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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchAstronomers have measured how 31 nearby subgiants, giants and supergiants fade from their bright centers to their edges, a phenomenon called limb darkening. Using the six-telescope CHARA Array, the team found that the effect weakens toward longer near-infrared wavelengths, but by about 38% between 1.6 and 2.2 microns—substantially more than the 17–22% decrease predicted by the atmosphere models they tested. The result gives stellar physicists a sharper test of how real stellar atmospheres behave and helps quantify a source of error in exoplanet measurements.
What limb darkening is
Limb darkening is the gradual decrease in a star’s apparent brightness from the center of its disk to its visible edge, or limb. Looking straight toward the center, an observer sees deeper, hotter layers of the stellar atmosphere. Near the edge, the line of sight travels through shallower layers that are cooler, so the edge appears fainter. The star is not necessarily emitting less energy at its surface edge; the difference is mainly the result of viewing different atmospheric depths.
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The effect is measurable in the Sun and is expected in other stars, but its exact strength depends on wavelength, temperature, gravity, chemical composition, atmospheric structure and surface magnetism. Those details matter whenever astronomers convert an observed brightness pattern into a stellar radius or use a star as the backlight for a transiting planet.
How astronomers resolve a star that is too distant to photograph
Most stars are so small in the sky that a conventional telescope records them as points. Long-baseline optical interferometry gets around that limit by combining light collected at widely separated telescopes. The CHARA Array on Mount Wilson combines six telescopes. As the separation between telescopes changes, the contrast of the resulting interference fringes—called visibility—changes in a way that depends on the star’s angular diameter and on how its brightness is distributed across that disk.
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By fitting visibility measurements at multiple baselines and wavelengths, astronomers can infer a limb-darkened diameter and a mathematical brightness profile without producing an ordinary photograph. The same principle was used by the VLTI/PIONIER instrument to resolve Alpha Centauri A and B in the near infrared. As CHARA director Gail Schaefer explained, “By combining light from telescopes across the mountaintop, we can image stars with enough detail to see what their surfaces actually look like.” Georgia State University describes the CHARA observations here.
What the CHARA survey found
The 2026 Georgia State University report covers 31 bright nearby stars in late evolutionary stages: subgiants, giants and supergiants. Observations at two near-infrared wavelengths tested how rapidly brightness falls from each star’s center toward its limb.
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| Quantity | Result | What it means |
|---|---|---|
| Stars observed | 31 | Nearby evolved stars, including subgiants, giants and supergiants |
| Wavelength comparison | 1.6 to 2.2 microns | Near-infrared measurements at two bands |
| Observed change in limb-darkening strength | About 38% decrease | The gradient became substantially weaker at 2.2 microns |
| Change predicted by tested atmosphere models | About 17–22% decrease | Models captured the direction but underestimated the wavelength dependence |
The observations therefore support the broad expectation that limb darkening weakens at longer infrared wavelengths, while showing that the change is stronger than several model atmospheres predict. The team reported no evidence for large starspots or unresolved companion stars in the sample, reducing two possible sources of a misleading center-to-edge signal. Lead author and CHARA staff scientist Narsireddy Anugu said, “We are not just measuring how large these stars are. We are measuring how their light is distributed across the stellar disk, which directly tests stellar-atmosphere models.” The university’s report provides the survey context and quoted statements.
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A peer-reviewed VLTI/PIONIER study observed the nearby solar-type pair Alpha Centauri A and B in the H band at 1.65 microns. It measured limb-darkened angular diameters of 8.502 ± 0.038 milliarcseconds for Alpha Centauri A and 5.999 ± 0.025 milliarcseconds for Alpha Centauri B.
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| Star | Limb-darkened angular diameter | Fitted power-law exponent |
|---|---|---|
| Alpha Centauri A | 8.502 ± 0.038 milliarcseconds | 0.1404 ± 0.0050 |
| Alpha Centauri B | 5.999 ± 0.025 milliarcseconds | 0.1545 ± 0.0044 |
| Sun (observed reference) | Not applicable in this comparison | 0.15027 |
The two fitted exponents bracket the observed solar value. The authors found weaker limb darkening than both their one-dimensional and three-dimensional atmosphere models predicted, and called for improved calibration of those models. Their results are published in Astronomy & Astrophysics.
Why the model mismatch matters for exoplanets
Limb-darkening coefficients are built into analyses of interferometric diameters, eclipsing-binary light curves, spotted stars and exoplanet transits. During a transit, a planet crossing the stellar center blocks a different average brightness than one crossing near the limb. If the assumed stellar profile is too steep or too shallow, the fitted planet radius, impact parameter and atmospheric spectrum can shift.
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A 2024 Nature Astronomy study reports that many atmosphere grids predict a steeper center-to-limb drop than measurements from the Kepler and TESS missions. Its three-dimensional MURaM experiments found that adding surface magnetic fields can reconcile the discrepancy; a field of roughly 100 gauss reproduced the mean Kepler offset in the model experiment. The study details the magnetic-field simulations and transit implications.
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What astronomers need to improve next
The CHARA and Alpha Centauri results point to a common requirement: atmosphere models must reproduce brightness profiles, not just total spectra or average temperatures. Useful comparisons need to keep several variables explicit:
- Wavelength: the CHARA comparison spans 1.6 and 2.2 microns, while Alpha Centauri was measured at 1.65 microns.
- Evolutionary class: the CHARA targets are evolved stars; Alpha Centauri A and B are solar-type dwarfs.
- Angular resolution and baseline: interferometric visibility depends on the telescope spacing and the angular size being resolved.
- Brightness law: fitted power laws or other limb-darkening prescriptions can produce different coefficients.
- Magnetism: models that omit surface magnetic fields may overestimate the center-to-edge contrast.
More multi-wavelength interferometry, combined with magnetically informed three-dimensional simulations and precise transit data, should show whether the stronger infrared dependence is widespread among evolved stars or reflects particular atmospheric conditions.
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