Magic: The Gathering - Card ability collision?

I have, on my battlefield, Galvanoth with the ability "At the beginning of your upkeep you may look at the top card in your library. If it's an instant or sorcery card, you may cast it without paying its mana cost.". My next upkeep, I draw Red Sun's Zenith (X)(R), with the effect "Red Sun's Zenith deals X damage to target creature or player...". What should happen?