Referencing an instantiated object with a global variable

I'm instantiating an object and plan to destroy it in another function, so i'm trying to reference it through a global variable. I'm currently doing

void DrawScore() {

     GameObject a;
     GameObject b;
     GameObject c;

     Vector3 star1Pos = GUIdrawCentre - new Vector3(+GUIdrawOffset,0,0);
     Vector3 star2Pos = GUIdrawCentre;
     Vector3 star3Pos = GUIdrawCentre - new Vector3(-GUIdrawOffset,0,0);

     if(starCount == 0)
     {

     }

     if(starCount == 1)
     {
         a = Instantiate(GUIstar, star1Pos, Quaternion.identity) as GameObject;

         _stars[0] = a;
     }

}

then in another scripts, looping through the array and destroying each member.

_stars[] is an array of GameObjects declared globally

For whatever reason, _stars[0] is always set to "none" and never gets overwritten with anything.

Any ideas why this is happening? I've used the same method in other projects but this time it just isn't working