Molecular Cloud

A molecular cloud is a vast, cold, and relatively dense region of interstellar space where hydrogen exists primarily in its molecular form, H2, rather than as individual atoms, and where conditions are just right for new stars to eventually form. These clouds are dramatically colder than most of the interstellar medium, often only 10 to 20 degrees above absolute zero, and while they seem dense by the standards of the near-vacuum of space, they are still far less dense than any laboratory vacuum on Earth; it is really their enormous size, sometimes hundreds of light-years across and containing enough material to build hundreds of thousands of Sun-sized stars, that matters most. Within a molecular cloud, gravity slowly pulls denser clumps of gas and dust together; if a clump becomes dense and massive enough, its own gravity causes it to collapse further, heating up at its core until nuclear fusion ignites and a new protostar is born. The famous pillars of gas and dust seen in the Eagle Nebula, and the broader Orion Molecular Cloud Complex, which contains multiple active stellar nurseries, are among the most iconic and well-studied examples visible from Earth. Because visible light cannot penetrate their thick dust, astronomers rely heavily on infrared and radio observations, which can see through the obscuring dust, to study the star-forming activity happening deep inside molecular clouds.