How tides, mergers and thermonuclear explosions forge and disperse iron across the Universe
Type Ia supernovae are among the brightest stellar explosions in the Universe. They occur when a white dwarf (the compact remnant of a Sun-like star) undergoes runaway thermonuclear burning in a binary system. These explosions are major producers of iron (the same in our blood!) in the Universe and play a central role in the chemical evolution of galaxies.
The traditional picture involved a white dwarf steadily accreting gas from an ordinary companion until it approached the Chandrasekhar mass, about 1.4 times the mass of the Sun. However, observations suggest that this “single-degenerate” route cannot explain most Type Ia supernovae. Recurrent helium flashes may also eject part of the accreted material, making it difficult for all but relatively massive white dwarfs to reach the required mass.
Two white dwarfs, two paths to explosion
Many Type Ia supernovae may instead originate from systems containing two white dwarfs. Gravitational waves gradually shrink their orbit until the less massive star fills its gravitational boundary and begins transferring matter. Because a white dwarf expands as it loses mass, this transfer can rapidly become unstable: the smaller star is torn apart by tidal forces.
What happens next is crucial. A prompt detonation during this violent interaction produces a “sub-Chandrasekhar” explosion: although the combined binary may exceed the Chandrasekhar mass, the exploding white dwarf has not settled into a single object near that limit. Alternatively, the disrupted companion may form a hot disc or envelope around the surviving white dwarf. If this material is accreted more gradually, the primary can approach the Chandrasekhar mass and explode later.
Cosmic factories of essential elements
The iron is not produced directly. During the explosion, carbon and oxygen burn at enormous temperatures, creating large quantities of radioactive nickel-56. This decays first into cobalt-56 and then into iron-56. Sub-Chandrasekhar explosions can also produce substantial iron, but near-Chandrasekhar events reach higher densities and are especially important for neutron-rich isotopes such as manganese and stable nickel. And the list is not complete… Type Ia supernovae also produce calcium, the same material in our own bones (although core-collapse supernovae are probably the dominant Galactic source) along with significant amount of silicon, sulfur, copper and much more.
References
Ruiter, A.J., Seitenzahl, I.R. Type Ia supernova progenitors: a contemporary view of a long-standing puzzle. Astron Astrophys Rev 33, 1 (2025).
U. Battino, J. D. Keegans, M. Allen, F. K. Röpke, F. Herwig, A. Best, R. Hirschi, L. Piersanti, O. Straniero, S. A. Sim, C. Travaglio and P. A. Denissenkov; A&A, 703 (2025)
