Giant Bubbles Erupt on Betelgeuse

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The twinkling stars we observe in the night sky do not always reflect their true nature. The sparkling appearance we perceive is not an inherent quality of these distant suns, but rather the result of our planet’s starlight-lit atmosphere. These stars, for the most part, emit a constant, unwavering brightness, unaffected by external factors, at least under normal circumstances.

Betelgeuse, a renowned red supergiant located in the celestial body in the constellation Orion, has deviated from this conventional behavior in recent times. Although a significant portion of its brightness, visible to the naked eye, is still influenced by elements in our night sky, some of the fluctuations in its brightness originate from the star itself. The star is in a perpetual state of agitation, similar to a pot of soup boiling on the stove, with surface activity contributing to its visual variability. In a notable incident in 2019, Betelgeuse experienced a sudden and dramatic dimming following the ejection of a large chunk of material into space, leading to speculation about a potential imminent supernova eruption. This event, known as the Great Dimming, had a lasting impact on Betelgeuse, leaving its surface in a turbulent state, similar to a malfunctioning washing machine stuck in a rapid spin cycle.

Numerous surveys conducted over the past thirty years have consistently revealed that Betelgeuse is spinning at a speed significantly greater than expected, exceeding the norm for a supergiant star by at least a factor of 100. A recent simulation conducted by an international team of astrophysicists proposes that This unusually high rotation rate can be misleading, a consequence of observers being fooled by the sheer enormity of the star’s bulky, foamy structure.

J. Craig Wheeler, an academic at the University of Texas at Austin not affiliated with the research mentioned above, emphasizes the critical importance of determining Betelgeuse’s true rotation speed. A discrepancy in the star’s rotation speed would require a reevaluation of the prevailing hypothesis, suggesting that Betelgeuse accelerated its rotation after absorbing a Sun-like star in the recent past. Furthermore, a hypothesis based on bubble-related phenomena could potentially unravel the enigma surrounding the excessively high rotation rates observed in several other supergiant stars.

The exact dimensions of Betelgeuse’s bubbles remain uncertain due to ambiguities regarding the star’s size (estimated to be between 700 and 800 times larger than our sun) and its distance from Earth (approximately 500 to 650 light-years or more). Despite these uncertainties, one indisputable fact is the colossal scale of these bubbles, with each hot plasma convective cell (referred to more formally) boasting a diameter comparable to Earth’s orbit around the sun. Some of these bubbles may even expand further, reaching sizes equivalent to the orbit of Mars, large enough to encompass our inner solar system.

Although details regarding size and distance are unclear, scientists express a greater level of confidence in their measurements regarding the speed at which material moves in and around Betelgeuse. Determining such speeds can be achieved by examining peculiarities in the spectrum of rainbow-like light emitted by a star. For example, if one side of a star’s face, as seen from Earth, is observed to be moving rapidly toward our planet while the opposite side is moving away, this phenomenon is interpreted as an indication that the star is rotating. This pattern aligns with what countless observations have consistently revealed about Betelgeuse.

When the Atacama Large Millimeter/submillimeter Array (ALMA) captured fuzzy images of Betelgeuse in 2015, a prominent spinning motion was detected. This observation led astronomers to deduce that the star is rotating at an astonishing rate of five kilometers per second. This presents a puzzle because, over time, as stars evolve into supergiants by expanding their atmospheres to considerable sizes, their rotation is expected to slow proportionally, in the same way that an ice skater slows down when extending its arms. Wheeler notes that it seems highly unlikely that Betelgeuse, as a single star, would achieve such a rotation. However, if it accelerated its rotation due to swallowing a neighboring star, unraveling clues to this event would be extremely challenging, explains Wheeler, highlighting the complexity of this process.

In February, a new simulation was presented in a paper published in Astrophysical Journal Letters, offering an alternative explanation by simulating five virtual years of Betelgeuse’s behavior in three dimensions. The simulation tracked how convective bubbles can cause the star’s surface to wobble back and forth. The findings illustrated clusters of hot plasma bubbles regularly erupting and dissipating at speeds of up to 30 km per second. When one set of bubbles rises on one side of the star’s visible surface while another set descends on the opposite side, the overall impression is that of a rapidly rotating star.

