A type of object first predicted 40 years ago has been found.
In the mid-1970s, theoretical astrophysicist Kip Thorne, working with collaborator Anna Zytkow, postulated the existence of a bizarre form of star. Now known as Thorne-Zytkow objects (TZOs), these bodies were the product of the merger of two separate stars: one a giant star, the second a neutron star. They were able to calculate several likely properties of these stars, making predictions for what they might look like. But in the intervening years, none have been discovered.
Anna Zytkow, however, did not give up the search. And now, 40 years later, she may have spotted one. She and three collaborators (Phil Massey, Nidia Morrell, and Emily Levesque) have reported what may be the first observational evidence that TZOs exist.
Neutron stars are the cores of massive stars that have undergone a supernova. Their massive gravity compresses matter so much that an object the mass of the Sun can squeeze into a sphere about 20 km across. At these densities, matter is compressed down to neutrons—and possibly even a sea of subatomic particles.
To form a star massive enough to undergo a supernova generally requires a dense cloud of gas, which often forms additional stars. These companions can exchange mass with the neutron stars in various ways, but Thorne and Zytkow suggested that they may do more than interact—the companion can swallow the neutron star. Many stars evolve through a giant phase in which their envelope expands significantly. Should the neutron star be orbiting close enough, this expansion could cause the giant star to envelop its companion.
Once this happens, the neutron star's orbit would rapidly be slowed down by its interactions with the gas, causing it to spiral toward the star's center. Once there, it would displace the normal core of the star, in effect taking over the center of the object. Although its intense gravity would draw matter in, its equally intense heat would drive it off, creating a stable balance—the TZO.