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 red-hot disk around the dwarf, which could unpredictably undergo a spontaneous thermonuclear explosion as the red giant’s hydrogen trap becomes denser and reaches a tipping point. These fireworks between companion stars are fascinating to astronomers because they provide insight into the physics and dynamics of stellar evolution in binary systems. Credit: NASA, ESA, Leah Hustak (STScI)
Hubble telescope revisits a galaxy that is still extraordinarily hot
If we could look down at the beautiful spiral structure of our Milky Way If we were to view the galaxy from far above us, compressing millions of years into seconds, we would see brief flashes of light, like the flashes of cameras going off at a stadium event. These are novae, where a burned-out star, a white dwarf, is taking in gas from a bloated red giant companion that it is orbiting. One of the strangest of these events occurred in 1975, when a nova called HM Sagittae became 250 times brighter. It never really faded as novae usually do, but retained its brightness for decades. The latest Hubble observations show the system has gotten hotter, but paradoxically faded a bit.
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 together for Hubble to resolve. 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 blur is evidence of stellar wind. The nebula is about a quarter of a light-year in diameter. Credit: NASA, ESA, Ravi Sankrit (STScI), Steven Goldman (STScI), Joseph DePasquale (STScI)
Hubble Space Telescope finds surprises around a star that burst out 40 years ago
Astronomers have revisited one of the strangest binary star systems in our Milky Way – 40 years after it burst onto the scene as a bright and long-lived nova – using new data from NASA‘s Hubble Space Telescope and the retired SOFIA (Stratospheric Observatory for Infrared Astronomy), as well as archival data from other missions. A nova is a star that suddenly becomes extremely bright and then fades back to its previous obscurity, usually within a few months or years.
The unusual behavior of HM Sge
Between April and September 1975, the binary system HM Sagittae (HM Sge) brightened 250 times. Even more unusual is that it did not fade quickly as novae usually do, but retained its brightness for decades. Recently, observations show that the system has become hotter, but paradoxically a bit faded.
HM Sge is a particular type of symbiotic star in which a white dwarf and a bloated, dust-producing giant companion star are in an eccentric orbit around each other, and the white dwarf absorbs gas flowing from the giant star. That gas forms a red-hot disk around the white dwarf, which can unpredictably undergo a spontaneous thermonuclear explosion as the giant’s hydrogen influx grows closer to the surface until it reaches a tipping point. These fireworks between companion stars fascinate astronomers because they provide insights into the physics and dynamics of stellar evolution in binary systems.
“When I first saw the new data, I thought, ‘wow, this is what Hubble UV spectroscopy can do!’ – I mean, it’s spectacular, really spectacular.”
— Ravi Sankrit, astronomer
Changes observed in 2021
“In 1975, HM Sge went from an unremarkable star to something that all the astronomers in the field were watching, and at some point that wave of activity subsided,” says Ravi Sankrit of the Space Telescope Science Institute (STScI) in Baltimore. In 2021, Steven Goldman of STScI, Sankrit and collaborators used instruments on Hubble and SOFIA to see what had changed at HM Sge over the past 30 years at wavelengths of light from infrared to ultraviolet (UV).
Hubble’s 2021 ultraviolet data showed a strong emission line of highly ionized magnesium that was not present in previously published spectra from 1990. Its presence shows that the estimated temperature of the white dwarf and accretion disk rose from less than 400,000 degrees. Fahrenheit in 1989 to over 450,000 degrees Fahrenheit today. The highly ionized magnesium line is one of many that can be seen in the UV spectrum. Analyzed together this will reveal the energies of the system, and how it has changed over the past thirty years.
“When I first saw the new data,” Sankrit said, “I thought – ‘wow, this is what Hubble UV spectroscopy can do!’ – I mean, it’s spectacular, really spectacular.”
SOFIA soars over the snow-capped Sierra Nevada mountains with the telescope door open during a test flight. SOFIA is a modified Boeing 747SP aircraft. SOFIA reached full operational capability in 2014 and completed its final scientific flight on September 29, 2022. Credit: NASA/Jim Ross
Data from SOFIA
Using data from NASA’s flying telescope SOFIA, which will be retired in 2022, the team was able to detect the water, gas and dust flowing in and around the system. Infrared spectral data show that the giant star, which produces large amounts of dust, has resumed normal behavior within just a few years of the explosion, but also that it has dimmed in recent years, which is another puzzle to be explained.
Using SOFIA, astronomers were able to see water moving at a speed of about 30 kilometers per second, which they believe is the speed of the blistering accretion disk around the white dwarf. The gas bridge connecting the giant star to the white dwarf currently has to span approximately 3.2 billion kilometers.
The team has also teamed up with the AAVSO (American Association of Variable Star Observers) to collaborate with amateur astronomers from around the world who help keep telescopic eyes on HM Sge; their continued monitoring reveals changes not seen since the eruption 40 years ago.
A Hubble Space Telescope image of the symbiotic star Mira HM Sge with compass and scale bar. 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 together for Hubble to resolve. 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 blur is evidence of stellar wind. The nebula is about a quarter of a light-year in diameter. Credit: NASA, ESA, Ravi Sankrit (STScI), Steven Goldman (STScI)
The rarity and significance of HM Sge
“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 for decades,” said Goldman.
The first results of the team’s research have been published in the Astrophysical Journaland Sankrit presenting research focused on UV spectroscopy at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.
Reference: “A Multiwavelength Study of the Symbiotic Mira HM Sge with SOFIA and HST” by Steven R. Goldman, Ravi Sankrit, Edward Montiel, Sean Garner, Nathan Wolthuis and Nicole Karnath, January 11, 2024, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad12c9
The Hubble Space Telescope has been in business for more than thirty years and continues to make groundbreaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, Colorado, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, Maryland, operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.