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FG Sagittae

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FG Sge is a very peculiar variable star whose antics have shed light on the very late stages of stellar evolution.  The object is believed to be a star in the process of ejecting multiple shells of matter, forming the planetary nebula that surrounds it as we watch.  Historical data from the 19th century suggested it rose from obscurity to approach near constant brightness where it remained through the mid 20th-century.  Then in 1992 it underwent a dramatic drop in brightness again -- similar to those we see in the R Coronae Borealis stars -- from which it hasn't recovered.  It's hypothesized that the bright phase (up through 1992) is a late thermal pulse, where helium inside the stellar remnant briefly ignites, temporarily putting the proto-white dwarf back onto the asymptotic giant branch.  It's unclear whether the most recent dimming episode will be followed by another thermal pulse, or if so, when that might happen.  But it is clear that FG Sge's days as a star are numbered.  You can continue following its progress in larger aperture telescopes, but be careful not to confuse nearby stars (or the planetary nebula itself) as the star.

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Prepared By: Matthew Templeton
AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484