2007 marks the 20th anniversary of the discovery of presolar grains. These tiny particles of stardust form when a star ejects elemental atoms from its core into interstellar space by stellar winds. As the interior of a star is too hot for molecular chemistry to take place, it is only outside the stellar core where elements can begin to combine. Stardust coalesces in the circumsolar shell, some of it eventually becoming preserved in asteroids.

The stardust is referred to as "presolar grains," since it must have been present in the molecular cloud which formed the sun, earth, and the rest of the solar system, including asteroids.

Some asteroids fall to earth as meteorites. Through analysis of these, researchers are able to isolate the presolar grains. The first such discovery was of presolar diamonds in a meteorite in 1987. More recently, the hunt was extended into space when a silicate grain from a foreign star inside a comet in our solar system was returned by the STARDUST space mission in 2006.

A nebula such as RCW49, pictured here, is a nursery for newborn stars and exists in circumstellar space, where chemistry first takes place