Despite all the optical horsepower afforded by the Hubble Space Telescope and cutting-edge observatories on the ground, astronomers have been able to resolve the disks of only a handful of stars. One of the few successes is Betelgeuse, the enormous red supergiant marking the shoulder of Orion.Astronomers first measured this star's angular diameter in 1920, but I remember getting my first true appreciation for Betelgeuse's size in 1996, when observers released an image of this stellar behemoth taken with the Hubble telescope's Faint Object Camera.
Everything about Betelgeuse is supersize. More than 900 times the Sun's diameter, this star would comfortably engulf the entirety of Mars's orbit and the asteroid belt. It also outshines the Sun by 135,000 times — a dazzling 1st-magnitude beacon despite its distance of some 640 light-years.
And it has practically the largest angular diameter of any star in our sky: about 55 milliarcseconds, depending on what wavelength you use to observe it.
Now two teams of observers have resolved the star as never before, and in doing so they've found that Betelgeuse is throbbing, churning, and spewing shells of its outer layers into the space around it. The new views show that this mass loss, an Earth's worth every year, isn't a nice even flow like our Sun's solar wind. Instead, Betelgeuse is hacking up hairballs of hot gas this way and that at 25,000 miles (40,000 km) per hour.
These spasms aren't a sign of stellar health. Only a few million years old, Betelgeuse is already dying. Astronomers predict that it's doomed to explode as a supernova relatively soon, astronomically speaking, an event that will be spectacular for Earth's future inhabitants.The new observations made use of the powerful telescopes atop Cerro Paranal high in the Chilean Andes. In January, Pierre Kervella (Paris Observatory) and six colleagues used an adaptive-optics system on one of the giant eyes of the Very Large Telescope to achieve a resolution of 37 milliarcseconds at near-infrared wavelengths. They've submitted their results to Astronomy & Astrophysics.
A second team, led by Keiichi Ohnaka (Max Planck Institute for Radio Astronomy), coupled the output from three 1.8-meter telescopes at the VLT site to create an near-infrared interferometer with a virtual aperture some 157 feet (48 meters) across. Using this setup's resolution of just 9 milliarcseconds, Ohnaka and his team found that parts of Betelgeuse's atmosphere are heaving up and down violently. Their results will also appear in Astronomy & Astrophysics.
Betelgeuse has long been recognized as a semiregular variable star. In fact, a third team of observers recently reported that its diameter seems to have shrunk by some 15% since 1993.