HM Sagittae’s Historic Nova Event

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In 1975, the binary star system HM Sagittae (HM Sge) underwent a dramatic increase in brightness, becoming 250 times brighter and capturing global attention. Unlike typical novae, which fade within months or years, HM Sge’s brightness persisted for decades. Recent observations by NASA’s Hubble Space Telescope and the retired Stratospheric Observatory for Infrared Astronomy (SOFIA) have revealed new insights into this unusual star system, which consists of a white dwarf and a giant companion star.
The white dwarf in HM Sge consumes gas from the giant star, forming a hot disk around it. This can lead to a thermonuclear explosion when enough hydrogen accumulates. Unlike typical novae, HM Sge’s luminosity has remained high for decades, and recent data show that while the system has become hotter, it has also paradoxically dimmed. SOFIA’s findings indicated that the giant star resumed normal activity shortly after the explosion but has mysteriously dimmed recently.
One notable discovery is the detection of water molecules moving at high speeds, indicating a dynamic and complex environment around the white dwarf. The gaseous bridge between the two stars spans 2 billion miles. Continuous monitoring and collaboration with organizations like the American Association of Variable Star Observers (AAVSO) have been crucial in studying this rare and valuable astrophysical phenomenon. The research underscores the importance of ongoing observation and the intricate nature of binary star systems.