²⁶Al(β⁺ν)²⁶Mg

²⁶Al in its ground state lives for about a million years. It decays to an excited state of ²⁶Mg, which at once emits a γ-ray of 1809 keV. Space telescopes see this γ-ray line across the Milky Way: a map of where nuclei were made in the last few million years. ²⁶Al also has a second, excited form, the isomer ²⁶ᵐAl. It decays within seconds, straight to the ground state of ²⁶Mg, and gives no 1809 keV γ-ray. Only ²⁶Al made in its ground state feeds the line.