Plate Nº 35 · recorded October 10, 2026
Space & AstronomyReported finding
Nearby Stars Fade Toward Edges More Than Models Predict
Georgia State astronomers measured limb darkening across 31 evolved stars with the CHARA Array. The 38% drop from 1.6 to 2.2 microns nearly doubles the 17–22% that stellar-atmosphere models predict.
By Nathan Brooks3 min read598 words
In brief
- Astronomers measured limb darkening in 31 nearby evolved stars using the CHARA Array on Mount Wilson, California.
- Observed limb-darkening strength dropped about 38% from 1.6 to 2.2 microns, nearly double the 17%–22% predicted by stellar-atmosphere models.
- The team used H-band and K-band near-infrared interferometry across six combined telescopes.
- No evidence of large starspots or hidden companion stars appeared in the sample.
- The study appeared in The Astronomical Journal in 2026 (DOI: 10.3847/1538-3881/ae9878).
Astronomers measured limb darkening — the gradual fading from a star's center to its edge — in 31 nearby evolved stars and found it drops about 38% as the near-infrared wavelength rises from 1.6 to 2.2 microns. Widely used stellar-atmosphere models predict a smaller drop of 17% to 22% over the same range.
The discrepancy, described in The Astronomical Journal, gives researchers a fresh empirical yardstick for testing how the outer layers of stars distribute light.
What is limb darkening, and why does it matter?
Stars are not uniformly bright disks. Light reaching an observer from the center of a stellar disk comes from deeper, hotter atmospheric layers. Light from the rim exits from shallower, cooler layers and passes through more material, dimming further.
That center-to-edge gradient influences two practical tasks: pinning down stellar diameters and interpreting exoplanet transit signals. The depth of the dip depends on how brightness spreads across the parent star's face.
"We are not just measuring how large these stars are," said lead author Narsireddy Anugu, a staff scientist at Georgia State University's CHARA Array. "We are measuring how their light is distributed across the stellar disk, which directly tests stellar-atmosphere models."
How did the team measure it?
The researchers used the CHARA Array on Mount Wilson, California, an interferometer that combines light from six telescopes spread across the observatory grounds. By interfering the beams, the array achieves the resolving power of a much larger telescope.
The team observed the 31 targets simultaneously through two near-infrared filters: the H-band at 1.6 microns and the K-band at 2.2 microns. Comparing the two filters let them track how the center-to-edge fading shifts with wavelength.
"This study demonstrates the powerful capabilities of our facility," said Gail Schaefer, director of the CHARA Array. "By combining light from telescopes across the mountaintop, we can image stars with enough detail to see what their surfaces actually look like."
Why target evolved stars?
The sample focused on evolved subgiants, giants, and supergiants — stars in the late stages of life whose outer layers have expanded outward. Their extended atmospheres host large convective flows that can sculpt complex brightness patterns.
Key findings at a glance:
- 31 bright, nearby stars observed in H and K bands
- Limb-darkening strength fell about 38% from 1.6 to 2.2 microns
- Tested models predicted a 17%–22% drop over the same range
- No evidence of large starspots or hidden companion stars
Where do the models fall short?
All tested stellar-atmosphere codes captured the broad behavior: limb darkening weakens at longer near-infrared wavelengths. None, however, reproduced the full magnitude of the change.
The gap points to missing physics in how the models handle the temperature and opacity structure of cool, extended stellar atmospheres. The mismatch carries practical weight for exoplanet science.
Transit depth estimates for planets around giant and supergiant stars depend on the limb-darkening law the observer assumes. Underestimating the gradient skews the inferred planet radius and, in some cases, the atmosphere's apparent composition.
What's next?
The team plans broader wavelength coverage, from visible light into the near infrared. It will also push toward smaller main-sequence stars — sun-like targets that are harder to resolve but central to characterizing transiting exoplanets around Sun analogs.
The paper, "Empirical H- and K-band Limb Darkening for 31 CHARA Stars: A Near-infrared Benchmark for Stellar-Atmosphere Models," was led by Anugu and published in The Astronomical Journal in 2026. Its DOI is 10.3847/1538-3881/ae9878.
via Phys.org Space & Astronomy (Source)
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