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Sunspots revealed
on Betelgeuse

Posted: 13 January 2010

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Two giant bright spots have been revealed on the surface of nearby red giant star Betelgeuse by a team of astronomers using interferometry.

Betelgeuse is one of the 'shoulder' stars in the constellation Orion, the hunter; it is 600 times larger than our own Sun and radiates an impressive 100,000 times more energy. Just as our Sun has sunspots, new research shows that so too does Betelgeuse, although on a much larger scale – these two giant spots have diameters on the order of the size equivalent of the Earth-to-Sun distance, some 150,000,000 kilometres.

The surface of Betelgeuse in near infrared at 1.64 micron in wavelength, obtained with the IOTA interferometer (Arizona). The image has been re-constructed with two different algorithms, which yield the same details. Image: Copyright 2010 Haubois / Perrin (LESIA, Observatoire de Paris).

The observation marks the first direct indication of the process of convection – transport of heat by moving matter – occurring on another star.

Led by astronomers from Paris Observatory, the international team simultaneously used the three telescopes of the Infrared Optical Telescope Array (IOTA) interferometer on Mount Hopkins in Arizona to make high-precision measurements of the star's surface. Applying computer models to convert the observations into images revealed the two bright spots near the centre of the star.

Analysis of the two spots reveal that the largest spot has a dimension equivalent to the quarter of the star's diameter (one-and-a-half the Earth-Sun distance) and that the spots show a variation of 500 degrees compared to the star's average temperature of 3,600 degrees kelvin. This is in stark difference to the Sun where the convection cells rarely exceed 1/20th of the solar radius (or a few Earth radii).

The convection cells may play an important role in the explanation of the mass-loss phenomenon that has seen the star steadily shrink by about 15 percent over the last 15 years, and in the gigantic plume of gas that is being expelled from Betelgeuse out to a distance comparable to the separation gap of the Sun and Neptune.

Further study will allow the astronomers to determine the spots' physical attributes, as well as the lifetime of these dynamic features, in order to better understand the structure and the evolution of supergiants.