  We present the first deep learning model for segmenting galactic spiral arms
and bars. In a blinded assessment by expert astronomers, our predicted spiral
arm masks are preferred over both current automated methods (99% of
evaluations) and our original volunteer labels (79% of evaluations). Experts
rated our spiral arm masks as `mostly good' to `perfect' in 89% of evaluations.
Bar lengths trivially derived from our predicted bar masks are in excellent
agreement with a dedicated crowdsourcing project. The pixelwise precision of
our masks, previously impossible at scale, will underpin new research into how
spiral arms and bars evolve.
