Presolar grains
Presolar grains are microscopic dust particles that formed around other stars before the Sun existed and survived inside primitive meteorites.
Presolar grains are stardust in the literal sense. They condensed in the outflows of ageing stars or in the debris of exploding stars, drifted through interstellar space, and were incorporated into the cloud that formed the Solar System. Their defining feature is isotopic: ratios of isotopes such as carbon-12 to carbon-13, or of oxygen and silicon isotopes, can differ from solar values by large factors, far beyond anything produced in the Solar System.
Known types include nanodiamonds, silicon carbide, graphite, oxides such as corundum and spinel, and silicates. Each grain is a sample of an individual star, so these grains let astrophysicists test models of nucleosynthesis, the making of elements inside stars, with laboratory precision. They are scarce, typically present at parts-per-million levels, and survive best in primitive, little-heated meteorites.
The Murchison CM2 chondrite and the Allende CV3 chondrite have been major sources of presolar silicon carbide and nanodiamonds. Silicates are only found by searching uncut sections grain by grain. Nothing about presolar grains is visible by eye: they are isolated by chemical dissolution of the host rock or located by isotopic imaging with a nanoscale secondary ion mass spectrometer.
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