Astronomy Picture of the Day . ... Credit: Yohkoh Project , SXT Group , NASA , ISAS Explanation: On the Sun , S marks the spot. Solar explosions have been discovered to explode preferentially from regions marked with this letter. ... If the two regions pass close enough and in just the right way, however, an X-ray bright S-shaped region called a sigmoid forms and quickly explodes in a Coronal Mass Ejection (CME). ... NASA Official: Jay Norris. ... Publications with keywords: Sun - surface . ...