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On the most nearby scales (within several hundred times the distance from the Earth to the Sun), the local interstellar medium (ISM) provides the boundary conditions for our Solar System, and the specific properties of the local ISM (as we pass through it) may have significant implications for the viability of life on the Earth.
More globally, the ISM is an active participant in the overall life cycles of galaxies and stars. Stars are formed out of interstellar gas and dust (primarily hydrogen and helium), synthesize heavier elements from those initial raw materials, and return material to the ISM during their lives (and, in may cases, as they die). The abundances of elements heavier than hydrogen and helium found in the Galactic ISM thus reflect their production in successive generations of stars in the Galaxy.
Understanding the abundances and physical conditions in the interstellar media of our Galaxy and of other nearby galaxies (looking for both similarities and differences) also provides a basis for understanding the absorption-line systems (ALS) seen toward distant QSO's (quasi-stellar objects). The QSOALS exhibit absorption similar (in some respects) to the interstellar absorption features found in those more local contexts, and they are thought to be associated with distant galaxies (at earlier stages in their lives). At redshifts of 0.5-4.0, the QSOALS sample a fair fraction of the history of the Universe, and provide perhaps the best opportunity for tracing the global evolution of heavy element abundances from very early times to the present -- with significant implications also for the formation and evolution of galaxies.
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Last modified 17 Sep 1997