Why are binary stars so different?

Binary stars are usually formed simultaneously from the same gas and dust clouds. Therefore, if they have the same mass, they should evolve similarly. However, sometimes this is not the case at all, and scientists were able to find out the reasons for this situation.

Binary stars. Source: NOIRLab/NSF/AURA/J. da Silva (Spaceengine)/M. Zamani

How binary stars form

A team of astronomers using the Gemini South telescope has confirmed for the first time that differences in the composition of binary stars may be the result of chemical variations in the cloud of stellar material from which they formed. The results help explain why stars born from the same molecular cloud may have different chemical compositions and planets in their orbits, and also cast doubt on modern models of the formation of such systems.

It is estimated that up to 85 percent of stars exist in binary star systems, and some even in systems with three or more luminaries. These pairs are born together from a single molecular cloud from a common large number of chemical building blocks, so astronomers expect them to have almost identical compositions and planetary systems. 

However, this is not the case for many binary systems. Although some proposed explanations link these differences to events that occurred after the evolution of stars, the team of astronomers confirmed for the first time that they could occur even before the stars began to form.

Using the Gemini Observatory’s new High-Resolution Optical Spectrograph (GHOST), the team studied the different wavelengths of light, or spectra, emitted by a pair of giant stars and found significant differences in their chemical composition.

What is the reason for the differences between the stars?

Previous studies have suggested three possible explanations for the chemical differences between binary stars. Two of them are related to processes that could occur during their evolution: atomic diffusion, or the deposition of chemical elements in gradient layers depending on the temperature and surface gravity of each star, and the absorption of a small rocky planet that could introduce new chemical elements into a thermonuclear furnace.

The third possible explanation refers to the beginning of star formation and suggests that the differences originate from primary regions of nonuniformity in the molecular cloud. Simply put, if a molecular cloud has an uneven distribution of chemical elements, then the stars born in this cloud will have a different composition depending on which elements were available in the place where each of them formed.

So far, studies have concluded that all three explanations are possible, but these studies have focused exclusively on main sequence binary stars. These include most of the luminaries in the universe, including the Sun.

Binary stars reveal their secrets

During a new study, Carlos Saffe and his team observed a system consisting of two giant stars. They have extremely deep and highly turbulent outer layers or convective zones. By studying their properties, scientists were able to exclude two of the three possible explanations for this case.

The continuous swirling of the liquid in the convective zone makes it difficult for the material to settle in layers, which means that giant stars are less sensitive to the effects of atomic diffusion than previously thought, and this excludes the first explanation. The thick outer layer also means that the absorption of the planet would not have changed the composition of the star much, since the absorbed material would have dissolved quickly, which excludes the second explanation.

This leaves the initial inhomogeneities within the molecular cloud as the only possible explanation. “This is the first time astronomers have been able to confirm that differences between binary stars begin at the earliest stages of their formation,” says Saffe.

Consequences of scientists’ research

Three implications of this study are of particular importance. First, these results provide an explanation for why astronomers see binary stars with such different planetary systems. Different planetary systems can mean very different planets: rocky, Earth-like, ice giants, gas giants. They can orbit their luminaries at different distances and where the potential for life support can be very different.

Secondly, these results seriously challenge the concept that the different chemical composition of stars means that they were formed from different gas and dust clouds. Now it is obvious that even luminaries born together can be very different.

Finally, the observed differences previously attributed to the falling of planets on the surface of stars will need to be reconsidered, since they can now be considered as having existed from the very beginning of the existence of the luminaries.

According to phys.org

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