A technetium star, or more properly a Tc-rich star, is a star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 whose stellar spectrum contains absorption lines of the light radioactive metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 technetium

Technetium

Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...

. The most stable isotope

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 of technetium is ⁹⁸Tc with a half-life

Half-life

Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 of 4.2 million years, which is too short a time to allow the metal to be material from before the star's creation. Therefore, the detection in 1952 of technetium in stellar spectra provided unambiguous proof of nucleosynthesis in stars

Stellar nucleosynthesis

Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...

, one of the more extreme cases being R Geminorum.

Stars containing technetium belong to the class of so-called asymptotic giant branch

Asymptotic Giant Branch

The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....

 stars (AGB) — stars that are like red giants, but with a slightly higher luminosity

Luminosity

Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

, and which burn hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

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

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 shell burning with an interval of some 100,000 years, in so-called "dredge-ups". Technetium stars belong to the classes M, MS, S

S-type star

A star with spectral type S is a late-type giant star whose spectrum displays bands from zirconium oxide in addition to titanium oxide which is characteristically exhibited by K and M class giant stars...

, SC and C-N

Carbon star

A carbon star is a late-type star similar to a red giant whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds,...

. They are most often variable

Variable star

A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

 stars of the long period variable

Long period variable

A long period variable is a type of variable star in which variations in brightness occur over long timescales of months or years. Long period variables are giant stars and brighter, from spectral class F and redwards, but most are red giants and AGB giants, meaning spectral class M, S or C...

 types.

Current research indicate that the presence of technetium in AGB stars occurs after some evolution, and that a significant amount of these stars do not exhibit the metal in their spectra. The presence of technetium seems to be related to the so-called "third dredge-up

Dredge-up

Dredge-up refers to a period in the evolution of a star where a surface convection zone extends down to the layers where material has undergone nuclear fusion...

" in the history of the stars.