Hubble Uncovers Surprises Around a Star That Erupted 40 Years ago

By Lydia Amazouz Published on June 11, 2024 01:30
Hubble Uncovers Surprises Around A Star That Erupted 40 Years Ago

Astronomers have revisited one of the strangest binary star systems in our galaxy, HM Sagittae (HM Sge), using new data from NASA’s Hubble Space Telescope and the retired SOFIA (Stratospheric Observatory for Infrared Astronomy).

This star system burst onto the scene 40 years ago as a bright and long-lived nova. Recent observations have revealed intriguing changes in the system's behavior and characteristics, providing new insights into stellar evolution.

The Unique Nature of HM Sagittae

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. The white dwarf ingests gas flowing from the giant star, forming a blazing hot disk around itself.

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This disk 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 dramatic interactions between companion stars provide valuable information about the physics and dynamics of stellar evolution in binary systems. Ravi Sankrit, an astronomer at the Space Telescope Science Institute (STScI), remarked, "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."

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

Observations Over the Decades

Between April and September 1975, HM Sagittae increased in brightness by 250 times. Unlike typical novae that fade rapidly, this system has maintained its luminosity for decades. In 2021, researchers including Steven Goldman of STScI and Ravi Sankrit used instruments on Hubble and SOFIA to observe what had changed with HM Sge in the last 30 years.

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The ultraviolet data from NASA's Hubble showed a strong emission line of highly ionized magnesium, indicating that the temperature of the white dwarf and accretion disk increased from less than 400,000 degrees Fahrenheit in 1989 to more than 450,000 degrees Fahrenheit now.

Insights From SOFIA and Infrared Observations

Data from SOFIA, which retired in 2022, provided additional insights into the system. The team detected water, gas, and dust flowing in and around HM Sge. Infrared spectral data showed that the giant star, which produces copious amounts of dust, returned to its normal behavior within only a couple of years of the explosion.

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However, it has dimmed in recent years, adding another puzzle for astronomers to solve. SOFIA also revealed water moving at around 18 miles per second, likely the speed of the accretion disk around the white dwarf. The bridge of gas connecting the giant star to the white dwarf spans approximately 2 billion miles.

Continuous Monitoring and Future Research

The team has collaborated with the AAVSO (American Association of Variable Star Observers) to engage amateur astronomers worldwide in monitoring HM Sge. Their observations have revealed changes that haven't been seen since the star's 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 Steven Goldman.

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The initial results from the team's research were published in the Astrophysical Journal, with further findings to be presented at the 244th meeting of the American Astronomical Society in Madison, Wisconsin. The Hubble Space Telescope, operational for over three decades, 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 supports mission operations at Goddard, and the Space Telescope Science Institute in Baltimore, Maryland, conducts Hubble science operations for NASA.

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