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More than meets the eye

© Rodolfo Astrada

Equilibrium is established by the balance of two opposing forces; pressure resulting from the enormous weight of the upper hydrogen pileup pushing inward, against intense radiation from burning trying to tear outward.

Understandably, a comparatively large star makes for greater weight and pressure on the combustion zone, implying a brisker throttling of the fusion engine for equilibrium. Equilibrium that is self-regulating; if combustion falters a little, the decrease in radiation pressure frees more hydrogen thus increasing fuel flow. Conversely, if heat builds up above equilibrium, radiation pressure restrains hydrogen feed bringing down combustion rate.

Shiny black body

When we look at the sun, the impression of combustion taking place just on the surface is overwhelming. Eclipses reinforce the illusion featuring gigantic flames streaming outwards, but as said before, actual burning takes place far below. What we see is a large ball of very hot gas, so hot it actually shines in the dark.

The physical archetype of something that shines because of temperature is what at first glance misleadingly is called a black body. What this name suggests is something that perfectly absorbs or radiates all colors without distinction.
A black object is seen black because it does not reflect light. A green leaf looks green because it absorbs most colors except green, and this rejected tone is what we see. A red hot iron, for its part, casts an ever whiter glare the higher its temperature, this time not reflected but emitted.

An ideal black body emits a well-defined spectrum, a range of colors with varying intensities in a shape directly related to temperature. A star's hot hydrogen envelope behaves as a fair approximation to a black body, which brings us after a lengthy digression to where we wanted.

The emission spectral shape of a certain star betrays its surface temperature.

Temperature in turn, relates to the rate of hydrogen combustion, and this can be modeled fairly well, which means we can gauge the size of a star (remember, size determines combustion rate) only by looking at its spectrum (well, to a first approximation, at least). This is the way we learned Rigel is a blue supergiant while Sirius is a relatively small star, only it happens to be much nearer making it outshine the famous Orion cornerstone.

Indian physicist Subrahmanyan Chandrasekhar was the first to develope a detailed model of stellar workings worth a Nobel prize, while the Chandra X-Ray Observatory was baptized in his memory.