Nova explosions are among the most fascinating and energetic events in the universe, occurring within binary star systems where a white dwarf—a compact remnant of a once-massive star—continuously siphons matter from a neighboring companion. This process results in a dramatic thermonuclear eruption once enough material accumulates on the white dwarf’s surface, reaching temperatures that trigger a sudden and powerful outburst. These eruptions are usually one-time events in observable timescales, but some novae have been found to erupt multiple times, earning them the classification of recurrent novae. Their study provides valuable insight into stellar evolution, nuclear reactions, and the impact of environmental conditions on astrophysical phenomena.
Unlike typical novae, which erupt once and fade away, recurrent novae undergo multiple explosions over varying intervals, ranging from a single year to several decades. These rare objects are of particular interest to astronomers because they offer a window into the processes governing mass transfer in binary systems and the conditions necessary for repeated thermonuclear detonations. While only a handful of recurrent novae have been identified within the Milky Way, a significantly larger number have been observed in extragalactic environments—those beyond our galaxy. Studying extragalactic novae is crucial in understanding how different galactic conditions, such as metallicity and interstellar medium properties, influence nova activity.
One of the most remarkable discoveries in the field of recurrent novae was LMC 1968–12a (LMC68), the first extragalactic recurrent nova ever observed. It resides in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, and has a remarkably short eruption cycle of just four years—the third-shortest known among all recurrent novae. LMC68 consists of a white dwarf and a red subgiant companion, a star significantly larger than the Sun. First identified in 1968, its eruptions have been systematically recorded since 1990, making it one of the best-studied recurrent novae beyond our galaxy.
In August 2024, LMC68 underwent another anticipated eruption, captured first by the Neil Gehrels Swift Observatory, which has been monitoring it closely since its last recorded explosion in 2020. Given its known four-year cycle, astronomers were eagerly waiting for this event, and LMC68 erupted precisely as predicted. This provided an excellent opportunity for scientists to conduct detailed follow-up observations and analyze the eruption with advanced telescopes and instruments.
Approximately nine days after the eruption, astronomers used the Magellan Baade Telescope at the Carnegie Institution to perform spectroscopic observations, allowing them to examine the nova’s high-energy emissions. Later, 22 days after the explosion, the Gemini South telescope, part of the International Gemini Observatory, conducted further spectroscopic analysis. These observations focused on the near-infrared light emitted by LMC68, capturing critical details about the high-energy processes occurring during the explosion’s most intense phase.
The use of spectroscopy enabled astronomers to study the nova’s chemical composition and extreme temperatures. During the early post-eruption phase, LMC68’s light faded rapidly, but Gemini South’s FLAMINGOS-2 instrument still detected a strong signal from ionized silicon atoms, particularly those stripped of nine of their 14 electrons. This degree of ionization requires an extraordinary amount of energy, either from intense radiation or violent particle collisions, suggesting that LMC68’s explosion was exceptionally powerful.
One of the most astonishing findings came from the earlier spectrum captured by Magellan, which revealed that the ionized silicon emission alone was 95 times brighter than the total light output of the Sun across all wavelengths—including X-ray, ultraviolet, visible, infrared, and radio. While the signal had faded by the time Gemini South observed it a few days later, silicon emission still dominated the nova’s spectrum, standing out as the most significant feature.
“This ionized silicon, shining at nearly 100 times the brightness of the Sun, is truly unprecedented,” said Tom Geballe, an emeritus astronomer at NOIRLab and co-author of the research published in the Monthly Notices of the Royal Astronomical Society. “What’s equally surprising is not just what we see, but what we don’t see.”
Typically, novae in the Milky Way exhibit a wide range of near-infrared signatures from various highly-excited elements. However, LMC68’s spectra contained only the ionized silicon feature. “Normally, we would expect to detect additional signatures from elements such as sulfur, phosphorus, calcium, and aluminum,” explained Geballe.
“This unexpected absence, coupled with the extraordinary strength of the silicon emission, suggests an unusually high temperature in the expelled gas,” added Sumner Starrfield, Regents Professor of Astrophysics at Arizona State University. Their analysis confirmed that during the nova’s early phase, the ejected material reached a temperature of 3 million degrees Celsius (5.4 million degrees Fahrenheit)—making it one of the hottest novae ever recorded.
This extreme temperature hints at a highly violent eruption, leading astronomers to theorize that the unique environment of the Large Magellanic Cloud plays a major role in LMC68’s exceptional characteristics.
The Large Magellanic Cloud (LMC) differs from the Milky Way in several key ways, one of the most important being its low metallicity—a term astronomers use to describe the abundance of elements heavier than hydrogen and helium. In high-metallicity environments like the Milky Way, these heavier elements help trap heat on the white dwarf’s surface, causing nova eruptions to occur earlier in the accretion process.
However, in low-metallicity systems such as the LMC, fewer heavy elements are present to retain heat, meaning more material accumulates on the white dwarf before reaching the critical temperature needed for ignition. As a result, the explosion is far more violent, releasing an immense amount of energy. Additionally, when the ejected gas collides with the extended atmosphere of the companion red subgiant, it produces powerful shock waves that further elevate temperatures, intensifying the explosion.
Interestingly, before these observations were conducted, Starrfield had predicted that the accretion of low-metallicity material onto a white dwarf would lead to more extreme nova explosions. The new data collected from LMC68 strongly supports this hypothesis, marking a significant step forward in our understanding of how chemical composition influences stellar explosions.
“With only a handful of recurrent novae discovered within the Milky Way, our knowledge of these systems has progressed slowly,” explained Martin Still, an NSF program director for the International Gemini Observatory. “Expanding our search beyond our galaxy using cutting-edge telescopes like Gemini South allows us to dramatically increase our data and study how these objects behave in different chemical environments.”
By studying extragalactic novae, scientists can gain a deeper understanding of how nova eruptions are influenced by their surroundings, shedding light on the evolution of binary star systems, the role of metallicity in stellar processes, and even the potential fate of white dwarfs in different galactic environments.
As astronomical technology advances, more extragalactic novae will be detected and studied with increasing precision. The case of LMC68 demonstrates that these stellar explosions are not just localized events but key to understanding the broader physics that govern stellar evolution across the cosmos. Future observations will likely continue to reveal surprising and unprecedented phenomena, pushing the boundaries of our knowledge about the dynamic and explosive universe we inhabit.
More information: A Evans et al, Near-infrared spectroscopy of the LMC recurrent nova LMCN 1968-12a, Monthly Notices of the Royal Astronomical Society (2024). DOI: 10.1093/mnras/stae2711