What was the name of the great star that fell at the third trumpet?

Answer: Wormwood

The star's name is Wormwood. A third of the waters turned bitter, and many people died from the bitter waters.

“Then the third angel sounded his trumpet, and a great star burning like a torch fell from heaven and landed on a third of the rivers and on the springs of water. The name of the star is Wormwood. A third of the waters turned bitter like wormwood oil, and many people died from the bitter waters.”— Revelation 8:10-11 (BSB)