Raw materials for star formation
Earth was originally made from gases condensed to solids such as nitrogen, oxygen and ammonia and rocks from the calcium, silicon, magnesium and iron groups. Life depends on the complex chemistry of organic compounds built around carbon atoms. The stars were the sites where these materials were created. Stars manufacture heavy elements in the course of nuclear fusion. At the end of their lives, massive stars explode and less massive stars slowly shed gas enriched with these heavy elements that in turn coalesces to form other stars. In each cycle of star birth and destruction, the proportion of heavy elements increases as the byproducts of nuclear burning in the center of stars is added to the mix. Over billions of years, complex chemistry and biology have evolved from their simple beginnings in the first stars and galaxies. Now astronomers are traveling back through time to wit- ness crucial steps in our origin.
Figure 1. Two mid-Infrared spectra of pow- dered materials from a primitive meteorite are shown. A CAI is a complex of minerals formed very early in our solar system, the fine grained matrix is material produced at a later stage. The horizontal bars are band position found in the astronomical spectra of the dust in the circumstellar disk around Beta Pictoris (Fig. 3, Knacke et al, 1993).
The molecular clouds generated by stars at the end of their lives provide the raw material for the formation of new stars and planetary systems such as our own. When the tem- perature in the circumstellar disk becomes low enough, a dust con- denses out of the gas. A rich range of chemical reactions occurs, includ- ing creation and depletion of heavy elements onto dust grains and into ices. Interstellar molecular clouds are also the principal formation sites of organic matter such as CH4, CH3OH, and H2CO. The composition of the dust provides valuable in- formation on the conditions when the dust was formed and helps de- termine subsequent steps involved in star formation. Comprehensive understanding of heavy element creation and depletion and the important role of dusty material in star and planetary system formation is needed to understand the chemical conditions from which life on our planet arose. These investigations can be pursued with spectroscopic studies of molecular clouds and dust that exists throughout the universe. Several telescopes have been launched into space that permit mid- to far-Infrared spectroscopy can be used determine the compo- sition, temperature, density and velocity of the molecular clouds.
A major problem needs to be over- come. The Infrared spectra generated by platforms such as the Infrared
Space Observatory (ISO) in the late 1990s and the Spitzer Space Infrared Telescope Facility which is operating now, are often unknown. One of the most important discoveries of the ISO was that crystalline silicates exist outside our own solar system. This is significant because the relatively sharp features of crystal- line silicates are sensitive to com- positional changes, in contrast to broad and smooth amorphous sili- cate features. To increase our understanding of the crystalliza- tion process, we need to positively identify the composition of the wide range of crystalline silica spectrum that has already been identified in molecular clouds. “Just like human fingerprints uniquely identify their owner, Infrared spectroscopy provides a spectral fingerprint that uniquely identifies a chemical compound,” Morlok said. “The challenge facing scientists is that, just like it is im- possible to match a fingerprint to a person unless you have previously taken that person’s fingerprint, it’s impossible to determine the compo- sition of a compound based on its spectra until you have analyzed an identical sample on earth. But the chemistry in molecular clouds is, to a large degree, unknown on the earth; and it will be a long time before we are able to travel to distant stars and galaxies to analyze the many com- pounds that we have already gen- erated Infrared spectra from.”