How stars shine: Infrared observations expose gaps in models of how stars distribute their light

The CHARA Array at Georgia State University has found that nearby stars darken toward their edges more strongly at near-infrared wavelengths than predicted by several widely used stellar-atmosphere models. The research team measured how the brightness of 31 nearby stars changes from the center of each stellar disk to its edge, revealing limb darkening effects. The observations show that limb darkening changes more strongly at near-infrared wavelengths than predicted by models. The study demonstrates the powerful capabilities of the CHARA Array, which combines light from six telescopes to achieve the resolving power of a much larger telescope. The team measured how the brightness of nearby stars changes from the center of the stellar disk to the edge, providing a new empirical benchmark for testing stellar-atmosphere models in the near infrared.

• The study focused on 31 bright stars in late stages of their lives, when their outer layers have expanded outward. These evolved subgiant, giant and supergiant stars provide especially valuable tests of stellar-atmosphere models because the large convective motion in their extended atmospheres can produce complex brightness profiles.
• The amount of light blocked by the exoplanet during a transit depends on the brightness across the stellar disk. Across the sample, the limb-darkening strength decreased by about 38% as the near-infrared wavelength increased from 1.6 microns to 2.2 microns. The atmosphere models tested in the study predicted the same overall trend—weaker limb darkening at longer infrared wavelengths—but a smaller decrease of about 17% to 22%.

The results show that current models capture the broad behavior but do not fully reproduce the wavelength dependence. The study, published in The Astronomical Journal, offers a new empirical benchmark for testing stellar-atmosphere models in the near infrared. Additionally, the team found no evidence for surface features like large starspots or hidden companion stars in the sample. Future work will extend the comparison to broader wavelength coverage, from visible to near-infrared light. The team also plans to image smaller main-sequence stars, which are more difficult to resolve but are especially important for understanding transiting exoplanets around sun-like stars.

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