The  Star's "Light Sphere"

This is a short discussion of the structure of a star known as the photosphere - important, since it is the structure that emits virtually all of the light we see from a star.

First, let us briefly review the definition of a star. A star is a hot, dense, ball of hydrogen and helium gas (with a tiny bit of the heavier elements). As such it is an opaque source of light that emits a continuous, thermal radiation (or blackbody) like1, spectrum. Something that is opaque inhibits the flow of light through it (brick walls are opaque to visible light, though a window is transparent). As we've discussed in class, the density and temperature of the gas diminish outwardly from the star's center.

Now, back to our story.

In our Sun, as in other stars, roughly 99.9% or so of all light emitted is emitted in a thin layer known as the photosphere, or light sphere. This is explained as follows. Interior to the photosphere the gas is ever denser and becomes far too opaque for any photon to emerge directly from that layer. Exterior to the photosphere is the Sun's transparent outer atmosphere (the so-called chromosphere and corona), but here the density of gas is so low that it cannot emit much light - less than 0.1% of the light from our Sun. Its spectrum is NOT a thermal radiation (or blackbody) spectrum, but an emission line spectrum (as an excited, low-density gas). Simply put, there is too little matter to emit much light. This leaves a thin layer in between, about 500 km thick, where the gas is dense enough to emit an intense thermal radiation spectrum, yet not so opaque that we cannot see the light emanating from it. We can see no further into a star than its photosphere.

Think of a piece of hot, glowing charcoal...where does the emitted light you see come from? (Answer: a very thin layer at its surface)

But how can we say the photosphere of our Sun is 500 km thick? Look at this visible (white) light image of our Sun's photosphere. Notice that the photosphere's intensity is brightest at the center and slowly dims toward the edge or limb of the Sun (ignoring the sunspots). This effect is called limb darkening. Why does it occur? Well, as we said above, the photosphere is opaque, but not totally opaque. When we look dead-center, we see straight down into the photosphere - and so we can see deeper, where it is hotter (6400K). A hotter thermal radiator emits more light per unit area per second (i.e., it's surface is brighter). Of course, we can't see any deeper than the layer that becomes totally opaque. Now, when we look along the limb of the Sun we cannot see as deep into the photosphere because our line of sight skims across the top layers of the photosphere before becoming too opaque to see any deeper in. The temperature in these upper layers is lower (5000 K) and so emits less light. The physical thickness of photosphere is derived from a detailed (though straightforward) analysis of this effect.

1More accurately, the spectrum of a star is an absorption line spectrum, which is a combination of a continuous thermal radiation spectrum with absorption of light occurring at specific wavelengths, known as absorption lines.  In the simplest way of looking at it, atoms and ions in the star's lower-density upper photosphere and lower chromosphere absorb light emanating from below at wavelengths corresponding to a change in energy state (from low to high) a bound electron can make when absorbing a photon with the appropriate energy.

Kirk Korista
Professor of Astronomy
Department of Physics
Western Michigan University