The reason that the stars do not show up on the film is that the stars are so dim that the camera cannot gather enough of their light in a short exposure. Our eyes are a lot more sensitive to light than photographic film. A good example of this is when we take a picture with a camera that is back lighted. Any picture that you may see of stars are from time-lapse photos. To take a time-lapse photo of the stars, the shutter must be left open on the camera in order for the lens to focus enough light on the film for the image to show up.
Longer times allow more photons to enter the camera and record the image.
The image is built over time from the total number of photons striking the film. The dimmer the object, the longer the film must be exposed because there are fewer photons per unit of time reaching the camera than for a brighter object. The brightness of an object is directly related to the number of photons that reach a recording device such as your eye or a camera. For example, to get a decent photo of the full moon, the shutter should be open for about a second or two. To record the image of a star, the shutter must be open from several minutes to several hours in order for enough photons to hit the film and make an image.