How did the great light of the first day form the firmament on day two and the stars on day four?

You could have electrolysis, but it may also be thermolysis, which is faster and more efficient; electrolysis is slow and usually happens in a laboratory, and in that early environment, with the great light providing enormous heat, thermolysis is the better candidate for breaking the water down. As the oxygen comes out it is under great pressure, about sixteen pascals at the limit, so the hydrogen freezes into a black solid metal, and if anything could act as a firmament to hold up the billions of miles of water above it, it is solid hydrogen under that pressure. Most of the oxygen dissipates into space, but because oxygen is heavy it falls down to the earth. The stars of the fourth day have nothing to do with the second day. Once the firmament is put in place and the water beneath it is saved for the earth, God stretches out the firmament, which Scripture says in about ten places across six books that He stretched out the heavens. As it stretches radially it carries the light and the water with it, changing from a dark solid into an ethereal hydrogen gas in which the planets and stars can sit, and the stretching breaks the great light apart into stars. That was the majority opinion of the Fathers, and I worked it out mathematically: the great light was about ten thousand times the size the sun would be, and given the time to break apart it is quite feasible that it formed the stars, and the sun as well. What was left over kept dissipating out to the rim of the universe, forming the underbelly of the water spread out on the first day, which is now ice, and that residual first-day light under the rim is where we get the 2.75 Kelvin microwave background radiation today. About ninety percent of outer space is hydrogen, which is the firmament, and the lighter elements like oxygen, nitrogen, carbon, iron, and calcium were made in the great light, while the heavier of the ninety-two elements were made in the earth.