Hundreds of stars have disappeared from the sky, where have they gone?
Our sky is full of "transient" phenomena. That is, astronomical events that are
to some extent temporary: among them are counted, for example, supernovae,
the stars that explode at the end of their lives and appear to our eyes as incredibly bright for a
few weeks or months before disappearing forever. Or as well as the more elusive gamma-ray bursts.
How do we discover these things? Simple: by comparing images of the
same area of sky taken at different times. Astronomers therefore often find themselves
comparing photographic plates from many decades ago with digital images made recently.
Which is also what an international team of astronomers did to discover any sources that
disappeared or appeared over the last century. The result? Apparently, hundreds of stars have
potentially disappeared from the sky over the past 70 years.
The researchers compared the position of millions of stars in a catalog published
in the last century with the position they have in a contemporary catalog. Net of all
the mismatches attributable to the proper motion of those stars and artifacts of various kinds,
in the research completed in 2019 a hundred light sources appear to have disappeared
into thin air. This number rose in 2021 to the extraordinary quota of 800 missing stars.
Several natural occurrences could account for their disappearance, but,
alongside them, scientists also consider the search for "technosignatures", that is,
clues that can be traced to artifacts created by
extraterrestrial technological civilizations OK, guys…let's see what it is about.
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What is nobler and enduring 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.
Altogether they formed the sky of the fixed stars, distant and unchanging.
Modern science, especially thanks to the achievements of astronomy and astrophysics
in the last century, has taught us, however, that the stars are anything but eternal and unchanging.
Like everything under the sky, they move and are subject to an evolution:
they are born, grow old and die. But they do it with very long times, millions or, more often,
billions of years: astronomical times, in fact. That said, it seems quite improbable that,
in the short span of human life, stars can suddenly disappear or appear from nowhere.
Certainly, they can increase or decrease in brightness, as happens to the many types of
variable stars cataloged by astronomers. Or they can explode as supernovae, but in that case,
they would leave a very visible trace, such as the Crab Nebula, which is the
nebular remnant of a star that exploded in 1054. In any case, it is very difficult for them to
suddenly appear or disappear as if they were light bulbs turned on or off with a switch.
However, the scientific method also teaches that we should never take anything for granted
and that even the most obvious phenomena should be subjected to empirical verification.
And it is precisely with this intent that in 2017 the VASCO project was born,
an acronym for Vanishing & Appearing Sources during a Century of Observations.
The purpose of the project is in fact to compare photographs of the last century with
more recent ones, looking for stars that were there and are no longer there or, conversely,
stars that were not there and today there are. The first results were published in December 2019,
in an article that appeared in The Astronomical Journal:
the first in a series of annual summaries planned as part of the project.
The article provided the first official data on the outcome of the research carried out so far
and described the results obtained by comparing two star-catalogs made about 70 years apart:
the US Naval Observatory Catalogue (USNO), whose observations date back to the 1950s,
and the Pan-STARRS1 survey catalog, published at the end of 2016.
As you can imagine, the work done on these two databases was only possible thanks to
the enormous computing power that computer tools make available to scientists today.
In fact, the authors have selected as many as 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,
that only very powerful computers could have "milled" in acceptable times.
The comparison was set up to verify, for each of the light sources taken from the USNO catalog, the
existence of a counterpart in Pan-STARSS1 within a maximum angular distance of 30 seconds of arc.
From this first skim, 426,975 mismatches emerged:
a number equal to 0.074% of the total number of objects examined.
The researchers then filtered the set of mismatches, eliminating those
that depended on errors due to different coverage of the sky by the two surveys.
In the end, of the 600 million objects initially considered,
about 100 mismatches survived all attempts to track down the missing object in recent catalogs.
There is a first clue: analysis of the magnitude and color of these 100 "missing" objects
showed that they are almost all very dim and very red.
The color and the fact that they have a point-like appearance suggests that they are not Solar
System objects: if they had been, for example, asteroids, they would have left a linear trace,
due to the displacement accumulated during the exposure (which in the USNO catalog photographic
plates was about 50 minutes). Also, Solar System bodies are typically much bluer
(because they reflect sunlight) than the 100 or so candidates discovered.
So what? Is the disappearance of these stars a mystery or a trivial consequence
of already known phenomena? This could be discussed endlessly...
When comparing vast star fields photographed at different times,
the probability of finding point sources visible in one image and
not in the other is very high. And this can be due to a large number of good reasons.
