Light from a streetlamp passing through blinds flickers: the stripes shift, and brightness rises and falls. From that rhythm, you can guess how wide the lamp is. The new method for measuring stars works similarly: instead of capturing a sharp image, astronomers monitor the "twinkling" of light collected by two or more telescopes simultaneously.
Usually, the size of a distant star is determined by combining light from different telescopes into one sharp picture, as if building a giant mirror as wide as the distance between them. But there's another way: don't combine the beams, just record how quickly brightness changes in each telescope. From the speed of these changes, you can compute the star's apparent diameter. This technique is called "intensity interferometry," and it doesn't require perfect synchronization — ordinary clocks suffice.
The study shows: for the star HD 17652, a red giant, two hours of observations at an infrared wavelength can yield a size with an error of less than one percent. This uses a baseline, the distance between telescopes, of about one hundred meters, like the existing PIONIER instrument. If you observe at shorter wavelengths, a second "peak" appears in the signal, providing an independent check that is nearly unaffected by how we model the limb darkening of the star's disk.
The main advantage of the new approach is the ability to use many telescopes and multiple color filters simultaneously. For instance, the future Cherenkov Telescope Array will consist of dozens of mirrors spread over kilometers. This would cut observation time to a few minutes per star. Then the method becomes practical for checking the sizes of a whole sample of red giants, which serve as "standard candles"—reference points for measuring distances in the universe.
as written by the authors arXiv:2602.02870
The surface-brightness-color (SBC) relationship for Red Clump stars provides a critical foundation for precision distance ladder measurements, including the 1\% distance determination to the Large Magellanic Cloud. Current SBC calibrations rely on angular diameter measurements of nearby Red Clump stars obtained through long-baseline optical interferometry using the Very Large Telescope Interferometer. We explore the application of intensity interferometry to measure limb-darkened angular diameters of Red Clump stars, offering a complementary approach to traditional amplitude interferometry. We describe the framework for extracting angular diameters from squared visibility measurements in intensity interferometry, accounting for limb darkening through the stellar atmosphere models. For the Red Clump star HD~17652, we show that intensity interferometry in the $$H$$ band at baselines matching PIONIER ($$\sim$$100~m) could achieve $$<1$$\% angular size uncertainties in 2-hour exposures by measuring the primary peak of the visibility function, enabling direct comparison with existing measurements. Critically, observations at shorter wavelengths probe the secondary visibility maximum, providing independent checks of both measurement and systematic errors that are largely insensitive to limb-darkening assumptions. Exploiting the multiplex advantage of simultaneous multi-bandpass observations and the large number of baselines available with telescope arrays such as the Cherenkov Telescope Array Observatory can reduce observing times to practical levels, making intensity interferometry a viable tool for validating the angular sizes for a subset of the Red Clump star calibration sample.
Intensity interferometry was first used in the 1950s to measure the angular size of Sirius, and it worked without lasers or ultra-precise clocks, just two searchlight mirrors and photomultipliers.
In Stanislaw Lem's novel "Solaris," the planet's ocean is studied with probes that register not images but changes in physical fields — remotely similar to how here the star's shape is reconstructed from light flickering.