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Unveiling the true face of a mammoth star

Colossal bubbles are key for pushing material out of the star's atmosphere into space before the star explodes as a supernova.
Provided by the Max Planck Institute, Garching, Germany
Betelgeuse
Artist's impression showing how a vast amount of material is flung out from Betelgeuse into space.
ESO/L. Calcada
An international team of astronomers, led by Keiichi Ohnaka at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, has taken the sharpest view yet of a dying mammoth star. The image shows how gas moves in different areas over the surface of a distant star. This view was made possible by combining three 1.8-meter telescopes as an interferometer, giving the astronomers the resolving power of a gigantic 48-meter telescope.

Using the European Southern Observatory's (ESO) Very Large Telescope Interferometer (VLTI) in Chile, the group discovered that the gas in the dying star's atmosphere is vigorously moving up and down, but the size of such "bubbles" is as large as the star itself. These colossal bubbles are a key for pushing material out of the star's atmosphere into space before the star explodes as a supernova.

In a clear winter night sky, it is easy to spot a bright, orange star on the shoulder of the constellation Orion the Hunter, even in light-flooded big cities. That is Betelgeuse. It is a mammoth star big enough that, if it were in our solar system, it would reach the orbit of Jupiter, swallowing the inner planets Mercury, Venus, Earth, and Mars. It is also glaringly bright, emitting 100,000 times more light than the Sun. Betelgeuse is a red supergiant, and it is approaching the end of its short life of several million years. Red supergiants shed a large amount of material made of various molecules and dust, which are recycled for the next generation of stars and planets, possibly like Earth. Betelgeuse loses material equivalent to Earth's mass every year.

How these mammoth stars can lose mass, which would normally be bound to the star by the gravitational pull, is a long-standing mystery. The best way to tackle this issue is to observe the scene where the material is ejected from a star's surface. Although Betelgeuse is such a huge star, it looks like a mere reddish dot even with today's largest telescopes because the star is 640 light-years away.

Astronomers use a special technique to overcome this problem. By combining two or more telescopes as an interferometer, astronomers can achieve a much higher resolution than individual telescopes provide. The VLTI is one of the world's largest interferometers. The astronomers observed Betelgeuse with the telescope's AMBER instrument operating at near-infrared wavelengths.

"Our AMBER observations mark the sharpest view ever made on Betelgeuse," said Keiichi Ohnaka at the MPIfR. "And for the first time, we have spatially resolved the gas motion in the atmosphere of a star other than the Sun. Thus, we could observe how the gas is moving in different areas over the star's surface."

The AMBER observations have revealed that the gas in Betelgeuse's atmosphere is moving vigorously up and down. The size of these "bubbles" is also gigantic, as large as the supergiant star. While the origin of these bubbles is not yet entirely clear, the AMBER observations have shed new light on the question about how red supergiant stars lose mass. It also means that the material is not spilling out in a quiet, ordered way, but is flung out more violently in arcs or clumps.

Cosmic fireworks known as a supernova, like the famous SN1987A, will accompany the death of the mammoth star, which is expected in the next few thousand to hundred thousand years. However, as Betelgeuse is much closer to Earth than SN1987A, the supernova can be clearly seen with the unaided eye, even in daylight.
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