What is reflection nebula?

In astronomy, reflection nebulae are clouds of interstellar dust which might reflect the light of a nearby star or stars. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. The nebula is famous in astronomical history because the first Herbig-Haro object was discovered immediately adjacent to it (it lies just outside the new Hubble image). Herbig-Haro objects are now known to be jets of gas ejected from very young stars.
The brightest reflection nebulae are places where new stars are being formed. Here the gas and dust is thick and shines by the light of new, bright stars. Sometimes the gas is so thick that the new stars cannot be seen.
Grains reflect light almost as well as fresh snow, more because of their favourable size (which promotes scattering rather than absorption) than their chemical composition. Calculations show that even graphite, which is black in bulk, reflects visible light well when dispersed into small particles.