Red Supergiant Remnants from Luminous Red Nova Mergers Revealed by JWST

Luminous Red Novae: When Stars Collide and Explode

The Mystery Resolved: Observing Merger Remnants Through Dust

1. Why Previous Observations Failed to Reveal the Remnant

   Answering the question of what remains after a luminous red nova has proven remarkably challenging. The fundamental obstacle is dust. When two stars merge, the explosion ejects vast quantities of stellar material—dust equivalent to 300 times the entire mass of Earth. This ejected dust forms a massive, optically thick cloud that completely obscures the underlying stellar remnant. The densest, brightest phase of a luminous red nova is actually produced by light from the hot, expanding shell of ejected material, not by the remnant star beneath it. This bright shell, although temporary, obscures the view of what lies beneath. As the ejected material expands and cools over years to decades, the cloud becomes increasingly transparent, but new dust particles continuously form as the cooling gas undergoes chemical reactions. By the time the dust cloud finally becomes transparent enough to see through it, the initial brightness has faded dramatically. Conventional telescopes observing in visible light could not penetrate this dust. Older infrared instruments, like the Spitzer Space Telescope, could partially see through the dust but lacked the resolution to distinguish individual stars or measure their temperatures precisely. It required the unprecedented infrared sensitivity and resolution of the James Webb Space Telescope to finally pierce through the dust and reveal the merged remnants with sufficient clarity to characterize them.

2. JWST's Infrared Vision and Dust Penetration

   The James Webb Space Telescope is uniquely positioned to observe objects obscured by dust. Designed to observe the infrared universe, JWST observes at wavelengths where dust is largely transparent. Dust grains are efficient absorbers of visible light—the grains are comparable in size to visible-light wavelengths—but at infrared wavelengths, especially the longer infrared wavelengths where dust becomes increasingly transparent, JWST can observe even heavily obscured sources. The JWST's extraordinary sensitivity means that even weak infrared radiation from cool objects far away can be detected. By combining JWST infrared observations with visible-light images from the Hubble Space Telescope and data from the Spitzer Space Telescope, the research team created a comprehensive multi-wavelength picture of the luminous red novae and their environment. For AT 2011kp, observations were conducted twelve years after the initial merger explosion in 2011. By 2023-2024, when JWST turned its gaze on the system, sufficient time had elapsed that the densest dust shell had partially dissipated. The infrared observations revealed the underlying star: a cool, massive, highly luminous red supergiant. For AT 1997bs, observations spanned 27 years after the merger event. With nearly three decades of cooling and dust dispersal, the stellar remnant was even clearer to observe. In both cases, the infrared data combined with visible and ultraviolet observations revealed the chemical composition of the surrounding dust.

3. Spectroscopic Analysis and Dust Chemistry

   By analyzing the spectrum—the intensity of light as a function of wavelength—from the luminous red novae, the research team could identify the chemical composition of the dust surrounding the merger remnants. The spectra showed prominent absorption and emission features characteristic of specific molecules. Analyzing these spectral features, the team identified the presence of graphite and other carbon-based compounds (carbonaceous dust) as the dominant components of the dust around both AT 2011kp and AT 1997bs. These carbon-rich compounds are crucial for understanding stellar nucleosynthesis—the process by which massive stars synthesize heavy elements during their lifetimes. In the early universe, the only elements present were hydrogen, helium, and trace amounts of lithium. All heavier elements—carbon, oxygen, nitrogen, iron, and beyond—were synthesized by stars and subsequently ejected into space, where they became incorporated into new generations of stars and planets. The discovery that luminous red novae produce significant quantities of carbon-rich dust reveals that these merger events play a more important role in cosmic chemical enrichment than previously appreciated. With estimated frequencies of 0.1-several per galaxy per year, these events collectively contribute substantial amounts of carbon to interstellar space.

