Stellar Fireworks: Hubble Revisits a Strange Star’s 40-Year Nova Mystery

Nova in Binary Star System HM Sagittae

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. Credit: NASA, ESA, Leah Hustak (STScI)

Hubble Telescope Revisits a Star System That is Still Extraordinarily Hot

If we could gaze down upon the magnificent spiral structure of our Milky Way galaxy from far above, and compress millions of years into seconds, we would see brief bursts of light, like the flashes from cameras popping off at a stadium event. These are novae, where a burned-out star, a white dwarf, ingests gas from a bloated red giant companion it is orbiting. One of the strangest of these events happened in 1975, when a nova called HM Sagittae grew 250 times brighter. It never really faded away as novae commonly do, but has maintained its brightness for decades. The latest Hubble observations show that the system has gotten hotter, but paradoxically faded a little.

Symbiotic Star Mira HM Sge

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 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. Credit: NASA, ESA, Ravi Sankrit (STScI), Steven Goldman (STScI), Joseph DePasquale (STScI)

Hubble Space Telescope Finds Surprises Around a Star That Erupted 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 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 increases its brightness tremendously and then fades away to its former obscurity, usually in a few months or years.

The Unusual Behavior of HM Sge

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.

HM Sge is a particular 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 ingests gas flowing from the giant star. That gas forms a blazing hot disk around the white dwarf, which can unpredictably undergo a spontaneous thermonuclear explosion as the infall of hydrogen from the giant grows denser on the surface until it reaches a tipping point. These fireworks between companion stars fascinate astronomers by yielding insights into the physics and dynamics of stellar evolution in binary systems.

“When I first saw the new data, I went – ‘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 being a nondescript star to something all astronomers in the field were looking at, and at some point that flurry of activity slowed down,” said 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 with HM Sge in the last 30 years at wavelengths of light from the infrared to the ultraviolet (UV).

The 2021 ultraviolet data from Hubble 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 temperature of the white dwarf and accretion 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.”

NASA SOFIA

SOFIA soars over the snow-covered Sierra Nevada mountains with its telescope door open during a test flight. SOFIA is a modified Boeing 747SP aircraft. SOFIA achieved full operational capability in 2014 and concluded its final science flight on September 29, 2022. Credit: NASA/Jim Ross

Data From SOFIA

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 behavior 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 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 2 billion miles.

The team has also been working 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 that haven’t been seen since its outburst 40 years ago.

Mira HM Sge Compass Image

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 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. 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 spanning decades,” said Goldman.

The initial results from the team’s research were published in the Astrophysical Journal, and Sankrit is presenting research focused on the 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, 11 January 2024, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad12c9

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking 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, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

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