A technetium star, or more properly a Tc-rich star, is a star whose stellar spectrum contains absorption lines of the light radioactive metal technetium. The most stable isotope of technetium is 98Tc with a half-life of 4.2 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 stars (AGB)—stars that are like red giants, but with a slightly higher luminosity, 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 indicate 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.
- Barium star – Spectral class G to K giants, whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium
- Carbon star – Star whose atmosphere contains more carbon than oxygen
- Mercury-manganese star – Type of star with a prominent spectral line due to absorption from ionized mercury
- S-type star – A cool giant with approximately equal quantities of carbon and oxygen in its atmosphere
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- T. Lebzelter & J. Hron. (2003). "Technetium and the third dredge up in AGB stars. I. Field stars". Astronomy & Astrophysics. 411 (3): 533–542. arXiv:astro-ph/0310018v1. Bibcode:2003A&A...411..533L. doi:10.1051/0004-6361:20031458. S2CID 18879265.