A molecular cloud is a cold, dense region of interstellar gas where hydrogen exists as molecules rather than single atoms, and where nearly all star formation happens. Temperatures fall to around ten degrees above absolute zero, cold enough for gravity to overcome internal pressure and pull knots of gas into collapse. The largest, called giant molecular clouds, hold millions of solar masses and stretch across hundreds of light-years. Dust inside them blocks visible light, which is why infrared telescopes are needed to watch stars being born.