A team of scientists has analyzed the positions of 600 million stars from a catalog published in the previous century and compared them with a more recent catalog. After accounting for stars’ movements and various artifacts, they found that about a hundred stars appear to have vanished. While natural phenomena could explain this disappearance, researchers are also investigating the possibility of "technosignatures," which are signs of advanced extraterrestrial technologies.

In ancient cosmologies, stars were viewed as eternal, fixed points in the sky, revolving around a central Earth. Yet modern astronomy reveals that stars are dynamic entities; they form, evolve, and ultimately perish over vast timescales—often spanning millions or even billions of years. Given these astronomical timelines, it seems improbable that stars could simply vanish or appear within the brief span of human life. While stars can vary in brightness or explode as supernovae, leaving remnants like the Crab Nebula, sudden disappearances akin to light bulbs being turned off are far less likely.

Nonetheless, the scientific method urges us to question even the most evident assumptions. This led to the creation of the VASCO (Vanishing & Appearing Sources During a Century of Observations) project. Its aim is to cross-reference historical star catalogs with contemporary ones, searching for stars that have disappeared or emerged over time.

On December 12, the initial findings of this ongoing research were published in The Astronomical Journal, with Beatriz Villarroel from the Swedish Nordita as the lead author. The study compares two star catalogs from approximately 70 years apart: the US Naval Observatory catalog (USNO) B1.0, compiled in the 1950s, and the Pan-STARRS1 survey catalog, released in late 2016.

Utilizing advanced computing power, Villarroel and her team sifted through 600 million entries in the digitized USNO catalog to find their counterparts in the Pan-STARRS1 catalog. This monumental task required substantial computational resources to process the immense datasets efficiently.

The researchers established a system to verify if each of the 600 million light sources from the USNO catalog had a corresponding entry in the Pan-STARRS1 catalog within a 30-arc-second radius. This initial comparison revealed 426,975 mismatches, amounting to 0.074% of the total examined objects.

To refine their findings, the team filtered out mismatches caused by discrepancies in the sky coverage between the two surveys, ultimately narrowing the unmatched objects down to 151,193.

One reason a star might be absent from a specific region of the sky after 70 years is due to significant proper motion, where a star shifts its position. To account for these cases, the authors utilized precise data from the Gaia astrometric satellite, as well as comparisons with the Sloan Digital Sky Survey (SDSS) version 12. After this additional analysis, the mismatches were reduced to 23,667.

The researchers then conducted a visual inspection of the remaining cases using three distinct online image databases. This meticulous comparison allowed them to eliminate false positives—artifacts and cases of incomplete coverage. The total mismatches were subsequently reduced to 1,691.

Further investigation of these final candidates revealed that many discrepancies were not due to actual star disappearances, but rather slight shifts in celestial coordinates, indicating that the stars recorded in the USNO catalog did not align with those in the more recent catalogs.

Ultimately, out of the original 600 million objects, approximately 100 mismatches persisted after all verification attempts. Analysis of these "missing" stars showed that they are predominantly faint and red. Their color and point-like nature suggest they are not Solar System objects; had they been asteroids, they would have left a linear trace due to their movement during the USNO's photographic exposure.

Another potential explanation for the missing stars could be that they are variable stars with significant brightness fluctuations. However, none of the 100 candidates are listed in the General Catalogue of Variable Stars, even when expanding the search radius.

Astrophysical phenomena that could lead to the disappearance of point-like light sources over time include:

• Flaring red dwarfs
• Tidal disruption events (stars being torn apart by black holes)
• Nova eruptions in binary systems
• Supernovae in distant galaxies
• Inactive active galactic nuclei (AGN)
• Failed supernovae, where massive stars collapse into black holes without exploding

The likelihood of encountering a failed supernova within a 70-year timeframe is estimated to be less than one in 90 million. However, flaring red dwarfs could plausibly account for many of the 100 missing objects. Notably, a recent case reported an M-dwarf whose brightness surged by ten magnitudes due to a powerful flare, suggesting that the vanished objects may be M-class stars that were highly active at the time of the USNO observations but later faded below the detection threshold of Pan-STARRS1 and SDSS.

The search for natural explanations is just one aspect of the VASCO project's scope. Researchers are also exploring the possibility of encountering "impossible effects," which refer to the disappearance of light sources potentially caused by artifacts from an advanced extraterrestrial civilization.

From this perspective, a red point source in the USNO catalog with no matches in recent catalogs could represent a "technosignature," a mark of an advanced civilization's technology. The authors note:

>An intriguing aspect of our findings is that a monochromatic interstellar laser operating at 600–680 nm for about an hour could appear as a point source detected only once in a single image, due to the brief duration of the laser's operation.

The technology required to generate such a signature is not beyond humanity's current capabilities; we possess the means to produce short laser pulses lasting mere nanoseconds, with brightness levels exceeding 5,000 times that of the Sun.

Another speculative explanation for a star's disappearance could involve its dimming due to alien megastructures akin to the theoretical Dyson spheres proposed by physicist Freeman Dyson in the 1960s. These colossal orbiting panels could harvest a star's radiation to provide virtually unlimited energy for a more advanced civilization.

However, the researchers of the VASCO project contemplate even more extreme scenarios than a system of orbiting panels designed to capture the energy of a single star. At the core of most galaxies lies a supermassive black hole, surrounded by vast accretion disks that generate temperatures and brightness levels billions of times greater than that of the Sun. An advanced civilization might have figured out how to partially harness such colossal energy by constructing Dyson spheres scaled to the size of active galactic nuclei (AGNs).

At this stage, all these ideas remain speculative. Nevertheless, Villarroel and her team plan to publish a detailed analysis of the approximately 100 identified objects soon, aiming to ascertain the cause of each mismatch between the USNO and Pan-STARRS1 catalogs. The results may reveal whether any light sources remain unexplained by natural causes alone.