Cone Nebula. Optical image of the dark Cone Nebula (lower left) and an emission nebula NGC 2264 (red, upper left) in the constellation Monoceros. North is at top. The Cone Nebula is the dark triangular intrusion into the lower edge of NGC 2264. The Cone Nebula is at least 6 light years long and is a site of star formation. NGC 2264 glows red due to the ionisation of hydrogen by radiation from the hot young star cluster (bright patch) within it. The brightest star in this cluster (above and left of the main glow) is the blue supergiant star S or 15 Monocerotis. This region is around 2600 light years distant.