分子云是星际云的一种，它的密度和大小允许分子──最常见的是氢分子（H₂）──的形成。

氢分子很难被直接侦测到。通常是利用一氧化碳（CO）侦测氢分子。一氧化碳辐射的光度与分子氢质量的比例几乎是常数。不过在对其他星系的观测中有理由怀疑这样的假设。

A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H₂). This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.