In Revelation, what might the "Star of Wormwood" refer to?
Note: As this is soliciting an opinion of how prophecy should be interpreted, I feel that it is very presumptuous to claim that a conclusive answer can be given.
2. Probable Answer, Perhaps the Angel: "Abaddon / Apollyon":
Although interpretations vary, it does not seem reasonable to infer another identity - other than Abaddon - from this context, alone.
Yes, it is correct that "Star" is consistently used metaphorically for "angel", (Revelation 1:20; Revelation 9:1; Revelation 12:4).
Revelation explicitly states that the "star" is an Angel:
NASB, Revelation 9 - Then the fifth angel sounded, and I saw a star from heaven which had fallen to the earth; and the key of the bottomless pit was given to him. 2 He opened the bottomless pit, and smoke went up out of the pit, like the smoke of a great furnace; and the sun and the air were darkened by the smoke of the pit. 3 Then out of the smoke came locusts upon the earth, and power was given them, as the scorpions of the earth have power.
The Angel being referred to, in this context, is Abaddon:
NASB, Revelation 9:11 - They have as king over them, the angel of the abyss; his name in Hebrew is Abaddon, and in the Greek he has the name Apollyon.
Note: Given that the mention of this name is within the same "trumpet blast", it seems reasonable to make this association.
Wormwood is Symbolic of the Pangs of Judgment:
Consistently, in Scripture, Wormwood is compared to "poison", the "state" of bitterness, anguish, and toxicity following the perversion of Justice.
NASB, Amos 6:12 - Do horses run on rocks? Or does one plow them with oxen? Yet you have turned justice into poison And the fruit of righteousness into wormwood,
This is consistent with the historical association between "Abaddon" and the Judgment of Hell/Sheol:
Abbadon, Wikipedia: The term abaddon appears six times in the Masoretic text of the Hebrew Bible; abaddon means destruction or "place of destruction", or the realm of the dead, and is accompanied by Sheol.