Want to know why your SNES turned yellow?
It wasn't aliens, ultraviolet light or your mom's terrible cleaning habits.
For the real reason, read this article from vintage gaming. It sounds WAY more complicated than the simple answer: shitty plastic choice by Nintendo for the initial batch of machines that were made.
This link also has an extensive list of repair & restorations choices including ways to prevent "The YELLOWING" from occuring in the first place.
http://www.vintagecomputing.com/index.php/archives/189