Stars and atoms — Edition Insights

Balancing Atoms on a Beam of Light

In the section on the Sun's chromosphere, Eddington describes how calcium atoms are held aloft by radiation pressure. The atoms absorb light at two specific wavelengths, creating the H and K lines in the solar spectrum. The residual light at the center of these lines is just strong enough to support the atoms against gravity. Eddington explains that if an atom moves away from the Sun, the Doppler effect shifts its absorption to a slightly different wavelength, where the light is more intense. This imbalance accelerates the atom outward, potentially allowing it to escape into space. He calls this a 'precarious' balance, and notes that the theory predicts a limiting velocity where the atom would encounter another absorption line and 'stick.' The argument is speculative, but it illustrates how astronomers use spectral details to infer dynamics.

The Red Glow of Betelgeuse

Betelgeuse, a red supergiant, serves as Eddington's example of a star whose low surface temperature implies a vast size. He reasons that a star's apparent brightness depends on both its surface area and its surface temperature. A white-hot star like Sirius can be bright with a small disk, but a red star must be enormous to appear bright. Betelgeuse, being both red and bright, is the best candidate for detecting a visible disk. Eddington notes that no star's disk can be resolved with existing telescopes; a mirror of about 20 feet aperture would be needed. This line of reasoning—linking color, brightness, and size—is typical of his method: he uses observable quantities to deduce hidden properties, always grounding his inferences in physical principles.

Mass as the Key to Stellar Luminosity

Eddington presents a relationship between a star's mass and its brightness, derived from the balance between radiation pressure and gravity. He explains that the opacity of stellar matter determines how easily radiation can escape, and that this opacity depends on the state of ionization of atoms inside the star. The interior temperatures are so high that atoms are stripped of their electrons, creating a plasma that is surprisingly transparent. Eddington's calculations show that a star's luminosity is roughly proportional to the cube of its mass. This 'mass-luminosity relation' is one of the central results of his work. He does not present it as a final truth, but as a consequence of the physical model he has built, acknowledging that the theory involves approximations and assumptions about the opacity.

From the Atom to the Star and Back

Eddington emphasizes the reciprocal relationship between astronomy and atomic physics. The study of matter in stars has contributed to understanding atomic structure, and vice versa. He describes how spectral lines reveal the presence of 'unknown atoms'—elements that were identified in the Sun before they were found on Earth. The story of the companion of Sirius, a white dwarf, illustrates how extreme density challenges the atomic model. Eddington notes that the companion's density is thousands of times greater than ordinary matter, implying that atoms must be crushed and their electrons forced into new configurations. He treats these discoveries as part of a continuous dialogue between observation and theory, where each new measurement forces a refinement of physical laws.

Eddington's lectures reward a reader willing to follow chains of inference. He does not simplify the physics, but he clarifies the logic. The book is best approached as a series of interconnected arguments rather than a survey of facts. Readers may find it useful to pause at each calculation and ask what assumption is being made. The pleasure lies not in the conclusions alone, but in watching a skilled physicist construct a bridge between the unimaginably large and the invisibly small.

That rainy afternoon, Eddington’s calcium atoms drifting on sunlight felt almost tender—matter obeying invisible rules with quiet grace. I closed Stars and Atoms and found myself reaching for something humbler, a long-ago plea for patience and precision. The same devotion to watching, waiting, counting, surfaced in A Plan for Securing Observations of the Variable Stars — A Closer Reading, and I sat there, letting both bookends of careful wonder settle into the grey afternoon.