Free Astronomy Magazine May-June 2025

39 MAY-JUNE 2025 ASTRO PUBLISHING ionized silicon atoms, specifically sil- icon atoms that have been stripped of nine of their 14 electrons, which requires incredible amounts of en- ergy in the form of radiation or vio- lent collisions. In the earlier spectrum from Magel- lan, the near-infrared light from just the ionized silicon alone shined 95 times brighter than the light emit- ted by the Sun added up across all its wavelengths (X-ray, ultraviolet, visible, infrared, and radio). When Gemini observed the line several days later the signal had faded, but the silicon emission still dominated the spectrum. “The ionized silicon shining at al- most 100 times brighter than the Sun is unprecedented,” says Tom Geballe, NOIRLab emeritus as- tronomer and co-author of the paper appearing in the Monthly No- tices of the Royal Astronomical So- ciety. “And while this signal is shocking, it’s also shocking what’s not there.” Novae found in the Milky Way typi- cally emit numerous near-infrared signatures from highly-excited ele- ments, but LMC68’s spectra con- tained only the ionized silicon feature. “We would’ve expected to also see signatures of highly ener- gized sulfur, phosphorus, calcium and aluminum,” says Geballe. “This surprising absence, combined with the presence and great strength of the silicon signature, implied an un- usually high gas temperature, which our modeling confirmed,” adds co- author Sumner Starrfield, Regents Professor of Astrophysics at Arizona State University. The team estimates that, during the nova’s early post- explosion phase, the temperature of the expelled gas reached 3 million degrees Celsius (5.4 million degrees Fahrenheit), making it one of the hottest novae ever recorded. This extreme temperature suggests a highly violent eruption, which the A team of astronomers have for the first time observed a recurrent nova out- side of the Milky Way in the near-infrared wavelength range. This nova, named LMC 1968-12a, is located in the Large Magellanic Cloud — a satellite galaxy of the Milky Way. It erupts about every four years, which is the third- shortest recurrence timescale of any nova. The team estimates that, during the nova’s early post-explosion phase, the temperature of the expelled gas reached 3 million degrees Celsius (5.4 million degrees Fahrenheit), making it one of the hottest nova ever recorded. This video summarizes the discovery. [Images and Videos: International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick, M. Za- mani,ESO/L. Calçada, M. Kornmesser, P. Horálek (Institute of Physics in Opava), N. Bartmann (NSF NOIRLab) − Music: Stellardrone - Airglow] ! team theorizes is due to the condi- tions of the nova’s environment. The Large Magellanic Cloud and its stars have a lower metallicity than the Milky Way, meaning it contains a lower abundance of elements heavier than hydrogen and helium, referred to as metals by as- tronomers. In high-metallicity sys- tems, heavy elements trap heat on the white dwarf’s surface such that eruptions occur early in the accre- tion process. But without these heavy elements, more matter builds up on the white dwarf’s surface be- fore it gets hot enough to ignite, causing the explosion to erupt with far greater violence. Additionally, the expelled gas collides with the atmosphere of the companion red subgiant, causing a huge shock that elevates the temperatures in the collision. Prior to collecting their data, Starrfield predicted that the accretion of low-metallicity material onto a white dwarf would result in a more violent nova explosion. The observations and analysis presented here are broadly in agreement with that prediction. “With only a small number of recur- rent novae detected within our own galaxy, understanding of these ob- jects has progressed episodically,” says Martin Still, NSF program di- rector for the International Gemini Observatory. “By broadening our range to other galaxies using the largest astronomical telescopes available, like Gemini South, as- tronomers will increase the rate of progress and critically measure the behavior of these objects in differ- ent chemical environments.”