Narrow visible light emission Below left is a familiar visible-light view of the Trifid nebula. The box indicates the region shown below right, which glows with a narrow-line visible light emission at about 657nm. This glow indicates that this nebula is rich in Image credits: Trifid Nebula by Deddy David Dayag licensed under CC BY-SA 3.0. Red Trifid by David McDavid .