In Chapter 12, Huck describes his and Jim's life on the raft,
We catched fish, and talked, and we took a swim now and then to keep off sleepiness. It was kind of solemn drifting down the big, still river, laying on our backs, looking up at the stars, and we didn't ever feel like talking loud, and it warn't often that we laughed--only a little kind of a low chuckle. We had mighty good weather, as a general thing, and nothing ever happened to us at all--that night, nor the next, nor the next.
In another passage from Chapter 18, Huck continues his reflection on the raft:
We said there warn't no home like a raft, after all. Other places do seem so cramped up and smothery, but a raft don't. You feel mighty free and easy and comfortable on a raft.
As they travel down the Mississippi river, Huck and Jim enjoy their idyllic life on the river, that refuge from the evils of society. While on the raft, the world is good, there is no inequality between Jim and Huck, there are no conflicts with Pa or others. Life on the raft is a solemn experience; it would seem disrespectful to talk loudly on this sanctuary from civilization with its corrupt institutions. In the natural world of the river in the open air with only the stars as their ceiling, the souls of Huck and Jim can expand--they look to the heavens as an expression of this feeling of expansion and delight in nature.
I think that the answer you are looking for can be found in Chapter 19. It is found at the end of the part of the book where Huck and Jim have things pretty good. They are going to meet the "King" and the "Duke" in a little bit and that will mess their lives up.
Anyway, they are lying on the raft looking up at the stars and wondering about them. They are talking about whether the stars were made by someone or something or whether they just happened to be there. Jim thinks that they were made. He thinks maybe the moon laid them. Huck thinks that there were too many of them to be made so he thinks they just happened.