In relation to the interstellar medium, the term dense cloud is only vaguely defined. It is sometimes used to refer to that portion of molecular clouds with densities exceeding a few 100 atoms per cubic centimeter. Some authors use dense to refer to “dense cores,” those regions in molecular clouds with densities of 10⁵ atoms per cc or greater, where stars may form or are forming. The gas phase of a dense interstellar cloud consists almost entirely of molecular hydrogen (H₂), with an admixture of interstellar dust. Since molecular hydrogen lacks an electric dipole moment and hence has no allowed rotational transitions, the gas in dense clouds is usually traced by observing the millimeter-wavelength rotational transitions of CO, the second most abundant gas phase constituent (typically about 1 part in 10,000 by number, relative to H₂).