Surprise, the “black hole” closest to the Earth was ultimately not one

ESO astronomers thought they were dealing with a black hole, but they caught a star in the act of vampirism.

In 2020, a team from the European Southern Observatory made a startling discovery. They announced the discovery of a black hole located about 1000 light years from Earth, which is exceptionally close for an object of this type. Today, in the light of new studies spotted by ScienceAlert, it appears that this cosmic glutton was not one, or rather, not of the expected type: it was indeed not a black hole, but a a vampire star that has fed on its neighbour.

Astronomy is sometimes a thankless science. Unlike other disciplines such as biology, observers of the cosmos do not have the leisure to set up dozens of experimental protocols to test hypotheses one after the other. Very often, they are dependent on signals that are not only weak, discreet and difficult to capture, but also systematically incomplete and regularly distorted.

When astronomers observe a curious phenomenon, the challenge is therefore to come up with an explanation that is as coherent as possible based on these few chewed pieces of an old puzzle; they remain attentive to the fact that it is almost always an extrapolation studded with gray areas.

Overall, with advances in theory and technology, researchers have become pretty good at formulating strong hypotheses. But to err is human and no one is immune to misinterpretation, and that’s what happened to ESO researchers with a cosmic object dubbed HR 6819.

© NASA – Goddard Space Flight Center

A casting mistake to begin with

When it was first spotted in the 1980s, astronomers at the time were convinced it was a Be star. It is a somewhat special type of star which is distinguished by a few peculiarities in its emission spectrum, but also and above all by its immense speed of rotation.

It is a type of object that is comparatively uncommon to observe, especially at the time. The scientific community has therefore kept an eye on this curiosity. Over the course of observations, it appeared that it could be not a single celestial body, but a pair of stars located in the vicinity of each other.

When they are interested in a celestial body, astronomers generally engage in a little game: they try to model the behavior of the objects in question from the information at their disposal. If their model is consistent with observable reality, the starting postulate is probably quite close to reality; but in the contrary case, this difference often hides an important parameter which has still eluded researchers.

In this specific case, it was the oscillations of this Be star that challenged the ESO team. They found that variations in orbital parameters did not match their models; they therefore deduced that a third object located nearby was disturbing the tango of these two planets. Their instincts told them it was a black hole, and based on the evidence at their disposal, they published a research paper presenting the closest black hole ever spotted.

A story of distance

But this conclusion did not satisfy all their colleagues; some of them were skeptical, and continued to defend another hypothesis. For them, the particularities observed are not necessarily linked to the influence of a black hole; it could also be a direct consequence of the interaction between the two stars.

ESO has therefore teamed up with a group of researchers based at KU Leuven, Belgium, to try to make sense of things. It was the latter who provided the element that made it possible to decide; a “simple” matter of distance finally made it possible to exclude the presence of a black hole.

In a joint press release, the two teams explain that if a black hole was indeed present in this system, the two stars would necessarily be quite far from each other. They therefore waited patiently for an observation window to try to determine the distance which separates them. They recently achieved this using ESO’s Very Large Telescope (VLT), and the result was against the “black hole” team.

Indeed, this observation confirmed that the two stars were very close to each other, much closer than they could be in the presence of such a cosmic monster; the two stars do not revolve around a black hole, but rather around each other Tatooine style in Star Wars. Or, more precisely, around a common center of gravity; we, then, speak of a binary system.

© Dinu J Nair – Unsplash

A real cosmic black widow

This is not a disappointment, quite the contrary; it’s a great opportunity to observe two stars at a critical but still very poorly documented moment in their lives. Indeed, the two stars indulge in a cataclysmic tango against a background of requiem. One of the two stars is near death; it is more or less short of the resources necessary to sustain the thermonuclear reactions that define the star.

And there is no need to look far to find the cause of this shortage. The culprit is all found: it is the second star of the couple which, promiscuity obliges, has quite simply engulfed a large part of the gases of its neighbor like a real cosmic black widow.

It is a relatively common cosmic-scale event, and researchers expect this process to play a central role in the life cycle of stars and therefore in the overall dynamics of the universe. But it is also difficult and therefore rare to succeed in observing it; this work particularly excites astronomers, since it is full of potential information on the evolution and end of life of stars.

It is extremely difficult to surprise two stars in this state shortly after this interaction, because the observation window is very short.”, welcomes Abigail Frost from KU Leuven in the press release. “This makes our discovery very exciting, because it makes it a perfect candidate to study how this vampirism affects the evolution of stars, and by extension the formation of associated phenomena, including gravitational waves and supernovae.

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