CORONA, in Astronomy

luminous envelope surrounding the sun, outside the chromosphere. Its density is less than one billionth that of the earth's atmosphere. The corona is visible only at the time of totality during a total eclipse of the sun. It then appears as a halo of light with an irregular outer edge, often with streamers radiating from the sun's surface and contrasting with the dark lunar disk that it borders. It is divided into the inner corona, a ring of pale-yellow light against which crimson prominences are outlined, and the outer corona, a pearly white halo that extends far out into space. The corona consists of ionized gas at a temperature of 1 million degrees Celsius. By means of the coronagraph, the innermost part of the corona can be studied and photographed in full daylight. Although the visible corona extends a few solar radii above the sun, because of its high temperature it produces a continual flow of electrically charged particles called the solar wind that move outward through the solar system.

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