Can I capture star trails with a smartphone by recording video?

Yes—if your phone can record a long enough video and remain completely still, you can create a star-trail image from the video frames. Put the phone on a tripod and record the night sky, then combine the frames afterward using a maximum/lighten-style stack. One example given was using ImageMagick: convert video.avi -evaluate-sequence max output.jpg.

A tripod is essential so the frames stay aligned. If the camera doesn’t move, you generally don’t need alignment, and stars can even confuse some alignment tools. When stacking, use a method like max (or add), since mean or median blending will tend to make the stars disappear instead of forming trails.

Light pollution mainly affects how bright the sky background becomes, so in brighter skies you may need to avoid overexposing the background while still recording long enough to build visible trails. The key idea is: stable camera, continuous capture, and frame stacking afterward.