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Dara and Brian's journey through the night sky continues with a look into the distant past, explaining how it is possible to chart the history of the universe by looking million of light years out into the depths of space. Meanwhile, Mark Thompson reveals what can be learned from the colours of the stars. At NASA, Liz Bonnin meets the team building the largest space telescope in the world, an instrument that's 100 times more powerful than the Hubble.
