Dr. Sutherland (RIC. p.105), notes of Caligula's gold and silver "...the portraits often equalled those of the aes in their almost brutally strong character."
The two stars on the reverse possibly refer to Augustus and Tiberius. The former had already been deified, and Dio Cassius tells that Caligula was intending to bestow the same honours on Tiberius. It has been noted that on some examples the reverse portrait does indeed closely resemble Tiberius. Caligula was frustrated in this however by very strong opposition from the Senate.