black hole is that it has 40 thousand solar masses in size. 40 00:04:16,000 --> 00:04:20,000 It means it is a lot bigger than the stellar mass black holes that we find in our Galaxy, 41 00:04:20,000 --> 00:04:25,000 but is much smaller than the supermassive black holes that we find at the centre of galaxies. 42 00:04:25,000 --> 00:04:31,000 It means that nature makes black holes in a continuous mass range, not just in small and big sizes. 43 00:04:31,000 --> 00:04:34,000 So, what’s the big deal? Why is it so important? 44 00:04:34,000 --> 00:04:41,000 Well, first of all this is only the second black hole that we find in a globular cluster, so this is a very nice reassuring thing. 45 00:04:41,000 --> 00:04:47,000 But also, we know that there are seeds required to grow the supermassive black holes from scratch. 46 00:04:47,000 --> 00:04:53,000 And if we find many of these, then it will be a very nice source for the seeds to grow the supermassive black holes. 47 00:04:56,000 --> 00:05:02,000 One implication of this discovery is that it is very likely that Omega Centauri is not a globular cluster at all, 48 00:05:02,000 --> 00:05:07,000 but rather some kind of dwarf galaxy that has been stripped of its outer stars and dark matter, 49 00:05:07,000 --> 00:05:11,000 as some scientists have suspected for a few years. 50 00:05:11,000 --> 00:05:19,000 More than two thousand years after Omega Centauri was wrongly classified as a star, it’s true nature is finally coming to light. 51 00:05:19,000 --> 00:05:25,000 But I wonder, does Omega Centauri have more surprises in store for us? 52 00:05:25,000 --> 00:05:28,000 This is Dr J signing off for the Hubblecast. 53 00:05:28,000 --> 00:05:32,000 Once again nature has surprised us beyond our wildest imagination … 54 00:05:33,000 --> 00:05:37,000 Hubblecast is produced by ESA/Hubble at ESO in Germany. 55 00:05:37,000 --> 00:05:42,000 The Hubble mission is a project of international cooperation between NASA and the European Space Agency.