Yep. You can do it tons of different ways. But the most simple is
$game_player.character_name = "#{@a}_star"
And some side advice, when you are just creating a simple string (no special characters or anything), use single quote marks instead of double quotes. From what I have been told (and it makes sense), single quotes are easier on the interpreter than double quotes. So instead of "_star", '_star' That is just a bit picky, but its a good habbit to form.
So you got everything figured out?