An extraordinary phenomenon was observed by the Hubble Space Telescope in a binary star system located about 3,400 light years away. The event involved a symbiotic system known as HM Sagittae (HM Sge), where a white dwarf star was feeding off a red giant companion star. The stolen material formed an accretion disk around the white dwarf, leading to a thermonuclear explosion on its surface, causing the system to brighten in a phenomenon referred to as a “nova.”
The nova event occurred in the constellation of Sagitta, the Arrow, between April and September 1975, during which HM Sge significantly brightened in the night sky. Unlike most novas that fade after a few days, HM Sge remained at peak brightness for years before slowly fading. Despite this, it has only dimmed to about magnitude +12 as of now.
Astronomers have been closely monitoring HM Sge since the 1975 nova event, with recent observations in 2021 revealing new insights into the system. The dimming of HM Sge since 1985 has been linked to the behavior of the red giant star, which is classified as a Mira variable due to its periodic pulsations. The separation between the white dwarf and red giant in the system is currently around 40 astronomical units.
New findings from Hubble and SOFIA observations in 2021 have shed light on the evolving nature of HM Sge. The detection of emission lines from ionized magnesium and water vapor in the system provided valuable information on its properties and evolution. Despite the overall fading in brightness, the white dwarf in HM Sge has experienced a rise in temperature, making it one of the hottest white dwarfs known.
Researchers conclude that HM Sge has settled into a “new normal” after the 1975 nova explosion, with its brightness slowly decreasing over the years. The system’s evolution may continue for many more years, potentially leading to another nova event when the white dwarf and red giant come close in their orbit.
Finally, the fate of the white dwarf and red giant in HM Sge offers a glimpse into the future of the system. Both stars were once sun-like in a binary system, with the more massive star evolving into a red giant and eventually revealing its white dwarf core. The gravitational interactions between the two white dwarfs may lead to a Type Ia supernova in the distant future.