What is a primary characteristic of dwarf galaxies?

Dwarf galaxies are defined by their small size and relatively low luminosity. The primary characteristic that distinguishes them from regular or larger galaxies is their star count; they contain significantly fewer stars. This characteristic contributes to their overall dim appearance and makes them challenging to observe compared to more massive galaxies.

Dwarf galaxies typically feature a variety of morphological types, including irregular and spheroidal shapes, and they often reside in the outskirts of larger galaxy groups or clusters. Their smaller stellar populations can lead to lower rates of star formation, although some dwarf galaxies can be actively forming stars under certain conditions.

The other options you presented relate to aspects that do not accurately define dwarf galaxies. For instance, while some dwarf galaxies may form from large-scale structures and have gauge variations in star formation rates, they do not generally outshine larger galaxies, nor do they possess a higher brightness than those larger counterparts. Thus, the defining trait that effectively captures what distinguishes dwarf galaxies is indeed their significantly fewer stars than regular galaxies.