When we look towards the Orion nebula, we see a young OB association—a cluster of young, massive stars—next to large dark clouds of gas and dust. This juxtaposition is not random, for the stars in this association were born from these clouds. The clouds are molecular clouds, and they are found throughout the Galactic disk. Nearby molecular clouds are relatively small—only about a dozen parsecs or less in size and containing several thousand solar masses of gas—but the more distant giant molecular clouds, those found along the spiral arms of our Galaxies, extend for hundreds of parsecs and contain from 100,000 to 10 million solar masses of gas. Molecular clouds are dense and cold, and this combination makes them the progenitors of stars.
Much of the interstellar medium is divided between warm, tenuous regions and cool, dense clouds that are in pressure equilibrium. These regions are uniformly heated by ultraviolet radiation. The cold molecular clouds, however, differ fundamentally from the cool clouds of atomic hydrogen by being opaque to visible and ultraviolet light and by being gravitationally bound. These two properties give the molecular cloud a turbulent, complex structure, with outer regions of densities and temperatures similar to the cool atomic hydrogen clouds and cores of much higher densities and lower temperatures.
The cores of molecular clouds are the coldest regions of our Galaxy, with temperatures in the 8°K to 20°K range. These cores are also the densest regions of the interstellar medium, with the number of molecules exceeding 105 per cubic centimeter. These extremes in temperature and density are coupled by radiative cooling; the rate at which a molecular gas cools by the emission of infrared radiation increases both with temperature and density, so if the radiative cooling rate is held constant, and the density of the gas increases, then the temperature of the gas must drop.
The temperature of the interstellar medium is a balance between cooling by the emission of microwave and infrared radiation and heating by starlight, cosmic rays, and shock waves. In most regions of space, ultraviolet radiation from young stars is the principal heat source, and in these regions, the temperatures are 100°K or greater. The center of a molecular cloud, however, can cool to a much-lower equilibrium temperature because the densities of these clouds are so much higher than in other regions of the galaxy. They are also cooler because dust in the cloud block starlight from entering the cloud, so that the most significant source of heat are the cosmic rays—atomic nuclei and fundamental particles moving at close to the speed of light. They strike dust grains deep within the cloud, knocking loose electrons that heat the gas. Per gram of material, this heating is much less than the heating by ultraviolet radiation of the transparent interstellar medium. While the dust keeps out the visible and ultraviolet starlight, it does not prevent the escape of infrared and microwave radiation, so the gas deep in a molecular cloud cools to a temperature that is within 6° of the cosmic microwave background temperature of 2.7°.
The extreme cold and the absence of starlight within a molecular cloud permit a much richer chemistry than is found in open space. As implied by the name, a molecular cloud is composed of molecules rather than neutral atoms. The primary constituent is molecular hydrogen (H2), but many other molecules composed of hydrogen, carbon, nitrogen, and oxygen are also constituents, ranging from simple molecules such as ammonia (NH3) and carbon monoxide (CO) to complex organic molecules.
The extremely low temperatures also affect the nature of dust within a molecular cloud. Unlike in the warmer surroundings, the dust grains in a molecular are covered with ices of water, ammonia, and carbon dioxide, which are chemically created from molecules that precipitate onto the grain from the gas. This precipitation of molecules changes the composition of the gas cloud, causing the gas to be depleted of carbon and oxygen. The ice-encrusted dust grains should stick together, creating fluffy clumps that are like dirty snow flakes.
Gravity is the critical factor that permits molecular clouds to have much higher densities than the surrounding interstellar medium. Molecular clouds are bound by their self-gravity. The gas pressure within a molecular cloud counters both this gravitational force and the pressure exerted by the external interstellar medium. In contrast, the gas pressure within the cool regions of the interstellar medium only need counter the gas pressure of the surrounding warm regions. The addition of self-gravity means that the pressure within a molecular cloud is much higher than in the surrounding interstellar medium. It is this higher pressure that causes the density within a molecular cloud to be higher than in the surrounding medium.
The size and mass of a gravitationally-bound gas cloud are directly linked to the temperature and the mass density of the gas. These characteristic values are called the Jeans length and Jeans mass. The Jeans length is proportional to (T/ρ)1/2, where T is the gas temperature and ρ is the gas mass density. The Jeans mass is proportional to T 3/2ρ−1/2. What these equations tell us is that the smaller the temperature and the higher the density of a cloud of gas, the smaller the mass necessary to gravitationally-bind a cloud together.
A cool cloud with a temperature of 100°K and a density of 30 hydrogen atoms per cubic centimeter has a Jeans length of approximately 50 parsecs and a Jeans mass of approximately 365,000 solar masses. This goes a long way to explain why the molecular clouds are typically over one hundred parsecs in size, because it is on this scale that cool clouds can collapse to form gravitationally-stable molecular clouds. For a temperature of 10°K and a density of 105 hydrogen molecules per cubic centimeter, the Jeans length is only 0.14 parsecs and the Jeans mass is 50 solar masses. The molecular cores therefore have masses that are comparable to the masses of the largest stars. These mass estimate and the proximity of young stars to molecular clouds are the spur that drove astronomers to the theory that stars are born in molecular clouds.
Bergin, Edwin A., and Tafalla, Mario. “Cold Dark Clouds: The Initial Conditions for Star Formation,” in Annual Review of Astronomy and Astrophysics, vol. 45. Palo Alto: Annual Reviews, 2007: 339–396.