Optical CCD (Charge Coupled Device) image of the Trifid nebula M20 (NGC 6514) in the constellation of Sagittarius. The nebula is formed by a cloud of hydrogen ionised and lit up by the ultraviolet radiation of the bright central star. The temperature of the ionised gas is close to 10,000 Kelvin and the density may vary from 10,000,000 to 1,000,000,000,000 ions per cubic meter. These nebulae are also known as HII regions and are considered to be the by-product in an area where star formation takes place. The nebula is crossed by dark lanes of interstellar dust which divide M20 in three sectors.