What is more noble and lasting than the stars? In the cosmographies of the pre-scientific era, they were imagined as incorruptible bodies, set for eternity in transparent spheres that revolved around the Earth, seen as the center of the Universe. They formed the sky of the fixed stars, distant and immutable.

Modern science, especially thanks to the last century’s achievements of astronomy and astrophysics, has taught us, however, that the stars are anything but eternal and unchanging. Like everything under the sky, they move and are subject to evolution: they are born, grow old, and die. But they do so with very long times, of millions or, more often, of billions of years: astronomical times, precisely.

That said, it seems rather unlikely that, in the short span of human life, stars could suddenly disappear or appear out of nowhere. Of course, they can increase or decrease their brightness, as happens to the numerous types of variable stars cataloged by astronomers. Or they can explode as supernovae, but in that case, they leave a visible trace, such as the Crab Nebula, the remnant of a star exploded in 1054. In any case, it is highly improbable that they suddenly appear or disappear as if they were bulbs switched on or off.

However, the scientific method also teaches us that nothing should ever be taken for granted and that even the more obvious hypotheses must be empirically verified. It is the reason why the VASCO [1] project was born. The project aims to compare star catalogs of the last century with catalogs published in recent years, looking for stars that were and are no longer there or, on the contrary, stars that were not there and are there today.

On December 12, the first of a series of studies planned for the project appeared in The Astronomical Journal. The article, signed as the lead author by Beatriz Villarroel of the Swedish Nordita (Nordic Institute for Theoretical Physics), provides the first official data on the outcome of the research carried out so far by the group of astronomers participating in the initiative.

Astrophysicist Beatriz Villarroel, first author of the study published in The Astronomical Journal

The article describes the results obtained by comparing two star catalogs separated by an interval of about 70 years: the US Naval Observatory catalogue (USNO) B1.0, whose observations date back to the 50s of the last century, and the Pan-STARRS1 survey catalog, published at the end of 2016.

The work done on these two databases by Villarroel and colleagues is the result of the enormous computing power available to scientists today. The authors have selected 600 million objects from the digitized version of the USNO catalog, looking for their analogs within the Pan-STARRS1 catalog — a cyclopean amount of data, which only very powerful computers can “grind” in a reasonable amount of time.

The comparison was set up in such a way as to verify, for each of the 600 million light sources taken from the USNO catalog, the existence of a counterpart in Pan-STARSS1 within a radius of 30 arc seconds starting from the position shown in USNO. From this first major skimming, 426,975 mismatches (that is, lacked correspondences) emerged: a number equal to 0.074% of the examined objects.

Researchers then filtered the set of mismatches, eliminating those that depended on errors due to different sky coverage by the two surveys. It meant removing all light sources with declination below −30° because natively not included in Pan-STARSS1. In all, 151,193 items remained not matched on the initial total of 600 million items.

One of the reasons why a star can disappear from a particular region of the sky after 70 years is because it has a high proper motion and has thus shifted considerably in the sky, ending up so far away from where it was seven decades before that it seemed to have disappeared. Therefore, to eliminate from the list of missing stars all cases attributable to a fast proper motion, the authors of the study used the very precise data provided by the astrometric satellite Gaia as well as a series of comparisons performed on the database of another large recent astronomical catalog, that of the Sloan Digital Sky Survey (SDSS) in its version number 12. After this further step, the number of mismatches has decreased to 23,667 in all.

One of the cases of missing stars presented in the study published in the Astronomical Journal. In the two top panels, taken from observations dating back to the last century, at the exact center of the inset, there is a faint star that forms a triangular asterism with two other nearby stars, a brighter one and an equally dim one. In the two panels below, taken from recent surveys (SDSS12 on the left and PanSTARRS1 on the right), the faint central star disappeared, while the other two remained [Beatriz Villarroel et al 2020 AJ 159 8]

The researchers then decided that the time had come to resort to visual inspection of the remaining cases. They used for accomplishing this task three different image databases available online [2]. This painstaking work of comparison led to the elimination of all false positives recognizable through direct inspection (various types of artifacts and cases of missing coverage in one of the databases examined). The total of mismatches was thus reduced to 1,691 cases.

Further comparison and in-depth analysis of these last 1,691 candidates led to the discovery that, in many cases, the mismatches were to be attributed not to the sudden disappearance of the stars, but to a slight offset of the celestial coordinates, by virtue of which those reported in the USNO catalog did not correspond to those of the most recent catalogs.

