Free Astronomy Magazine September-October 2024

50 SEPTEMBER-OCTOBER 2024 ASTRO PUBLISHING A Hubble Space Telescope image of the symbiotic star Mira HM Sge. Located 3,400 light-years away in the constellation Sagitta, it consists of a red giant and a white dwarf companion. The stars are too close to- gether to be resolved by Hubble. Material bleeds off the red giant and falls onto the dwarf, making it extremely bright. This system first flared up as a nova in 1975. The red nebulosity is evidence of the stellar wind. The nebula is about one-quarter light-year across. [NASA, ESA, Ravi Sankrit (STScI), Steven Goldman (STScI) − Joseph DePasquale (STScI)] tronomers by yielding insights into the physics and dynamics of stellar evolution in binary systems. “In 1975 HM Sge went from being a nondescript star to something all astronomers in the field were look- ing at, and at some point that flurry of activity slowed down,” said Ravi Sankrit of the Space Telescope Sci- ence Institute (STScI) in Baltimore. In 2021, Steven Goldman of STScI, Sankrit and collaborators used in- struments on Hubble and SOFIA to see what had changed with HM Sge in the last 30 years at wavelengths of light from the infrared to the ul- traviolet (UV). The 2021 ultraviolet data from Hub- ble showed a strong emission line of highly ionized magnesium that was not present in earlier published spectra from 1990. Its presence shows that the estimated tempera- ture of the white dwarf and accre- tion disk increased from less than 400,000 degrees Fahrenheit in 1989 to greater than 450,000 degrees Fahrenheit now. The highly ionized magnesium line is one of many seen in the UV spectrum, which analyzed together will reveal the energetics of the system, and how it has changed in the last three decades. “When I first saw the new data,” Sankrit said, “I went – ‘wow this is what Hubble UV spectroscopy can do!’ – I mean it’s spectacular, really spectacular.” With data from NASA’s flying tele- scope SOFIA, which retired in 2022, the team was able to detect the water, gas, and dust flowing in and around the system. Infrared spec- tral data shows that the giant star, which produces copious amounts of dust, returned to its normal behav- ior 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 18 miles per second, which they sus- pect is the speed of the sizzling ac- cretion disk around the white dwarf. The bridge of gas connect- ing the giant star to the white dwarf must presently span about 2 billion miles. The team has also been working with the AAVSO (American Associ- ation of Variable Star Observers), to collaborate with amateur as- tronomers from around the world who help keep telescopic eyes on HM Sge; their continued monitor- ing reveals changes that haven’t been seen since its outburst 40 years ago. “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 for astrophysicists spanning decades,” said Goldman. !