THE SOUTHWORTH PLANETARIUM

70 Falmouth Street Portland, Maine 04103

43.6667° N 70.2667° W Altitude: 10 feet below sea level Founded January 1970

2021-2022: LXXXIX

"So comes snow after fire. and even dragons have their ending!"

-J.R.R. Tolkien

THE DAILY ASTRONOMER Wednesday, March 9, 2022

Thorne-Zytkow Objects

We've written it before and will likely write it again: if nothing else, the Universe is the quintessential artist. It is creating, experimenting, sculpting, planning, discarding, destroying, combining and melding every single blessed nanosecond throughout the hollows and niches scattered about its unfathomably vast expanse. Given enough time, the cosmos will likely work through every single possible permutation of form and function. After all, we observe this prodigious creativity on Earth, from the beguilingly cute Gollum-like red-eyed tree frog to the lumbering and zoologically perplexing platypus. It is possible that eventually nature will contrive even more fantastic works of biological improbability: phosphorescent giraffes, winged horses, Chaucer-quoting Daschunds, and New Englanders who secretly believe that football is only a game.

Even astronomers have discovered such stunning diversity out in the boundless black beyond. One such strange entity is known as a Thorne-Zytkow object, named for American astrophysicist Kip Thorne and Polish astrophysicist Anna Zytkow,* who, in 1977, postulated the existence of such an object. What is it? It is a highly massive star that contains a neutron star** at its core. Yes, a highly dense stellar remnant trapped within the confines of a massive star.

Is such a thing possible?

Well, it's certainly possible and a few candidates have even been identified, such as HV 2112 and HV 11417. both of which reside in the Small Magellanic Cloud. Yet, how does such a hybrid star form and how can such an object be identified?

Simply, a Thorne-Zytkow object forms when a highly massive star collides with and then "ingests" a neutron star. Recall that a neutron star, though highly dense, is vanishingly small, about the size of a major city: a mere kernel compared to a highly massive star. The collision could only occur by a chance encounter between an active star and a neutron star or by having a neutron star form in a binary and then eventually collide with the still active companion. Because chance encounters between stars are so rare out here in comparatively diffuse spiral arms, they likely occur only in globular clusters or in galactic regions of high stellar density.

Identifying such an object requires detection of unusually high abundances of molybdenum and rubidium that would not ordinarily be observed in such massive stars. It is also possible to identify a Thorne-Zytkow object by detection of the neutron star's gravitational wave and simultaneous observation of the star's optical spectrum. However, no such observation has yet been made.

While astronomers have not yet positively identified a Thorne-Zytkow object yet, the existence of such an entity is highly probable. Let us not forget that black holes, which are now a dime a dozen in any reputable astronomy shop, were merely hypothetical objects for the longest time.

Heavens above, do we live in a strange place!

*The Z actually contains an "overdot." An overdot Z indicates a voiced retroflex fricative, a lovely term you can shout if you've recently had a nightmare about Hell and are therefore too frightened to swear. This overdot instructs us to pronounce the "z" as one would pronounce the "g" in "mirage."

**Neutron stars form after a highly massive star explodes as a supernova.

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