Radio images of the pulsar-powered supernova remnant G21.5-0.9, from 2006 and 1991, visually showing the expansion over the 15-year period. The two images are on the same scale, and the larger and smoother of the two is that from 2006, with the smoother appearance being due to the higher signal-to-noise.  Between 1991 and 2006, we find that the average expansion rate of the remnant is 0.11 +/- 0.02 %/year, corresponding, for a distance of 5 kpc, to a speed of 910 +/- 160 km/s wrt. the centre of the nebula. Assuming undecelerated expansion, this expansion speed implies that the age of G21.5-0.9 is 870 (+200,-150) yr, which makes PSR J1833-1034 one of the youngest, if not the youngest, known pulsars in the Galaxy.

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