The interstellar medium. Infrared image of a region in Chamaeleon showing delicate and complex patches of interstellar medium (ISM), the diffuse gas and dust spread between the stars. The ISM accounts for less than 10% of the total mass in our galaxy. It is mainly formed by hydrogen, grains of dust and smaller amounts of helium and heavier elements. Dust grains shield atoms in the ISM from the destructive effects of interstellar photons enabling the formation of molecules such as H2O and CH4. The temperature of the ISM is very low (10 to 100 Kelvin) and it is better seen at infrared or sub-millimetre wavelengths. The image was taken by the IRAS satellite.