NASA scientists have calculated that the Sun's atmosphere extends some 8 million kilometres above its surface. That's 20 times the distance between the Earth and the Moon.
This figure, which is the equivalent of 12 solar radii, was determined using information from NASA's Solar Terrestrial Relations Observatory, or 'STEREO'. STEREO is a solar observation mission that launched two almost identical spacecraft in 2006 to orbit around the Sun and provide stereoscopic imaging of its activities, including coronal mass ejections and solar flares. In 2011, these two spacecraft managed to position themselves exactly 180 degrees apart, which allowed scientists to view the entire Sun all at once for the first time. Read more.