In relation to the interstellar medium, this is only a vaguely defined term. It is sometimes used to refer to that portion of molecular clouds with densities exceeding a few hundred 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), and with an admixture of interstellar dust.