What IS the sun?

"The Sun Is A Mass Of Incandescent Gas..."

"The sun is a mass of incandescent gas, a gigantic nuclear furnace, where hydrogen is built into helium at a temperature of millions of degrees." So begins a song written in 1956, originally recorded to promote nuclear power. In actuality, the sun is a ball of luminescent hydrogen, helium, and other gases, already burning itself away for billions of years.



A diagram of the sun showing (1) the Photosphere, (2) the Chromosphere, and (3) the Corona.

For the reason of temperature alone, it is fortunate that Earth only receives one two-billionth of the total energy radiated by the sun. Most of the visible energy, light, is radiated from the photosphere (1, above), the outside shell of the sun. The incandescent gases here measure a depth of about 300 kilometers (200 miles), but unlike the surface of Earth, the pressure here is only about 10 millibars (1000 Pascals), a mere 1/10 of the pressure at the surface of Earth.

The chromosphere (2, above) is above the photosphere, and its luminescence is rarely visible, despite the fact that these gases also glow. During the correct moments of a solar eclipse, however, the overpowering glow of the photosphere is hidden, and, for a few minutes, one can see the chromosphere as a thin red circle about the sun using special filter. The chromosphere is only a few thousand kilometers thick, and it is almost amazing that it can be detected at all at this great distance.

Outside the chromosphere, at a depth of about a million or so kilometers, are the ionized gases of the corona (3, above). This gossamer layer of the Sun is only visible during a solar eclipse, and it emits light about half as bright as that of a full moon. (Note that care should be used when viewing the corona; cobalt welder's glass is recommended.)

Solar Wind

The outermost particles of the corona , mainly ions, move so fast that they are able to escape the gravity of the Sun, and these charged atomic particles form the solar wind, traveling away from the sun; much of the solar wind is lost to the vacuum of space, but some particles travel far enough to affect planets and their satellites. An increased solar wind can also interact with the Earth's magnetic field and ions in our upper atmosphere, and we see this interaction as the Aurora Borealis in the northern hemisphere (often called the "Northern Lights"), and the Aurora Australias in the southern hemisphere.


The Sun itself operates on an eleven-year cycle, its activity ebbing and growing in a steady rhythm. This cycle was calculated after much observation in the early 1800s; the main object of examination in this experiment was the frequency of sunspots, dark spots visible on the surface of the sun. (Again, special filters are required.) There have been many attempts at connecting solar activity to climate on Earth, but there is no conclusive evidence that sunspot activity is related to terrestrial climate.

Solar Flares

A solar flare occurs when the area above a sunspot brightens and releases huge amounts of energy in the forms of ultraviolet, radio, x-ray radiation, and high-speed atomic particles. These particles intensify the solar wind and yield auroras, while the radiation may interfere with radio and television reception.