Please bear in mind that I haven't really encountered the phrase ""Himmel auf(reißen)" in daily life (and I would be very surprised to find out that it is commonly used). An educated guess would be that it refers to sky (but with a slight religious/otherworldly connotation regarding "aufreißen"). Why? 1. The lyrics contain other words (Kreuze, Paradies, Teufel) with religious implications. 2. As alread mentioned, there really aren't many uses of that phrase, the most important one may be in the translation of the book of Jesaja: "63,19 Uns geht es, als wärest du nie unser Herrscher gewesen, / als wären wir nicht nach deinem Namen benannt. Reiß doch den Himmel auf und komm herab, / sodass die Berge zittern vor dir." I would find it strange if it would mean "tear open the heavens" and not "tear open the physical sky", though it could also mean "tear down the barrier between the sky/the physical world and heaven". I think the meaning behind it is to have some kind of epiphany, to suddenly see the light at the end of the tunnel. Honestly. don't worry too much about that one phrase. You will never use it.

"Himmel auf" is the end of the phrase "Wann reißt der Himmel auf?" in the song by Silbermond. This is a split verb (aufreißen = to tear open, to rip open). It's also an idiom for "the sky clears up, blue sky appears" (imagine the clouds being teared open). You can find translations for such idioms on linguee: http://www.linguee.de/deutsch-englisch/search?source=auto&query;=himmel+rei%C3%9Ft+auf The song text plays with both of the meanings (kalter Himmel -> cold sky full of clouds, Paradies -> heaven). Lyrics like these are difficult to translate, there's no "right" translation. You usually try to find a similar ambiguous phrase, or you try to place some of the meanings into different phrases. This never completely works, and translating lyrics like these is an Art.