How to Truly See the Stars: Resolving Stellar Surfaces with Interferometry

The key difference between the Sun and the stars that fill the night sky is simple: distance. The Sun is close enough that it appears as a disk and we can study its surface features—sunspots, faculae and granulation—in detail. Most other stars are so far away they look like unresolved points of light.

Even physically enormous stars subtend only tiny angles on the sky. The nearest star, Proxima Centauri, is roughly 280,000 times farther from us than the Sun, so even huge red supergiants appear minuscule through ordinary telescopes. For scale: some nearby giants measure less than 0.00002° across—about the same angular size as a U.S. quarter seen from 100 km away. The Sun, by contrast, spans roughly 0.5°—over 30,000 times larger in apparent diameter.

Earth’s turbulent atmosphere blurs fine detail and sets a practical limit on ground-based resolution. Astronomers use several techniques to fight that blur. Adaptive optics rapidly reshapes telescope mirrors to compensate for atmospheric motion. Speckle imaging freezes turbulence by combining many extremely short exposures. These methods helped astronomers create the first sharp images of nearby large stars in the 1970s and remain essential today.

Why aperture matters—and how to beat the limit

A telescope’s angular resolution is traditionally set by the diameter of its aperture: larger mirrors or lenses resolve finer detail. Building ever-larger single telescopes is costly and technically challenging, and atmospheric effects often limit the practical gains.

An elegant workaround uses the wave nature of light: interferometry. Light is an electromagnetic wave with crests and troughs, and overlapping beams of light produce interference patterns. Those fringe patterns encode the source’s size, shape and brightness distribution across its surface. By combining light collected at separated telescopes and recording the interference, astronomers can reconstruct high-resolution images. Crucially, an interferometer’s resolving power depends on the separation between collecting apertures—the baseline—not the size of each aperture.

Interferometry is routine at radio wavelengths and has been extended to optical and infrared bands with impressive results. Facilities such as the European Southern Observatory’s Very Large Telescope (VLT) and the CHARA array demonstrate just how powerful the technique can be. The VLT uses several 8.2 m telescopes working together across baselines of more than 100 m; CHARA links six 1 m telescopes with baselines up to ~330 m, reaching resolutions finer than one millionth of a degree. Most of the resolved optical images of stellar surfaces to date have come from CHARA.

What we’ve learned

Interferometric imaging has revealed surprising and sometimes bizarre stellar structures. Examples include:

• π1 Gruis: Convective cells the size of tens of millions of kilometers—each covering a large fraction of the star’s disk—have been directly imaged.
• Altair: Rapid rotation flattens the star into an oblate, egg-like shape, with a measurable temperature gradient from equator to pole.
• RW Cephei and Betelgeuse: Irregular, time-varying shapes and large dust-ejection events have been observed, helping to probe how massive stars shed material and form dust that seeds the galaxy.

These observations illuminate fundamental processes—convection, rotation, mass loss and dust formation—in ways that spectroscopy or unresolved photometry alone cannot.

Limits and the road ahead

The ultimate limits of optical interferometry are set by engineering: how far apart we can place collecting apertures, how precisely we can control optical paths and phases, and how rapidly and accurately we can process the enormous data streams. Radio arrays have already pushed the concept to its extreme: the Event Horizon Telescope combined radio dishes around the globe to make images with an effective Earth-sized aperture. Optical and infrared interferometry face tighter tolerances but continue to improve as detectors, delay-line technology, adaptive optics and computing power advance.

As instruments and algorithms progress, astronomers will be able to resolve the surfaces of many more stars—revealing their faces and dynamics much as we study our own Sun and unlocking new insights into stellar physics and the life cycles of stars.