Gravitational waves and the birth of black holes

Black holes, the most mysterious objects in the Universe, literally mock astronomers. They can have masses ranging from a few to billions of suns and arise from the death of stars. But how exactly do stars turn into black holes? And why do some black holes have mass, while others do not?

Scientists from different countries are trying to answer these questions using a new tool - gravitational wave astronomy. We are talking about space-time fluctuations that occur when two very massive objects merge.

Gravitational waves were first discovered in 2015, and since then we have made some progress in understanding the physical nature of black holes.

In their new paper, published in the latest edition of The Astrophysical Journal, the researchers analyse data on nearly 100 black hole merger events that have been recorded by the LIGO and Virgo gravitational-wave detectors.

According to the report, the mass distribution of black holes has several peaks, meaning that there are certain mass values that occur more often than others. These peaks are located at approximately 9, 16, 30 and 57 solar masses.

To explain the results, the scientists used computer models of the evolution and explosion of stars. They also took into account various factors, such as the initial mass of the star, its chemical composition, and whether it was alone or paired with another star.

It now becomes clear when and why a star loses its outer shell of hydrogen due to strong winds or the flow of matter to a neighbouring star.

If the luminary loses its shell early, it will form a lighter core of carbon and oxygen, which will explode more easily during the collapse and leave behind a neutron star.

If the birth of a supernova is delayed or even fails to explode, a heavier core will emerge. And it is this core that may turn into a black hole.

This means that there is only a theoretical distinction between the two models - the "neutron star" and the "black hole". Usually, there is only a narrow range of nucleus masses at which a star will form a black hole with a certain mass. Therefore, in practice, we cannot be sure what we are observing: an astronomical object can be both. It all depends on our own interpretation of the data.

Let us add: the more carbon in the nucleus, the more energy is released during its combustion. And the greater the chance of an explosion.

The more oxygen there is, the more neutrinos take heat from the nucleus and the less chance of an explosion.

The researchers also took into account that stars with different chemical compositions have different wind strengths that blow away their envelopes.

By comparing their models with data on gravitational waves, the scientists were able to reconstruct the history of the formation of black holes with different masses. They found that a peak of 9 solar masses corresponds to black holes that formed from stars with a metallicity (the proportion of elements heavier than helium) of about 0.1 of the solar mass.

Usually, such stars lose their envelope in a pair with another star after the end of hydrogen burning in the core, but before the start of helium burning.

The peak of 16 solar masses corresponds to black holes that formed from stars with a metallicity of about 0.01 of the solar mass. These stars lost their envelope after the end of helium burning in the core.

But the peak of 30 solar masses corresponds to black holes with a metallicity of 0.001 of the solar mass. Stars thus remain lonely for the rest of their lives.

The 57 solar mass peak is the merger of two black holes in dense star clusters.