All About Betelgeuse: A Boiling and Asymmetrical Star

Over three decades of research utilizing advanced telescopes and interferometers have revealed intricate details about Betelgeuse's photosphere and its extensive atmosphere. This red supergiant exhibits a strikingly different form compared to our Sun. Lacking spherical symmetry, it resembles a constantly boiling pot, where enormous plasma bubbles emerge on its surface.

Betelgeuse, a red supergiant that has moved beyond the main sequence, is at a significantly more evolved stage than the Sun. The nuclear processes occurring within it have led to an immense expansion, causing the outer layers of the star to become exceptionally thin. Intense convective currents transport the hottest material from its core to the photosphere, resulting in the formation of gigantic "bubbles" that create the star's irregular surface.

This stark contrast to the Sun's appearance has been corroborated by numerous studies conducted over recent decades, employing the most powerful observational tools available.

In a 1990 study, Buscher, Haniff, Baldwin, and Warner noted an asymmetrical feature on Betelgeuse's photosphere, utilizing the 4.2-meter William Herschel telescope with a masking technique that mimics an interferometer:

> We present high-resolution images of the M-supergiant Betelgeuse taken in February 1989 at wavelengths of 633, 700, and 710 nm, employing the non-redundant masking method. At all these wavelengths, there is clear evidence for an asymmetric feature on the star's surface, contributing 10–15 percent of the total observed flux. This could be attributed to a nearby companion passing across the stellar disc or, more plausibly, to large-scale convection within the stellar atmosphere.

The authors concluded, expanding on the convective motion hypothesis:

> These surface zones are expected to be so large that only a few would be present on the surface at any one time [...]

The First Direct Image of a Star's Disk Beyond the Sun

The presence of this asymmetry was validated six years later with the publication of the first direct image of a star’s surface other than the Sun, Betelgeuse, in May 1996 in The Astrophysical Journal.

The choice of Betelgeuse for this landmark observation was not coincidental; it is the star with the largest angular diameter visible from Earth after the Sun.

Thanks to Hubble's capabilities, the presence of a significantly extended chromosphere around Betelgeuse was identified, indicating that the diameter of the stellar disk measured in ultraviolet light was approximately double that observed in visible light. Authors Gilliland and Dupree focused on a bright region evident in Hubble's image:

> The appearance of one dominant bright feature in Betelgeuse's atmosphere is dramatically different from the UV image of the Sun, which typically exhibits a mottled appearance with numerous active regions present. The feature on Betelgeuse might consist of an unresolved cluster of bright regions. However, during solar minimum, such a significant active region has not been observed. Thus, this supergiant’s atmosphere offers evidence of a novel physical phenomenon in luminous stellar atmospheres, promising insights into structure and energy balance in low-gravity environments.

A Vastly Extended Atmosphere

In 2001, Timothy, Horch, and Valenti published the first direct image of Betelgeuse captured in the far ultraviolet using the Hubble space telescope's spectrograph. Data from the STIS revealed that Betelgeuse's atmosphere extended more than three times its visible light diameter.

The average diameter of the stellar atmosphere recorded in the far ultraviolet was around 260 milliarcseconds, equating to 8.6 billion km, placing Betelgeuse approximately 724 light-years from Earth. If this colossal red supergiant were situated where the Sun is, its atmospheric edges would nearly reach Neptune's orbit!

The image obtained by Hubble in the far ultraviolet confirmed the inhomogeneity of Betelgeuse’s atmosphere. Two large regions appeared significantly brighter than the average, and the lack of spherical symmetry was apparent.

Recent studies have reinforced the findings of asymmetry. For instance, Betelgeuse was observed using the PIONIER instrument of the Very Large Telescope Interferometer in Chile from 2012 to 2014, with results reported in a 2016 study published in Astronomy & Astrophysics.

A Bright Spot on the Photosphere

As illustrated in the following image from the aforementioned study, a gigantic bright spot was detected on Betelgeuse's surface, offset from the star's physical center. This hot spot migrates over time and complicates precise measurements of the star's angular diameter. This hot spot is approximately 1,000 K hotter than Betelgeuse's average effective temperature.

The study's authors interpreted this bright spot as an outlet of a massive convection cell—high-temperature plasma that, due to its lower density, rises to the photosphere from within the star. While convection cells, or granules, are also present on the solar photosphere, their diameters are around 1,000–1,500 km. In contrast, the enormous bubbles emerging from Betelgeuse's surface can be as large as Mars's orbit.

The red supergiant resembles a giant boiling pot, lacking a definite shape due to the continual evolution of the large plasma bubbles that surface. This evident asymmetry starkly contrasts with the nearly perfect spherical shape of the Sun, and observations from the ALMA interferometer have revealed even more pronounced forms of asymmetry.

Betelgeuse Observed with ALMA and SPHERE

Betelgeuse was observed with ALMA on November 9, 2015, for 75 minutes within the frequency range of 275 to 373 GHz, utilizing 47 of the 66 antennas of the large Chilean interferometer. The results are illustrated in the following image.

ALMA's image highlights the extended atmosphere of the red supergiant, encompassing the chromosphere that reaches beyond the photosphere.

Obvious asymmetries are visible, including a bright spot in the upper left quadrant and a bump in the middle of the disk on the same side. Both features have temperatures around 1,000 K higher than the average temperature at 1.3 stellar radii, which is 2,760 K (nearly 1,000 degrees cooler than the photospheric temperature).

These hot regions in the stellar atmosphere suggest effects from heightened local magnetic activity, driven by the immense convective motions that extend to Betelgeuse's photosphere. Another intriguing aspect revealed by ALMA is the temperature inversion in the star's atmosphere. At 1.3 stellar radii, the chromosphere's temperature is 2,760 K, increasing to 3,600 K at 2 stellar radii (similar to the photosphere's temperature), before dropping to 1,400 K at a distance of 6 stellar radii.

In summary, Betelgeuse's atmosphere is far from uniform or homogenous. Its asymmetric structure and temperature inversion illustrate that the physical phenomena influencing it are markedly different from those observed on the Sun.

The extended atmosphere of Betelgeuse, as depicted in the ALMA image, is roughly 1,400 solar diameters, nearly 2 billion km—sufficient to reach halfway between Jupiter and Saturn if Betelgeuse occupied the Sun's position.

Recent observations of Betelgeuse's variability in shape and brightness were presented in two images released recently by ESO. Captured in visible light with the SPHERE instrument of the Very Large Telescope in Chile, these images by a team led by Miguel Montargès from the University of Leuven show the star in January and December of 2019. A comparison reveals two key points:

1. A noticeable decrease in brightness of the star (beginning in late 2019);
2. The persistent asymmetry of Betelgeuse, which changes shape over months but consistently deviates from the typical sphericity of the Sun.

If We Could Observe Betelgeuse Up Close...

One final curiosity: what would this enormous star look like if viewed from the distance of Pluto from the Sun, adequately shielded from its dazzling brightness? Likely, it would appear similarly to how it is visualized in the simulations created by Bernd Freytag, an astronomer from the Center de Recherche Astronomique in Lyon.

Notes

[1] Due to the extreme rarefaction of the outer layers, the gravity of a red supergiant at the photosphere level is exceptionally low, around 0.5 cm/s², significantly less than solar gravity.

This article is part of a five-part series. Read the *other four parts here:*

• All About Betelgeuse: The Most Conspicuous Drop in Brightness in a Century
• All About Betelgeuse: How Far Is It?
• All About Betelgeuse: How Big Is It?
• All About Betelgeuse: Live Fast, Die Young