Betelgeuse: a Review

Abstract: Betelgeuse has fascinated people since they first looked at the sky. Here we present a contemporary summary of the observations and theory that lead to our understanding of Betelgeuse as a massive red supergiant doomed to collapse and explosion. At only ~200 parsecs from Earth, Betelgeuse can be spatially resolved yet uncertainties in its distance remain a critical impediment to deeper understanding. The surface of Betelgeuse is rent with a complex structure as deep convective eddies arise to the surface affecting most of its measured physical properties. Determination of the equatorial rotation velocity is critical since some current estimates indicate that Betelgeuse is rotating anomalously rapidly, a property that cannot be explained by single-star evolutionary models. Betelgeuse is also moving through space at relatively high velocity that indicates that it received a boost, likely via collective interaction with other stars in its birth cluster. A bow shock and other structure in the direction of the star's motion suggest that it has affected the organization of the circumstellar and interstellar medium. Betelgeuse varies in brightness on a variety of time scales with 200, 400 and 2000 days being prominent. Betelgeuse is probable to have been born in a binary system, and the high space velocity and apparent rotation have been related to binary star evolution. One possibility is that Betelgeuse underwent common envelope evolution culminating in a final merger with the core of a massive primary. Such merger models have been invoked to account for the anomalous rotation velocity. Betelgeuse underwent a Great Dimming in 2020 that received widespread attention. Explanations have focused on large cool spots on the surface and the expulsion of a cloud of dust that obscured the surface. We sketch the nature of the explosion to come and discuss perspectives for further research.

• The paper presents a comprehensive synthesis of observational data and theoretical models to address uncertainties in Betelgeuse's structure and evolution.
• It highlights the star's rapid rotation, possibly resulting from a merger event, which challenges classical stellar evolutionary models.
• The analysis of Betelgeuse's variability and historical color changes offers actionable insights into mass loss processes and pre-supernova behavior.

Analysis of "Betelgeuse: a Review"

The review paper on Betelgeuse offers a comprehensive examination of this well-studied red supergiant star, exploring the complexity of its structure, evolution, and past events. This paper serves as an extensive resource by synthesizing observational data, historical context, and theoretical models, addressing the uncertainties and contradictions inherent in the study of Betelgeuse.

Observational Characterization

Betelgeuse is characterized as a massive red supergiant located approximately 200 parsecs from Earth. Despite its proximity, significant uncertainties persist regarding its fundamental parameters, such as distance, luminosity, radius, and surface gravity, which affect the determination of its mass and evolutionary state. The star's surface is marked by convective eddies contributing to a complex atmospheric structure, affecting elements like the photosphere and chromosphere, along with mass loss mechanisms.

Rotation and Kinematics

Anomalously, Betelgeuse exhibits a rapid rotation velocity near the threshold of rotational breakup, a feature not aligned with the classical single-star evolutionary models. Hypotheses regarding this high rotation involve a merger with a former binary companion, while other conjectures include dynamical interactions resulting in its high space velocity. The notable bow shock observed in the circumstellar medium suggests historical interactions, possibly tied to past binary interactions or cluster dynamics in its birth environment.

Variability and Historical Color Changes

Betelgeuse exhibits variability on multiple timescales, with prominent periods around 200, 400, and 2000 days. Theories surrounding its variability are intertwined with convective and pulsational processes. A significant point of contention is the historical color shift from yellow to red over two millennia, potentially indicating rapid evolution up the red supergiant branch, as suggested by certain historical analyses. This notion conflicts with recent studies that position Betelgeuse high on the red supergiant branch based on pulsation periods.

Theoretical Studies and Merger Hypothesis

The hypothesis that Betelgeuse might have undergone a merger plays a central role in interpreting its rotational and evolutionary properties. Merger models suggest that such an event could explain the anomalously high rotation without invoking atypical internal processes. However, the nature of mixing and effects on surface and internal compositions remain important areas for further study. Common envelope evolution during the merger is anticipated to manifest in observable effects, demanding high-resolution 3D simulations to elucidate its dynamics.

The Great Dimming and Current State

The Great Dimming of 2019-2020, where Betelgeuse's brightness decreased significantly, offers a case study in the complexity of mass loss and surface activity. The event likely combined large-scale convective activity, transient dust formation, and spots leading to temperature variations across its surface, a phenomenon not entirely understood and still under study.

Future Perspectives

The eventual supernova explosion of Betelgeuse, notwithstanding its uncertain timing, is anticipated to provide valuable insights into late-stage stellar evolution of massive stars. Continuous monitoring of Betelgeuse offers a unique empirical window, presenting opportunities to refine our models of mass loss, envelope dynamics, and pre-supernova behavior in red supergiants. Such observations are critical to resolving the outstanding questions related to the star's evolutionary trajectory and past interactions.

In conclusion, this paper not only highlights the complexities associated with Betelgeuse but also underscores the opportunities for advancing our understanding of massive star evolution by integrating multi-faceted observational data with refined theoretical models.