Our environment is composed of "stardust", the chemical elements formed long ago in stellar interiors and supernovae. This process of nuclear fusion leads to the emission of gamma rays, which easily reach us from all regions of the Milky Way Galaxy. An international team of scientists led by Roland Diehl of the Max Planck Institute for Extraterrestrial Physics in Garching, Gera number of now has been using ESA's INTEGRAL satellite to determine that gamma rays from radioactive aluminium (26Al) originate from the central regions of the Galaxy. This implies that production of new atomic nuclei is an on-going process and occurs in star forming regions galaxy-wide. From those new observations, the astronomers estimate that the total amount of radioactive 26Al in the Galaxy is equivalent to three solar masses. This amount of production corresponds to a galactic rate of supernovae from gravitational collapse of about one every 50 years.

We are familiar with radioactive isotopes from medical radiology tests and therapys. Astrophysicists use penetrating gamma rays emitted during radioactive decay to obtain direct messages from cosmic nuclear fusion reactions, through special telescopes operated in near-Earth space. Gamma-rays from decaying 26Al were detected in 1978, and because of its known half life of 720, 000 years, this provided direct proof of currently-ongoing nucleosynthesis. Supernova 1987 in the Large Magellanic Cloud galaxy was then observed through short-lived radioactive gamma rays. This led researchers to think that these nuclei had been produced within this supernova event.

Astrophysicists from the Max Planck Institute for Extraterrestrial Physics in Garching were part of the pioneering sky study on such radioactive gamma rays. Roland Diehl and his MPE colleagues were able to show in the mid-1990s that this relatively long-lived radioactivity is present over large regions along the plane of the Galaxy. Hence, production of new atomic nuclei is common in the Galaxy. This was a scientific surprise, because at the end of the 1970s,traces of 26Al decay had been found in meteorite samples originating from the early solar system. This finding had been interpreted as evidence that the 26Al radioactivity was a key ingredient in the formation of planetary bodies of the solar system; radioactive heat is a necessary ingredient to melt cometary material to form rocks. Therefore it was usually believed at that time that 26Al radioactivity was intimately correlation to the early solar system; now, however, we have signals of currently-decaying 26Al all over the Galaxy. A unifying picture emerged from nucleosynthesis theories of the 1950s, which claimed that all nuclear species were produced inside stars, novae, and supernovae. 26Al could be the result of such stellar processing, occurring, with some enhancement, near the formation site of the solar system 4.5 billion years ago. Alternatively, special conditions during the formation of the solar system were thought to cause high-energy particle collisions, which could produce 26Al locally. These two competing scenarios are still debated and remain an unsolved puzzle. Eventhough gamma rays clearly show widespread cosmic nucleosynthesis, it remains to be understood if only this, or additional local high-energy reactions, has produced the amount of 26Al inferred for the early solar system. One key to answering this question is the determination of the total 26Al content of the Galaxy.



Source: Max-Planck Institute

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