Betelgeuse: The incredible shrinking star?

Pinned prominently on Orion's shoulder, the bright red star Betelgeuse hardly seems like a wallflower. But a new study suggests the giant star has been shrinking for more than a decade.

Betelgeuse is nearing the end of its life as a red supergiant. The bright, bloated star is 15 to 20 times more massive than the sun. If it were placed at the centre of the solar system, the star would extend out to the orbit of Jupiter.

But the star's reach seems to be waning. New observations indicate the giant star has shrunk by more than 15 per cent since 1993. This could be a sign of a long-term oscillation in its size or the star's first death knells. Or it may just be an artefact of the star's bumpy surface, which may appear to change in size as the star rotates.

Betelgeuse is enshrouded by vast clouds of gas and dust, so measuring its size is difficult. To cut through this cocoon, Charles Townes of the University of California, Berkeley, and colleagues used a set of telescopes that are sensitive to a particular wavelength of the star's infrared light.

The team used these instruments to measure the size of Betelgeuse's disc on the sky. Over a span of 15 years, the star's diameter seems to have declined from 11.2 to 9.6 AU (1 AU, or astronomical unit, is the distance from the Earth to the sun).

Ejected gas
The cause for this reduction is unknown, as it is unclear how red supergiants behave near the end of their lives.

"Maybe there's some instability in the star and it's going to collapse or at least go way down in size or blow off some material, but who knows," Townes, who shared a 1964 Nobel Prize for the invention of the laser, told reporters on Tuesday at a meeting of the American Astronomical Society in Pasadena, California.

The shrinking size could also be evidence of an as-yet-unidentified pulsation in the star, says Graham Harper of the University of Colorado in Boulder, who was not affiliated with the study.

The surface of Betelgeuse is known to wobble in and out, fed in part by the roiling energy of convection beneath its surface. Two such pulsations are already known – one seems to start anew each year, the other every 6 years. Since this observation shows a progressive decrease in the size of the star over 15 years with a consistent set of measurements, Harper says: "I think this is a very nice indication that [Betelgeuse] is getting smaller."

Bumpy surface
But he notes the change in size could be an illusion. Simulations suggest temperature differences in red supergiants can make their surfaces extraordinarily bumpy, causing the star to appear to be a different size when viewed from different angles.

"Often if you look at the simulations, the star is not spherical. It looks like a bad potato," Harper told New Scientist. Betelgeuse is thought to rotate every 18 years or so, which might suggest an especially narrow part of the star recently rotated into view.

Another possibility, Harper says, is that the team is not measuring the surface of the star but a layer of dense molecular gas that some astronomers suspect may hover above it.