The article states, "By studying historical documents, the researchers found that Betelgeuse went through this phase two millennia ago. The findings could help researchers better understand the life cycles of stars. One of the sources used by the team was Chinese court astronomer Sima Qian who wrote about star colors in 100 BCE commenting "white is like Sirius, red like Antares, yellow like Betelgeuse, blue like Bellatrix."
The reference paper does references Sirius 17x. So, Betelgeuse changed color from yellow to red, but Sirius did not (red to white or blue) because some texts indicated this white/blue color while others do not support this? Reports 2,000 years ago or earlier indicated Sirius was red in some texts. Such ancient reports cause problems for modern stellar evolution models.
The paper cited, Colour evolution of Betelgeuse and Antares over two millennia, derived from historical records, as a new constraint on mass and age,
https://academic.oup.com/mnras/article/516/1/693/6651563?login=false
"Small, secular variations in brightness and/or colour over millennia are expected for evolving stars, but were rarely considered: Hearnshaw (1999) found that, statistically, there have been no significant brightness changes for giants and supergiants compared to Ptolemy’s Almagest (2nd century AD). One or a few individual stars might have evolved in brightness, but this would be hard to prove because the magnitudes given in ancient star catalogs such as the Almagest scatter around today’s values by about ±1 mag (Hearnshaw 1999; Schaefer 2013). There has been a debate on a possible colour change in Sirius, which is reported as ‘somewhat reddish’ in the Almagest, whereas it is given as blue or white in several other ancient works. The colour in the Almagest is likely due to either strong scintillation (some red rays) or as a late addition by a copying scribe (Ceragioli 1995 and references therein), rather than to an observation at low altitude."... In the original Greek, the term used in the Almagest for the colours of these six stars is the compound adjective ‘hypokirros’ (Heiberg 1903). Let us consider the meanings of the two components: ‘hypo’ means ‘below, pale, weak, somewhat’; ‘kirros’ means something like ‘pale yellow’ (German: ‘blaßgelb’, Gemoll, Vretska & Kronasser 1954), ‘orange-tawny’ (Liddell & Scott 1940), or ‘rose’ (by comparison with wine colours give by Galen from the 2nd century AD, like Ptolemy). For ‘hypokirros’, we found ‘somewhat yellow’ (Liddell & Scott 1940); in the Almagest, this word was translated to German as ‘rot’ (red) or ‘rötlich’ for reddish (Manitius 1898) and to ‘(orange-)rot’ for orange-red (Kunitzsch 1974, pp. 230 & 267) – as well as to ‘reddish’ (Toomer 1984). These translations have a purely philological quality. We can now try to quantify the colour index and brightness ranges or limits for the use of ‘hypokirros’ in the Almagest (and Tetrabiblos, see below). That Sirius was included here could be due to the fact that it sometimes briefly does appear with red rays due to scintillation at large brightness, or possibly as a late addition by a copying scribe (Ceragioli 1995). The five other stars listed as ‘hypokirros’ in the Almagest are all indeed K- or M-type giants or supergiants, and their B − V colour indices range today from 0.97 (Pollux) to 1.84 mag (Antares). They are among the brightest stars in the sky."..." In sum, Ptolemy’s Tetrabiblos, Cleomedes, Germanicus, and Manilius all appear to list the brightest and reddest stars down to their respective magnitude limit, but they do not mention Betelgeuse, even though it is now one of the brightest and reddest stars. The least red star listed by the above is Arcturus (B − V = 1.14 mag), which sets the upper limit in colour index for Betelgeuse for this time range (1st/2nd century AD). We list the limits on Betelgeuse in Table 3. Except Betelgeuse, all stars listed by these authors as red/reddish were indeed constant in colour since the last few millennia according to their location in the CMD and their MIST tracks (Figs 1 and 3). The largest B − V colour index range for yellow-to-red stars is found in the Almagest (namely B − V = 0.80–1.84 mag), so that both Pollux and Betelgeuse were included (plus Sirius, which is in a different category)."
Interesting how ancient sources known today may or may not support some stellar evolution model changes.
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