I built some pine shelves. They're pretty, but I don't want them to darken. I'm told I can apply polyurethane to ensure they stay nice and bright.

My question is this: do I need to apply some kind of stain, even "natural," before polyurethane? Or can I sand and then apply the polyurethane directly?

If it's okay to apply polyurethane directly, can I sand 60-grit and then do poly? Or do I still need to work up to 200-something-grit?