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The Hubble sequence sorts and characterizes or classifies galaxies by their shape. It was suggested by Edwin Hubble back in 1926.

One drawback, or criticism, of this scheme is that Hubble had a limited data set of galaxies at hand, compared to today's data. He "only" had a few hundred galaxies in visible light from which he could build the sequence.

So, adding today's knowledge of way more galaxies imaged in all different kinds of spectra how correct is Hubble's scheme? Is it skewed because of this originally limited selection of data? Are there other shapes missing in the sequence?

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The only way the Hubble sequence is truly incomplete is that it assumes such a classification applied to all stages of the universe. We now have telescopes that can see much further in space—and thus in time—and so we can see many more galaxies at different periods of time in the universe. Observations show that the Hubble sequence is still valid with one modification.

As this Hubble Space Telescope article explains, peculiar galaxies—so named because they exhibit odd features, including sizes and shapes—appear to constitute over 50% of distant galaxies observed by Hubble, while they make up only about 10% of local galaxies. Elliptical and lenticular galaxies made up roughly the same fractions (3-4% and 13-15%) in both periods, while spiral galaxies make up 72% of local galaxies and only 31% of distant galaxies.

The Hubble sequence classification still seems to organize galaxies well in this earlier period, with the glaring exception of peculiar galaxies. The conclusion the team has drawn is that many of the peculiar galaxies, one way or another, became spiral galaxies through collisions or other processes.

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