When viewed with an X-ray telescope, like the one on the Yohkoh satellite, the Sun sometimes looks as it does here. This image shows the hot outer atmosphere of the Sun, the corona. The Sun's magnetic field plays a big role in determining how this picture will look. The bright features represent hot, dense gas that is trapped by the magnetic field of the Sun. In contrast, the dark, empty regions are where the Sun's magnetic field extends out into space allowing the hot gas to escape.
These regions, known as coronal holes, contain very little material and are not very hot. They often appear near the Sun's poles as seen above. An X-ray image like this one can only view material that is greater than 1 million degrees. Because the coronal holes do not contain much hot material they appear dark.
Image Credit: Yohkoh Science Team