Astronomy 162: Professor Barbara Ryden
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.
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.
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...
Updated: 2003 Jan 9
Copyright © 2003, Barbara Ryden