corona
Sun
outermost region of the Sun's atmosphere, consisting of plasma (hot ionized gas). It has a temperature of approximately 2,000,000 K and an extremely low density. The corona extends more than 13,000,000 kilometres (8,000,000 miles) from the photosphere (solar surface). It has no definite boundaries, continually varying in size and shape as it is affected by the Sun's magnetic field. The solar wind (q.v.), which flows radially outward through the entire solar system, is formed by the expansion of the coronal gases.
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 (q.v.).