The Sun Is Smaller Than We Thought It Was

sun photo

Advancements in astrophysics, based on the interpretation of internal sound waves, suggest a downward revision of the Sun’s radius. This could lead to solar eruptions occurring differently from what astrophysical models predict. These insights also apply to other stars.


This new measurement, derived from a detailed analysis of sound waves traversing the Sun’s core, presents scientists with an unexpected reality that challenges prevailing theories. If confirmed, this revision could prompt a partial reinterpretation of the mechanisms governing solar dynamics and, by extension, similar stars. The study is available as a preprint on the arXiv platform.

The Traditional Method Overshadowed?

The size of the Sun has traditionally been deduced from direct observation of the photosphere, the visible outer layer of the star, especially during solar eclipses. This conventional method is questioned by recent work by Masao Takata and Douglas Gough, who utilized the internal sound waves of the Sun (p-modes) to deduce its size.

The authors suggest that the solar radius is actually slightly smaller than previous measurements indicate. Their study challenges the accuracy of f-modes; these sound waves, unlike the deeper p-modes, do not propagate to the end of the photosphere, potentially introducing bias in evaluating the solar radius.

A Potential New Sound Model for the Sun

The method used to reassess the Sun’s size relies on analyzing sound waves generated within the star, the p-modes. These waves result from pressure fluctuations in solar plasma. As they propagate, they encounter the star’s surface, where they are reflected inward, their trajectory bending through turbulent plasma.

By measuring these waves, scientists can obtain a more precise image of the internal structure, leading to a finer estimation of its radius. To grasp this concept, the authors explain in a New Scientist article that one should imagine the Sun as a continuously resonating bell. However, this resonance is not due to a single impact but rather constant “strikes” by myriad tiny particles.

This incessant seismic activity generates millions of sound waves or “modes” in oscillation, which scientists can detect and measure remotely. Besides p-waves, which exert compressive and expansive actions, there are g-modes, generated by gravitational force, manifesting as vertical movements. These g-modes become f-modes when occurring near the stellar surface. With increasing stellar density, new modes can emerge, providing additional clues about the star’s characteristics.

Unlike f-modes, limited by a kind of “phantom surface” and not extending to the end of the photosphere, p-modes offer a more complete view, less disturbed by magnetic fields and turbulence in the solar convection zone. Exploiting these waves has led to an estimation of the Sun’s radius lower than traditionally accepted.

Toward a Renewed Understanding

The reassessment of the Sun’s size, if confirmed, could impact our understanding of solar dynamics. Indeed, the star’s radius is a key factor in modeling solar eruptions, energy explosions that can have repercussions even on Earth. A better understanding of the correlation between the Sun’s size and its internal functioning is crucial. This extends beyond the Sun to the analysis of other stars in the universe.

Precisely understanding the Sun’s internal structure could provide insights into the mechanisms at play in other stellar systems. However, regardless of the magnitude of the error, “changing the more traditional model to accommodate such discoveries would not be a trivial matter,” says astrophysicist Emily Brunsden in the New Scientist article.