A technetium star, or more properly a Tc-rich star, is a
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
whose
stellar spectrum
Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light, ultraviolet, X-ray, infrared and radio waves that radiate from stars and othe ...
contains absorption lines of the light
radioactive metal technetium. The most stable
isotope of technetium is
97Tc with a
half-life of 4.21 million years, which is too short a time to allow the metal to be material from before the star's formation. Therefore, the detection in 1952 of technetium in stellar spectra provided unambiguous proof of
nucleosynthesis in stars,
one of the more extreme cases being
R Geminorum.
Stars containing technetium belong to the class of
asymptotic giant branch
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...
stars (AGB)—stars that are like
red giants, but with a slightly higher
luminosity
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...
, and which burn
hydrogen in an inner shell. Members of this class of stars switch to
helium shell burning with an interval of some 100,000 years, in "dredge-ups". Technetium stars belong to the classes
M, MS,
S, SC and
C-N. They are most often
variable stars of the
long period variable types.
Current research indicates that the presence of technetium in AGB stars occurs after some evolution and that a significant number of these stars do not exhibit the metal in their spectra. The presence of technetium seems to be related to the "third
dredge-up" in the history of the stars. In between the
thermal pulses of these RGB stars, heavy elements are formed in the region between the hydrogen and helium fusing shells via the slow neutron capture process; the
s-process
The slow neutron-capture process, or ''s''-process, is a series of reactions in nuclear astrophysics that occur in stars, particularly asymptotic giant branch stars. The ''s''-process is responsible for the creation (nucleosynthesis) of approximat ...
. The materials are then brought to the surface via deep convection events.
[ 99Tc, an isotope with a half-life of only 200,000 years, is produced in AGB stars and brought to the surface during thermal pulses. Its presence is taken as a reliable indicator that a ]third dredge-up A dredge-up is any one of several stages in the evolution of some stars. By definition, during a ''dredge-up'', a convection zone extends all the way from the star's surface down to the layers of material that have undergone fusion. Consequently, th ...
has taken place.
See also
*
*
*
*
References
External links
*
{{Star
Star types
Technetium