The star / comet on the reverse probably refers to Halley's Comet, which appeared in the night sky during Elagabal's reign (Halley's Comet: A Visual Record on Coins of Elagabalus by Michael Kodysz). Interestingly, this symbol differs from the standard depictions of stars on ancient coins - it has one ray much longer, which may indicate the representation of a comet.
Obverse: IMP ANTONINVS PIVS AVG
Laureare, cuirassed and draped bust right.
Reverse: SACERD DEI SOLIS ELAGAB
Elagabalus, in Syrian priestly robes, standing right, sacrificing out of patera in right hand over lighted altar, holding upright club in left hand; star.
Silver, diameter 17,5 x 20 mm, weight 3,26 g.
