The Horsehead nebula is actually two clouds of interstellar hydrogen. The nearer, foreground cloud (containing the ghostly horsehead figure) is not illuminated and appears dark. Behind this dark cloud lies a cloud of glowing hydrogen. The horsehead shape is silhouetted against the glowing background cloud. Notice the star density in the upper half of the photo compared to the lower half of the photo. The foreground cloud is blocking out the light of the distant stars.
