This artist’s concept shows the HM Sagittae (HM Sge) nova system, where the white dwarf is pulling material from its red giant. This creates a glowing hot disk around the dwarf, which can unpredictably undergo a spontaneous thermonuclear explosion as the inflow of hydrogen from the red giant thickens and reaches a tipping point. These fireworks between companion stars are fascinating to astronomers by providing insight into the physics and dynamics of stellar evolution in binary systems. Credit: NASA, ESA, Leah Hustak (STScI)
The Hubble Telescope revisits a star system that is still extremely hot
If only we could look down on our beautiful spiral structure Milky Way galaxies from far above and compress millions of years into seconds, we would see brief flashes of light, like the flashes from cameras popping up in a stadium. These are novae where the star is burned out, and white dwarf, consumes gas from a bloated red giant companion orbiting it. One of the strangest events happened in 1975 when a nova called HM Sagittae brightened 250 times. It never actually faded, as novae normally do, but retained its brightness for decades. The latest HST observations show that the system has warmed up, but paradoxically faded a bit.
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 detect. Material bleeds from the red giant and falls onto the dwarf, making it extremely bright. This system first flared up as a nova in 1975. The red nebula is evidence of a stellar wind. The nebula is about a quarter of a light year across. Credits: NASA, ESA, Ravi Sankrit (STScI), Steven Goldman (STScI), Joseph DePasquale (STScI)
The Hubble Space Telescope has found a surprise around a star that exploded 40 years ago
Astronomers have revisited one of the strangest binary star systems in our galaxy — 40 years after it burst onto the scene as a bright, 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 increases in brightness tremendously and then fades back to its former obscurity, usually in a matter of 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, it did not disappear quickly, as novae normally do, but retained its luminosity for decades. Recently, observations show that the system has warmed up, but paradoxically faded a little.
HM Sge is a special kind of symbiotic star where a white dwarf and a bloated, dust-producing giant companion star are in an eccentric orbit around each other, and the white dwarf absorbs the gas flowing from the giant star. This gas creates a glowing hot disk around the white dwarf that can unpredictably undergo a spontaneous thermonuclear explosion as hydrogen from the giant condenses on the surface until it reaches a tipping point. These fireworks between companion stars fascinate astronomers by providing insight 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’s UV spectroscopy can do!’ – I mean it’s spectacular, really spectacular.”
— Ravi Sankrit, astronomer
Changes observed in 2021
“In 1975, HM Sge went from being a nondescript star to something that every astronomer in the field was looking at, and at some point the flurry of activity slowed,” said Ravi Sankrit of the Space Telescope Science Institute (STScI) in Baltimore. In 2021, STScI’s Steven Goldman, Sankrit and co-workers used the instruments on HST and SOFIA to see what has changed with HM Sge over the last 30 years at wavelengths of light from infrared to ultraviolet (UV).
Ultraviolet data from the 2021 Hubble showed a strong emission line of highly ionized magnesium that was not present in previously published spectra from 1990. Its presence indicates that the estimated temperature of the white dwarf and accretion disk has increased from less than 400,000 degrees Fahrenheit in 1989 to over 450,000 degrees Fahrenheit now. The highly ionized magnesium line is one of many observed in the UV spectrum that, together, will reveal the energetics of the system and how it has changed over the past three decades.
“When I first saw the new data,” Sankrit said, “I thought – ‘wow, this is what Hubble’s UV spectroscopy can do!’ – I mean it’s spectacular, really spectacular.”
SOFIA hovers 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 science flight on September 29, 2022. Credit: NASA/Jim Ross
Data from SOFIA
With data from NASA’s SOFIA flyby telescope, which will be retired in 2022, the team was able to detect water, gas and dust flowing in and around the system. Infrared spectral data show that the giant star, which produces large amounts of dust, returned to its normal behavior within just a few years of the explosion, but also that it has dimmed in recent years, another puzzle to be explained.
With the help of SOFIA, astronomers were able to see water moving at about 18 miles per second, which they say is the speed of the sizzling accretion disk around the white dwarf. The gas bridge connecting the giant star to the white dwarf must currently span about 2 billion miles.
The team also works with the AAVSO (American Association of Variable Star Observers) to collaborate with amateur astronomers from around the world who help maintain the telescopic eyes on HM Sge; their continued monitoring reveals changes not seen since its explosion 40 years ago.
Hubble Space Telescope compass and scale 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 detect. Material bleeds from the red giant and falls onto the dwarf, making it extremely bright. This system first flared up as a nova in 1975. The red nebula is evidence of a stellar wind. The nebula is about a quarter of a light year across. Credits: NASA, ESA, Ravi Sankrit (STScI), Steven Goldman (STScI)
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 decades-long treasure trove for astrophysicists,” said Goldman.
The initial results of the team’s research were published in Astrophysical Journaland Sankrit presents UV spectroscopy research at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.
Reference: “Multiwavelength Study of the Symbiotic Mira HM Sge with SOFIA and HST” Steven R. Goldman, Ravi Sankrit, Edward Montiel, Sean Garner, Nathan Wolthuis and Nicole Karnath, 11 January 2024, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad12c9
The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is an international collaborative project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland manages the operation of the telescope and the mission. Lockheed Martin Space, based in Denver, Colorado, also supports missions 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.