A bright prominence at the edge of the Sun that reached a height of 149 000 km on 3 April 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved
Serious observers of the Sun like Monty Leventhal OAM of the Sydney City Skywatchers use special filters called hydrogen alpha filters. These are safe to use as they cut out all light from the Sun except for the red light of hydrogen atoms. Hence these filters emphasise features that radiate at that wavelength, which are those composed of hot hydrogen atoms. Features on the Sun that can be seen with a hydrogen alpha filter includes prominences, filaments and flares.
A prominence at the edge of the Sun reaching 93 000 km on 19 March 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved
Prominences are hot clouds of gas travelling along lines of magnetic field. They can exhibit all sorts of shapes such as arches and loops and can sometimes stay above the edge of the Sun for days. Others can detach from the Sun’s visible surface and float away.
It should be noted that the Sun’s visible surface is not solid. Nothing on the Sun is solid as it is a gas even towards its central regions. The visible surface is a region of surface temperature around 5500°C with the deeper regions beyond it too hot and opaque to be visible.
A long filament viewed in the red light of hydrogen atoms stretching 297 000 km across the Sun on 3 April 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved
When prominences are seen against the bright solar disc instead of the darkness at the edge of the Sun they appear as filaments – long dark lines snaking across the Sun.
The most exciting and the rarest events on the Sun are flares. These are explosions on the Sun that can be seen as the brightening of regions of the Sun near sunspot groups. They can last from for just a few minutes to a few hours. Satellites such as the GOES satellites provide continuous measurements of the X-rays emitted by the Sun and so provide complementary information to what can be seen visually.