1 00:00:01,000 --> 00:00:06,000 Everyone has heard echoes — but have you ever seen one? 2 00:00:06,000 --> 00:00:09,000 Hubble has. 3 00:00:09,000 --> 00:00:12,000 By watching the variable star RS Puppis 4 00:00:12,000 --> 00:00:19,000 Hubble has captured a beautiful, and useful, phenomenon known as a light echo. 5 00:00:35,000 --> 00:00:39,000 Episode 71: Visible echoes around RS Puppis 6 00:00:42,000 --> 00:00:52,000 For most of its life a star is quite stable, slowly consuming the hydrogen at its core to keep it shining brightly. 7 00:00:52,000 --> 00:00:56,000 But when most of the hydrogen has been consumed, 8 00:00:56,000 --> 00:01:04,000 some stars evolve into very different beasts known as variable stars. 9 00:01:04,000 --> 00:01:08,000 They become unstable and start pulsating, 10 00:01:08,000 --> 00:01:17,000 expanding and shrinking over a number of days or weeks and growing brighter and dimmer as they do so. 11 00:01:21,000 --> 00:01:29,000 RS Puppis is one such variable star, a type known as a Cepheid variable. 12 00:01:29,000 --> 00:01:40,000 It varies in brightness by almost a factor of five every 40 days or so, and is engulfed in a thick shroud of cosmic gas and dust. 13 00:01:41,000 --> 00:01:49,000 Hubble gazed at RS Puppis over a period of around 5 weeks back in 2010, 14 00:01:49,000 --> 00:01:55,000 observing it growing brighter and dimmer within its murky surroundings. 15 00:01:55,000 --> 00:02:06,000 This enabled scientists to create a time-lapse video that appears to show the gas around the star expanding outwards. 16 00:02:06,000 --> 00:02:14,000 However, this gas is not actually moving — it is an optical illusion known as a light echo. 17 00:02:16,000 --> 00:02:25,000 The dusty environment around RS Puppis allows us to see this light echo with stunning clarity. 18 00:02:26,000 --> 00:02:33,000 As the star expands and brightens, some of the light does not reach Hubble directly 19 00:02:33,000 --> 00:02:41,000 but is first reflected off progressively more distant shells of dust and gas surrounding the star. 20 00:02:41,000 --> 00:02:51,000 This reflected light has further to travel, and so arrives at the Earth later than the light that has travelled straight from star to telescope. 21 00:02:53,000 --> 00:03:03,000 Hubble also observed a light echo around variable star V838 Monocerotis in 2002. 22 00:03:04,000 --> 00:03:08,000 This is very much like the experience of an audible echo 23 00:03:08,000 --> 00:03:18,000 where the listener hears a sound once and then again as the second wave takes a longer path bouncing off surrounding objects. 24 00:03:23,000 --> 00:03:32,000 Beyond the striking beauty of this effect, there is an important scientific reason to observe Cepheids like RS Puppis. 25 00:03:33,000 --> 00:03:40,000 The period of their pulsations is known to be directly connected to their intrinsic brightness 26 00:03:40,000 --> 00:03:45,000 which allows astronomers to use them as cosmic distance markers. 27 00:03:47,000 --> 00:03:58,000 Astronomers have measured the distance to RS Puppis, calculating it to be around 6500 light years — 28 00:03:58,000 --> 00:04:03,000 the most accurate distance to such a star ever measured. 29 00:04:10,000 --> 00:04:14,000 Hubblecast is produced by ESA/Hubble at the European Southern Observatory in Germany. 30 00:04:14,000 --> 00:04:19,000 The Hubble mission is a project of international cooperation between NASA and the European Space Agency. 31 00:04:20,000 --> 00:04:24,000 www.spacetelescope.org 32 00:04:27,000 --> 00:04:31,000 Transcribed by ESA/Hubble. Translation --