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Published online by Cambridge University Press: 07 February 2017
The clearest evidence that galaxies evolve comes from the Milky Way: here we see stars with ages and lifetimes differing by 4 orders of magnitude, and with chemical compositions indicating a hundred-fold enrichment in metals between the times of formation of the halo and most disc stars. the general resolution of Olbers' Paradox also gives evidence for cosmological change: the sky is dark because galaxies turned on a finite time ago (Harrison, 1974). Here I discuss some of the further evidence for galactic evolution that is emerging from observations probing billions of years in lookback time.