Astronomers have recently revisited one of the most peculiar binary star systems in our galaxy, shedding new light on a cosmic phenomenon that unfolded four decades ago. The star system in question is HM Sagittae (HM Sge), which burst onto the scene as a bright and long-lived nova back in 1975. Unlike typical novae that flare up and then fade away within months or years, HM Sge has maintained its luminosity for an astonishing 40 years.
The Unusual Behavior of HM Sge
HM Sge is a symbiotic star, a captivating celestial dance between two stellar companions: a white dwarf and a bloated, dust-producing giant star. These two stars orbit each other in an eccentric dance, with the white dwarf siphoning gas from its companion.
As the gas accumulates around the white dwarf, it forms a searing-hot disk. Occasionally, this disk undergoes a spontaneous thermonuclear explosion when the infalling hydrogen from the giant star reaches a critical density, resulting in a dazzling display of fireworks in space.
A Nova That Defied Expectations
Between April and September 1975, HM Sge experienced an unprecedented surge in brightness, growing 250 times brighter than its usual state. What makes this event truly remarkable is that HM Sge didn’t fade away as expected. Instead, it continued to shine brightly for decades, defying the typical nova behavior.
Recent observations reveal that the system has become even hotter, yet paradoxically faded slightly. The mystery deepens as astronomers grapple with the underlying physics and dynamics of this binary system.
Hubble’s Spectacular Insights
In 2021, a team led by astronomers Steven Goldman and Ravi Sankrit used instruments on NASA’s Hubble Space Telescope and the retired SOFIA (Stratospheric Observatory for Infrared Astronomy) to investigate HM Sge.
Their ultraviolet data revealed a strong emission line of highly ionized magnesium, absent in earlier spectra from 1990. This finding indicates that the white dwarf and its accretion disk have heated up significantly, with temperatures exceeding 450,000 degrees Fahrenheit.
The Hubble data provided a stunning glimpse into the heart of this enigmatic binary system, showcasing the power of UV spectroscopy in unraveling cosmic mysteries.
“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,” remarked astronomer Ravi Sankrit of the Space Telescope Science Institute (STScI) in Baltimore.
As we continue to study HM Sge, its enduring brilliance challenges our understanding of stellar evolution and the intricate interplay between companion stars. The cosmic ballet of HM Sge reminds us that the universe still holds surprises, waiting to be unveiled by our telescopic eyes.
