What characteristic distinguishes a molecular cloud from other forms in the interstellar medium?

The characteristic that distinguishes a molecular cloud from other forms in the interstellar medium is its density. Molecular clouds are often referred to as "stellar nurseries" because they contain a high concentration of gas and dust, allowing for the formation of stars and other celestial bodies. Typically, the density in these clouds can range from 10 to several hundred particles per cubic centimeter, which is significantly higher than in other regions of the interstellar medium, such as diffuse interstellar clouds where the density is much lower.

This increased density is critical for the cooling processes that allow molecules to form and eventually lead to star formation. In contrast, regions of lower density cannot support the same level of molecular interactions, which is essential for star birth.

While high temperature, magnetism, and color are important factors in the study of the interstellar medium, they do not uniquely characterize molecular clouds. For example, many other structures within the interstellar medium can also exist at varying temperatures, and magnetism can be found across different cloud types. Color variations may result from different processes and compositions but do not specifically define a molecular cloud's identity. Thus, density stands out as the key attribute that differentiates molecular clouds from other interstellar mediums.