Analysis I: Red Supergiant Remnants and Theoretical Predictions

1. Surprising Mismatch Between Theory and Observation

   The discovery of red supergiant remnants from luminous red nova mergers came as a significant surprise to the astrophysical community. "We didn't expect to find this type of object as a result of the merger," Andrea Pastorello, a team member at INAF, stated. "Rather, we would have expected that the system, going from two stars of a certain mass to a single one with a mass almost equal to the sum of the two (net of the material expelled by the collision), would have stabilized on a hotter and more compact source." Theoretical models had predicted that when two stars merge, the resulting object would settle into a state of hydrostatic equilibrium with properties reflecting the combined mass. For two stars with combined mass of, say, 5-10 solar masses, the theoretical expectation was that the merger remnant would become a hot, compact star—perhaps a blue supergiant or Wolf-Rayet star. Yet the observations revealed a different outcome: the merger remnants appeared as cool, extended red supergiants. Red supergiants are rare, evolved massive stars near the ends of their lives, characterized by their enormous radii (often hundreds of times the Sun's radius) and cool surface temperatures (around 3,000-4,000 Kelvin). The mismatch between theoretical expectation and observation is profound and demands explanation. One possibility is that the theoretical models are incomplete, failing to account for some physical process that causes the merger remnant to expand and cool rather than contracting and heating. Alternatively, the merger process itself may be more complex than simple models assume, with the collision energy and angular momentum playing roles not fully captured in existing theoretical frameworks.

2. Massive and Luminous: Characteristics of the Merger Remnants

   The red supergiant remnants identified by the JWST observations exhibit remarkable properties. Their luminosities—the total power they radiate across all wavelengths—place them among the brightest stars known, exceeded only by the rarest and most massive stars in the universe. Their masses, estimated from orbital dynamics and theoretical stellar models, correspond to the combined masses of the two progenitor stars (minus the material ejected during the explosion). This implies that the merger process conserved the mass of the system—the two stars combined to form a single object containing nearly the combined mass of both progenitors. The radii of the red supergiant remnants are enormous, spanning hundreds to thousands of solar radii. To place this in perspective, if a red supergiant with a radius of 1,000 solar radii were placed at the center of our solar system, its surface would extend nearly to Jupiter's orbit. Such enormous radii are consistent with the extended envelopes of red supergiants, produced by hydrogen burning in a thin shell at the base of a tenuous atmosphere. The color of the remnants—their spectral type—confirms they are indeed cool red supergiants, not hot blue supergiants as theoretical models might have predicted. The infrared colors measured by JWST indicate surface temperatures around 3,000-4,000 Kelvin, cool enough that carbon monoxide and other molecules can condense in the upper atmosphere, producing the observed absorption and emission features in spectra.

3. Implications for Stellar Merger Theory

   The discovery of red supergiant remnants forces a reconsideration of stellar merger theory. Binary star evolution—the processes by which binary systems change over time due to mass transfer, orbital decay, and eventually merger—must explain not only the observation of luminous red novae but also the properties of the merger remnants they produce. One theoretical possibility is that the merger remnants begin in a state of rapid rotation, having acquired angular momentum from the merger process. This rapid rotation could support the massive merged star against gravitational collapse, allowing it to expand to the enormous radii observed. As the merger remnant cools and rotates slower due to magnetic braking, it might eventually contract to more normal dimensions, but the timescales involved could be millions of years or longer. Another possibility involves the role of convection and the structure of the stellar interior immediately following merger. The violent energy release during merger creates a highly mixed, energetic environment. As this energy dissipates and the system reaches equilibrium, the resulting internal structure might favor expansion and cooling rather than contraction and heating. Future theoretical work will be essential to fully understand these mechanisms and reconcile observation with theory.

Analysis II: Carbon Production and the Origin of Life-Essential Elements

1. Graphite and Carbon Compounds: Building Blocks of Life

   The discovery that luminous red nova explosions produce vast quantities of carbon-rich dust—equivalent to 300 Earth masses per event—has profound implications for the chemical composition of the universe and the origins of life itself. Carbon is the central element in all known life. Every living organism—from the smallest bacterium to the largest whale—is fundamentally a collection of carbon-based organic molecules: proteins, nucleic acids, lipids, and carbohydrates. The origin of this carbon traces back to stellar nucleosynthesis: the production of heavy elements in stars. In the cores of massive stars, hydrogen and helium burn through a sequence of nuclear reactions, producing progressively heavier elements: carbon, oxygen, nitrogen, silicon, iron, and beyond. When stars reach the ends of their lives and explode as supernovae, or as in this case, when they merge and explode as luminous red novae, they eject this chemically enriched material into the interstellar medium. The carbon ejected by stars is the ultimate source of the carbon in all living things on Earth. The discovery that luminous red novae produce carbon-rich dust reveals that these merger events are more significant contributors to cosmic carbon enrichment than previously appreciated. The graphite and carbon-based dust particles produced by luminous red nova mergers incorporate into the molecular clouds that collapse to form new stars and planetary systems. Some of this carbon-rich dust becomes incorporated into planetary atmospheres, rocks, and eventually into organic molecules and living organisms.

