Can a star disappear without a trace?

Where are the thousands of stars photographed in the sky in the middle of the last century and that no longer appear in modern photos?

In a recently published study, led by researchers from the Center for Astrobiology (CSIC-INTA), it was discovered that thousands of objects detected on photographic plates taken in the 1950s have disappeared in much more recent observations.

A study carried out with the Virtual Observatory (a kind of astronomical Google that allows us to quickly and efficiently find and analyze everything related to any astronomical object) has confirmed that objects previously detected on photographic plates are not detected in observations much more recent ones made with much more powerful telescopes and instrumentation.

In the new study thousands of images, millions of objects have been analyzed and counterparts have been sought in dozens of astronomical files, each of them also with millions of objects.

The reasons for these disappearances can be diverse and the authors of the study, published in the academic journal Monthly Notices of the Royal Astronomical Society, weigh and analyze them one by one.

Detection of an astronomical object on a photographic plate taken in the 1950s (top left), an object that does not appear in more modern observations taken in both the visible (top right and bottom left) and infrared ranges (Bottom right). (Image: CAB / INTA / CSIC)

The work has been directed by Enrique Solano, who is a researcher at the Center for Astrobiology (CAB), dependent on the Higher Council for Scientific Research (CSIC) and the National Institute for Aerospace Technology (INTA), all of these entities in Spain.

For Solano, this list of objects is, without a doubt, a treasure to be explored in the short or medium term with new telescopes, both from the ground and from space. “As an example, a preliminary analysis has made it possible to identify a brown dwarf, a type of object that already appeared in observations in the 1950s and for which, however, it was necessary to wait another 40 years for its official discovery.”

Among the explanations for these “disappearances” is the possibility that the objects identified on the photographic plates were never real, but merely defects. The fact that the original plates are treated as true astronomical relics and their access is strictly limited, makes it very difficult to verify this hypothesis. However, it is highly unlikely that all the identified objects are plate defects and that they all have the spherical shape expected for real stars.

Another possible explanation is that the object is still visible, but has changed position. In this category would fall those that are known by the name of stars with high proper motion. To identify them, the study has made use of data from the Gaia mission, which has generated the most complete census of positions and velocities of stars to date. And, although there could be objects with high proper motion that had not been cataloged by Gaia, it is not expected to find many.

A third option is to posit that the object has not disappeared: it is still there but its brightness has dimmed so much that it is not detectable even with modern telescopes and instrumentation. In this case, the ones known as variable stars, stars that change their brightness, naturally enter.

Specifically, stars of spectral type M with flares would explain a high percentage of these thousands of objects that have disappeared in recent decades. They would be objects that experienced a flare at the time they were observed in the 50s and that, at the time in which the most modern mapping took place, were in a much calmer state and with brightness below the level of light. detection.

Continuing with the explanations, another possibility is that the object has disappeared due to an unknown astrophysical process. According to some models, very massive objects (more than 15 solar masses) could collapse in a way that would make them become a black hole directly without going through the explosive and very showy supernova phase. These objects are known as “failed supernovae”. An object of this type visible in the 1950s would not be detectable in modern surveys. However, although different failed supernova candidates have been proposed, we are still far from understanding their true nature (if they really exist).

There are other hypotheses that have been ruled out. One of these is that the missing object is an artificial satellite. Keep in mind that the first, Sputnik-1, was launched into space in October 1957, while 99% of the observations analyzed in the study were made between 1949 and 1956. Another hypothesis ruled out is that the object was hidden by a technologically advanced extraterrestrial civilization. Although this last hypothesis is very difficult to refute 100%, experience (for example, the discovery of the first pulsar or the anomalous brightness changes in objects such as the star of Tabby or Betelgeuse, which also raised the suspicion of a intervention of aliens, tells us that it is much more reasonable to associate the disappearance of these astronomical objects to natural causes, than to develop other alternative hypotheses.

In conclusion, the team states that it is not certain what these objects really are. They could be low-mass flare stars, other types of variable stars, uncatalogued high proper motion objects, previously unidentified asteroids, high redshift supernovae, extragalactic objects, or even unknown astrophysical processes. The adventure of discovery has only just begun. (Source: CAB / INTA / CSIC)