The stars aren't just for sci-fi geeks. Poets love them, too. In Thomas's poem, the cosmos represent a world beyond our physical selves. It's where we'll all "rise up" after death and become one with those groovy stars and moons. So in a way, all the cosmic imagery makes us think of a world that's much bigger than our earthly selves.
Lines 2-3: Those "dead men naked" will be one with the "man in the wind and the west moon." So we get the feeling that man and cosmos are kind of on the same plane after death.
Line 5: We sense that unity of man and cosmos even more when we imagine dead guys with "stars at elbow and foot." It's almost like we'll all become our own constellation. Far out.
Line 26: All the rising of dead folks will persist until "the sun breaks down." The relationship between man and cosmos is not only powerful but also unbreakable. The sun will literally have to "break down" before this relationship breaks.