2. Dust Production and Interstellar Enrichment

   The quantity of dust produced by luminous red nova mergers is staggering. For each event, dust equivalent to 300 times Earth's mass is ejected—a total of roughly 6×10²⁷ grams. Given that luminous red novae occur at rates of 0.1 to several per galaxy per year, and considering the billions of galaxies in the observable universe, the total cosmic rate of luminous red nova dust production is enormous. Over cosmic time, these events cumulatively contribute a significant fraction of the carbon-rich dust that pervades the interstellar medium. This dust plays multiple crucial roles in the cosmos: it absorbs ultraviolet radiation that would otherwise destroy fragile molecules; it catalyzes chemical reactions that produce complex organic molecules; it permits the formation of planets by providing the "seeds" around which dust grains accumulate. By understanding that luminous red nova mergers are major carbon producers, we gain new insight into how the universe self-enriches with the chemical elements necessary for life. The universe began with only hydrogen and helium. Through billions of years of stellar evolution, nuclear burning, and stellar explosions, the universe has progressively accumulated heavier elements—a process called galactic chemical evolution. Luminous red nova mergers, now recognized as significant carbon producers, contribute meaningfully to this process.

3. Implications for Planetary System Formation and Habitability

   The carbon-rich dust ejected by luminous red nova mergers affects planetary systems on multiple scales. On large scales, this dust is incorporated into the interstellar medium and becomes part of the gas and dust clouds from which new star systems form. The carbon abundance in these clouds affects the chemistry of the resulting star systems and the planets that form. On intermediate scales, the dust can directly influence protoplanetary disks—the rotating disks of gas and dust around newly formed stars from which planets condense. Planets rich in carbon (often called "carbon planets") have different properties from planets like Earth, with compositions favoring carbides and graphite rather than silicate rocks. The abundance of carbon in a stellar system thus influences the types of planets that can form and their compositions. On small scales, the carbon-based molecules in this dust are the precursors to complex organic chemistry. When incorporated into primitive planetary atmospheres and supplied with energy (from stellar radiation, lightning, or chemical reactions), carbon-based molecules can undergo reactions that produce the prebiotic organic molecules that preceded the origin of life. The discovery that luminous red novae are major carbon producers thus connects directly to questions about the origin of life and the likelihood of finding life-bearing planets throughout the cosmos. The more carbon-rich dust a planetary system has inherited, the greater the likelihood of complex organic chemistry and the eventual emergence of life.

Discussion: Stellar Mergers as Cosmic Crucibles of Life

1. The Role of Violent Events in Enriching the Cosmos

   The discovery of red supergiant merger remnants surrounded by carbon-rich dust illustrates a profound truth about cosmic evolution: the universe's most violent events are often its most creative. Supernovae, which rip apart stars and disperse their nucleosynthesized elements, are essential for enriching galaxies with heavy elements. Gamma-ray bursts, among the most energetic phenomena known, may play important roles in shaping galactic environments. And now, luminous red nova mergers—the violent collision of two entire stars—emerge as significant contributors to the universe's inventory of carbon and other life-essential elements. This perspective reframes our understanding of stellar mergers from mere curiosities, rare accidents of stellar dynamics, to important cosmic processes that materially shape the chemical composition of the universe and influence the likelihood of life elsewhere. The observation that stellar mergers produce carbon-rich red supergiants, rather than hot compact stars, also offers a reminder of how incomplete theoretical predictions can be. Despite sophisticated computational models of stellar physics, the merger of two stars produces an outcome that surprised experts. This humility before nature motivates continued observational effort to constrain and improve theoretical models.

2. Future Observations and Population Studies

   The current study examined two well-documented luminous red novae observed at different epochs after their mergers. Future work will expand this sample through systematic searches for additional luminous red novae in archival JWST data and through targeted follow-up observations of newly discovered events. By studying a larger population of merger remnants and observing them at various times after merger, astronomers will constrain the properties of these systems—the masses of progenitor stars, the masses of merger remnants, the timescales of dust formation and dispersal, and the chemical compositions of ejected material. Such population statistics will enable tests of theoretical models of stellar mergers and provide essential constraints on the contribution of luminous red novae to galactic chemical enrichment. Additionally, observations of nearby galaxies—the Andromeda Galaxy, the Large and Small Magellanic Clouds, and others—will permit detailed studies of luminous red nova rates and properties across different galactic environments. These population studies promise to transform our understanding of stellar mergers from isolated curiosities to fundamental processes in cosmic evolution.

Conclusion: When Stars Collide, Life's Elements Are Born