Betelgeuse After the Fall — Analysis of 185-yrs of Photometry: Evolution of Light Variations up to and beyond the 2020 Great Dimming — What’s next?

Betelgeuse (α Ori; M2.5–4.0 Iab; V ~ +0.2-1.6 mag) is the brightest red supergiant (RSG) seen from Earth, and along with Antares, is one of the nearest RSGs. RSGs have evolved from late-O-early B-type main-sequence ( ~10-25 Mo) stars and now most are undergoing helium fusion. Typically, RSG stars end their short (but brilliant) lives as core-collapse Type-II supernovae. Betelgeuse has been observed for from 1837–2022 (~185 yrs) by professional and amateur observers and undergoes semi-regular (multi-periodic) brightness variations. Except for the well-publicized 2020 “great dimming”, Betelgeuse typically shows brightness variations between ~0.2 to +1.2 mag with two prominent periods of ~2100–2250 d and ~390–430 d (see Harper et al. 2021). Also, sometimes a ~190-d periodicity is present. From over a decade of simultaneous photometry and radial velocity measures from Ganzer et al. 2021, the brightness and radial are well-correlated for both the short-period and long-period and indicate stellar pulsation (together with mass motions of giant (~1-2 AU) size convective cells) are the probable driving mechanisms.

We report on period studies conducted over this 185-yr interval utilizing the Period Analysis Software (Peranso-3) package. We employed the CLEANest (Discrete Fourier Transform) algorithm to detect and characterize multi-periodic signals. The Weighted Wavelet Z-Transform (WWZ) program also was employed to study the evolution of the star’s periods over time. In addition to the ~430-d and ~2200-d periods, a hitherto unreported “periodicity” of ~19.4-yrs was also returned. Sufficient time has not passed since the 2020 “Great Dimming” but with better (seamless) coverage from daytime photometry, shorter period low amplitude light variations appear present. We discuss the photometric behavior of the Betelgeuse over its 185-yr photometric history and discuss its behavior since the “great dimming”. Has the star returned to business-as-usual or have significant changes taken place?

We thank the AAVSO for organizing and vetting much of the photometry.