Tau4 Serpentis Explained

Tau⁴ Serpentis, Latinized from τ⁴ Serpentis, is a variable M-type giant star in the constellation of Serpens, approximately 710 light-years from the Earth.

With a spectral classification M5IIIa, Tau⁴ Serpentis is a cool red giant star. The spectrum varies, and some sources classify it between M4IIIe and M6IIIe.^[1] Some of its spectral lines show an inverse P Cygni profile,^[2] where cold infalling gas on to the star creates redshifted hydrogen absorption lines next to the normal emission lines.^[3] It is classified as a semiregular late-type variable, and its magnitude varies between +5.89 and +7.07 with a period of approximately 100 days.

τ⁴ is unique among the stars with the Bayer designation τ Serpentis as being the only one with no HR catalog number.

Notes and References

1. 2014yCat....1.2023S. VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009-2016). VizieR On-line Data Catalog: B/Mk. Originally Published in: Lowell Observatory (October 2014). 1. Skiff. B. A.. 2014.
2. Kolotilov . E. A. . Russev . R. M. . Inverse P Cyg Profile of Halpha in the Spectrum of the Red Giant HD 139216 = tau4 Ser . Information Bulletin on Variable Stars . 1730 . 1 . January 1980 . 1980IBVS.1730....1K.
3. Web site: Lecture 7: The Collapse of Cores and Infall . Galactic Star and Planet Formation Research Group . Department of Physics and Astronomy, University of Toledo . 17 July 2015.