What happens when you put a star inside a star?

Throwing one star into another into another star does not bode well for either star. However, given the right conditions and the right types of stars this can lead to the stars merging and forming one single object. If one of the stars is a neutron star (the dense stellar remnant after a supernovae) it can sink to the center of the other star replacing that star’s core. Such objects are called Thorne-Żytkow objects (TŻOs) as they where first proposed by Kip Thorne and Anne Żytkow. Now an international team of astrophysicists led by the Max Planck Institute for Astrophysics (MPA) has re-evaluated what these TŻOs look like and whether we can find them.

Irrespective of the original star, the resulting object would expand and become a red supergiant, an object so large that it would consume Jupiter if our Sun became such a supergiant. This gigantic object would then be supported by the neutron star at its center, which is the size of the city of Munich. For the first time, a team at MPA  was able to simulate these objects in detail and determined what they would look like.

Inside the TŻO, material continuously falls onto the neutron star providing the energy to keep the star supported. These conditions can produce very exotic nuclear nuclei, so unstable that they would not survive for more than a second. However, some of the exotic elements produced are stable and can migrate to the surface of the TŻO, where they potentially become observable. Unfortunately it is likely harder to find a TŻO in our own Galaxy than previously assumed, so the team was looking for alternative ways to find them.

One avenue could be stellar pulsations. Many stars pulsate as their surface moves inwards and outwards, and TŻOs are no exception. For the first time, the team could show that the TZO’s should have both very long (>1000 day) pulsation periods and shorter ~500 day periods. Combining these different pulsation periods creates an unique fingerprint that can be used to identify a TZO. Based on the pulsation properties of several candidates observed in our galaxy and the nearby Magellanic clouds, the team was able to rule out that these are indeed TŻOs.

TŻOs represent an exotic class of stars that have undergone an extreme form of stellar evolution. The MPA study now provides models for what they should look like, and help astronomers to try to find them. Even though they are probably very rare objects, they do have some distinct features that make them stand out from other stars. Thus, the results presented in this study are only the first step in the - hopefully - eventual detection of these unusual objects.