In the end, of the 600 million objects initially considered, around 100 mismatches survived all attempts to trace the missing object in recent catalogs. The analysis of the magnitude and color of the “disappeared” objects has shown that they are almost all very faint and very red. The color and the fact that they have a point-like appearance suggests that they are not objects of the Solar System. If they had been, for example, asteroids, they would have left a linear trace, due to the movement accumulated during the exposure (which, in the photographic plates of the USNO catalog, lasted for about 50 minutes). Furthermore, the bodies of the Solar System are typically much bluer (because they reflect sunlight) than the approximately 100 candidates discovered by Villarroel and colleagues.

The celestial coordinates of the 28 most interesting objects, among the approximately 100 mismatches that survived the long control work carried out by the researchers of the VASCO project [Beatriz Villarroel et al 2020 AJ 159 8]

Another possibility is that the disappeared objects are variable stars with large differences in magnitude between their maximum and minimum luminosities. But none of those 100 objects is listed in the General Catalogue of variable stars, even widening the search to a radius of 30 arc seconds starting from the position indicated in the USNO catalog.

Other astrophysical causes for which point-like light sources can disappear after years or decades include transient events such as:

flaring red dwarfs;

tidal destruction events (stars torn apart by black holes);

novae eruptions in binary systems;

supernovae exploded in distant galaxies;

active galactic nuclei ( AGN ) that have stopped accreting matter;

) that have stopped accreting matter; failed supernovae, that is, massive stars that, once they used up the available nuclear fuel, collapse directly into black holes, without a supernova explosion.

The latest cause seems to be particularly unlikely. Villarroel and colleagues calculate that the odds of running into a failed supernova in a 70-year time window are less than one in 90 million. On the other hand, flaring red dwarfs could instead be the most plausible explanation for many of the 100 missing objects. In this regard, it is worth noting that at least one case has recently been reported in which the brightness of an M-dwarf has increased by ten magnitudes due to a super-powerful flare. The alleged disappeared objects could, therefore, be M-class stars that were very active at the time when the observations were made for the USNO catalog and which later faded to the point of falling below the minimum magnitude detectable by Pan-STARSS1 and SDSS.

However, the search for natural explanations, whatever they are, is only part of the range of causes considered by the authors of the study. The VASCO project was not born, in fact, only to find stars that disappeared as a result of natural causes, but also to probe what are the chances of coming across what Villarroel and colleagues call “impossible effects.” By this, they mean the disappearance of a light source due to artifacts created by an extraterrestrial technological civilization operating on a stellar or even galactic scale.

From this point of view, a red point source in the USNO catalog without matches in recent stellar catalogs could also be considered a “technosignature,” that is, the technological signature of an advanced civilization. The authors of the study explain:

An attractive feature about the list we have produced is that a monochromatic interstellar laser at 600–680 nm that shines for about one hour may well present itself as a point source detected only once in one image, due to the short time when the laser operated.

The technological level for producing such a signature is not far from what humanity has achieved. We already have technologies capable of producing short laser pulses, lasting nanoseconds, with a brightness equal to about 5,000 times that of the Sun.

One of the most interesting cases of missing stars. The object near the center of the inset in the upper left panel, taken from an image acquired with a red filter in the 50s of the last century, has no correspondence nor in the image in the upper right, dating back to the 80s, nor in the two images below, taken from the recent SDSS and Pan-STARRS1 surveys. What happened to that star? [Beatriz Villarroel et al 2020 AJ 159 8]

Another possible technological cause that could explain the disappearance of a star is its darkening by alien megastructures on the type of the so-called Dyson spheres, imagined in the 60s of the last century by the physicist and mathematician Freeman Dyson. They would be gigantic orbiting panels, spread over several astronomical units, capable of seizing the radiation of a star to supply energy in almost inexhaustible quantities to a civilization much more advanced than ours.

But researchers from the VASCO project envision even more extreme possibilities than a system of orbiting panels used to sequester the radiation of a single star. In the heart of almost all galaxies, supermassive black holes are hidden, surrounded by immense accretion disks, inside which temperatures of billions of degrees and brightness thousands of billions of times higher than solar are generated. There is no more enormous localized energy source in the Universe than that produced in active galactic nuclei or AGNs. A truly advanced technological civilization may have found a way to capture, at least partially, such an immense amount of energy by building Dyson spheres suitable for the scale of AGNs.

For the moment, all this is and remains pure speculation. However, Villarroel and colleagues have announced that, in a study to be published soon, they will analyze in detail the approximately 100 objects identified during this research, to try to understand, for each of them, the cause of the mismatch between the USNO and Pan-STARRS1 catalogs. It will be interesting, then, to find out whether at least one light source has remained on the list, the disappearance of which cannot be explained by exclusively natural causes.