FG Sagittae

Lore & Background

FG Sagittae was first noted as a variable star in 1943 and designated AN 377.1943. Its variations were initially described as irregular, but it soon showed a steady increase in average brightness, brightening by about two magnitudes between 1943 and 1970 before beginning to fade. Old photographic observations revealed it had been brightening since at least 1900, with extrapolations suggesting a minimum around 1880. As it faded, FG Sagittae began to show periodic variations, at first a period of 80 days, but increasing to 130 days.

Reader's Guide

FG Sagittae is significant as a rare example of a star undergoing a late thermal pulse (LTP) of helium fusion after having left the asymptotic giant branch (AGB) and moving toward the white dwarf cooling track. This thermal pulse is believed to have revived the aged star to briefly behave again as an AGB star. Its evolution on a timescale almost unheard-of for a star—from a small, very hot post-AGB star becoming a white dwarf, to a hot supergiant, then a cool supergiant—makes it a key object for studying stellar evolution. The star's effective temperature dropped from about 25,000 K in 1930 (possibly 45,000 K in 1890) to about 5,500 K by 1975, and its bolometric luminosity increased from around 2,700 L☉ at the end of the 19th century to over 10,000 L☉ by 1965. Its radius expanded from about one R☉ in 1900 to about 184 R☉ by 1992. The deep fades since 1992, caused by dust formation, have obscured the star's surface, but models suggest the underlying stellar luminosity remained essentially constant until at least 2001. FG Sagittae is compared to other presumed late thermal pulsing objects such as Sakurai's Object (V4334 Sgr) and V605 Aquilae.

Did You Know?

• Its pulsation period increased from 15 days when it was an A-type star to over 100 days later.
• The star is the central star of the planetary nebula Henize 1-5.
• S-process elements became at least 25 times more abundant between 1967 and 1974.

Discovery and Early Recognition

FG Sagittae first caught astronomers' attention in 1943 when a star catalogued as AN 377.1943 was flagged as a previously unknown variable. It was initially given the provisional designation CSV 5066 before earning the confirmed variable star name FG Sagittae. At the time of discovery, its brightness fluctuations appeared irregular, though observers quickly noticed the star's average luminosity was climbing steadily. It gained roughly two magnitudes of brightness between 1943 and 1970 before beginning to dim. Looking back through old photographic plates revealed the star had actually been brightening since at least 1900, with extrapolations pointing to a minimum around 1880. As the star entered its fading phase, it began exhibiting periodic variations—first with an 80-day cycle that stretched to 130 days over time. This combination of long-term brightening, subsequent fading, and emerging periodicity made FG Sagittae a genuinely puzzling object that defied easy classification in its early years of study.

A Dramatic Spectral Transformation

Perhaps the most striking aspect of FG Sagittae is how radically its spectral identity shifted over a single human lifetime. When first catalogued as a variable, it presented as a blue supergiant, with the earliest reliable classification placing it at B0 in 1930. Extrapolating brightness and colour data backward suggests it may have been an O3 star as early as 1890, with an effective temperature possibly reaching 45,000 K. By 1975, that temperature had plummeted to roughly 5,500 K, and detailed 1980s analyses showed it drifting as low as 5,280 K. The spectral class settled into the G or K range for the remainder of the twentieth century. Alongside this cooling, elemental abundances shifted dramatically: s-process elements surged by at least a factor of twenty-five between 1967 and 1974, iron-peak elements faded from view, and carbon-rich dust became prominent after 1992. The star's radius swelled from about one solar radius around 1900 to roughly 184 solar radii by 1992, while its bolometric luminosity climbed from around 2,700 solar luminosities to over 10,000.

The 1992 Collapse and R Coronae Borealis Mimicry

In 1992, FG Sagittae underwent a sudden and dramatic transformation. Its periodic variations, which had persisted with a period stretching from 80 to 130 days, abruptly ceased, and the star's brightness plunged by five magnitudes in a mere two months. What followed was a pattern of deep, episodic fading events and partial recoveries that closely resembled the behaviour of R Coronae Borealis-type variables. This resemblance is reinforced by a hydrogen deficiency in the star's atmosphere, a hallmark of that class. The visual dimming was accompanied by a comparable increase in infrared brightness, pointing to the formation of obscuring dust rather than a true reduction in stellar output. Models of the dust envelope suggest the luminosity dropped sharply for a few hundred days as dust formed and was heated, while the underlying stellar luminosity remained essentially constant at least through 2001. Observing the star's spectrum after 1992 has proven difficult because of the dust, though s-process and rare-earth elements appear to have continued growing in abundance.

The Late Thermal Pulse and Nebular Legacy

The accepted explanation for FG Sagittae's extraordinary behaviour is a late thermal pulse—a brief reignition of helium fusion in a shell that had been dormant since the star departed the asymptotic giant branch. This event effectively revived the ageing star, causing it to temporarily re-enact the behaviour of an AGB star before it resumes its long journey toward the white-dwarf cooling track. The star is now shedding mass at a rate of about one solar mass per million years, and a dust shell is forming around it that may eventually produce a second planetary nebula. The first such nebula, the extremely faint Henize 1-5 at visual magnitude 23, was created when FG Sagittae originally left the AGB. The entire sequence—from a compact, very hot post-AGB object, through a hot supergiant phase, to a cool supergiant—unfolded on a timescale virtually unheard of in stellar evolution. Models can approximate the star's behaviour, though detailed discrepancies remain, and FG Sagittae stands alongside objects like Sakurai's Object as one of the few known examples of this rare evolutionary episode.