Massive collections of molecular gas and dust that can contain 100,000 to several million solar masses of material and span 50-300 light-years across. These are the largest gravitationally bound structures in the galaxy and the sites of most star formation.
The term was coined in the 1970s when radio observations revealed that molecular clouds come in a hierarchy of sizes, with the largest examples being truly gigantic compared to smaller molecular clouds. The 'giant' designation distinguishes these massive star-forming regions from smaller molecular cloud complexes.
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