The Sun in X-rays
With a surface temperature of 5800 K, the Sun should normally emit no X-rays at all, and so should look completely dark on an X-ray image.
Experience reveals that this is not the case, as demonstrated first by X-ray telescopes onboard the Skylab space station, and exemplified here by two X-ray images of the Sun taken on different days with an X-ray telescope on board the Japanese satellite Yohkoh. While large portions are indeed dark, small, very bright regions are also quite conspicuous. X-ray bright regions indicate heating to temperatures in excess of 2 million degrees Kelvin. Comparisons of such images with, for example, white light images (slide 1) taken simultaneously reveal that the brightest X-ray emitting regions are almost always overlying sunspots or active regions. Sequences of X-ray images occasionally reveals very sudden and short lived increases in brightness, known as flares. During such events, the X-ray brightness of a flaring active region often exceeds the total X-ray brightness of the rest of the Sun. Flares are also visible as brightening in Hα, and sometimes even in white light. They are one of the manifestations of the class of phenomena grouped under the heading of solar activity. The reconfiguration and dissipation of magnetic fields via reconnection is believed to be the energy source of solar flares. Other noteworthy features on these images are the very dark regions located near the south solar pole, called coronal holes. Coronal holes are usually located above the solar poles, but sometimes may extend down to lower latitudes, as illustrated by these two images.
Written By P. Charbonneau and O.R. White–April 18, 1995