From the February 2011 issue

Is it possible to see a star’s disk? I’ve heard that Betelgeuse has been imaged, but what about the Alpha Centauri system or other stars?

Guillermo Ciancio, Ricardone, Argentina
By | Published: February 21, 2011 | Last updated on May 18, 2023
Using interferometers, astronomers have imaged Altair to study its rapid rotation, which makes the star more oblate than spherical; imaged Betelgeuse’s disk and found that it’s shrinking; and imaged the mysterious transit of Epsilon Aurigae. Altair: Ming Zhao, University of Michigan; Betelgeuse: Xavier Haubois (Observatoire de Paris), et al.; Epsilon Aurigae: John D. Monnier, University of Michigan
In 1920–21, scientists Albert Michelson and Francis Pease measured the first stellar diameter, of Betelgeuse, at the Mount Wilson Observatory. Today we refer to the technique they used as “astronomical optical interferometry,” although it isn’t restricted to visible light — infrared radiation works, too. The term optical distinguishes the technique from the slightly different one radio interferometers use.
You might recall Thomas Young’s famous double-slit experiment: Light coming from a source travels through two small openings (slits in the original experiment) and produces physical phenomena referred to as “interference fringes.” Today’s astronomical interferometers exploit this technique by using many separated telescopes to collect radiation and recombine the light. The combined light produces interference fringes, which scientists can mathematically invert to measure anything from the diameter of a star all the way to creating complex images. The angular scales we can study depend on the wavelength of light and how distant the telescopes are from one another.

In the past decade, astronomers routinely make images of stars to analyze stellar shapes (they are not all spheres), star spots, and evidence for mass flowing off the star itself (like dust and gas). Because of astronomers’ common love of great images, you can find many of these examples online. Some of the exciting ones include rapidly rotating stars like Regulus and Altair, and spots and mass loss from Betelgeuse and CW Leo. Astronomers have imaged disks around young stars like LkHa 101 and IRAS 13481–6124, and also binary systems like Wolf-Rayet 104, Epsilon Aurigae, and Beta Lyrae. Our typical “unit of measure” for these types of images is the milliarcsecond, or one one-thousandth of an arcsecond. (An arcsecond is one part in 3,600 of a degree.)

Interferometers in the Southern Hemisphere have examined Alpha Centauri, but as far as I am aware, no image of the system has yet been made. More than one dozen optical interferometers have been built across the world, although some have closed in recent years. — Michelle Creech-Eakman, New Mexico Institute of Mining and Technology, Socorro