Colour evolution of Betelgeuse and Antares over two millennia, derived from historical records, as a new constraint on mass and age

TL;DR

This study uses historical star colour records to constrain the evolutionary stages, mass, and age of Betelgeuse and Antares, revealing significant colour changes over two millennia that inform stellar evolution models.

Contribution

It introduces a novel method of using historical colour observations to constrain the evolutionary status, mass, and age of red supergiants like Betelgeuse and Antares.

Findings

Betelgeuse was historically recorded as less red, similar to Saturn, indicating a significant colour change.

The colour evolution constrains Betelgeuse's position to less than a millennium past the red giant branch.

Evolutionary models suggest Betelgeuse has a mass of about 14 solar masses and an age of approximately 14 million years.

Abstract

After core hydrogen burning, massive stars evolve from blue-white dwarfs to red supergiants by expanding, brightening, and cooling within few millennia. We discuss a previously neglected constraint on mass, age, and evolutionary state of Betelgeuse and Antares, namely their observed colour evolution over historical times: We place all 236 stars bright enough for their colour to be discerned by the unaided eye (V$\le$3.3 mag) on the colour-magnitude-diagram (CMD), and focus on those in the Hertzsprung gap. We study pre-telescopic records on star colour with historically-critical methods to find stars that have evolved noticeably in colour within the last millennia. Our main result is that Betelgeuse was recorded with a colour significantly different (non-red) than today (red, B$-$V=$1.78 \pm 0.05$ mag). Hyginus (Rome) and Sima Qian (China) independently report it two millennia ago as appearing like Saturn (B$-$V=$1.09 \pm 0.16$ mag) in colour and `yellow' (quantifiable as B$-$V=$0.95 \pm 0.35$ mag), respectively (together, 5.1$\sigma$ different from today). The colour change of Betelgeuse is a new, tight constraint for single-star theoretical evolutionary models (or merger models). It is most likely located less than one millennium past the bottom of the red giant branch, before which rapid colour evolution is expected. Evolutionary tracks from MIST consistent with both its colour evolution and its location on the CMD suggest a mass of $\sim$14M$_{\odot}$ at $\sim$14 Myr. The (roughly) constant colour of Antares for the last three millennia also constrains its mass and age. Wezen was reported white historically, but is now yellow.