Ὁ θρόνος σου ὁ θεὸς εἰς αἱῶνα τοῦ αἰῶνος. ὁ θεὸς is the ordinary vocative in Hellenistic Greek. This use of the nominative for the vocative is sometimes scornful in classical Greek (as in χαῖρε ὁ βασιλεὺς τῶν Ἰουδαίων), but is used in Hellenistic in direct addresses, comp. Luke 12:32 μὴ φοβοῦ τὸ μικρὸν ποίμνιον, 8:54 ἡ παῖς ἔγειρε. The quotation is from Ps. 45:6, 7 (LXX.)
F.W. Farrar, The Epistle of the Apostle Paul to the Hebrews (1893) 37.