Lagoon Nebula, bright, diffuse nebula in the southern constellation Sagittarius; cataloged as M8 or NGC 6526. It is visible to the naked eye and has an angular area larger than that of the full moon. The central parts are extremely bright, and some stars can be seen embedded in the nebulosity. Because of the nebula's large size, light from its stars cannot illuminate all of the associated interstellar gas and dust. Thus, parts of it appear blacker than the surrounding sky.