The essential concept is called, simply enough, stacking and averaging. The technique is often used in astrophotography where the photographer takes a long series of short exposures which are then combined to form the final, and somewhat smaller, image. The reason they do this is to handle the rotation of the earth without an equatorial mount and to reduce noise. You will also find it used in macro photography to increase the depth of field of the image, each layer providing a slightly different plane of focus.
Why do I tell you this? Well, the software they use can be used by you to achieve a similar purpose. If you Google "image stacking software to reduce noise" you will find quite a few hits. Some will give you guidance for doing it in photo editing software, some will be dedicated software, etc. The dedicated options will usually provide some assistance in lining up the images which, I think, is the biggest problem you face.