The Trifid Nebula (designated Messier 20 and NGC 6514) is a star-forming region (H II) about 40 LY in diameter, located in the Sagittarius Arm of the Milky Way about 5,200 LY from the Solar System. It projects into the constellation Sagittarius and is estimated to be only 300,000 years old. The Trifid (“three-lobed”) Nebula is an unusual combination of a red, emission nebula with a young open star cluster near its center, surrounded by a blue, reflection nebula, particularly noticeable at north end, and a dark nebula (the apparent "voids" within the emission nebula causing the trifid appearance) also known as Barnard 85. The hydrogen molecules in the emission nebula at the center of the Trifid Nebula are heated by hundreds of bright young stars that cause them to emit red light. The dense part of the nebula is a stellar nursery filled with embryonic stars. This star cluster, known as C 1759-230, may be the youngest star cluster in the Milky Way. The reflection nebula's blue color comes from cosmic dust grains that preferentially reflect the blue component of starlight while scattering light from new stars that have formed nearby. The largest and hottest of these stars shine brightest in the hot, blue part of the visible spectrum. In some parts of the nebula there are so many grains of dust that they are created in the atmospheres of cold giant stars and in the debris of supernova explosions, that they hide the incandescent gas, producing the dark absorption streaks. Within these dark alleys, the remnants of previous stellar births and deaths continue to collapse under the gravitational pull. The increase in density, pressure and temperature within these dark spots will eventually trigger the formation of new stars. A few arc-minutes NE of M20 is open cluster M21 (or NGC 6531). This cluster contains about sixty stars and has a remarkable central concentration of stars for this type of object, being the distance between the stars close to a light year. The brightest stars (magnitude 8) are very young: the age of the cluster is estimated to be less than 5 million years. The distance of the cluster from the Solar System is thought to be about 4,000 AL. The stars of M21 are believed to be part of the Sagittarius OB1 association, a group of young, hot and massive stars of spectral class O and B (hence the name) which are found within a narrow region of space and which emit a large amount of ultraviolet light which ionizes the surrounding gas.