Cosmic monsters: searching for a star within a star (aka Thorne-Zytkow object)

Thorne-Zytkow object is a cosmic monster: a red supergiant star with a neutron star in the core

Every time we learn something new about Space we think: how much weirder can things get? A star 1500 times the radius of the Sun? Fine. An object with gravity so strong that not even light can escape it? OK! But… a star within a star? That’s where we draw a line. This is seriously weird!

But do such things even exist?

Thorne-Zytkow object

Back 1975-1977 two astronomers, Kip Thorne and Anna Zytkow, predicted the existence of a bizarre star, now known as Thorne-Zytkow object, or TZO. Thorne-Zytkow objects are red giants or red supergiant stars (RSG) that have neutron stars in their cores. From the outside they look just like normal red supergiants, but inside they “work” differently and “do” things that normal RSGs should not do. Because of their bizarre hot interiors they produce heavy elements, like Lithium, Rubidium and Molybdenum, that by no means should be made in ordinary cool supergiants!

For the past 40 years astronomers have been searching for TZOs but so far no definite detection has been made. We still don’t know whether these cosmic monsters exist.

But let’s start from the beginning…

How to make a Thorne-Zytkow object

A TZO is a “two things in one”. Therefore to make a TZO you have to start with two components: neutron star and a supergiant. Of course, the two objects can just happen to be nearby, but we are more likely to find them together in a binary system.

Binary system

To make a Thorne-Zytkow object you need two massive stars in a binary system. As the two big stars evolve, two different scenarios can lead to a star-within-star situation.

  1. One of the stars undergoes a core collapse (i.e. goes Supernova) and turns into a neutron star. At the same time the second stellar companion grows as supergiant and gets so big that it swallows the neutron star. The neutron star inside the supergiant keeps spiraling inwards towards its center and finally reaches the core. It’s a TZO!
  2. One of the stars in a binary is already a supergiant while another is about to go supernova. The thing about the supernovae explosion is: it is asymmetrical. So when the explosion does happen, the remnant (i.e. neutron star) gets a kick in some particular direction. If the kick happens to be in the direction of the supergiant companion (and you don’t even have to aim very hard as supergiants are VERY big and hard to miss), the neutron star will smash into the supergiant, sink in and settle in the core. It’s a TZO!


Occasionally the stars do collide, especially those in the overpopulated globular clusters. Therefore in theory a red giant and a neutron star can smash together and make a TZO. But given how rare these events are (and remember, we need not just any but two particular stars to collide), the chances of finding a TZO born this way are fairly small.

Summarizing the above, it i most likely that TZOs are the result of  the evolution of close binary systems.

Observations of Thorne-Zytkow objects

TZO’s were predicted back in the 1977, but for the next 40 years no observations confirmed the existence of these strange stars. In 2014 Anna Zytkow, one of the TZO “creators”, approached a team of astronomers (led by Emily Levesque) that was studying red supergiants and encouraged them to look for TZOs. The team picked around 60 objects from their database that  looked like potential TZO candidates  and observed them again. One of the stars, HV 2112 in the Small Magellanic Cloud, showed an unusually high abundance of Li, Rb and Mo and looked very much like a Thorne-Zytkow object. Until recently HV 2112 was considered the most likely TZO candidate. Unfortunately the new study did not confirm this claim. 

Up to date it remains a mystery whether the exotic Thorne-Zytkow objects exist, but astronomers do not stop looking!

Future of the TZOs

Thorne-Zytkow object, if real, is just a stage in a life of a weird star. What will it turn into at the end of its life? Nobody knows. Many researchers think that massive TZOs can collapse into black holes. This process might be accompanied by a big explosion that will send the heavy elements far into space! Could it be another way to “seed” the Universe with heavy elements? We’ll see!

Still curious?

  • Watch the talk presented by Dr Emily Levesque for the Denver Astronomical Society about weird stars and TZOs.
  • Visit one of our inflatable planetarium space shows to learn more about stars and stellar evolution.
  • Stay tuned for our future space blog posts!
  • Questions? Comments? Let our mobile planetarium team know.