SA part of Betelgeuse investigation

South African astronomers have joined an international team investigating a mysterious shell of dense gas encircling the red star Betelgeuse.

Betelgeuse is a red supergiant star in the constellation of Orion, located approximately 600 light years from us, and is one of the brightest stars in the night sky. It is a colossal star with a radius about 1000 times that of our own sun: if it were at the centre of our solar system, it would extend all the way out to Jupiter. The star is in the final stages of its life, blowing off huge amounts of gas and dust from its outer layers. Its eventual death as a supernova will be visible with the naked eye during the day and night, and according to the soon to be published paper by a team of astronomers based in South Africa, Germany, Russia and the UK, the explosive event is likely to be highly dramatic, approaching the brightness of the full moon, not once but twice over the space of a few years.

The 2012 discovery of a mysterious shell of dense gas encircling Betelgeuse prompted this investigation. The gas is ejected from the surface of the star, but its flow is somehow disrupted by an unknown force and it piles up in a shell around the star. Using computer simulations, the authors showed that energetic interstellar radiation (ultraviolet light that is almost everywhere in space) can ionize and heat up the gas ejected by the star. This abrupt change in temperature drives a strong shock wave back towards the star. This shock wave has sufficient force to decelerate the gas so it slows down and accumulates in a shell around the star. The shell is known as a photoionization-confined shell. It can capture up to 35% of the mass lost by the red supergiant, keeping the gas close by until the star explodes.

“When we apply our model to the wind of Betelgeuse, we find that it predicts the properties of the shell very well. This is the first convincing explanation of how this shell of gas has formed,” said Dr. Jonathan Mackey from the Argelander Institute for Astronomy in Bonn, Germany.

When Betelgeuse explodes as a supernova, its debris, moving at thousands of kilometers per second, will crash into the nearby shell, and the force of this collision will light up the shell, resulting in a second increase in the brightness. Other red supergiants that are losing mass 10 to 100 times faster than Betelgeuse may be even more extreme: in some cases, the collision of the debris with the shell will be much brighter than the original supernova explosion.

“What is exciting about the model is that it not only agrees well with the observations of Betelgeuse, but it also provides clues to a long standing problem related to some supernovae that appear over-luminous or that show an unexpected second peak in brightness. In our model, the extra luminosity comes from the violent collision of the supernova debris with these shells,” explained Dr. Shazrene Mohamed based at the South African Astronomical Observatory (SAAO) in Cape Town, South Africa.

In galaxies like ours, astronomers expect a supernova to go off every 100 years or so. The last supernova observed in our galaxy, the Milky Way, was in 1604, and is referred to as Kepler’s supernova after the German astronomer who described it in detail. It was also a very bright event and visible to the naked eye.

“Kepler’s supernova went off more than 400 years ago, so a supernova explosion in our galaxy is long overdue. We don’t know when Betelgeuse will explode. It could happen tomorrow or in 100 000 years, actually, it may have gone supernova already as we have to wait a few hundred years for the light to reach us. I hope that is it soon, the view from earth will be absolutely spectacular!” says Dr. Shazrene Mohamed.

Animation explaining the phenomenon: www.saao.ac.za/~shazrene/nature_animation.mov

Betelgeuse, often seen as the shoulder of Orion or the ferocious lion, is one of the brightest stars in the night sky. The star drives a bow shock ahead of it as it flies through space. Only in the last two years has it been discovered that a second shell of material has been piled up much closer to the red supergiant. When Betelgeuse explodes as a supernova, debris from the star will be expelled outwards towards

this shell with unimaginable speed. There will be a powerful collision between the debris and the shell, that should be visible from Earth with the naked eye.

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