A molecular cloud is a cold dense region of the interstellar medium in which hydrogen is predominantly molecular and stars can form.
Rotational carbon-monoxide line emission traces cold molecular clouds because abundant molecular hydrogen is difficult to observe directly. A conversion factor relates integrated CO intensity to molecular-hydrogen column density.
For one optically thin LTE carbon-monoxide component, and . Thus for the 3–2/1–0 luminosity ratio. The temperature then determines the population fraction and hence the CO mass. Multiple excitation components, optical depth or subthermal excitation invalidate a single-temperature inference.
