[2] Members of the genus are commonly found in eutrophic freshwater ponds and lakes, and have a cosmopolitan distribution.

[1] Traditionally, Actinastrum was placed within the family Coelastraceae based on morphology (spherical, three-dimensional colonies and reproduction by autospores).

However, molecular phylogenetic studies have shown that the type species Actinastrum hantzschii is closely related to Chlorella, another autospore-forming genus in the family Chlorellaceae.

[3][4] Actinastrum consists of eight (sometimes two, four or 16) cells, which are attached together at a common point to form a star-shaped colony, termed a coenobium.

However, the genus is very morphologically, and the cell shape changes considerably during its life cycle and in response to external conditions.