A technetium star, or more properly a Tc-rich star, is a star whose stellar spectrum contains absorption lines of the light

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

technetium Technetium is a chemical element with the symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. All available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous ...

. The most stable

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...

of technetium is ⁹⁷Tc with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...

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 R Geminorum (R Gem) is a Mira variable and technetium star in the constellation Gemini. It is located approximately away. R Geminorum pulsates with an average period of 369.9 days, varying by up to eight magnitudes at visual waveleng ...

. Stars containing technetium belong to the class of asymptotic giant branch stars (AGB)—stars that are like

red giants A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around o ...

, but with a slightly higher luminosity, and which burn

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

in an inner shell. Members of this class of stars switch to

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

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 Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

stars of the

long period variable The descriptive term long-period variable star refers to various groups of cool luminous pulsating variable stars. It is frequently abbreviated to LPV. Types of variation The General Catalogue of Variable Stars does not define a long-period vari ...

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 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, t ...

" in the history of the stars. In between the

thermal pulse 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) l ...

s 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 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

s. Its presence is taken as a reliable indicator that a third dredge-up has taken place.

References

External links

* {{Star Star types Technetium

See also

References

External links