A STAR WITHIN A STAR?
We speak to astrophysicist Cole Miller about hypothetical hybrid stars known as Thorne–żytkow objects
What is a Thorneżytkow object? A hypothetical object in which a red giant or supergiant – with a radius on the order of the Earth-sun distance – contains a neutron star in its core. Neutron stars and regular stars can orbit each other, and the suggestion is that when the regular star evolves into the red giant phase it might sometimes swallow the neutron star.
What’s the history behind this weird-sounding idea?
An early suggestion was by the great Soviet physicist Lev Landau, who proposed in 1938 that a small neutron star core at the centre of normal stars could provide their power sources. That turns out not to work, but in 1977 Kip Thorne and Anna Żytkow of the California Institute of Technology made the suggestion that bears their names. Landau hoped that the idea would be amazing enough to save him from arrest as a dissident in Stalinist Russia, but it didn’t work and he spent a year in prison.
Are these objects purely theoretical, or have they actually been observed?
The evidence isn’t clear because differences from standard red giant stars are subtle. As Thorne and Żytkow noted, such stars “are thoroughly hidden from the prying eyes of the astronomer by the huge, tenuous red-giant envelope”. It could be that the abundances of certain isotopes would be different than normal in such stars. There was a report in 2014 that a star named HV 2112 has anomalous abundances of the element rubidium and the expected very high luminosity – about 100,000 times that of the Sun. However, in 2018 a reanalysis by another group found a lower luminosity and no excess of rubidium. At the same time, the 2018 paper proposed its own stellar candidate, HV 11417, so there is still hope.