The largest and most luminous asymptotic-giant branch (AGB) star is VX Sagittarii. Wikipedia states that its spectral type is M4eIa-M10eIa (citing https://arxiv.org/pdf/astro-ph/0608438.pdf which in turn cites the General Catalog of Variable Stars). Additionally, VX Sagittarii's temperature ranges from 2400-3300 K, and 2400 K is well within the range of an M8/M9 class star. Why is its spectral type cited as M10, instead of something more common, like M9.5 or L0 instead?

  • $\begingroup$ Can you provide a non-wikipedia reference please. $\endgroup$ – ProfRob May 18 at 6:57

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