Rare Stellar Transformation Offers Real-Time View of a Star’s Final Stages
Kaposvár, Hungary – Amateur astronomers have a unique opportunity this month to observe FG Sagittae, a variable star undergoing a rare “late helium flash” that is dramatically altering its life cycle, according to the Hungarian Astronomical Society (MCSE).Onc a faint, unremarkable star, FG Sagittae is now exhibiting unusual behavior as it transitions from a white dwarf back into a red giant, offering scientists and enthusiasts a real-time glimpse into the final stages of stellar evolution.
The star’s transformation, documented over recent decades, involves increasing brightness coupled with decreasing surface temperatures. This process is linked to a core helium flash, re-inflating the star and triggering the ejection of outer layers, forming a planetary nebula – currently approximately 30 arcseconds in diameter (Hen 1-5).
“This unusual behavior received an explanation when it turned out that the star was experiencing a rare ‘late helium glance’,” explains the MCSE in its October 2025 Heavenly Calendar article.”This core process has re-inflated the celestial body that had become a white dwarf and apparently pushed it back into the red giant state.”
In the 1990s, FG Sagittae began to fade as a dust cloud formed from carbon-rich material, leading to increasingly deep minima in its light curve - characteristic of RCB (R Coronae Borealis) variables. While once readily visible through binoculars, the star now primarily requires photometric observation due to its dimness, though occasional brighter phases still offer viewing opportunities.
The MCSE encourages observers to monitor FG Sagittae,noting that it periodically emerges from its dust cloud,offering a rare chance to witness a star “almost ‘resumes its past’ in the final stage of their life.”
The article was contributed by BGB, SGR, KVM, and Kaposvári Z.
