A Cosmic Necklace with a Carbon Conundrum express
Deep in space, there's a cloud of gas that looks like a necklace with bright beads. This is the Necklace Nebula — the remains of a star that once swelled up and swallowed its neighbor. At the center, a pair remains: a small, hot star and its bloated companion, rich in carbon.
Scientists pointed the Hubble Space Telescope at the nebula and measured how much carbon lies in its inner region. It turned out that carbon is scarce there, as if someone had scooped it out with a spoon. But the companion star nearby is literally stuffed with carbon. How did that happen?
Likely, the first star expelled carbon-rich gas before its death, and the companion pulled it in. But then why is the inner part of the nebula poor in carbon? Perhaps that gas drifted farther away, leaving the center "empty." Or we are only seeing part of the picture.
This mystery is like a detective story: the clues contradict each other, but together they piece together a tale of how one star gave its substance to another. And how, from that chaos, a beautiful cosmic necklace was born.
Original abstract
as written by the authors arXiv:2601.19382
Context: The Necklace nebula is a bipolar, post-common-envelope planetary nebula, the central star of which has been shown to have a dwarf carbon star companion. Aims: We aim to understand the origins of the Necklace and its dwarf carbon central star. Methods: We study the carbon abundance of the nebula through far ultraviolet spectroscopy obtained with the Hubble Space Telescope. Furthermore, through simultaneous modelling of multiband light and velocity curves, we attempt to constrain the parameters of the central star system. Results: Puzzlingly, we find that the region of the inner nebula observed with the Hubble Space Telescope is seemingly not carbon-rich, at odds with the dwarf carbon star nature of the companion of the central star. The initial mass of the nebular progenitor was likely very close to the limit to become carbon-rich, perhaps experiencing a very late thermal pulse. The dwarf carbon star companion is found to be significantly inflated with respect to that expected for an isolated main sequence star of the same mass. Conclusions: The properties of the central binary are consistent with the progenitor having become carbon-rich and its companion having accreted a significant amount of that carbon-enriched material. However, it is unclear how this evolutionary hypothesis can be reconciled with the inner nebula potentially being carbon poor.
The Necklace Nebula is located about 15,000 light-years from Earth in the constellation Sagitta.
In Stanislaw Lem's novel "Solaris," the planet's ocean also behaves unexpectedly, much like this nebula hiding its secrets.