26Al

26Al, or aluminum-26, is a radioactive isotope of aluminum with atomic number 13 and mass number 26. It decays by beta-plus emission to magnesium-26, with a half-life of about 717,000 years. The decay produces a characteristic gamma-ray photon of 1.809 MeV, which is used to trace the isotope’s distribution in the galaxy.

Natural production of 26Al occurs primarily in stars. The dominant channels involve proton captures on magnesium-25 in massive stars and explosive nucleosynthesis in supernovae, with additional production in asymptotic giant branch stars and novae. Cosmic ray spallation also generates smaller amounts. The isotope is ejected into the interstellar medium by stellar winds and supernova explosions, where it can mix into future generations of stars and planetary systems.

The 1.809 MeV gamma-ray line emitted by 26Al has been observed across the Milky Way, indicating ongoing production in star-forming regions and remnants of massive stars. Space-based gamma-ray observatories have produced maps showing 26Al distributed along the Galactic plane, consistent with continuous nucleosynthesis in the galaxy.

In meteoritic material, 26Al acts as a short-lived chronometer for early solar system events. The inferred initial 26Al/27Al ratio is about 5 × 10^-5, suggesting that 26Al was present when solids condensed about 4.57 billion years ago. The heat released by its decay likely contributed to melting and differentiation of some planetesimals and early planetary bodies.

Today, 26Al remains an important tracer of nucleosynthesis and galactic chemical evolution. Its presence in the early Solar System provides constraints on the timing and proximity of nearby massive-star events relative to solar system formation.