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Star exploded into a brilliant nova 40 years ago and has still not dimmed

In 1975, the binary star system HM Sagittae (HM Sge) experienced an extraordinary event. The system grew 250 times brighter, captivating astronomers worldwide. Unlike typical novae that fade within months or years, HM Sge’s brightness persisted for decades.

Recent observations using NASA’s Hubble Space Telescope and the retired Stratospheric Observatory for Infrared Astronomy (SOFIA) have unveiled surprising details about an enigmatic star system.

HM Sagittae and symbiotic star systems

A nova occurs when a white dwarf star ingests gas from its companion, causing a sudden increase in brightness.

Symbiotic stars, such as HM Sge, consist of a white dwarf and a giant companion star in an eccentric orbit around each other.

In this system, the white dwarf consumes gas from the giant star. This process creates a hot disk of material around the white dwarf.

When the hydrogen gas from the giant star accumulates on the white dwarf’s surface, it can reach a critical density, igniting in a thermonuclear explosion.

These interactions between the stars provide valuable insights into the processes of stellar evolution and the dynamics of binary star systems.

Hubble captures HM Sagittae system

HM Sge’s 1975 outburst transformed it from an obscure star into a focal point for astronomers.

“In 1975 HM Sge went from being a nondescript star to something all astronomers in the field were looking at,” noted Ravi Sankrit of the Space Telescope Science Institute (STScI).

Unlike typical novae, HM Sge’s brightness did not diminish rapidly. Instead, it maintained its luminosity for decades, puzzling scientists.

Symbiotic star Mira HM Sge in nova system HM Sagittae. Credit: NASA
Symbiotic star Mira HM Sge in nova system HM Sagittae. Credit: NASA

In 2021, Steven Goldman of STScI, Sankrit, and collaborators revisited HM Sge using Hubble and SOFIA. They found that the system had become hotter but paradoxically dimmer.

The new ultraviolet (UV) data from Hubble revealed a strong emission line of highly ionized magnesium, absent in earlier spectra from 1990.

This indicated an increase in temperature from less than 400,000 degrees Fahrenheit in 1989 to over 450,000 degrees Fahrenheit now.

“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,” said Sankrit.

Hubble and SOFIA reveal binary star secrets

SOFIA’s observations provided a deeper look into the complex environment around HM Sagittae.

Infrared spectral data indicated that the giant companion star, the source of the outflowing gas and dust, quickly recovered from the nova-like explosion, resuming its normal activity within a few years.

However, it has mysteriously dimmed in recent times, posing another enigma for astronomers to unravel.

One of the most intriguing findings from SOFIA is the detection of water molecules moving at a remarkable speed of 18 miles per second.

Star HM Sagittae pulling material from its red giant companion star. Credit: NASA
Star HM Sagittae pulling material from its red giant companion star. Credit: NASA

This rapid motion is likely indicative of the swirling accretion disk surrounding the white dwarf, where the gas from the giant star accumulates and fuels the system’s activity.

The immense scale of this stellar interaction is highlighted by SOFIA’s data, revealing that the gaseous bridge connecting the giant star and the white dwarf stretches an astonishing 2 billion miles across.

These findings underscore the dynamic and intricate nature of HM Sagittae and the complex processes at play within this extraordinary binary system.

Additional collaborations

The team also collaborated with the American Association of Variable Star Observers (AAVSO) and amateur astronomers worldwide. Their continued monitoring revealed changes that had not been detected since the 1975 outburst.

“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.

Implications and future research

The findings from HM Sge offer valuable insights into the physics of binary star systems. The team’s research and Sankrit’s presentation at the 244th meeting of the American Astronomical Society highlight the importance of continuous observation.

The Hubble Space Telescope, a project of international cooperation between NASA and European Space Agency (ESA), has been instrumental in these discoveries.

HM Sagittae’s story is a testament to the ever-evolving nature of the universe. From its unexpected outburst in 1975 to the recent discoveries of its increasing temperature and changing dynamics, HM Sge continues to captivate astronomers.

The study is published in The Astrophysical Journal.


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