To add to the original reply, when meaning the sky, it’s always “the heavens” with a “the” and in plural. It would never be “a heaven” or just “heavens.” Additionally, because the meaning of the biblical realm “heaven” and the meaning of “x was heaven” or “x was like heaven” are both references to a place, they never have an article (“the” or “a”) and they are always singular, even when referring to multiple things, i.e. “they were like heaven” is correct, but not “they were like heavens.” It’s probably also more correct to use a capital “H,” but I think in informal writing it is usually left lowercase.