The familiar structure of the Lyra Nebula (or Ring Nebula) has just revealed an unexpected feature. At the heart of this celestial jewel, well-known to astronomers, lies a rectilinear formation composed of iron atoms, whose origin constitutes a genuine enigma for astrophysicists. This discovery, made possible by an innovative instrument, calls into question established models about the end of life of stars similar to our Sun.
This image of the Ring Nebula, taken by the Hubble Space Telescope (NASA/ESA) in 2013, has since become one of the observatory's most famous.
An international team identified this bar of ionized iron using the WEAVE spectrograph, recently installed on the William Herschel Telescope in the Canary Islands. This instrument captures, for the first time, a complete light spectrum across the entire surface of the nebula. By analyzing this data, researchers isolated the spectral signature of iron, revealing a linear geometry that contrasts with the overall ring-like structure. This singular configuration indicates a formation process distinct from that of the rest of the surrounding gas.
The characteristics of an unprecedented discovery
The iron bar is located in the inner region of the Lyra Nebula. Its dimension is colossal, equivalent to several hundred times the orbit of Pluto around the Sun (approximately 0.25 light-years or about 1.5 trillion miles). The total mass of iron is estimated to be of the same order of magnitude as that of the planet Mars. This significant concentration in such a localized form puzzles scientists.
Unlike the other chemical elements detected in the nebula, whose distribution follows the ring or halo shape, the iron adopts exclusively this bar-like form. The observations, published in
Monthly Notices of the Royal Astronomical Society, indicate the absence of other elements sharing this same structure.
The contours of the recently discovered iron structure.
Credit: NASA/ESA
The technology of the WEAVE spectrograph was decisive. Its "integral field" mode allows for the simultaneous acquisition of thousands of spectra across the entire object, creating a three-dimensional chemical map. This approach unveiled details previously lost in global observations. The researchers indicate that without this capability of integral field spectral imaging, the discovery would have been impossible.
The hypotheses to explain the origin of the iron
The first considered clue links the bar to the ejection mechanisms of the dying star's outer layers. During its final phase, the star undergoes pulsations and stellar winds of different speeds and temperatures. The bar could materialize a shear structure or a particular interaction between these gaseous flows, where the iron became trapped and ionized.
A second, more speculative hypothesis involves the destruction of a planetary body. The star, as it transformed into a red giant, might have swallowed and vaporized a rocky planet in orbit. However, the mechanisms explaining the linear and confined shape of the debris remain to be elucidated.
To decide between these scenarios, scientists plan new observations with higher spectral resolution. The goal is to detect any other chemical elements associated with this bar, such as nickel or silicon, whose presence would point toward a planetary origin. The search for similar objects in other planetary nebulae is also underway to verify if this phenomenon is unique or common.
Article author: Cédric DEPOND