A new observational method offers a look inside the guts of a star, with surprising results.
Image: Laurent Gizon et al/Max Planck Institute for Solar System Research/Mark A Garlick
Across the universe, perfect spheres are things that should not be. This is simply because stars and other astronomical objects spin as they move through space, a consequence of angular momentum inherited from the expanding universe's turbulent youth. And objects that spin experience centrifugal forces—their equatorial regions pull away from the center of rotation, leaving even the most round-seeming stars and black holes a bit squished and flattened (more wide than they are tall).
In the current issue of Science Advances, astronomers from the Max-Planck Institute, New York University, and others describe observations made of the star KIC 11145123. Thanks to an alternative measurement technique based on asteroseismology, they've found that this star is far less flat than should be expected. This is so much so that the study authors can claim that KIC 11145123 is the most spherical known astronomical object: an extraordinarily perfect ball of plasma.
Since the 1920s, astronomers have been measuring stars based on a technique known as optical interferometry. Here, incoming light from an astronomical object is sampled at two or more locations (by two or more optical telescopes) and these signals are recombined at the actual interferometer instrument. Putting these light waves back together results in interference patterns that provide levels of detail unattainable by simply spying on a star with a single telescope.
Interferometry, however, turns out to have a limit when it comes to precision and this is where asteroseismology comes in. Generally, this is the study of the oscillations that occur in stars. Stars don't just sit out there in states of comfortable equilibrium and are instead pulsating through a variety of mechanisms. Stars like our own Sun— Alpha Centauri A and B, for example—experience solar-like oscillations, which are excitations that result from turbulent convection occurring in their outer layers.
The team behind the current paper was able to make asteroseismological observations of KIC 11145123 by separating out the frequencies of acoustic waves emanating from the star's interior. Using these waves to visualize the guts of the star, they found that KIC 11145123's exterior layers are rotating faster than its core. This is what is likely causing the unusually round (or less "oblate") shape—because of the disconnect between surface and core, the star is not spinning quite as much as may appear just by looking at it from the outside (that is, by using interferometry).
There aren't a whole lot of explanations for why this might be so. The current study suggests that it's likely the result of a weak magnetic field surrounding the star. "Waves propagate faster in magnetized regions, so surface magnetic fields at low latitudes will make a star appear less oblate to acoustic waves," it explains.
"Other than a magnetic field, there are few alternative explanations for the reduced oblateness," the authors continue. "At this level of precision, the physics of stellar oscillations may need to be studied in more detail."
Indeed, what the paper suggests is much bigger than the geometric weirdness of one particular star. It's a proof-of-concept of asteroseismology itself, along with the promise of a much deeper understanding of star guts in the future.
"This work is a first step in the study of stellar shapes through asteroseismology," the study concludes. "The method demonstrated here will be applied to other stars, including more rapidly rotating stars and stars with stronger magnetic fields, where deformations will be greater."