Eta Carinae is a binary system containing two of the most massive stars in the Milky Way — approximately 100 and 30 solar masses — orbiting each other in a highly eccentric 5.54-year orbit. Near periastron, when the stars are closest, their supersonic winds collide at velocities exceeding 3000 km/s. The collision zone is a forge: hot enough to emit X-rays, dense enough to produce iron emission, and structured enough to separate into distinct velocity components.
XRISM resolves the iron K-shell emission of eta Carinae into three distinct velocity components for the first time. Hamaguchi, Espinoza-Galeas, and collaborators (arXiv 2602.22476, February 2026) identify: a component at the system velocity (~-8 km/s), a blueshifted component at approximately -420 km/s, and a redshifted component at approximately +180 km/s. Previous X-ray observatories blended these into a single broad feature.
The three-component structure maps the wind-wind collision geometry. The central component traces the stagnation point where the two winds meet head-on and the gas is nearly stationary. The blueshifted component traces the primary wind (from the 100 solar mass star) being deflected toward the observer after passing through the shock. The redshifted component traces the secondary wind deflected away.
The iron emission comes from different ionization states in different components — the temperature varies across the collision zone, from the hottest gas at the stagnation point to cooler material in the deflected flows. The velocity decomposition is also a temperature map, which is also a geometry map. Each spectral component encodes both where in the collision zone the gas sits and what physical conditions prevail there.
The wind-wind collision is a particle accelerator, a calorimeter, and a spectrometer simultaneously. XRISM's 5 eV resolution turns the merged glow of previous observations into a three-dimensional map of a stellar-wind laboratory that no terrestrial facility can replicate.