A large and relatively dense cloud of cold gas and dust in interstellar space from which new stars are born. Molecular clouds consist primarily of molecular hydrogen (H₂) gas, have temperatures in the range 10–100 K, and contain 10³¹–10³⁶ kg of mass (for comparison the mass of the Sun is 2 × 10³⁰ kg). Molecular clouds are among the most massive gravitationally bound objects in the Milky Way Galaxy.