While I’ve been going through early extraterrestrial ideas like those of Ronald Bracewell I’ve run back into that most unpronounceable of stellar objects, Przybylski’s Star. This one is worth a return look and I was reminded of it by author and futurist John Michael Godier on his Event Horizon podcast. I do few interviews but I’ve always admired John and Ross’s work on Event Horizon so much that I made an appearance last week. It was John who summoned up Przybylski’s Star as we moved into the broader topic of technosignatures.
Image: Antoni Przybylski in the early 1960’s. Credit: Mike Bessell (via Charles Cowley’s site).
David Kipping does a gallant job of pronouncing Przybylski in one of his Cool Worlds videos, and Wikipedia recommends pʂɨˈbɨlskʲi, which is itself a challenge. Try jebilskee, which is what University of Michigan astronomer Charles Cowley heard when he asked Przybylski himself how to say his name way back in 1964 (the reference is now offline, as Cowley unfortunately passed away in 2024). In any case, suppress the initial ‘p.’ I can’t resist reprinting an old pre-X Twitter post on the matter:
PRZYBYLSKI'S STAR (HD 101065) Blue dwarf with a peculiar spectrum showing an almost complete absence of vowels.
— FSVO (@FSVO) November 22, 2012
This star is more than a curiosity. As a matter of fact, if I were to declare the one most intriguing object in the technosignature hunt, it’s this one, although I’ll hasten to add that we’d need a lot more evidence before making that call. Przybylski’s Star is roughly 350 light years out in Centaurus, discovered in 1873 but gaining attention in 1961 when the Polish astronomer Antoni Przybylski examined its spectrum to discover that it didn’t fit our normal stellar classification scheme. I’ve seen it pegged as an F3-class star but also as an F0p, with the p standing for peculiar. If we go by effective temperature, we come up with early F-class, but its spectrum separates it from all else in that category.
It’s also referred to as an Ap star (this is Kipping’s preference), and whereas F0p is a designation based on temperature, Ap refers to stars larger and hotter than the Sun and possessed of intense magnetic fields and slow rotation rates. What to make of the star’s spectrum? It’s laced with oddball elements like europium, gadolinium, terbium and holmium. Moreover, while iron and nickel appear in low abundances, the stellar atmosphere shows the presence of short-lived ultra-heavy elements like actinium, plutonium, americium and einsteinium.
The latter were identified in 2008. Called actinides, these are elements with atomic numbers from 89 to 103 on the periodic table. They force the question of how radioactive elements with half-lives on the order of centuries or even decades could be there. How are these reactions being sustained on the surface of a star? The reference here is an important if strangely obscure one. The work of a Ukrainian team under V. F. Gopka, the paper is “Identification of absorption lines of short half-life actinides in the spectrum of Przybylski’s Star (HD 101065)” (citation below). David Kipping (on an earlier Event Horizon podcast) and Jason Wright (Penn State) have both mused on the lack of follow-up to it, even though the work seems solid and has implications in terms of technosignature searches.
We also have a 2017 paper by Vladimir Dzuba (University of New South Wales) that offers an interesting solution. The idea is that the short-lived actinides in Przybylski’s Star are the result of undiscovered superheavy elements (a theoretical ‘island of stability’ on the periodic table) which can survive for millions of years. In this model, it is the decay of these elements that produce lighter ‘daughter’ products that are found here, including such things as plutonium and uranium. A possible origin for such superheavy elements is a nearby supernova explosion whose shockwave would have fed this matter directly into the forming star.
Image: Przybylski’s star, image center. By Vizzualizer – Own work, CC BY-SA 4.0,
What we’ve seen in intervening years is discussion of whether the spectral data have simply been misinterpreted, or whether a nearby neutron star might be bombarding the atmosphere of Przybylski’s Star, but there is no observational evidence for such a companion. The island of stability idea has yet to be confirmed in the laboratory, although this work continues. I’ll also mention the star HD 25354, another ‘peculiar’ star, this one in Perseus, which is now being investigated. It contains unstable radioactive elements in its upper atmosphere.
So we have an ongoing mystery, one with tantalizing reminders of a 1980 paper from Daniel Whitmire and David Wright called “Nuclear waste spectrum as evidence of technological extraterrestrial civilizations.” Here the concept is using a star as a repository for radioactive waste. The authors homed in on A stars as being likely candidates. I suspect they were thinking about Sagan and Shklovskii in their book Intelligent Life in the Universe (Delta, 1968), where the authors speculate on the possibility of ‘salting’ a star to call attention to it, a kind of interstellar beacon. Look, a civilization is saying, there is intelligence near this star.
I mention Sagan and Shklovskii pointedly because while both are frequently fused into a single entity in later descriptions of their era, the duo had profound disagreements on a lot of things, especially the kind of SETI embodied in the Drake Equation. On the matter of ‘salting’ a star, it’s Sagan who references Shklovskii as well as a separate Drake paper on the concept, not claiming it for himself. From the book:
Drake and Shklovskii envision the dumping of a short-lived isotope – one which would not be ordinarily expected in the local stellar spectrum – into the atmosphere of the star. In any case, the material of the marker should be of a type that is difficult to explain, except as a result of intelligent activity…. Remarkably enough, the spectral lines of one short-lived isotope, technetium, have in fact been found in stellar spectra… This example illustrates one of the difficulties with such a marker announcement of the presence of a technical civilization. We must know a great deal more than we do about both normal and peculiar stellar spectra before we can reasonably conclude that the presence of an unusual atom in a stellar spectrum is a sign of extraterrestrial intelligence.
So could the spectrum of Przybylski’s Star actually be a technosignature? You can see how difficult this problem is considering the rarity of this kind of star. Even so, a look at The Catalog of Ap, HgMn, and Am Stars reveals over 8,000 ‘peculiar’ stars, ranging across the temperature scale. I dug into the catalog with AI to learn that Ap and hotter Bp stars are the largest subgrouping, both characterized by unusually strong magnetic fields. There is much work here for aspiring graduate students because we need to learn whether the actinides at Przybylski’s Star are simply a rare natural phenomena or a technical marker.
Przybylski’s original paper on the star is “HD 101065-a G0 Star with High Metal Content,,” Nature Vol. 189, Issue 4755 (1961) 739 (abstract). Jason Wright’s three-part essay on Przybylski’s Star is well worth your time. The paper identifying actinides in this star is Gopka et al., “Identification of absorption lines of short half-life actinides in the spectrum of Przybylski’s star (HD 101065),” Kinematics and Physics of Celestial Bodies Vol 24, Issue 2 (April 2008) 89-98 (abstract). The Whitmire and Wright paper is “Nuclear waste spectrum as evidence of technological extraterrestrial civilizations,” Icarus Vol. 42, Issue 1 (April 1980), 149-156 (abstract). Vladimir Dzuba’s paper is “Isotope Shift and Search for Metastable Superheavy Elements in Astrophysical Data,” Physical Review A 95 (30 June 2017), 062515 (abstract).
