corona
/ kuh-ROH-nuh /
The corona is the Sun's outermost atmosphere, a faint, pearly-white halo of incredibly thin gas that streams far out into space. It is normally invisible because the bright disk of the Sun outshines it a million times over, but during a total solar eclipse, when the Moon blocks the disk, the corona blossoms into view as a ghostly crown of light — which is exactly what 'corona' means.
The corona is extraordinarily tenuous — far emptier than any vacuum we can make in a laboratory — yet astonishingly hot, reaching one to three million degrees Celsius, hundreds of times hotter than the photosphere just below it. Because it is so hot, it glows in ultraviolet light and X-rays, so spacecraft observe it in those invisible colours. Its shape is sculpted by the Sun's magnetic field: long graceful streamers, bright loops arching over active regions, and dark coronal holes where the field opens out and lets gas escape.
The corona matters because it is the source of the solar wind and the launch site of the most violent solar storms, and because its temperature is one of the great unsolved puzzles in solar physics. It also marks the very beginning of the Sun's influence on space: the corona does not simply end but blows outward to fill the entire solar system.
NASA's Parker Solar Probe has flown right through the corona, becoming the first spacecraft to 'touch' the Sun by passing inside this million-degree halo.
It survives because the corona, though millions of degrees, is so thin it carries little actual heat.
Temperature and heat are not the same thing. The corona's particles move extremely fast (high temperature), but there are so few of them that it would not feel scorching — much as a sparkler's sparks are very hot yet harmless.