From the January 2016 issue

Vega is said to rotate at about 87 percent of the speed that would tear it apart. If it ever reached that speed, what would an observer on Earth witness?

Ralph Heide, El Segundo, California
By | Published: January 25, 2016 | Last updated on May 18, 2023
Hot, rapidly spinning stars can sometimes break up as they expan, creating hot, gaseous disks.
Hot, rapidly spinning stars can sometimes break up as they expand, creating hot, gaseous disks.
Astronomy: Roen Kelly; after Mourard et al. 2015 (Astronomy & Astrophysics)
A hot star, such as Vega, will gradually increase its luminosity and also grow in size as it ages. As the star expands, it should actually spin slower to conserve angular momentum — just like an ice skater stretching out her arms. At the same time, the core is becoming more dense and compact as hydrogen is “burned” to helium ash, and thus the inner layers of the star should spin faster. Interestingly, observations show that the core and outer layers of the stars mostly synchronize their spin through viscosity so the speed of the star’s surface remains roughly constant during this time.

That is not the end of the story. The outer layers of the star feel less gravitational pull as they expand, making it easier for material to escape. If the star keeps expanding while maintaining constant surface speeds, eventually the material at the equator will fly off. We think some stars might be born spinning so fast that they do reach “breakup” during their hydrogen-burning lifetime.

Possible evidence for this picture comes in the form of hot, gaseous disks seen around some hot stars — the so-called “Be” stars, where B refers to the temperature spectral type and little “e” means that the star shows emission lines. We think gas at the equator of a fast rotating star gets nudged out of the photosphere and into orbit, pushing on material that left earlier until a little disk forms. The accompanying photo illustration is a real infrared image of such a gaseous disk we recently captured using the CHARA Interferometer on Mount Wilson. Occasionally amateur astronomers discover normal B stars that all of sudden become Be stars. But how can the gas disk appear so suddenly, within a few weeks? We someday hope to take a picture of this happening in real time with CHARA to find out.

John Monnier
University of Michigan