00:00Astronomers found an enormous green cloud glowing beside a distant galaxy.
00:04It was being flooded with radiation from something incredibly powerful.
00:09But when they searched the galaxy for the source,
00:11they found a quasar that had already faded thousands of years earlier.
00:16The mystery began in 2007 when Dutch teacher Hanny van Arkel
00:20noticed a bright green object beside the galaxy IC 2497
00:24while helping classify galaxies online.
00:28Hubble revealed twisting filaments, young stars,
00:31and what looked like a huge dark hole through the cloud.
00:34But the apparent opening may actually have been a shadow.
00:38The green cloud was only the visible part of a gas streamer
00:41stretching roughly 300,000 light years, several times wider than the Milky Way.
00:48Its green color came from highly ionized oxygen,
00:51evidence that something inside IC 2497 had once flooded the cloud with intense radiation.
00:57But the once-blazing quasar was no longer visible.
01:01Dust may have hidden it,
01:02yet X-ray observations still found a nucleus far too weak to explain the glowing cloud.
01:08That left a startling possibility.
01:10IC 2497 had once hosted a powerful quasar,
01:14fueled by matter falling towards its central supermassive black hole,
01:17and its radiation had struck the distant cloud.
01:20Because the cloud lies tens of thousands of light years from the galaxy's center,
01:25the radiation illuminating it began traveling long before the quasar faded.
01:31Hanny's vorwarp was showing astronomers the galaxy as it was in the past.
01:36The quasar's brightness had collapsed by at least 50 times in around 100,000 years,
01:41far faster than astronomers expected from a system powered by a supermassive black hole.
01:47But the cloud held another secret.
01:49It may be debris torn from a galaxy during an ancient collision,
01:53while an outflow from IC 2497 is forming new stars inside it.
01:58Then, astronomers found more glowing clouds around other galaxies.
02:03Hanny's vorwarp was not an isolated case,
02:06but evidence that quasars can fade long before their light has finished crossing space.
02:12The quasar had faded before anyone on Earth knew it had ever shown.
02:16Yet, its old radiation is still lighting the cloud beside IC 2497.
02:22What astronomers found was not the quasar itself,
02:26but the last bright echo of what it once was.
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