Astronomers at the W. M. Keck Telescope facility on Maunakea have have listened to the music of a nearby star, finding shocking irregularities in its size and age. In the finding, appearing in The Astrophysical Journal, we see indirectly into the internal processes of a star heretofore believed to be too tranquil to examine—proposing new tools to better understand the life cycles of stars and planets orbiting them.

The study conducted by Dr. Yaguang Li from the University of Hawai’i at Mānoa employed the Keck Planet Finder (KPF), one of the most advanced spectrographs to detect extremely small stellar motion. For four straight nights, the team obtained over 2,000 extremely accurate velocity measurements of HD 219134, which is a cool, orange-colored star located merely 21 light-years from us. Observing the star’s resonant vibrations—like musical notes—scientists were able to gaze inside its interior using asteroseismology.

Stars are far from silent; they vibrate with natural frequencies, much as musical instruments in an orchestra, said Li. If we listen to the vibrations, we’re able to calculate the mass, diameter, and even the age of a star with great accuracy.

Earlier, such “stellar songs” were captured mostly in hotter stars with space telescopes such as NASA’s Kepler and TESS. However, HD 219134’s vibrations were below brightness-based detection levels, and Doppler shift sensitivity by Keck Observatory was needed to detect them. This is the first time asteroseismology has been employed to determine the age and radius of a cool star from KPF, which solidifies the instrument’s role in unlocking stellar secrets.

A 10-Billion-Year Time Capsule

The vibrations confirmed HD 219134 is an incredible 10.2 billion years old—twice as old as the Sun and one of the oldest main-sequence stars ever to have been aged with asteroseismology. This discovery has significant implications for star-aging theory, specifically in understanding how stars decelerate rotation with age.

Artist’s concept of the HD219134 system. Sound waves propagating through the stellar interior were used to measure its age and size, and characterize the planets orbiting the star. The 10-second audio clip transforms the oscillations of HD219134 measured using the Keck Planet Finder into audible sound. The star pulses roughly every four minutes. When sped up by a factor of ~250,000, its internal vibrations shift into the range of human hearing. By “listening” to starlight in this way, astronomers can explore the hidden structure and dynamics beneath the star’s surface. Credit: OpenAI, based on original artwork from Gabriel Perez Diaz/Instituto de Astrofísica de Canarias

Astronomers have traditionally used gyrochronology, an approach which estimates the age of a star from its rotation period. Young stars rotate quickly, slowing as they shed angular momentum, much like the spinning top unwinding. HD 219134 is not playing by the rules, however, as its slowing suddenly stops in old age. This star is like the lost tuning fork of star clocks, said Li. It gives us a valuable reference for fine-tuning how stars slow down on the million-to-billion year time scale.

The finding defies expectations for magnetic braking, the mechanism by which stars lose rotational energy to stellar winds. If older stars such as HD 219134 resist the slowdown they should, it may require updates to models employed to age many thousands of stars—a consideration in the quest for habitable worlds, as older stars might harbor old life.

A Mysterious Size Inconsistency

Equally mystifying was the star’s dimensions. Whereas the method of interferometry, which uses multiple telescopes to gauge the diameter of a star, indicated HD 219134 to be some 4% larger, asteroseismology reported a smaller configuration. This inconsistency, one without resolution in current theory, is indicative of missing understanding about cool stars. It might reflect unmodeled atmospheric influence, magnetic fields, or broken theoretical assumptions.

The discrepancy is no trivial one, said co-author Dr. Daniel Huber. If our simulations are lacking something critical about stars like this one, it could propagate through all the way from planet-formation studies to habitability studies.

Precise measurements also revealed attributes of HD 219134’s five known planets, including the star’s two rocky super-Earths. New density estimates confirm the worlds probably have solid, Earth-like compositions—vital information for future habitability research.

KPF’s success opens doors to the study of cooler, quieter stars, which will be among NASA’s prime targets for its next Habitable Worlds Observatory. If we discover life elsewhere, Huber said, having the age of its host star will inform us about how long ago its life must have evolved. Asteroseismology provides us with the information.

SOURCES

W.M.Keck Observatory

Yaguang Li (李亚光), Daniel Huber, et al., K Dwarf Radius Inflation and a 10 Gyr Spin-down Clock Unveiled through Asteroseismology of HD 219134 from the Keck Planet Finder. ApJ 984 125. DOI:10.3847/1538-4357/adc737

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