How many irregular galaxy classes did Hubble define in the Hubble sequence? The answer is two.
Hubble divided irregular galaxies into Irr I and Irr II. Irr I galaxies have asymmetric shapes and often contain many individually visible young star clusters, while Irr II galaxies have smoother, more amorphous appearances without clearly resolved stars or stellar clusters.
Irregular galaxies do not show the orderly elliptical, lenticular, or spiral structures represented elsewhere in the Hubble sequence. The Large and Small Magellanic Clouds are familiar examples of nearby irregular galaxies; Hubble classified the Magellanic Clouds among the Irr I group.
A common confusion is between the number of irregular classes and the number of broad galaxy categories in the full tuning-fork diagram. The sequence includes ellipticals, lenticulars, spirals, barred spirals, and irregulars, but Hubble’s irregular branch itself contained two classes. Later classification systems refined and extended galaxy types beyond Hubble’s original scheme.