Lagoon nebula. Optical image of the Lagoon nebula (M8, NGC 6523). This large emission nebula lies around 3500 light years away in the constellation Sagittarius. It glows as the hydrogen gas it contains is excited by radiation from hot young stars embedded in it. The nebula, which is about 30 light years across, is the birthplace of thousands of new stars. At upper left is the nebula NGC 6559. This emission nebula is fringed with blue reflection nebulae, where cooler gas and dust reflect the light from nearby stars. The dark dots seen in both these nebulae are Bok globules. These are dense regions of gas and dust believed to be at an advanced stage of star formation.