The first that comes to mind is that the stars have not disappeared,
but much more simply moved. But if that were the case,
since the proper motion of the fastest stars is on the order of a few seconds of arc,
the stars could have moved at most a few minutes of arc and would have been easily detected.
The most likely reason is that the missing objects are variable stars with large differences
in magnitude between the peaks of maximum and minimum brightness. But after extensive checks,
it was found that none of those 100 objects were listed in the General Catalogue of Variable Stars.
They could be Novae stars, i.e. stars that for a certain period of their life
go through a paroxysmal increase in brightness and then return to their minimum levels.
But in that case, they would have found the residual gas ejected during the violent explosion
of the surface layers. Which did not happen. Completely discard the hypothesis of stars
exploded as supernovae. First, because if they had been galactic supernovae,
i.e. belonging to our Milky Way, they would have been visible to the naked eye... and second,
because even supernovae (even more so, since the explosion destroys them completely!)
leave very characteristic and evident nebulous residues.
On the other hand, red dwarf flares could be the most plausible explanation for many
of the 100 missing objects: it has been recently reported, for example,
at least one case where the brightness of an M-class dwarf increased by 10 magnitudes
after a super-powerful flare. The presumed missing objects could therefore be M-class stars that
were very active at the time the observations were made for the USNO catalog and that later
faded to the point of falling below the minimum magnitude detectable by the Pan-STARSS survey.
A fascinating explanation is that they may be "failed supernovae",
i.e. stars of at least 20 solar masses that, having exhausted their nuclear fuel,
collapse directly into black holes, without going through a real supernova explosion.
Traditionally, massive stars are expected to end their life cycle by triggering a supernova,
a violent event in which the stellar core collapses into a neutron star, potentially
followed by a further collapse into a black hole. During this process, a shock wave expands at great
velocity ejecting large amounts of matter into the surrounding space. Matter that will then
go on to form the so-called supernova remnant. In recent years, however, there have been growing
suspicions that not all massive star deaths are accompanied by such catastrophic events.
And in 2015, using the Large Binocular Telescope in Arizona, astronomers may
have obtained the first confirmation of their hypotheses. In the well-known galaxy NGC 6946,
in fact, a star about 25 times more massive than the Sun was seen to become brighter,
although not at the level expected for a supernova, and then disappear completely!
A truly unexpected phenomenon, which could soon lead to the confirmation of the existence of a new
class of celestial phenomena... But as interesting as this is, there are many doubts that this can
explain the hundreds of stars that disappeared in recent decades... Also considering the fact
that a hypothetical failed supernova should disappear within a galaxy well recognizable.
To finish, and to put some pepper in the story, there is obviously also the
possibility that these 100 missing stars are only real defects in the images ... who knows!
In the meantime, however, according to the authors of the research,
in 2021 the initial database of comparison has been greatly expanded and the stars vanished
into thin air have now become more than 800! The case begins to get really curious,
also because it is still far from a reasonable solution.
However, the search for natural explanations, whatever they are,
is only a part of what the research proposes. The VASCO project, in fact,
was not born only to find stars disappeared as a result of natural causes, but also to explore
what are the chances of running into what the authors call "impossible effects". By this,
they mean that although during the survey have never emerged evidence of an extraterrestrial
involvement of ET type, they do not exclude that the disappearance of a light source may occur
by extraterrestrial technological civilizations that operate on a stellar or even galactic scale.
In short, the authors consider possible the involvement of extreme
astrophysical phenomena not yet identified but do not rule out 100% the alien hand.
In case they were really involved alien civilizations, the phenomenon
of missing stars could be explained by the darkening by alien megastructures on the
type of the so-called Dyson spheres, imagined in the '60s of last century by the English
physicist and mathematician Freeman Dyson. These would be giant orbiting panels,
extended over several astronomical units, able to intercept all the radiation of a star to provide
energy in almost inexhaustible quantities to civilization much more advanced than ours.
But the researchers of the VASCO project imagine even more extreme possibilities. In the heart
of almost all galaxies are hidden supermassive black holes, surrounded by giant accretion disks,
inside which are generated temperatures of billions of degrees and brightness trillions
of times greater than the solar one. There is no localized source of energy in the Universe greater
than that produced in active galactic nuclei. A truly advanced technological civilization
could have found a way to use at least in part such an immense amount of energy,
building Dyson spheres appropriate to the scale of active galactic nuclei.
For the moment, all this is only pure speculation. What is certain is that we are facing a mystery
to be solved. Its solution could open the way to a whole new physics,
overturning many misconceptions about the life cycle of stars.