Nearby Stars Fade More Sharply at Their Edges Than Models Predict

Astronomers using Georgia State University’s Center for High Angular Resolution Astronomy (CHARA) Array on Mount Wilson in California have delivered a result that could ripple through nearly every corner of modern astrophysics. In a survey of 31 nearby stars, the team found that these stars darken toward their edges more strongly at near-infrared wavelengths than several widely used models of stellar atmospheres predict. The finding, published in The Astronomical Journal, provides a new empirical benchmark for understanding how stars are structured, and it matters because limb darkening quietly underpins some of the most celebrated measurements in astronomy, from stellar diameters to the characterization of planets orbiting other stars.

The effect at the heart of the study is called limb darkening, and it is exactly what the name suggests: a star appears fainter at its edge than at its center. This happens because stars are not uniformly bright disks. When we look toward the center of a stellar disk, our line of sight penetrates deeper into the star’s atmosphere, reaching hotter and brighter layers. Near the edge, the light we receive originates from shallower, cooler layers, and it must pass through more stellar material on its way out. The result is a gradual fade from a brilliant center to a dimmer rim, a signature that encodes the temperature and density structure of the star’s outer atmosphere.

Lead author Narsireddy Anugu, a staff scientist at Georgia State University’s CHARA Array, emphasized that the survey goes beyond simply sizing up stars. “We are not just measuring how large these stars are,” Anugu said. “We are measuring how their light is distributed across the stellar disk, which directly tests stellar-atmosphere models.” That distinction is crucial. Stellar radii can be measured in many ways, but the distribution of brightness across a stellar surface is a far more demanding test, one that probes the physics of how energy flows through a star’s outermost layers.

The CHARA Array itself is a marvel of engineering that makes such measurements possible. Located on Mount Wilson, the facility combines light from six telescopes positioned at different sites across the observatory grounds. By interfering the light together, CHARA achieves the resolving power of a much larger telescope, allowing astronomers to resolve details on the surfaces of stars that would otherwise remain forever blurred into points of light. Rather than capturing direct images, CHARA measures how the contrast of the interference pattern changes with the spacing between telescopes. These measurements reveal both the size of a star and how brightness changes across its surface, a technique that has made CHARA one of the most productive stellar-imaging facilities in the world.

Gail Schaefer, director of the CHARA Array, highlighted the facility’s unique capabilities in enabling the survey. “This study demonstrates the powerful capabilities of our facility,” Schaefer said. “By combining light from telescopes across the mountaintop, we can image stars with enough detail to see what their surfaces actually look like.” The observations were conducted by observing the same stars simultaneously through two different near-infrared filters, allowing the team to measure how the center-to-edge fading changes with wavelength. This dual-wavelength strategy is what transformed the survey from a collection of stellar portraits into a quantitative test of atmospheric physics.

The sample consisted of 31 bright stars in late stages of their lives, stars whose outer layers have expanded outward far beyond the dimensions they had during the prime of their existence. These evolved subgiant, giant, and supergiant stars provide especially valuable tests of stellar-atmosphere models because the large convective motions churning through their extended atmospheres can produce complex brightness profiles. Unlike the Sun, whose surface is a relatively thin layer governed by well-understood radiative processes, these puffed-up stars have atmospheres where convection dominates, making them natural laboratories for testing whether theoretical models can capture the messy reality of stellar surfaces.

The quantitative result is striking. Across the sample, the limb-darkening strength decreased by about 38 percent going from near-infrared wavelengths at 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 with a smaller decrease of only about 17 to 22 percent. In other words, the models capture the broad behavior correctly, but they do not fully reproduce how strongly the wavelength dependence plays out in real stars. The discrepancy means that current models may underestimate how stellar brightness changes across a star’s surface, a systematic error that could propagate into downstream measurements that astronomers rely on every day.

Why does this matter so much? Limb darkening directly affects measurements of stellar diameters, since the apparent edge of a star depends on how its brightness fades. More dramatically, it impacts how astronomers characterize exoplanets that transit across the surface of their parent star. When a planet crosses in front of a star, the amount of light blocked during the transit depends on the brightness across the stellar disk. A planet passing in front of the bright center blocks more light than one skimming the dimmer edge. If models misrepresent that brightness distribution, the inferred sizes and other properties of transiting exoplanets inherit the error. With thousands of confirmed exoplanets and missions like TESS and the upcoming PLATO survey depending on precise transit modeling, an empirical correction to limb darkening is a gift to the exoplanet community.

The survey also delivered a reassuring null result: the team found no evidence for surface features such as large starspots or hidden companion stars anywhere in the sample. That cleanliness matters, because spots and companions can mimic or mask limb-darkening signatures, and their absence confirms that the measured wavelength dependence is a genuine atmospheric effect rather than contamination from surface asymmetries or stellar multiplicity. The result strengthens the case that the discrepancy with models is real and systematic, not an artifact of a few unusual stars.

Looking ahead, the team plans to extend the comparison to broader wavelength coverage, stretching from visible light into the near-infrared, which will further constrain how well models reproduce the full spectral behavior of stellar atmospheres. The researchers also intend to image smaller main-sequence stars, which are far more difficult to resolve but are especially important for understanding transiting exoplanets around Sun-like stars, the primary targets in the search for habitable worlds. For now, the CHARA Array’s survey of 31 fading disks stands as a reminder that even the most familiar objects in the sky still hold surprises, and that pushing observational precision to its limits remains one of the most reliable ways to find where our theories fall short.

Subject of Research: Near-infrared limb darkening measurements of evolved stars using optical interferometry

Article Title: Georgia State Telescope array reveals how nearby stars fade from center to edge

Article References: Georgia State Telescope array reveals how nearby stars fade from center to edge. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: CHARA Array, limb darkening, stellar atmospheres, optical interferometry, evolved stars, near-infrared astronomy, exoplanet transits, stellar diameters, Mount Wilson, supergiant stars, stellar convection, The Astronomical Journal