Stars form from sparse material between other stars. In most cases, the region between stars is a near vacuum containing very thin, cold gas. Atoms and molecules number only about 106 per cubic meter. The temperature is typically around 10 to 20 K. However, certain regions of space have a density of gas and dust 10,000 times greater. With some 1010atoms per cubic meter, the atoms are close enough to collide frequently. Moreover, the temperature is low enough that atoms will stick together to form molecules. The most important molecular species is molecular hydrogen (H2), and it forms on dust grains. These regions are called molecular clouds. They are rich in molecular hydrogen, carbon monoxide (CO), water (H2O), and more complex forms such as formaldehyde (H2CO) and ethyl alcohol (C2H5OH). Such complex molecules are virtually absent in most parts of interstellar space. Although a molecular cloud is denser than most interstellar gas, it is still a near-perfect vacuum compared with ordinary room air, where there are about 20 trillion trillion (2 × 1025) molecules per cubic meter.
Molecular clouds are important because they are the places where most stars form. In these regions the atoms and molecules and dust grains are crowded close enough to begin to attract each other gravitationally — a key step in forming stars from a gas. Observations taken with a radio telescope can show the dynamic activity of molecules in the clouds. Spectra made at millimeter wavelengths show many spectral features caused by different states of molecular vibration and rotation. The low energy of the transitions explains why the spectral lines are seen at long wavelengths, in the millimeter or sub-millimeter part of the electromagnetic spectrum. Astronomers have found that the chemistry within a molecular cloud can be very complex.
Certain dense regions in molecular clouds display particularly spectral lines that are as intense and pure as the light from a laser! Astronomers have observed OH and H2O molecules that generate maser emission. Maser is an acronym for microwave amplification by the stimulated emission of radiation, analogous to the familiar phenomenon in optical light called a laser. Normally, gas in space is at such a low density that most of the atoms or molecules are in their lowest energy states. However, molecules in the dense cores of molecular clouds can be forced to overpopulate the excited energy levels. When this happens, a single photon can trigger a surge of radiation as the molecules drop into their lowest energy states.
Nearly 130 different molecules have been discovered in interstellar space, including ten containing ten or more atoms. Examples include benzene (C6H6), acetone ((CH3) 2CO), and ethanol (CH3CH2OH). One of the most remarkable molecules discovered in interstellar space is the amino acid glycine (NH2CH2COOH), one of the building blocks of life. In the 1950s, British astronomer Sir Fred Hoyle wrote a science fiction novel called The Black Cloud. In it, he speculated that life might form in the dark depths of a molecular cloud. Scientists do not think it likely that replicating molecules could form in the near vacuum of space, but the discovery of complex molecules such as glycine is certainly intriguing.
The Sagittarius B2 molecular cloud contains a substantial amount of ethyl alcohol, or ethanol. The American astronomer Ben Zuckerman noted that, if purged of impurities, this cloud would yield approximately 1028 fifths at 200 proof! This exceeds the sum of all human fermentation efforts in recorded history. A few years after this discovery, NASA received a proposal from a Texas businessman to "harvest" this alcohol. Molecular clouds are too diffuse to make this idea practical. It turns out that a spaceship pushing a funnel with a diameter of one kilometer at 10% of the speed of light would take 1000 years to collect enough ethyl alcohol to fill a shot glass.