How to make an image file flicker in a video? (stars twinkling)
Create 10 copies of your star field image. In an image-editing program (like Photoshop or GIMP), open the first image. Pick 10 stars at random, and make them darker/dimmer. Save that image. Open the next image.  Pick 10 different stars and make them dimmer.  Do that for all of your images.  In your video editor, cycle through the images, about 3/4 second each, dissolving from one to the next.