Who is dumping radioactive waste into Przybylski's star?
In April 1980, on the pages of Icarus, the prestigious planetology magazine directed
at that time by Carl Sagan, appeared an article really very ahead of its time.
The title?
"Nuclear waste spectrum as evidence of technological extraterrestrial civilizations".
The two authors, Daniel Whitmire and David Wright, considered the possible observational
consequences of galactic civilizations which utilize their local star as a repository for
radioactive fissile waste material.
Since then more than 40 years have passed, but the hope of being able to prove the existence
of technologically advanced alien civilizations by analyzing the atmosphere of stars and planets
is not yet dormant.
And indeed, in recent years, thanks to the increasingly extraordinary capabilities of
space telescopes, is becoming the main method of investigation on extrasolar planets.
But the question at this point arises spontaneously: have ever been found, so far, stars with some
anomalous concentration of chemical elements such as to suspect the activity of alien intelligence?
Let's try to discover it together!
Whoever asks this question, will be surprised to know that twenty years before the publication
of the article, in 1960, a star of that type had already been identified!
And that this star still represents one of the most exciting mysteries, both from the
astrophysical point of view and from the SETI research one.
The discovery of its extraordinary characteristics is due to Antoni Przybylski, an astronomer
born in Poland in 1913 and then moved to Australia in 1950, where he died in 1985 after a thirty-year
scientific career full of satisfactions.
On the day of his arrival at Mount Stromlo Observatory, near Canberra, when his colleagues
heard his name - pronounced a bit like "SheeBILski" - there was a pause, after which someone announced
that all those consonants were a bit too much, and that from then on they would call him
simply "Bill".
That was the case until his death.
His observational program consisted in reviewing stars that manifested unusual properties,
but putting himself to work he could never have imagined that he was going to meet something
that really went far beyond the unusual.
It all began on the night of April 26, 1960, when Przybylski, in the course of a spectroscopic
survey of stars with large proper motions, detected the spectrum of the eighth magnitude
star HD 101065, about 400 light years away in the direction of Centaurus.
Przybylski immediately realized that the spectrum of the star did not correspond to the general
framework of stellar classification.
In other words, the light of that star revealed the presence of elements much heavier than
iron, such as holmium, strontium, niobium, cesium, yttrium, scandium, and many others,
all belonging to the group of so-called "rare earths".
Elements that a "normal" star should not show in its spectrum, since the heaviest element
that could be able to synthesize should be iron...
No scenario explained the existence of a young star rich in rare earths.
And Przybylski's Star was surely not highly evolved: all evidence suggested that it was
an ordinary hydrogen-fusing star like the Sun.
With stellar-evolution theory enjoying success and momentum in the astronomical community,
Przybylski's Star became an embarrassing aberration.
The rare-earth lines actually blanket the spectrum, crowding against each other.
The lines of iron have been so hard to detect among the confusion that convincing evidence
that Przybylski's Star contains any iron at all did not come until 1983.
Modern abundance analysis indicates that, compared to the Sun, iron is about a factor
of 10 depleted, while the rare earths are factors of 1,000 to 10,000 enhanced.
But the strangeness does not end here.
In addition to its overabundance of rare elements, Przybylski's star is also the only star known
to contain ultra-rare elements called actinides.
These elements, which hang on the fringes of the periodic table occupied by atomic numbers
89 through 103, appear to exist within Przybylski's star as short-lived radioactive isotopes - which
doesn't make a lot of sense, since their short half-life means they should have long ago
decayed.
One possibility as to how they could still be there is they are themselves the decayed
forms of unseen ultra-heavy elements that haven't even been discovered by scientists
yet.
Another hypothesis is that Przybylski's star has a neutron star as a companion, capable
of carrying elements much heavier than iron in the atmosphere of Przybylski's star.
Unfortunately, the star's spectral lines appear very sharp and show no hint of radial velocity
changes induced by a nearby neutron star.
Could there be a relationship with another very strange feature, that of super slow rotation?
In fact, you should know that, in 2018, a new analysis led by researchers from the Leibniz
Institute for Astrophysics Potsdam in Germany has discovered that the star isn't just composed
of bizarre stuff - it also exhibits bizarre movement!
Using a tool called the High Accuracy Radial Velocity Planet Searcher on the ESO's 3.6-meter
telescope in Chile, the team took readings of its magnetic field, discovering the star's
rotation period - the time it takes to complete one revolution on its axis - stretches out
over almost two centuries!
More precisely, 188 years, which means that Przybylski's star rotates 2500 times slower
than our Sun!
We already know that "peculiar" stars like Przybylski have among their characteristics
to rotate slowly, but at the moment we do not see any connection with the chemical composition.
In short, many clues but no evidence.
And the mystery still continues.
So much that at this point someone of you will begin to ask: "But it will not be that
Whitmire and Wright, the authors of that famous article of 1980, were not all wrong with their
hypothesis to the Star Trek?
