Astronomers Found a Star Inside a Star, 40 Years After It Was First Theorized

Such Thorne–Żytkow objects promise to open up a whole new side to the universe that we’ve never known.

Sep 16 2014, 6:20pm

A render of a dead star passing near a red supergiant. Image: JPL/NASA

The universe is massive, and we can't see nearly all of it. That's the most exciting thing about space: The potential to find something completely unknown, something that brings fiction into fact, is ever-present. 

Case in point: A rather strange celestial body called a Thorne–Żytkow object (TZO). Originally predicted in the 1970s, the first non-theoretical TZO was likely found earlier this year, based on calculations presented in a paper forthcoming in MNRAS.

TZOs were predicted by astronomer Kip Thorne and Anna Żytkow, who was then postdoctoral fellow at CalTech. The pair imagined what might happen if a neutron star in a binary system merged with its partner red supergiant. 

This wouldn't be like two average stars merging. Neutron stars are the ancient remnants of stars that grew too big and exploded. Their cores remain small—about 12.5 miles—as they shed material out into space. Red supergiants are the largest stars in the galaxy with radii up to 800 times that of our sun, but they aren't dense.

So what would it look like for a massive star to merge with the tiny, super-dense core of another? 

This merger would create a system where a neutron star surrounded by a diffuse envelope of material would look almost identical to a regular red supergiant. But inside, most of the system's energy still comes from the star's core. 

How convective processes keep a TZO in action. Image: Mike Guidry/EPCC

Eventually, over the course of several hundred years, the core of the envelope and the neutron star would merge, yielding a larger neutron star or a black hole.

TZOs involve some pretty neat physics that astronomers would love to study, but identifying them is difficult because they essentially look like a regular red supergiant from the outside. Telling a TZO from a red supergiant means looking for a very chemical signature—specifically, analyzing its spectra for an over-abundance of lithium and other heavy metals.

This is what a group of astronomers led by Emily M. Levesque, which included Żytkow, did. They analyzed stars in the Milky Way and Magellanic Clouds from previous stellar surveys, paying special attention to where temperature and photometry data indicated the presence of a red supergiant. 

They then took this data to the Apache Point Observatory in New Mexico and the Magellan telescopes in Chile and used these scopes to look for 62 specific candidate TZOs. They were specifically focussing on analyzing the spectra for anomalies between the ratios of elements expected from a red supergiant, a sure sign of a TZO.

Among all the stars, one stood out, a star known as HV 2112 that lies in the Small Magellanic Cloud. Spectral analysis revealed the star held an unusually high concentrations of lithium, molybdenum, and rubidium. It was clear evidence that HV 2112 is a TZO and not a red supergiant.

The discovery in itself was exciting, but it's what this discovery means for astronomers going forward that's really interesting. Pending additional observations to confirm HV 2112's status as a TZO, it will become the archetype of a whole new kind of system. It will also help scientists answer some lingering questions, like what happens when massive binary systems with different types of stars collapse on each other.

And that's pretty amazing, isn't it? Finding a new type of astronomical object also emphasizes how little we actually know about the universe we live in, and just what we have to gain by exploring our corner of the cosmos.