Astraea

When Astraea became the constellation Virgo, her scales of justice became the constellation of Libra.

Actually, I'm pretty sure Astraea was the Greek Goddess of innocence, and her mother Themis was the Goddess of Justice.

Not quite. Themis' form of justice was more of the cosmic order, whereas Astraea's (in versions of the mythology where "Astraea" is just an epithet and her name is actually Diké) is human justice. The two domains differ.

In Greek mythology, Astraea is the goddess (and personification) of justice, along with her mother, Themis.