Technetium
One element, technetium, is of particular interest in astronomy because it is unstable and
does not occur in natural abundance, but it can be made in stars (and in nuclear
reactions on Earth). Thus, the detection of technetium in space is a sure sign that it has been
made recently in a star near where it is observed.
Technetium-99 is observed in the spectra
of red giant stars, which
is one of the strongest indications that the element production sequences we are about to
desribe actually take place in stars.