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Normal Stars
The heating of a star's corona and the flare phenomenon are related to the
important question of how energy is transported from the nuclear furnace in the
star's central regions to the surface.
In hot massive stars, the energy flowing out from the center of the star is so
intense that the outer layers are literally being blown away. Unlike a nova, these stars do not
shed their outer layers explosively, but in a strong, steady stellar wind.
Shock waves in this wind produce X-rays; from the intensity and distribution with
energy of these X-rays, astronomers can estimate the temperature, velocity and
density of this wind.
In medium-sized stars, such as the Sun, the outer layers consist of a rolling,
boiling turmoil called convection. A
familiar example of convection is a sea-breeze. The Sun warms the land more
quickly than the water and the warm air rises and cools as it expands. It then
sinks and pushes the cool air off the ocean inland to replace the air that has
risen, producing a sea-breeze. In the same way, hot gas rises from the subsurface layers that extend to a depth of about 200,000 kilometers, cools at the surface and descends again.
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