HRD showing the position of ε Ori (blue) in comparison to the β-Cep instability region (ℓ = 0; orange), several evolutionary tracks (black) from Paxton et al. (2015), and evolutionary track for the mass of ε Ori computed by us using the MESA code. Note: the instability was computed only up to 24 M_⊙, but the region most likely continues upwards, and the instability also occurs for the luminosity class Ia (log L/L_⊙ ≥ 5.3).