When you're photographing the night sky you'll want to capture as much light as you can while minimizing the movement of the stars in the image — unless you're deliberately trying to create star trails.
When you're photographing the night sky you'll want to capture as much light as you can while minimizing the movement of the stars in the image — unless you're deliberately trying to create star trails. The longer the focal length of your lens, the shorter the shutter speed will need to be to avoid capturing star trails.