Thursday, February 7, 2013
“Normal stars such as the Sun are hot balls of gas millions of kilometers in diameter. The visible surfaces of stars are called the photospheres, and have temperatures ranging from a few thousand to a few tens of thousand degrees Celsius. The outermost layer of a star’s atmosphere is called the “corona”, which means “crown”. The gas in the coronas of stars has been heated to temperatures of millions of degrees Celsius.
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.”