AG Pegasi

AG Pegasi is a symbiotic binary star in the constellation Pegasus. It is a close binary composed of a red giant and white dwarf, estimated to be around 2.5 and 0.6 times the mass of the Sun respectively.^[3]

Initially a magnitude 9 star, AG Pegasi brightened and peaked at an apparent magnitude of 6.0 around 1885 before gradually fading to magnitude 9 in the late 20th century. Its spectrum was noted by earlier observers to resemble P Cygni.^[5] The spectrum of the hotter star has changed drastically over 160 years, leading investigators Scott Kenyon and colleagues to surmise that its hotter component, originally a white dwarf, accumulated enough material from the donor giant star to begin burning hydrogen and enlarge and brighten into an A-type white supergiant around 1850. It had this spectrum and an estimated surface temperature of around 10000 K in 1900,^[3] with a likely radius 16 times that of the Sun,^[4] before becoming a B-class star in 1920, then an O-class star in 1940, and finally a Wolf-Rayet star in 1970,^[3] with a surface temperature of 95000 K since 1978. It has shrunk to star with a diameter 1.1 times that of the Sun in 1949, then 0.15 times in 1978 and 0.08 times that of the Sun in 1990.^[4] AG Pegasi has been described as the slowest nova ever recorded,^[3] with a constant bolometric luminosity of the hotter star over 130 years from 1850 to 1980. By the late 20th century, the hotter star has evolved into a hot subdwarf on its way to eventually returning to white dwarf status.^[1]

Vogel and colleagues calculated the hotter star must have been accreting material from the red giant for around 5000 years before erupting. Both stars are ejecting material in stellar winds.^[3] The resulting nebula contains material from both stars and is complex in nature.^[2]

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