Nikon D5200 Long Exposure Error. Star photos, red line across photo

Added (1). This was taken at various ISO sensitivities. All were 30 seconds at f/3.5. They all have red marks at the bottom and I don't know why! The ISO range I tested with was 3200-25600

Sensor heat would be my guess. Turn down the ISO to 400 or so, try again. You may still see this, which means that you need to shorten the exposure time until you lose the red area. It won't really affect your results, as you can stack multiple exposures to get the same image. Plenty of tutorials on the web on how to do that if you don't already know how.