How do Huck and Jim explain the stars?The Adventure of Huckleberry Finn by Mark Twain

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.