Betelgeuse

Betelgeuse’s Surface is Boiling Furiously

Of all the stars in the sky, betelgeuse must be among the most enigmatic. One of its many mysteries surrounds the speed of its rotation which is surprisingly fast for a supergiant star. If it were placed where the Sun was, then its photosphere (visible layer) would be out around the orbit of Jupiter and it would be moving at 5 km/s. A new study now hints that instead of high rotation, it may be that the surface is boiling so furiously that it has been mistakingly identified as fast rotation. 

Betelgeuse is one of the first stars an amateur astronomer will learn. Its distinctive red colour in the upper left corner of Orion makes it a prominent star, easy to find and identify and a great signpost to other constellations. We all know that stars are big but Betelgeuse takes this to a whole new level at 1.2 billion km across, almost 2,000 larger than the Sun. Stars of this size are usually expected to rotate slowly but observations revealed its high rotation speed, far higher than expected of a star at this evolutionary stage. 

Orion and the molecular cloud covering the region. Betelgeuse is the red star in the upper left. (Credit : Rogelio Bernal Andreo)

Observations from the Atacama Large Millimetre Array pointed at the rotation speed of Betelgeuse. The system, which is made up of 66 antennae is a radio interferometer that combines the signal from all dishes to increase its sensitivity. Using this instrument, astronomers had concluded that one hemisphere seems to be approaching while the other seems to be receding and the rate of this led to the conclusion of a 5 km/s rotation speed. If Betelgeuse was a perfect sphere then this would have been a reasonable conclusion however, the surface of Betelgeuse is not like that! 

Two of the Atacama Large Millimeter/submillimeter Array (ALMA) 12-metre antennas gaze at the sky at the observatory’s Array Operations Site (AOS), high on the Chajnantor plateau at an altitude of 5000 metres in the Chilean Andes. There is now a total of 66 antennae, 54 of them with 12-metre diameter dishes, and 12 smaller ones, with a diameter of 7 metres each. The ALMA project is an international collaboration between Europe, East Asia and North America in cooperation with the Republic of Chile. ESO is the European partner in ALMA.

Like all stars, convection is a prominent process in the photosphere that brings heat from the stellar interior. In the case of Betelgeuse the convection cells are massive, sometimes even as large as the Earth’s orbit around the Sun and they rise and fall at speeds around 30 km/s (that’s over twice the escape velocity of the Earth so is faster than any launching spacecraft).

Jing-Ze Ma PhD from the Max Planck Institute for Astrophysics now proposes that the dipolar velocity map which identified the approaching and receding hemispheres, may actually have been picking up convection cells instead. The theory postulates that the limited resolution of the ALMA system was observing (but not able to differentiate) convection cells rising on one side of the star and sinking on the other. 

To reach that conclusion, the team had developed a new processing technique to produce synthetic data from ALMA and in 90% of cases, the boiling motion was not clear and led to an interpretation of high rotational speeds. Further observations are now needed to explore this exciting possibility but instruments with greater resolution are required. to that end, higher resolution observations were made back in 2022 but the data is still being analysed but it will, it is hoped, start to reveal much more about the nature of Betelgeuse.

Source : A new spin on Betelgeuse’s boiling surface

Mark Thompson

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