The study’s lead author, Jing-Ze Ma, a PhD candidate at the Max Planck Institute for Astrophysics (MPA) in Garching, Germany, expressed that manipulating the simulated appearance of his star could potentially fool human vision, when discussing the concept of deceive your eyes. According to Ma and his colleagues, Betelgeuse’s rapid rotation would be logical if the star had a perfect spherical shape, with no irregular movements, however, the chaotic movements on its surface neutralize each other, resulting in a speed of 5 kilometers per second, which was misinterpreted as the star’s rotational speed. By distorting the simulated star to align with what ALMA would observe, it became evident that the star’s wobble closely matches the telescope’s observations. Looking ahead to the telescope’s upcoming data, Ma and his team predict that the star’s surface will exhibit significant differences compared to its appearance during the last close examination in 2015, due to the presence of rapidly moving cells. Study co-author Selma de Mink, an astrophysicist at MPA, confidently suggests that a revisit to Betelgeuse should reveal a completely altered surface if the explanation lies within bubbles, emphasizing the potential for drastic changes.

Initial glimpses from ALMA’s observations of Betelgeuse from two years ago, hailed as the clearest views of the star after a major telescope upgrade, appear to support the conclusions drawn in the new study, according to Ma and his colleagues. Despite the apparent alignment with his predictions, Ma acknowledges that there are discrepancies that require further examination, indicating a sense of anticipation for additional analysis. Despite the promising results proposed by the study, skepticism remains regarding the feasibility of vast convective bubbles accurately replicating rapid stellar rotation, as highlighted by Wheeler’s reservations about the statistical improbability of such precise bubble motions on the star’s surface.

The claim made in the study that Betelgeuse’s rotation speed is slower than previously estimated contradicts the findings of “three independent observations at three different times using three different techniques,” according to astrophysicist Andrea Dupree, who monitors Betelgeuse. at the Center for Astrophysics | Harvard & Smithsonian. Dupree, who is not involved in the new research, references Hubble Space Telescope data analyzed in a 1996 study, as well as corroborating studies from 2006 and 2018, which collectively support a fast rotation rate for the star. Ma and his team’s simulation proposes the constant presence of numerous large convective bubbles on Betelgeuse, resulting in a perpetually turbulent surface, but validation of this hypothesis through real observations remains pending. While she acknowledges the researchers’ efforts, Dupree expresses a desire to witness concrete evidence of her claims in observational data.

The recent study, described as a “thought experiment, which is good,” by Pierre Kervella, an astronomer at the Paris Observatory who led the 2018 investigation of the ALMA observations, appears to have some limitations, according to Kervella. He believes that not all arguments were fully considered in the study. It’s crucial to note that, based on the most reliable estimates available, Betelgeuse is approximately 10 to 20 times more massive than our sun. However, the study conducted by Ma and his colleagues simulated a star that is only five times heavier than our sun due to valid technical reasons. Kervella expresses her reservations about this decision, classifying it as questionable. The appendix of the new paper contains results from additional simulations using a significantly higher mass for the star. However, it is the weaker gravitational field of the simulated lower-mass star that generates the huge convective bubbles capable of mimicking rapid rotation, according to the study’s findings. While Kervella appreciates the team’s transparency in presenting the two models, he criticizes the reliance on one model that doesn’t align well with Betelgeuse, considering it an unfavorable approach.

According to Wheeler, replicating Betelgeuse’s turbulent surface in three dimensions poses a significant challenge to our computer models, which are based on the predictable behavior of our symmetrical sun. Despite undeniable advances in computational capabilities over the past decade, astronomers face increasingly complex challenges, as explained by Wheeler. He emphasizes that nature operates independently and it is our responsibility to keep up with its complexities. However, until we achieve this, even experienced astronomers like Dupree and Kervella, who have devoted years to studying Betelgeuse, struggle to predict the exact behavior of this tumultuous star or anticipate its future actions.

Reflecting on this uncertainty, Kervella shares her contemplation by stating: “Who knows? It’s a tortured star.” The enigmatic nature of Betelgeuse continues to confuse and fascinate astronomers, highlighting the intricate and unpredictable aspects of celestial phenomena.

Source:

https://www.scientificamerican.com/article/giant-bubbles-may-explain-betelgeuses-surprising-spin/

Tags: Giant Bubbles Erupt Betelgeuse

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