Total solar eclipse. The delicately structured glow of the solar corona—or solar …—Copyright AURA Inc./National Optical Astronomy Observatories/National Science Foundation

Outermost region of the Sun's (or any star's) atmosphere, consisting of plasma. The Sun's corona has a temperature of about 3.6 million °F (2 million °C) and a very low density. Extending more than 8 million mi (13 million km) from the photosphere, it has no definite boundaries, continually varying in size and shape as it is affected by the Sun's magnetic field. The solar wind is formed by expansion of coronal gases. Only about half as bright as the full moon, the corona is overwhelmed by the brilliance of the solar surface and normally not visible to the unaided eye, but a total eclipse permits naked-eye observations.

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