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In 1921, Carl Otto Lampland was the first to announce that he had seen changes in the structure of the Crab Nebula.

This announcement occurred at a time when the nature of the nebulas in the sky was completely unknown.

Their nature, size and distance was subject to debate.

Observing changes in such objects allows astronomers to determine if their spatial extension is “ small ” or “ large ”, in the sense that notable changes in an object as vast as our Milky Way cannot be seen over a small time period, such a few years, while such changes are possible if the size of the object does not exceed a few light-years.

Lampland's comments were confirmed some weeks later by John Charles Duncan, an astronomer at the Mount Wilson Observatory.

He benefitted from photographic material which has not changed since 1909 and in fact made the comparison with older snapshots easy, highlighting a general movement of expansion of the cloud.

The points were moving away from the centre, and did so faster as they got further from the centre.