We recognize presolar grains on the basis of their unusual isotopic ratios .
Isotopes are different atoms of the same chemical element that have slightly different mass.
Most of the isotopes of most of the elements are made by nuclear fusion inside stars, a process known as nucleosynthesis . Although different stars produce different relative proportions of the isotopes, the material from these stars gets mixed up in space. Therefore, when our solar system formed, the contributions from the different stellar sources were almost entirely homogenized and the isotopic ratios of the elements are almost identical throughout the solar system. Presolar grains, on the other hand, contain the original atoms from their parent stars. These atoms did not get mixed up with the atoms from other stars, so the isotopic ratios in presolar grains are remarkably different from solar system stuff, and can span a huge range.