Using interferometry, an international team led by an astronomer of l’Observatoire de Paris obtained an unprecedented image of the surface of the red supergiant Betelgeuse in the constellation Orion. The image reveals the presence of two giant bright spots whose size is equivalent to the Earth-Sun distance — they cover a large fraction of the surface. It is a first strong and direct indication of the presence of the convection phenomenon, transport of heat by moving matter, in a star other than the Sun. This result allows us to better understand the structure and evolution of supergiants.
Betelgeuse is a star quite different from our Sun — 600 times larger in dimension, it radiates approximately 100,000 times more energy. But following the Sun, this type of object also reveals a surface with bright and dark spots — hotter and colder spots. These structures would be mainly due to the phenomenon of convection — the transport of heat by matter currents. This phenomenon is observed every day in boiling water. On the surface of the Sun, these spots are rather well-known and visible. However, it is not at all the case for other stars and, in particular, supergiants. The size, physical characteristics, and lifetime of these dynamical structures remain unknown.
Betelgeuse is a good target for interferometry because its size and brightness make it easier to observe. Using simultaneously the three telescopes of the Infrared Optical Telescope Array (IOTA) interferometer on Mount Hopkins in Arizona (removed since then), the team, partly formed by three researchers from the l’Observatoire de Paris, could obtain a great number of high-precision measurements. These made it possible to reconstruct an image of the star surface thanks to two algorithms. A small number of computer programs are used by the few astronomers who use astronomical interferometry to produce images. Here, two different algorithms gave the same image. One was created by Eric Thiebaut from the Astronomical Research Center of Lyon (CRAL), and the other was developed by Laurent Mugnier and Serge Meimon from the French aerospace lab, ONERA. The final image reveals the star surface with unprecedented fine details. Two bright spots clearly show up next to the center of the star.
Other images of less quality of Betelgeuse’s surface had already been obtained in the past. They were primarily models of the surface constrained from interferometric data. Now, the researchers have a true image whose richness exceeds what is possible to imagine from a model. For the first time, one can say that two spots are present and determine the size of the largest. Perhaps this difference in dimension corresponds to different physical phenomena.
The analysis of the brightness of the spots shows a variation of 500° compared to the average temperature of the star is 5841° Fahrenheit (3227° Celsius). The largest of the two structures has a dimension equivalent to the quarter of the star diameter (or one and a half the Earth-Sun distance). This marks a clear difference with the Sun where the convection cells are much finer and reach hardly 1/20th of the solar radius (a few Earth radii). These characteristics are compatible with the idea of luminous spots produced by convection. These results constitute a first strong and direct indication of the presence of convection on the surface of a star other than the Sun.
Convection could play an important role in the explanation of the mass-loss phenomenon and in the gigantic plume of gas that is expelled from Betelgeuse. The latter was discovered by a team led by Pierre Kervella from l’Observatoire de Paris. Convection cells are potentially at the origin of the hot gas ejections. A new research field is opening. This has been made possible thanks to the Paris observatory researchers who now take advantage of their mastery in interferometry on the largest telescopes in the world — Keck I and II, Gemini, the Canada-France-Hawaii Telescope, and the European Very Large Telescope.
Use Astronomy.com’s interactive star chart, StarDome, to locate and observe Betelgeuse in your sky. Click on the StarDome graphic to the right. Be sure to find and set your location under “Location Settings.” After you’ve set your location, click on “Show Names” — located just to the lower right of the star chart — and select “Mark One Specific Object.” Scroll down to find “Betelgeuse” from the list of objects. Highlight it and click OK.
If Betelgeuse is visible from your location, it will appear on the map. Track the object’s motion across the sky by adjusting the figures under “Date and Time Settings.”
