Here we have another compound, which is comprised of hala, coming from “hali” meaning “tail” and stjarna which looks like it sounds: “star.” That leaves us with the idea of a comet being a “tail star” of sorts, trailing through space like an eel swimming through dark waters.
Halastjarna isn’t the only interesting compound involving “stjarna.” You also have “reikistjarna,” the word for planet that breaks down into reika and stjarna, collectively meaning “wandering/roaming star,” much like early astronomers described the planets before they realized they weren’t actually the massive, luminous spheres of plasma that we call stars.
A third one is “sjöstjarna,” broken down as sjö (seven) and stjarna. This “seven star” is the word for Pleiades, a cluster of seven stars named as the “seven sisters” of the solar system. This word also happens to be the name of a plant called the Chickweed Wintergreen, although its etymology and the beginning of this usage is unknown to me.
While on the topic of space, let us not forget to add a new word to our long list of compounds containing steinn or “stone.” In addition to dropasteinkerti, brennisteinn, augasteinn, and steinþegja, we have the lovely “loftsteinn,” meaning “meteorite” but containing loft (sky) and steinn, or “sky stone.”
If you dare to challenge yourself, here is a lovely wikipedia article in Icelandic about the reikistjarna.