Astronomers have discovered the first triple star system with a black hole in the history of observations.

The sensational “Three Body Problem” by the Chinese science fiction writer Liu Cixin clearly showed how unstable and therefore a rare phenomenon in the Universe a star system of three stellar mass objects (stars or a black hole) can be. All the more surprising was the discovery of such a system, in the center of which a black hole was discovered for the first time.

An artist’s impression of the V404 Cygni system (the black hole tearing apart a nearby star and holding a distant one). Image source: Jorge Lugo

The source of information for the discovery was data collected by the European astrometric satellite Gaia. This device creates a dynamic three-dimensional catalog of stars in the Milky Way and slightly beyond. “Gaia” determines the vector and speed of movement of stars, which makes it possible to determine gravitationally bound objects and identify double and even triple systems.

According to Gaia measurements, the system originally thought to be a binary system, V404 Cygni, includes a third star. The system is 7800 light years away from Earth. Not far from the compact center in the form of a black hole of stellar mass and a nearby star being torn apart by it with an orbital period of 6.5 days, there is a third star, apparently gravitationally connected to the system. The nuance is that this star is located at a distance of 3500 AU. from a black hole and makes a complete revolution around it in 70 thousand years. This star had been visible before, but only Gaia measurements were able to show its connection with the two central objects.

Actually, in such a configuration, the notorious three-body problem is solved in a positive way – such a system will be gravitationally stable for a conditionally infinitely long time. The essence of the discovery is different – the gravitational binding of the third distant star to the central pair is so weak that in this situation it seems impossible.

On the left is an image of the central pair and a distant star in optics, on the right – in the infrared range. Image source: Nature 2024

The fact is that the central black hole should have formed as a result of a supernova explosion, shed its outer shell and collapsed with its core. All these violent manifestations should have broken the weak gravitational connection with the third star. This would not have happened only if the collapse had occurred without a supernova explosion. Such a phenomenon is theoretically possible, but it is difficult to detect and confirm with observations (the supernova will be visible in any case).

Modeling of the situation with the Cygnus V404 system showed that the collapse of the central star inward is the most likely scenario to describe what astronomers saw in the Gaia data and subsequent observations of the system. The question immediately arose: is it just luck, or are triple systems with black holes an indispensable or frequently occurring stage in the evolution of black holes? Only subsequent observations can answer this question.