Well, the idea that a very advanced extraterrestrial civilization could leave some traces of its
existence in the spectrum of the star of belonging was not entirely new even in 1980...
Whitmire and Wright in their article spoke of a civilization that to get rid of nuclear
waste does not hesitate to throw them into the furnace of their sun, thus releasing dozens
of radioactive elements.
But already in 1966, in his book Intelligent Life in the Universe, Carl Sagan hypothesized
that an alien civilization could disseminate very rare elements in the atmosphere of a
star to attract the attention of any other galactic observers.
As if to say, "Hey, look how weird this star is...doesn't this suggest anything to you?"
And this could all make sense.
The scientific search for extraterrestrial intelligence has focused on listening for
radio signals voluntarily sent into space.
But if alien civilizations weren't listening, or if we weren't able to listen to them, their
biological activity could still produce traceable clues.
This type of research is called interstellar archaeology or cosmic archaeology.
A multitude of spectral anomalies are analyzed along with the possible presence of artificial
compounds released into the planetary atmosphere, such as stellar doping with nuclear waste
isotopes.
An extraterrestrial civilization could also reveal itself through colossal engineering
works called Dyson spheres, i.e. hypothetical structures used to collect solar energy.
A Dyson sphere would be partially or completely capable of blocking the visible light of a
star.
But because the sphere would be heated by it, it would emit infrared light in a way
that would be detectable from Earth.
Astronomers have found some candidates for Dyson spheres, but they could easily be just
hydrogen gas clouds, ancient stardust, or even asteroids.
The cosmic-scale search for archaeological artifacts, such as Dyson spheres or Kardashev
civilizations, is an interesting alternative to the conventional SETI project.
Discovering such an artifact does not require the intentional transmission of a signal by
alien civilizations.
In this regard, you will all remember the story of the so-called "Tabby's Star", so
named in honor of Tabetha Boyajian, the astronomer leading the group that first studied it.
In 2015, the Kepler Space Telescope, whose task was to reveal the presence of planets
by observing the dip in light that a star shows when they pass in front of it, revealed
irregular variations in the brightness of the star KIC 8462852, a star very similar
to the Sun, 1500 light years away in the Constellation of the Cygnus.
Tabby's brightness dropped (and still continues to drop) unpredictably by as much as 20%,
at irregular intervals, thereby indicating the presence of something covering a large
fraction of its surface (one-fifth of its surface, when, by comparison, Jupiter only
manages to cover 1% of the Sun's surface).
From 2015 to the present, numerous hypotheses have been put forward to make sense of these
observations: swarms of planets, comets, dust clouds, or intrinsic variations in the light
emitted by the star.
But to have undoubtedly struck most the imagination of the general public was the hypothesis of
an alien megastructure, something that completely wraps, albeit in a very irregular way, Tabby's
star.
The idea recalls that of a Dyson sphere, an inhabited structure that, completely surrounding
the star around which it orbits, collects all the energy.
The structure in our case would be only at the beginning of its construction, or would
represent a so-called Dyson swarm, composed of flat megastructures not connected to each
other.
The parts able to obscure the star would still be half a million kilometers wide, more than
the distance that separates us from the Moon.
A cyclopean construction then, that only an alien civilization much more advanced than
us would be able to face.
Although no astronomer has ever really believed in this hypothesis, the behavior of the star
continued to be sufficiently unique to push the group of Dr. Boyajian to continue the
observations.
And unfortunately for the alien hypothesis, the data then showed that the dimming appears
to be due to the interposition of interstellar dust, and not that of a solid structure.
The proof lies in the fact that the dust absorbs light very differently depending on the color.
Red light is absorbed less than blue light, and infrared light is absorbed even less.
In contrast, an opaque solid structure, such as a planet or an alien megastructure, would
block light regardless of its color.
Observations, made in light of various colors, have shown a very different absorption, about
twice as much in the blue color than in the infrared...
But for one hypothetical Dyson sphere being debunked, there's more to come.
Indeed, the triumphant technology of our time puts us in a position to detect an increasing
number of anomalies in the brightness and chemical composition of stars.
However, with few exceptions, these archaeological traces are still very difficult to "confirm"
for our current technological capabilities.
It is true that nuclear waste scattering in a star could release a large amount of rare
elements, such as technetium or neodymium, detectable in its spectrum.
But a detectable trace from space might require a colossal amount of material, for example
about 100,000 tons of technetium.
For comparison, the terrestrial reactors of technetium have produced since 1950 only 100
tons...
As you can understand, one thing is to find anomalies, and another thing is to identify
the cause, trying to discriminate between the "natural" and the "artificial".
For the time being, the star of Przybylski, which in this sense has ousted Tabby from
his throne, is considered by many the most "strange" star among all those we know.
Perhaps the only one, despite the skepticism of the majority of astronomers, able with
its chemical anomalies to keep alive the hope of artificial and alien contamination of its
atmosphere.
And hope, as we know, is always the last to die.