The sun's three major zones are the core (where nuclear reactions generate energy), the radiative zone (where energy travels outward by radiation), and the convection zone (where convection currents move energy to the surface). The flare, sunspots and prominence are clipped from SOHO images.

Courtesy: NASA/SOHO

This sketch illustrates how sound waves propagate through the Sun's interior. Only waves with specific combinations of period and horizontal wavelength resonated within the Sun. The precise combinations are related to the Sun's interior structure.... Measurements of the Sun's oscillations provide a window into the invisible interior of the Sun allowing scientists to infer the structure and composition as well as the rotation and dynamics of the solar interior. to the line of sight.

Courtesy NASA

Pure energy
The sun may look calm and placid, glowing yellow on a hazy afternoon, but placid hardly describes a ball of blistering-hot matter that's cranking out energy at the rate of 92 billion megatons every second.

You read that number right. A gigantic fusion engine in the sun's core is combining hydrogen atoms into helium atoms. And we're talking big. Every second, 700 million tons of hydrogen fuse into helium -- and five million tons is converted into energy, heating the core to about 16 million degrees centigrade.

Fusion releases so much energy that the rest of the sun has a major job in simply distributing that energy. Like a Vidalia onion, or the Earth, the sun is built of layers. The core is where fusion takes place; the energy created then passes as electromagnetic energy through the radiative zone. The convective zone, in turn, acts like molasses boiling in a moonshiner's kettle: The hotter fluid rises to the surface and the cooler fluid sinks back down to be reheated. These convection cells, as they are called, are about as big as Texas in area, but they're hotter than XXX Southwestern Special chili sauce.

Blast it! Once at the surface, most of that energy is released by radiation. But a small proportion of energy -- and a substantial amount of matter over billions of years -- is carried away by other magnetic phenomena:

The solar wind is a gentle stream of charged particles that, when it reaches Earth's orbit, is moving at about 1 million miles per hour.

Solar prominences are fiery arches that hang above the surface, sometimes for weeks on end. They are best seen on the edge of the sun's disk.

Solar flares are eruptions starting in the corona that disturb the chromosphere below it. The chromosphere got its name when it's reddish color was discovered in the late 1800s. It's a relatively cool 10,000 degrees Celsius.

Coronal mass ejections (CMEs) are giant blobs, often weighing one to ten billions tons (about the mass of Lake Erie) flung into space at a million or so miles an hour. Hot enough to create X-rays, they are formed in a low-density region called the corona. CMEs result in bulletins from the Space Weather Center. CMEs are almost always accompanied by flares, and indeed are probably caused by flares since CMEs seldom occur by themselves.

The relationship between this stuff is not exactly clear, and our view changes fast. "The most diplomatic way to put it is that flares and CME are both manifestations of the same event," says David McKenzie, who studies solar physics at Montana State University. In fact, says Charles Lindsey, an astronomer with the Solar Physics Research Corporation in Tucson, they are all magnetic phenomena, and probably related. "Almost everybody thinks that solar flares, wind, mass ejections, on any scale, have something to do with the magnetic field, but the variety and complexity seems enormous."

Howzee know?
To state the blindingly obvious, the sun is too hot to handle, and so the task of explaining how it worked fell, until recently, to theoreticians. Still, if you stare closely at the sun -- using instruments, not your bare eyes, silly -- you'll see some odd stuff. In 1610, Galileo and others first saw sunspots -- dark objects on the surface. Eventually, it became clear that sunspots wax and wane in an 11-year cycle -- as does the dispersal of solar mass -- the junk that can cause problems on Earth.

Now, after years of speculation, there's finally some actual evidence about the sun's structure, courtesy of the new discipline of helioseismology -- the study of "sunquakes."

Tell me. What goes on inside the sun?