Thanks to new data from NASA’s Hubble Space Telescope and the SOFIA observatory, astronomers have revisited one of the strangest star systems in our galaxy: HM Sagittae (HM Sge). This binary star system caught the attention of scientists 40 years ago, when it exploded as a bright, long-lasting nova. A nova is a star that suddenly increases in brightness and then fades within a few months or years.
A stellar explosion that lasts over time
Between April and September 1975, the HM Sge binary system became 250 times brighter. Contrary to expectations, it did not dim as quickly as common novae do, but retained its brightness for decades. Recent observations show that the system has become warmer, but paradoxically has lost some of its brightness.
HM Sge is a particular type of symbiotic star in which a white dwarf and a red giant, which produces a lot of dust, orbit each other. The white dwarf absorbs the gas flowing from the giant, forming a glowing disk around itself. This disk can undergo sudden thermonuclear explosions when the influx of hydrogen from the giant becomes sufficiently dense.
The latest discoveries
In 2021, Steven Goldman and Ravi Sankrit of the Space Telescope Science Institute (STScI) used Hubble and SOFIA to observe HM Sge in infrared and ultraviolet light wavelengths. Hubble’s 2021 ultraviolet data showed strong emission of highly ionized magnesium, not present in the 1990 spectra. This indicates that the temperature of the white dwarf and accretion disk increased from less than 400,000 degrees Fahrenheit in 1989 to above 450,000 degrees Fahrenheit today.
With SOFIA, the team detected water, gas and dust flowing in and around the system. Infrared data shows that the red giant returned to its normal behavior within a few years of the explosion, but has also dimmed recently.
Continuous monitoring
The team collaborates with the AAVSO (American Association of Variable Star Observers), involving amateur astronomers from around the world to monitor HM Sge. This monitoring revealed changes not seen since the explosion 40 years ago.
The team’s initial findings were published in The Astrophysical Journal, and Ravi Sankrit presented the ultraviolet spectroscopy research at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.
A treasure for astrophysics
“Symbiotic stars like HM Sge are rare in our galaxy, and witnessing a nova-like explosion is even rarer. This unique event is a treasure trove for astrophysicists that spans decades,” said Steven Goldman.
The Hubble Space Telescope, active for more than thirty years, continues to make discoveries that shape our understanding of the universe. Hubble is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center manages the telescope and mission operations, supported by Lockheed Martin Space and the Space Telescope Science Institute.
