Astronomy 162: Professor Barbara Ryden

Thursday, January 9


``Brightness falls from the air;
Queens have died young and fair.''
- Thomas Nashe, `In Time of Pestilence'

Key Concepts

(1) The visible surface of the Sun is called the photosphere.

Unlike the Earth (and other terrestrial planets), the Sun doesn't have a solid surface. Unlike Jupiter (and other jovian planets), the Sun doesn't have a cloud layer. Nevertheless, in pictures, the Sun does seem to have a sharp, well-defined, clean-cut surface. Why? What makes a `mass of incandescent gas' look more like a billiard ball than a fuzzy ball of gas?

When we look at the Sun, remember, we are looking at photons which have escaped from the hot ball of gas which constitutes the Sun. Photons scatter their way out of the Sun, reeling from one encounter to another in a drunkard's walk. Every photon that leaves the Sun must have undergone one final scattering before escaping to the transparent darkness of outer space. The layer in which the escaping photons scatter for the last time is called the photosphere, meaning ``sphere of light''.

Not every photons undergoes its last scattering at exactly the same distance from the Sun's center. It is true, however, that the photosphere is relatively thin. It's only 400 kilometers thick, a distance which is small compared to the distance of the photosphere from the Sun's center (700,000 kilometers). The relative thinness of the photosphere means that the Sun doesn't appear fuzzy or blurred to us. The average temperature of the gas in the photosphere is 5800 Kelvin; this is the temperature we measure from looking at the Sun's continuous spectrum. The top of the photosphere is significantly cooler, with a temperature of only 4400 Kelvin; thus, the cooler, low-density gas at the top of the photosphere produces absorption lines in the solar spectrum.

(2) The chromosphere and corona lie above the photosphere.

The layer of the Sun immediately above the photosphere is called the chromosphere, meaning ``sphere of color''. The chromosphere can be seen when the bright photosphere is hidden from view by the Moon during a total solar eclipse. The chromosphere gets its name from its distinctive pinkish-purplish color - the color characteristic of hot, low-density hydrogen.

During an eclipse in August 1868, astronomers saw unfamiliar emission lines in the spectrum of the Sun's chromosphere. Since the emission lines didn't correspond to any known element, the astronomer J. Norman Lockyer made the bold announcement that it came from a previously unknown element. Lockyer called this new element ``helium'', after the sun god Helios. The new element wasn't isolated in a terrestrial laboratory until 1895, 27 years later. Lockyer was still alive, and was pleased to see his bold guess vindicated.

The layer of the Sun immediately above the photosphere is called the corona, after the Latin word for ``crown''. The corona stretches away for several million kilometers from the chromosphere, but it's so faint it can only be seen, like the chromosphere, during total solar eclipses. (In the image below, the pinkish chromosphere can be glimpsed very close to the eclipsing moon, while the whitish corona stretches much farther away.)

The corona is very low in density (it's only a trillionth the density of the photosphere, which in turn is far lower in density than the Earth's atmosphere at sea level). However, it is extraordinarily high in temperature, with a temperature reaching as high as 2 million Kelvin.

(3) Some of the hot corona escapes to form the solar wind.

The corona, like the rest of the Sun, is made mostly of hydrogen and helium. The hydrogen and helium in the corona is ionized; thus the corona consists of hydrogen nuclei (alias protons), helium nuclei, and free electrons. The high temperature of the corona means that many of the protons, helium nuclei, and electrons are moving faster than the Sun's escape speed. No longer gravitationally bound to the Sun, these fast-moving particles stream away from the Sun into interplanetary space.

This constant flow of ionized gas away from the corona is called the solar wind. Every second, a million tons of gas flows away from the Sun. (This is less, remember, than the mass loss from nuclear fusion in the Sun's core, which is about 4 million tons a second.) During the Sun's lifetime, so great is the mass of the Sun, the mass loss from the solar wind has been negligibly small. (It takes about 200 million years to lose the equivalent of the Earth's mass -- and it takes 330,000 Earths to equal the Sun.)

The Sun is largely unaffected by losing a million tons of gas a second. However, the solar wind does have a significant effect on other bodies in the solar system. For instance, the interaction of the solar wind with the Earth's magnetic field and the Earth's upper atmosphere produces the aurora borealis. The interaction of the solar wind with a comet produces the comet's long ion tail.

The question ``How big is the Sun?'' is therefore an interesting one. The radius of the photosphere is 700,000 kilometers; thus, when we look at the Sun, we see a sphere 700,000 kilometers in radius. However, the tenuous corona of the Sun blends seamlessly into the solar wind which pervades the solar system.

A question which I have failed to ask so far is ``Why is the corona so darn hot?'' Below the photosphere, temperature decreases steadily with increasing distance from the Sun's center. However, going outward from the chromosphere to the corona, the temperature leaps upward spectacularly. Where does the energy come from to heat up the corona? Tune in tomorrow...

Prof. Barbara Ryden (

Updated: 2003 Jan 9

Copyright 2003, Barbara Ryden