coronagraph

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coronagraph

telescope

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Key People:: Bernard Lyot

Related Topics:: corona; solar telescope

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coronagraph, telescope that blocks the light of a star inside the instrument so that objects close to the star can be observed. It was invented in 1930 by the French astronomer Bernard Lyot and was used to observe the Sun’s corona and prominences.

When a coronagraph is used to observe the Sun, a round metal screen blocks the overwhelming brightness of the Sun’s central disk, or photosphere. Other screens and diaphragms block reflections of the photospheric image. The coronagraph must be used at high altitudes and on very clear days, when the atmospheric diffusion of light is at a minimum; imperfect lenses or dust inside the instrument will scatter enough photospheric light to obscure the dim solar corona.

This article was most recently revised and updated by Chelsey Parrott-Sheffer.

corona

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Total solar eclipse. The delicately structured glow of the solar corona—or solar atmosphere—seen during the March 7, 1970, total eclipse of the Sun. The corona is visible to the unaided eye only during an eclipse.

corona

Sun

Also known as: solar corona

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Key People:: Sir Edward Frankland

Related Topics:: solar wind; solar prominence; interplanetary magnetic field; coronal hole; reconnection

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corona, outermost region of the Sun’s atmosphere, consisting of plasma (hot ionized gas). It has a temperature of approximately two million kelvins and an extremely low density. The corona continually varies in size and shape as it is affected by the Sun’s magnetic field. The solar wind, which flows radially outward through the entire solar system, is formed by the expansion of the coronal gases and only ends at the heliopause.

In spite of its high temperature, the corona yields relatively little heat, because of its low density; i.e., the constituent gas molecules are so sparse that the energy content per cubic centimetre is substantially lower than that of the interior region of the Sun. The corona shines only about half as brightly as the Moon and is normally not visible to the unaided eye, because its light is overwhelmed by the brilliance of the solar surface. During a total solar eclipse, however, the Moon blocks out the light from the photosphere, permitting naked-eye observations of the corona. The corona can also be studied under noneclipse conditions with a special telescopic instrument called a coronagraph.

This article was most recently revised and updated by Richard Pallardy.