John Foster/Science Photo Library

Astronomers say that they have discovered the first example of a long-sought cosmic oddity: a bloated, dying star with a surprise in its core — an ultradense neutron star.

Such entities, known as Thorne-Zytkow objects, are theoretically possible but would alter scientists' understanding of how stars can be powered. Since Thorne-Zytkow objects were first proposed in 19751, researchers have occasionally offered up candidates, but none have been confirmed.

The latest work, reported on 6 January at a meeting of the American Astronomical Society outside Washington DC, focuses on a red supergiant star in the Small Magellanic Cloud, a neighbouring galaxy to the Milky Way. The star is enriched in lithium, rubidium and molybdenum. Elevated amounts of these elements are thought to arise as by-products of Thorne-Zytkow objects, which have to burn through unusual nuclear fusion pathways.

“What we found is the most compelling observational evidence for this model of stellar interior,” said Emily Levesque, an astronomer at the University of Colorado in Boulder. Levesque, who led a survey of 22 red supergiants using the Magellan telescopes in Chile, declined to name the star with the special elemental signature, because the results have not yet been published in a peer-reviewed journal.

Formation theories

Theorists have proposed several ways in which a Thorne-Zytkow object could form, but the most likely scenario involves a red giant swallowing an orbiting neutron star. There could be several to tens of them in the Milky Way, says Philipp Podsiadlowski, a stellar astrophysicist at the University of Oxford, UK. The presence of a neutron star — the corpse of a normal star left behind after a supernova — would disrupt the usual thermonuclear fires.

“The object is an excellent candidate, although it is perhaps not an open-and-shut case,” says Podsiadlowski. There is not quite as much of the three enriched elements as expected, he says.

The existence of such objects was first proposed by Kip Thorne, an astrophysicist at the California Institute of Technology in Pasadena, and Anna Zytkow, an astrophysicist at the University of Cambridge, UK. Thorne says that the star is the strongest candidate yet found.