How Did the Elemental Composition of the
Why do we observe the types and quantities of chemical elements in the Universe
revealed by astronomy? This is one of the most fundamental questions facing
astronomers. Determining how the elements were created - the field of
nucleosynthesis - draws from a wide range of studies: from the Big Bang to the
formation and evolution of our solar system.
Light elements (those up to lithium on the periodic table) were mainly created
in the Big Bang. Measurement of the abundances of these elements can give
scientists important clues to the nature and evolution of the early Universe. In
particular, measurement of the ratio of the normal isotope of hydrogen to its
heavier isotope known as deuterium will provide important insight into
All other elements are products of nuclear reactions occurring in stars
and supernova explosions. With certain exceptions, we have a
comprehensive understanding of how a star evolves, as it converts
hydrogen and helium into heavier elements. These heavy elements are
disseminated into the Universe by stellar winds and supernova
explosions. However, we need to understand just where and when the
chemical evolution happens during the lifetime of a typical galaxy, and
how well our models predict the observations.
The study of radiation due to the decay of nucleosynthetic products provides
direct information about the synthesis of elements. Due to its short lifetime
(about 1 million years), 26Al is known to be an excellent tracer of the
birthplace of elements. The 1.8 MeV 26Al emission line has been shown to be
concentrated on the galactic plane by observations of the COMPTEL instrument on
NASA's Compton Gamma-Ray Observatory.
Recent studies of this emission performed
by GRIS have revealed spectral line broadening indicative of matter moving at
about 500 km/second. This is much faster than any movement linked to the
galactic rotation. Although 26Al can be released at high velocities to the
interstellar medium by supernovae, it is hard to understand why it has not come
to rest after a million years, the radiative lifetime. This observation
constitutes a great challenge to our understanding of the origin and propagation
of the 26Al.