Astronomers have long noted unusual binary systems in which yellow dwarfs orbit safely and are in close proximity to black holes. The problem was not that a relatively small star could have such a dangerous companion, but how it could “live” until the moment of its emergence.

A stellar-mass black hole was once the core of a very massive luminary that was dozens of times more massive than the Sun. These do not “live” for long: sometimes in just 10 million years all the thermonuclear fuel in their cores is completely “used up”.

Then nothing can prevent gravity, which tends to compress matter. This compression causes the core to become incandescent, heating and blowing the surrounding layers. The star temporarily becomes a red giant and then explodes as a supernova, shedding its entire outer shell. At the same time, the core “collapses” into itself: it turns into a black hole.

This classical scenario, observed everywhere, makes it impossible, especially at close range, to imagine a black hole having a “alive and intact” dwarf companion. By all calculations, the massive partner should absorb it while it is still in the red giant phase: aging stars expand to very large diameters, and being near them at this time is certain death.

That’s why astronomers are so surprised by two systems in particular discovered by the Gaia space observatory: BH1 and BH2. BH, as you might guess, means black hole. Black hole No. 1 is 1,560 light-years away in the constellation Ophiuchus, No. 2 – It is located 3,800 light-years from Earth in the constellation Centaurus. Their masses are very similar: 9.6 and 8.9 masses of the Sun.

Both have companions, and in both cases they are stars that have about as much “weight” as our luminary. Both are still “alive”, but the second is already completing the main stage of its evolution and has turned into a red giant. But it is still a sun-like star of its kind.

It is interesting that these “suns” are located very close to their black holes: in BH1 the star is almost closer to the Earth than the Sun, in BH2 it is where Jupiter is in the Solar System. At the same time, the “progenitors” of these black holes were massive stars: for example, BH2 was the core of a luminary with a “weight” of 92 Suns, and BH1 certainly had a mass of more than 50 Suns. How they didn’t “eat” their comrades before they “died” or kick them out of the system after the supernova exploded is an interesting question.

A team of astronomers from Switzerland, the USA, Spain and Greece presented an answer to this question. In their paper, scientists shared calculations that revealed that these massive “progenitors” of black holes were never red giants: during the “burning” of the core, they “costly” lost most of their matter long ago. .

Such examples are observed in space and are called Wolff-Rayet stars (in memory of the pioneering French astronomers). These fixtures are initially heavy, but they seem unable to hold such a mass and will gradually push it out. One of the most spectacular examples is WR 124, which, although it did not explode as a supernova, is surrounded by a heavy “cloud” of its own ejected matter.

In such a scenario, BH2 would have to have only 11 solar masses when it ends up as a star, the astronomers explain. A few solar masses were thrown out, and the rest “collapsed” into a black hole.

Modeling showed that, in principle, all this allows the yellow dwarf companions to continue to “live” peacefully together with their giant “brothers” while all these transformations take place. Meanwhile, low-mass stars “burn” hydrogen inside themselves for billions of years, so for them the metamorphosis of weightlifters is a fleeting memory. The observed situation also raises a more interesting question: Does this mean that planets of sun-like stars could also have a similar neighborhood and then exist near a black hole?