SETI : Stars



		The interstellar medium The space between the star is not empty but is filled with a tenuous medium consisting mostly of hydrogen (99%) and dust (1%). Diffuse clouds have a density of 1-10 atoms/cm³, while denser (dark) clouds have 10⁶ atoms/cm³. Ultraviolet radiation from stars outside the clouds or embedded inside it, bathes diffuse clouds and provides a harsh environment for molecules to survive. Most molecules get split by this radiation, so most of the gas is made up of single atoms. On the contrary, dense clouds are so thick that UV radiation hardly penetrates and most of the gas in these clouds is in molecular form. More than 100 species of molecules have been identified, including alcohols, methane, ammonia and formaldehyde. There could be bigger and more complex molecules in dark clouds, but their abundance in the gas phase could be low, since they might be trapped on the surface of grains and wouldn't be easily deteced. Indeed, grains are important catalysts (i.e., promoters) for the formation of molecules in interstellar space (see sketch of a grain above). It is believed that the most important molecule of all, the mother of all molecules, H₂, is formed almost exclusively on the surface of grains and intervenes in the formation of virtually all other molecules. The composition of interstellar dust grains is somewhat uncertain, since they are very cold (10-15 K) and give off little radiation. It is believed that they are formed of carbon (amorphous carbon and graphite) and silicates (i.e., silicon plus oxygen and metal atoms, such as iron). The study of the physical and chemical properties of the interstellar medium is an active field of research. In order to measure how hydrogen and other molecules form on the surface of dust grains scientists work to replicate astrophysical environments in the laboratory, using ultra-high vacuum and low temperature (7-30 K) techniques as well as UV radiation fields. See: /research/surface_physics The interstellar medium plays a vital role in the in the birth of stars and in formation of complex molecules and dust. Recall that in the Big Bang only deuterium (heavy hydrogen, i.e. with a nucleus composed of a proton and a neutron), helium (He) and lithium (Li) were produced. All other elements, molecules and materials were made afterwards in the cores of stars or in supernova explosions (see Section Death of Stars). Elements (atoms) heavier than iron cannot be produced in stars, since nuclear fusion in stars stops with the synthesis of iron. All heavier elements, including copper, gold and uranium (to name a few) are made during supernova explosions when temperatures even higher than the core are reached. As the outflows from supernovae cool off, carbon, silicon, oxygen, iron and some other elements coalesce in tiny dust grains that are injected into the interstellar medium. The material in the interstellar medium becomes part of future stars. This is the reason why spectrographic measurements of the Sun will detect elements, such as Calcium, that are not produced in the Sun; these elements are remnants of earlier stars that died and ejected their material into the interstellar medium.

The interstellar medium