"Bright Star"-Apostrophe

"Bright star, would I were steadfast as thou are-"

Here, the speaker is directly addressing the star. He wants to be like the star in that it is undying. At first, this is just for the sake of living forever and being able to watch the world. However, he does not want to be unchanging by himself. The stars, although to us seem to be close together, are thousands of miles away from each other. The speaker does not want to be as alone as they are. He wants to be with his love forever. By addressing the star instead of simply saying his desire, it invokes the image of a plea to be immortal.