Star Betelgeuse shows its face

See the two giant bright spots? Each one of those is equivalent in size to the distance between our Earth and sun.

The image at the top of this post has particular appeal for me. It takes what many of us know best as a pinpoint of light in the sky – a star – and shows this star’s face in unprecedented detail. This image, which has been re-constructed via two different algorithms (or set of mathematical instructions) is the surface of the star Betelgeuse – a bright yet somber red light in the famous constellation Orion – in near infrared wavelengths.

See the two giant bright spots? Each one of those is equivalent in size to the distance between our Earth and sun.

It’s exciting to think about the immense sizes and distances in space, but astronomers – who think on those immense scales all the time – are excited about this image for another reason. They say it is a first strong and direct indication of the presence of the convection phenomenon – the same phenomenon you see in a pot of boiling water – in a star other than our sun. This result allows them to better understand the structure and the evolution of supergiant stars like Betelgeuse.

08jan10_250Betelgeuse is a red supergiant located in the constellation of Orion the Hunter. You can see it easily now, every evening, in the south as viewed from the northern hemisphere. See the diagram at right? Notice the three stars in the central part of Orion? These stars are known as Orion’s Belt, and they are very easy to spot in the sky. Betelgeuse is bright and reddish and approximately above them, if you are looking from the northern hemisphere.

This star is different from our sun. It is 600 times larger in dimension, and it radiates approximately 100,000 times more energy. But our sun has sunspots, as you probably know, and now we can see that Betelgeuse also has bright and dark spots, which indicate hotter and colder regions on the surface of this star. Astronomers say these structures would be mainly due to the phenomenon of convection. The size, physical characteristics, and lifetime of these dynamical structures remain unknown.

This image was obtained via the simultaneous use of the three telescopes of the Infrared Optical Telescope Array (IOTA) interferometer on Mount Hopkins in Arizona. Here’s more, from this morning’s press release from the American Astronomical Society

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 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 degrees compared to the average temperature of the star (3,600 Kelvins). 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 has been discovered by a team led by Pierre Kervella from Paris Observatory. 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 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.

Deborah Byrd

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