Nova in binary star system HM Sagittae (artist's concept)

Astronomers have used new data from the NASA/ESA Hubble Space Telescope and the retired NASA SOFIA observatory (Stratospheric Observatory for Infrared Astronomy) as well as archival data from other missions to revisit one of the strangest binary star systems in our galaxy – 40 years after it burst onto the scene as a bright and long-lived nova. A nova is a star that suddenly increases its brightness tremendously and then fades away to its former obscurity, usually in a few months or years.

Between April and September 1975, the binary system HM Sagittae (HM Sge) grew 250 times brighter. Even more unusual, it did not rapidly fade away as novae commonly do, but has maintained its luminosity for decades. Recently, observations show that the system has gotten hotter, but paradoxically faded a little.

The 2021 ultraviolet data from Hubble showed a strong emission line of highly ionised magnesium that was not present in earlier published spectra from 1990. Its presence shows that the estimated temperature of the white dwarf and accretion disk increased from less than 220,000 degrees Celsius in 1989 to greater than 250,000 degrees Celsius now. The highly ionised magnesium line is one of many seen in the UV spectrum, which analysed together will reveal the energetics of the system, and how it has changed in the last three decades.

With data from NASA's flying telescope SOFIA, which retired in 2022, the team was able to detect the water, gas, and dust flowing in and around the system. Infrared spectral data shows that the giant star, which produces copious amounts of dust, returned to its normal behaviour within only a couple years of the explosion, but also that it has dimmed in recent years, which is another puzzle to be explained. With SOFIA astronomers were able to see water moving at around 28 kilometres per second, which they suspect is the speed of the sizzling accretion disk around the white dwarf. The bridge of gas connecting the giant star to the white dwarf must presently span about 3.2 billion kilometres.

This artist’s concept shows the nova system HM Sagittae (HM Sge), where a white dwarf star is pulling material from its red giant companion. This forms a blazing hot disk around the dwarf, which can unpredictably undergo a spontaneous thermonuclear explosion as the infall of hydrogen from the red giant grows denser and reaches a tipping point. These fireworks between companion stars are fascinating to astronomers by yielding insights into the physics and dynamics of stellar evolution in binary systems.

Learn more about these results here.