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Stardust grains (also called presolar grains by meteoriticists) are contained within meteorites, from which they are extracted in terrestrial laboratories. Stardust was a component of the dust in the interstellar medium before its incorporation into meteorites. The meteorites have stored those stardust grains ever since the meteorites first assembled within the planetary accretion disk more than four billion years ago. So-called carbonaceous chondrites are especially fertile reservoirs of stardust. Each stardust grain existed before the Earth was formed. Stardust is a scientific term referring to refractory dust grains that condensed from cooling ejected gases from individual presolar stars and incorporated into the cloud from which the Solar System condensed. Many different types of stardust have been identified by laboratory measurements of the highly unusual isotopic composition of the chemical elements that comprise each stardust grain. These refractory mineral grains may earlier have been coated with volatile compounds, but those are lost in the dissolving of meteorite matter in acids, leaving only insoluble refractory minerals. Finding the grain cores without dissolving most of the meteorite has been possible, but difficult and labor-intensive (see presolar grains). Many new aspects of nucleosynthesis have been discovered from the isotopic ratios within the stardust grains. An important property of stardust is the hard, refractory, high-temperature nature of the grains. Prominent are silicon carbide, graphite, aluminium oxide, aluminium spinel, and other such solids that would condense at high temperature from a cooling gas, such as in stellar winds or in the decompression of the inside of a supernova. They differ greatly from the solids formed at low temperature within the interstellar medium. Also important are their extreme isotopic compositions, which are expected to exist nowhere in the interstellar medium. This also suggests that the stardust condensed from the gases of individual stars before the isotopes could be diluted by mixing with the interstellar medium. These allow the source stars to be identified. For example, the heavy elements within the silicon carbide (SiC) grains are almost pure S-process isotopes, fitting their condensation within AGB star red giant winds inasmuch as the AGB stars are the main source of S-process nucleosynthesis and have atmospheres observed by astronomers to be highly enriched in dredged-up s process elements. Another dramatic example is given by the so-called supernova condensates, usually shortened by acronym to SUNOCON (from SUperNOva CONdensate) to distinguish them from other stardust condensed within stellar atmospheres. SUNOCONs contain in their calcium an excessively large abundance of 44Ca, demonstrating that they condensed containing abundant radioactive 44Ti, which has a 65-year half-life. The outflowing 44Ti nuclei were thus still "alive" (radioactive) when the SUNOCON condensed near one year within the expanding supernova interior, but would have become an extinct radionuclide (specifically 44Ca) after the time required for mixing with the interstellar gas. Its discovery proved the prediction from 1975 that it might be possible to identify SUNOCONs in this way. The SiC SUNOCONs (from supernovae) are only about 1% as numerous as are SiC stardust from AGB stars. Stardust itself (SUNOCONs and AGB grains that come from specific stars) is but a modest fraction of the condensed cosmic dust, forming less than 0.1% of the mass of total interstellar solids. The high interest in stardust derives from new information that it has brought to the sciences of stellar evolution and nucleosynthesis. Laboratories have studied solids that existed before the Earth was formed. This was once thought impossible, especially in the 1970s when cosmochemists were confident that the Solar System began as a hot gas virtually devoid of any remaining solids, which would have been vaporized by high temperature. The existence of stardust proved this historic picture incorrect.
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