So,
星 does just mean "star" or more generally "heavenly body" with
流れる being intended to describe it as "flowing" or "streaming." I think this is supposed to tie in with the idea of the movement of the stars affecting fate, which ostensibly would also include shooting stars (but I don't think it needs to be exclusively referring to them.)
Typically the game uses
流星 to refer to meteors or shooting stars, which, as far as I can tell, does derive from the same root
流れ, so in this context it would fit anyway since Radahn seems to have specifically confronted
falling stars.
From what I can tell, the use of "constellations" appears to be a localisation flair on FrogNat's part to make the language more interesting. But, according to the dictionary, in one sense
星 when read せい (Sei)
can refer to the Chinese constellations, which is possibly what prompted FrogNat to localise it that way.