

Unveiling the true face

of Betelgeuse

DR EMILY BALDWIN

ASTRONOMY NOW

Posted: July 29, 2009



Using state of the art imaging techniques, astronomers have revealed a vast plume of gas and gigantic bubbles boiling on the surface of Orion's supergiant star Betelgeuse. The new observation, the first of its kind, will provide important clues to help explain how these behemoths shed material at such an impressive rate. State of the art observations reveal a vast plume of gas almost as large as our Solar System, and a gigantic bubble boiling on its surface. This artist's impression includes a scale in terms of the radius of Betelgeuse and the scale of the Solar System. Image: ESO/L. Calçada.

Betelgeuse rides on the shoulder of the constellation known as Orion the Hunter. At 1,000 times the size of our Sun it is one of the biggest stars known and also one of the most luminous, emitting more light than 100,000 Suns put together. But such mightiness comes at a cost, for Betelgeuse will meet its fate in a spectacular supernova explosion at an age of only a few million years. Giant stars like Betelgeuse shed the equivalent mass of the Earth every year, but the mechanism of how they do so is poorly understood. "We know relatively well how much mass supergiants loose, and how it ends up in the interstellar medium as planetary nebulae," Pierre Kervella of the Paris Observatory tells Astronomy Now. "However, the mechanism of this mass loss is currently poorly understood, i.e. how physically the material escapes the gravitational field of the star." Two teams of astronomers used ESO's Very Large Telescope to take steps closer to finding the answer. The first team used the adaptive optics instruments NACO, combined with the "lucky imaging" technique to obtain the sharpest view of the giant star ever obtained. Lucky imaging combines only the sharpest exposures to form an image much clearer than a single long exposure would provide. The resulting images have a resolution as fine as 37 milli-arcseconds, which is roughly the size of a tennis ball on the International Space Station, as seen from the ground. Orion is easily recognisable by the three stars that make up the Hunter's belt, with red Betelgeuse riding on the Hunter's shoulder, indicated by the marker (left image). Zooming in on Betelgeuse (middle) and the sharpest ever image of this supergiant star (right) was obtained with NACO on ESO’s Very Large Telescope. Image: ESO, P.Kervella, Digitized Sky Survey 2 and A. Fujii.

"Thanks to these outstanding images, we have detected a large plume of gas extending into space from the surface of Betelgeuse," says team leader Kervella. The plume extends out to a distance at least six times the diameter of the star, corresponding to the distance between the Sun and Neptune. The images show that the whole outer shell of the star is not shedding material evenly in all directions. Kervella suggests two possible mechanisms for the asymmetry, associated with either large scale gas motions caused by heating, or because of the star's rotation. "We think that convection at the surface, or the star's rotation can create sufficient momentum to eject the gas into space," he says. "The exact mechanism is however unknown for the moment. The convection is caused by vertical motion of material in the star. When it reaches the surface, it still has a significant vertical velocity, that can be sufficient to escape the star." Kervella also suggests that despite Betelgeuse's slow rotation – it has a period of about 17 years – it might have a hot spot at its poles that would create additional pressure on the gas, forcing it into space. "Our observations are the first to establish a link between the surface of the star and its envelope," he says. "This is clearly a step towards a good comprehension of the mass loss mechanism for evolved stars." A close look at Betelgeuse obtained with the NACO adaptive optics instrument on ESO’s Very Large Telescope. The image is based on data obtained in the near-infrared, through different filters. The field of view is about half an arcsecond wide, with north up and east left. Image: ESO and P. Kervella.

To probe Betelgeuse in even greater detail, Keiichi Ohnaka from the Max Planck Institute for Radio Astronomy in Germany and colleagues used the AMBER instrument on ESO’s Very Large Telescope Interferometer to obtain images equivalent to those taken with a 48-metre telescope. This provided even greater detail than the NACO images, equivalent to seeing a marble on the International Space Station from the ground. "Our AMBER observations are the sharpest observations of any kind ever made of Betelgeuse," says Ohnaka. "Moreover, we detected how the gas is moving in different areas of Betelgeuse’s surface the first time this has been done for a star other than the Sun."Ohnaka's unrivalled observations show that the gas in Betelgeuse's atmosphere is bouncing vigorously up and down in bubbles that are as large as the supergiant star itself, and could be responsible for the ejection of the massive plume into space. Kervella tells Astronomy Now that the behaviour seen at Betelgeuse could represent that at other red supergiant stars. "Many others have similar or even more extreme properties, so it is reasonable to expect similar properties in other stars," he says. "Betelgeuse has the advantage of being the nearest star of this kind." Because of its proximity to Earth, when the star does go supernova it will be clearly visible with the unaided eye, even in daylight. Betelgeuse also stole the show last month with the discovery that its core was shrinking, perhaps indicating the onset of a new phase of its evolution. See our news story and the August issue of Astronomy Now magazine for further information.