Should my camera be alright in the cold?

I left my nikon d5100 outside in the cold for a star lapse, its about 15 degrees Celsius. Should it be alright for an hour? Will it get damaged?

It'll be perfectly fine. Cameras are designed to be used and you would be surprised the kind of beating they can take. That is not to say they don't break. Much like a child, it is not meant to be coddled, but can be broken.

Personal anecdote, I've taken my canon 500d - Nikon equivalent of d3000, from cold to hot, to dusty to humid, to mountains and lakes. It never missed a beat. I also took it bare under a waterfall, the Niagra Falls. No rain cover. My socks were soaked through, the camera held up fine.

No problem.

You think 15 Celsius is cold? LOL.

Should be alright down to a few degrees below zero so long as you keep it dry.

They work in Europe and North America way down below freezing, your 15C is like 60F to us… Nice. Just don't take it from one extreme temperature to another (like from the cold into the warm humid atmosphere of a swimming pool) without putting it in a plastic or ziplock bag.

Or do you mean will the sensor overheat? It's best to shoot multiple 5 minute exposures and stack them. Longer than 10 mins and problems start to become obvious.

15 is really not that bad, you're probably thinking that celsius are similar to farenheitt so don't worry, especially since cameras nowadays are meant to resist all type of weather! Make sure it's not raining though