This supernova remained brilliantly bright 40 years after its explosion

An unusual event took place in our cosmic space in the year 1975. The binary star system HM Sagittae (HM Sge) grew 250 times brighter and captured the attention of astronomers across the world. 

A supernova's brightness fades away within months or years, however, the brightness of HM Sge had unusually persisted for decades.

The retired Stratospheric Observatory for Infrared Astronomy (SOFIA) and NASA’s Hubble Space Telescope recently observed the enigmatic star system and unveiled the surprising details.

A supernova happens when a gas from its companion is ingested in a white dwarf and causes a sudden increase in brightness.

Symbiotic stars like HM Sge have a white dwarf and a giant companion star which are placed in an eccentric orbit near each other.

The gas from the giant star was consumed by the white dwarf in this system. In this process,  a hot disk of material was created around the white dwarf.

When the giant star's hydrogen gas accumulates on the surface of the white dwarf, it can reach a critical density and lead to the ignition in a thermonuclear explosion.

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

In 1975, HM Sge’s outburst happened and it transformed from an obscure star to 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,” emphasised Ravi Sankrit of the Space Telescope Science Institute (STScI).

The brightness of HM Sge did not diminish rapidly unlike typical novae. For decades, the supernova maintained its luminosity which puzzled scientists.

With the use of the Hubble Telescope and SOFIA, Steven Goldman of STScI, Sankrit as well as the collaborators revisited HM Sge in 2021. The scientists figured that the system had turned hotter but paradoxically dimmer.

According to Hubble's new ultraviolet (UV) data, a strong emission line of highly ionized magnesium was absent in earlier spectra from 1990.

“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,” Sankrit said. 

The star, however, became mysteriously dimmer in recent times, which has now left astronomers curious and more puzzled.