The Sun's corona is usually impossible to see with the naked eye because its brightness is overwhelmed by the Sun's photosphere, which is the visible surface of the Sun. The photosphere is much brighter than the corona because it is much hotter and denser. The corona is only visible during a total solar eclipse, when the Moon blocks out the photosphere and allows the corona to be seen.

The corona is made up of extremely hot plasma, which is a gas that has been heated to such a high temperature that its electrons have been stripped away from its atoms. The plasma in the corona is so hot that it emits X-rays and ultraviolet radiation, but it does not emit much visible light. This is why the corona is usually invisible to the naked eye.

During a total solar eclipse, the Moon blocks out the photosphere and allows the corona to be seen. The corona appears as a halo of white or silver light around the Sun. The corona is usually only visible for a few minutes during a total solar eclipse.