Above all, it was the unusually strong drop in brightness of the star in the constellation Orion that fascinated astronomers. However, the darkening has most likely caused unusually large spots on the surface of Betelgeuse. This is what scientists at the Max Planck Institute for Astronomy (MPIA) in Heidelberg assume. The team headed by Thavisha Dharmawardena was able to rule out a previous assumption that it was is the ejected dust of the star.
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Betelgeuse: the pulsating red giant
Instead, temperature fluctuations in the photosphere – the shining surface of the star – are responsible for the drop in brightness, the scientists write in an article published in “The Astrophysical Journal Letters“The most plausible cause for such temperature changes are gigantic cool star spots that cover 50 to 70 percent of the surface. These star spots are very similar to sun spots.
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“At the end of their lives, stars become red giants,” Dharmawardena explains. “When the fuel supply runs out, the processes by which the stars release energy change. As a result, they inflate, become unstable and pulsate with periods of hundreds or even thousands of days, which we consider to be fluctuations in brightness.” Betelgeuse is a so-called red giant. It is about 20 times more massive and about 1,000 times larger than the sun.
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It remains exciting about the bright star in Orion
Due to this immense size, the attraction on the surface is less than on a smaller star with the same mass. Therefore, pulsations can eject the outer layers of such a star relatively easily. That is why astronomers assumed light-absorbing dust as the cause of the sharp drop in brightness, explains the international team of scientists. But it turned out that “a decrease in brightness in the sub-millimeter range is not due to an increase in dust production”, as Steve Mairs from East Asian Observatory who collaborated on the study. Instead, the star itself must have caused the change in brightness.
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Star spots are common in giant stars, but not on this scale, explain the researchers at the Max Planck Institute. “We know from the sun that the number of spots increases and decreases in an eleven-year cycle,” says team leader Dharmawardena. It is uncertain whether giant stars have a similar mechanism. “Observations in the coming years will show us whether the sharp decrease in the brightness of Betelgeuse is related to a point cycle.” The giant star, which is just 700 light years away, will definitely remain an exciting object for future studies.
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