There is more than one instance of a "star" being added to a plate number on US stamps.
To make my reply easier, I will assume you are referring to the 1922-1925 regular issue; let me know if otherwise.
The original plates used in that series were "narrow gauge" plates, referring to the distance from the center guideline (vertical/horizontal, depending on orientation of stamp) to the edge of the last stamp. In 1925, BEP experimented with plates with wider spacing, later referred to as "wide gauge" plates. Except for the earliest set of plates, later "wide gauge" plates were marked with the star. There are also varieties of the "star". These plates are also referred to as "star plates".
See, all my selvedge collecting wasn't a complete waste...
The star plates are briefly discussed in the Durland Plate Block catalog (see section on 1922-1925 Regular Issue). If you don't have a Durland, here is a link that also briefly discusses the star plates (see page 10 of the pdf):
(the real) khj