Star within a Star!

A team of researchers from the University of Colorado have confirmed the existence of a long-hunted type of galactic oddity: a star within a star called a Thorne-Żytkow object, this is specifically a red giant or supergiant that has swallowed a neutron star, and it was first proposed by the two named scientists more than 35 years ago. Now, after much skepticism and searching, a candidate has been confirmed: HV 2112, as the object is known, is almost certainly our first confirmed stellar hybrid.

A Thorne-Żytkow object is possible due to the properties of a neutron star. Usually less than 10 km across, neutron stars result from the collapse of a star too big to form a white dwarf, but to small to produce a black hole. They are made almost entirely of neutrons, and they are among the densest objects in the universe — so if one were to collide with a much larger star it could theoretically avoid being dissolved and disbursed. Once it’s been swallowed whole by the much bigger supergiant, the tiny neutron star will begin to orbit inside the other star, slowly spiraling in toward the center. When the neutron star hits the giant’s preexisting core, the two meld and become a larger neutron star — if the resulting neutron core isn’t itself big enough to trigger another supernova, the whole nested system ought to remain fairly stable.

The actual meeting between two stars also informs the search, as it will likely occur either from a random meeting between two lost wanderers or an inevitable collision inside an overly crowded binary system. The team directed the Magellan Clay telescopes in Chile to search densely populated areas where such meetings were most likely to occur, sweeping for candidates. 

Like any theory, the Throne-Zytkow hypothesis lays out a list of testable predictions that led to this discovery. Specifically, since neutron stars burn much hotter than the core of a normal red giant, seeming supergiants can betray their cores through abnormal concentration of certain isotopes. Specifically, HV 2112 displays above average concentrations of rubidium, lithium and molybdenum, which are considered telltale signs of a Thorne-Zytkow object. 

In fact, this object could provide a whole new inroad production of certain elements and isotopes. Most elements heavier than carbon are produced through supernovae; merely heating matter in a stable stellar furnace isn’t enough to drive production of, say, an atom of lead. Many mathematical models have been proposed over the years to explain how each atom we find in the universe might have come to be, either made directly when a star explodes or when a star-made atom undergoes radioactive decay. Hybrid stars provide a whole new type of atomic forge, one that could produce atoms totally off limits to conventional stars.

It’s worth noting that HV 2112 does not fit every prediction for a Thorne-Zytkow object, but the large generalities are there. The team points out that the theory underlying these objects has not been upgraded in a long while, and that modern physics might be able to model a hybrid star more accurately. Certainly, this announcement should prompt a much wider array of instruments to swing and examine the alleged nesting star. Whether or not it is a Thorne-Zytkow object, that sort of intense scrutiny ought to get at the truth.

This discovery comes in the Small Magellanic Cloud, a dwarf galaxy just 200,000 light years away. This makes it one of the Milky Way’s nearest major neighbors — implying that these bizarre stars might be more common than we think. Like finding Earth-like planets virtually everywhere we look, the sheer nearness of this discovery implies that similar oddities might be scattered all throughout the galaxy.