Hmm. There are lots of different ways. Using a smart object in photoshop is perhaps one of the simplest. If you search for Reducing Noise staking photos using smart media objects (something like that
http://www.tipsquirrel.com/noise-reduction-with-smart-object-stack-modes/ comes up on google, but there are lots). Astrophotography is a little more complex as you cant stack the stars that way (median stack removes all stars and mean stack produces a faint star trail); so there are some free tools like Sequator that do a good job of stacking and maintaining the stars in the sky. You can't stack an aurora shot however (other than the foreground) as the aurora is dynamic.