The city of Baltimore, Maryland, witnessed a rare celestial event when the aftermath of a thermonuclear explosion in a binary star system located about 3,400 light years away was observed by the Hubble Space Telescope. The system, known as HM Sagittae or HM Sge for short, is a symbiotic system where a white dwarf star feeds off a companion red giant star, creating an accretion disk with stolen material swirling around the white dwarf. If too much material falls from the disk onto the white dwarf, a thermonuclear explosion occurs on its surface, causing the system to brighten in what is called a “nova.”
In April and September of 1975, HM Sge experienced a nova event in the constellation of Sagitta, the Arrow, brightening in the night sky by six magnitudes. Unlike most novas that diminish in brightness after a few days, HM Sge remained at peak brightness for years, eventually starting to fade in the mid-1980s. However, even now, it has only faded to about magnitude +12.
Astronomers have been closely monitoring HM Sge to understand its unique behavior since the 1975 nova explosion. Recent observations using the Hubble Space Telescope and data from NASA’s SOFIA have revealed new insights into the system’s evolution. The dimming of HM Sge since 1985 has been linked to the behavior of the red giant star, which pulsates periodically, causing changes in the system’s brightness.
The separation between the white dwarf and red giant components of HM Sge has increased over time, leading to less material flowing between the two stars. This, along with other factors, has contributed to the system’s slow evolution and changes in its emission lines. Despite the overall fading in brightness, the white dwarf in HM Sge has become one of the hottest known, with its temperature rising over the years.
The findings from Hubble and SOFIA observations have been published in The Astrophysical Journal, shedding light on the evolution of the HM Sge system and its potential future. The system’s unique characteristics make it a valuable source of information for astrophysicists, offering insights into the life cycle of binary star systems and the fate of similar stars in the universe. While the system continues to evolve slowly, astronomers anticipate future changes that could lead to another nova event in the distant future.