But it's very much a focus for us, even if we mostly take it for granted. We've given it the shortest name of any star, and not many people seem amazed that a ball of fire crosses the sky each day. These days it makes the news mainly because we desperately want to exploit its clean energy, as it delivers one kilowatt to each square meter of Earth's surface.
Solar knowledge is now arriving in a flood, thanks to dedicated acronymic spacecraft such as SOHO, Trace, Stereo and Sorce. Scientific progress in understanding the sun is obvious to me when I reach for the beloved antique astronomy books in my collection and see what was taught 160 years ago. From an 1847 text: "The sun has generally been considered a vast body of liquid fire." Back then, many thought that William Herschel had the right idea: "It is highly probable, if not absolutely certain, that the great body of the sun consists of an opaque solid globe, with elevations and depressions, surrounded with a [bright] shell or covering [of] phosphoric clouds."
Later the text says, "Herschel and several others consider it not altogether improbable that the sun is peopled with rational beings. The interior stratum protects the inhabitants of the sun from the fiery blaze of the sphere of heat and light with which they are surrounded." What a hoot! The preeminent astronomer of his time thought that intelligent creatures are basking in the sky.
A decade later, Olmsted's School Astronomy, 1856 edition, repeats these same concepts to students. Bottom line: We hadn't a clue. Since a sun-sized lump of coal would burn itself out in just 2,000 years, nothing really made sense. It took until 1920 for Arthur Eddington to deduce correctly that four protons of hydrogen can fuse into an atom of helium in a self-sustaining process that can endure for ten billion years, changing matter into energy. Einstein's E = mc2 revealed how much of the sun must be sacrificed in this conversion: The sun's weight diminishes by four million tons every second.
The sun's Equator requires 25 days to make one spin. Since Earth orbits the same direction the sun rotates, it appears as if the sun turns in 28 days. Its polar regions do move much more slowly, requiring 34 days. Such differential rotation makes hot gas and plasma contact other layers in continuous swirling violence. But this chaos is confined to the outer 30 percent of the sun's body. The inner 70 percent spins like a solid ball, as if those old astronomers were right after all.
The steady inner sun-ball meets the variable outer sphere a quarter-million miles beneath the solar surface, in a newly discovered zone called the tachocline. This boundary is where the enormous solar magnetic field is created. Give it credit for the coronal mass ejections that cause auroras and some of the genetic mutations responsible for Earth's continual biological evolution.
All the juicy machinery happens far below, out of sight. This includes the innermost 20 percent of the sun: the H-bomb place of nuclear fusion. The sun's visible surface or photosphere is thin, lightweight fluff, just one percent as dense as our own sea-level air - interesting only because it's where each photon of light finally escapes the sun's interior, after struggling upward for between 17,000 and one million years.
As for the spots, the new cycle started just three months ago and will keep increasing as we approach the next solar maximum in 2011. But its arrival is never a "done deal." The 11-year cycle stopped cold for a full human lifetime, from 1645 to 1715. Since sunspot absence results in a one-watt decrease of sun-power on each square meter of Earth, it was a time of harshly frigid conditions. Why did the sunspot cycle fail?
We're back to guessing again. We're pretty sure that no "rational beings" hang out in the solar interior sipping lattes. And we understand why it shines, which is no small thing. Still, the sun remains more intricate and amazing than anyone ever imagined, then or now.