What is the star called Wormwood?
Wormwood is the name given to a great star that falls from heaven at the sounding of the third trumpet, poisoning a third of the rivers and springs of water so that many people die from the bitter waters.
When the third angel sounds his trumpet, a great star falls from heaven, burning like a lamp, and it strikes a third of the rivers and the fountains of water. That star is named Wormwood, and its fall causes a third of the waters to become bitter and poisonous.
Because of this poisoning, many people who drink from those waters die from the bitterness that the star brings upon them. This event is part of the sequence of trumpet judgments that unfold after the seventh seal is opened, moving from the burning of the earth and sea in the first two trumpets to the corruption of fresh water sources in the third.
The fall of a burning star tied to judgment on the heavens and waters echoes themes found elsewhere, such as Isaiah's description of the host of heaven being dissolved and falling like leaves, showing that this kind of heavenly disturbance and judgment has long been spoken of as part of God's dealings with the earth.
The scriptures
And the third angel sounded, and there fell a great star from heaven, burning as it were a lamp, and it fell upon the third part of the rivers, and upon the fountains of waters; And the name of the star is called Wormwood: and the third part of the waters became wormwood; and many men died of the waters, because they were made bitter.
And all the host of heaven shall be dissolved, and the heavens shall be rolled together as a scroll: and all their host shall fall down, as the leaf falleth off from the vine, and as a falling fig from the fig tree.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.