The asterisk is a star that began as an editor's flag. Its name says so: Greek asteriskos is a diminutive of aster, "star," so the word means "little star."
The mark itself is ancient. Ancient Greek scholars editing Homer marked a text with symbols rather than changing it. Wikipedia's page on those symbols says Aristophanes of Byzantium used a small star for passages that did not make sense. It adds that Aristarchus of Samothrace, who lived from 220 to 143 BCE, used it to flag a line repeated elsewhere in the text.
Sources disagree on the details. Wikipedia notes that one scholar argues the Aristophanes mark flagged nonsense, not duplicates. Lapham's Quarterly says Aristarchus used it to mark passages he thought came from another source, and Digital Medievalist credits both men with marking repeated lines. Either way, the star meant "look at this line."
Origen of Alexandria carried the idea into scripture in the third century. In the Hexapla, his edition of the Septuagint, Lapham's says he used the asterisk to mark text he had added to the Greek from the Hebrew. Digital Medievalist describes the same use as a sign for omitted passages. Medieval scribes later used it as a signe de renvoi, a mark that says where a correction or insertion belongs.
The word entered English in the late 14th century. Etymonline records it as asterich or asterisc, from Late Latin asteriscus. It defines the word as a figure used in printing to mark a footnote, an omission, or a conjectural reading. The verb to asterisk came later, in 1733.
The star's modern jobs grew from that same function. It points the eye to a note, a caveat or a missing piece. The name was never a metaphor for brilliance. It was a description of a tiny star-shaped mark, drawn by hand in the margin of a scroll.