These are examples of cosmic ray hits on the CCD/detector. Cosmic rays are typically charged particles or very energetic gamma rays. When they impact the detector the generate electrons (like the light from the stars) which are then detected. If the detector gets hit by protons or alpha particles (Helium nuclei) they usually result in brighter, straight streaks. Gamma rays tend to create more point like ‘cosmics’ which do look like stars sometimes – you can also get them when a proton impacts the CCD at 90deg. Again, we take the three images to median combine them and remove these features.