FG Sagittae


FG Sagittae, is a supergiant star in the constellation Sagitta at a distance of 8000 light-years. When first noted in 1943, it was identified to be a variable star, and its spectrum was registered as of B4Ieq in 1955. Since then it has expanded and reddened to G in 1991, and then further to about K. FG started to pulsate when becoming an A-type star with a period of 15 days, now the period has increased to over 100 days.
FG Sagittae is the central star of the planetary nebula Henize 1-5.
Since 1992 the star has exhibited fadings and recoveries similar to that of a R Coronae Borealis variable star; this behavior is emphasized by a hydrogen deficiency typical for this class of stars.
It has been proposed that this star has undergone a "late thermal pulse" of helium fusion after having left the asymptotic giant branch to move towards the hottest end of the "white dwarf cooling track". This thermal pulse is believed to have revived this aged star to once again, for a short time, behave as an AGB star.