Introduction

Interstellar molecules range from simple species such as H₂ and CO to complex organic molecules such as HC₁₁N and the amino acid glycine (H₂NCH₂COOH).

Interstellar molecules tell us about the local environment where they are found - the temperature, how fast they are moving, and the local gas density. Most of our information about our universe outside the solar system comes from indirect sources - because the distances involved are so large, we can't physically visit other galaxies. However, we can observe other galaxies spectroscopically - by collecting and analyzing the electromagnetic radiation (light, infrared radiation, microwaves and radiowaves) that reach us from outer space.

Unlike on Earth, where molecules are everywhere, in outer space there are relatively few molecules. In order to form molecules, the temperature has to be sufficiently low so that any molecules that are formed will not fall apart, and the gas density (the pressure) has to be sufficiently high to allow atoms to combine to form molecules or existing molecules to react to form new molecules.

On Earth, at sea level, the temperature is approximately 300 K and our atmosphere contains about 0.041 moles/liter of gas, or 2.4 x 10¹⁹ gas molecules/cm³. Most of space is composed of the gas and dust between stars - the interstellar medium. Here the average gas density is extremely low, only about 1 particle/cm³. Since hydrogen is by far the most abundant element in the universe (see table below), the interstellar medium consists mostly of H atoms and H₂ molecules. At these low densities, few collisions occur between atoms or molecules, so there is little chemistry.

Close to stars, gas densities are much higher, but there are few molecules found near stars because at temperatures above ~3000 K the heat causes most molecules to fall apart to atoms. Thus, most molecules in space are found in regions with temperatures below ~3000 K and gas densities significantly higher than 1 particle/cm³.

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