Bizarre “Hybrid” Star Found


It has been almost 40 years since Professor Kip Thorne and Dr Anna Zytkow first suggested it was possible for a red giant and a neutron star to merge. Now this theoretical, almost mythical, chimera has been found.

Neutron stars are the outcome of a supernova explosion. Red giants are the later stage of life for stars with between 0.3 and 8 times the mass of the sun. Having run out of hydrogen in their cores, red giants have taken to conducting fusion in a shell further out, which in turns causes their outer layers to expand thousands of times in size. They can often grow to the size of the Earth’s orbit, and in some cases much further.​

If the neutron star is in a close orbit with the red giant it would be swallowed by its companion as the outer layers expand. Frictional drag would cause the denser object to slowly spiral in to the center of the giant to become part of the core.

Thorne and Zytkow noted that such a star would have a very different chemical signature from an ordinary red giant, and would also be powered in an unusual way.

Now a team, including Zytkow herself, have announced in the Monthly Notices of the Royal Astronomical Society Letters the discovery of the first Thorne-Zytkow Object (TZO) in the Small Magellanic Cloud (SMC).

The SMC is a dwarf galaxy neighboring the Milky Way, and one of just three other galaxies visible with the naked eye. A recent burst of star formation left the SMC rich in large stars and their remnants such as X-ray binaries.

Examining the SMC’s red giants, Philip Massey of the Lowell Observatory, noted the unusual features of one, HV 2112, commenting, “I don’t know what this is, but I know that I like it!” The spectral lines of HV 2112 looked like nothing Massey had seen before, and on closer examination showed high concentrations of rubidium, lithium and molybdenum.

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