A group of large clouds of gas and dust around 1500 l.y. away in the constellation Orion, the nearest examples of giant molecular clouds. Each cloud is over 100 l.y. across and contains more than 100 000 solar masses of gas, mostly in the form of hydrogen molecules. The young stars of Orion formed from these clouds within the last 10 million years. New stars are still forming in dense molecular cores within the clouds, hidden from sight by the dust, but detectable as sources of infrared and molecular radiation. The best-known molecular cores are designated OMC-1 and OMC-2.