Even though our imagination picturing a binary system of stars usually involves Earth-like landscapes presenting two stars on the sky at the same time, it seems that we might not be so imaginative as previously thought as this reaction is just a bias towards something familiar, while the true picture shows universe as somehow out of the ordinary. According to astronomers, as much as half of the stars in the visible universe might be part of a binary system.

In order to understand how such objects react in the presence of one another, Ronald Taan of Northwestern University started a collaboration with NCSA, spanning the last seven years to study the phenomenons that take place during the envelope phase of a binary system evolution, mostly concentrating on the boundaries of the outer regions of the stars' atmosphere.

Such envelopes start to evolve as the stars become so close one another that they share parts of the atmosphere, enabling change of material between the two objects and determining radical changes in both stars' structures as well as violent interaction. Both suffer severe drops in angular momentum, the orbits around each other shorten from just a few months to a couple of days and in most cases the mass transfer occurs from the larger, more massive star to the smaller companion, which might eventually result in some material ejection from the common envelope of the system.

This scenario may result in two different outcomes as the two stars orbit each other; one in which the least massive star merges with its more massive companion, to form a single star, or the material sharing phase stops at one point and both survive the encounter which implies that one of the cores of the stars will undergo a phase that will result in the creation of a black hole or a white dwarf remnant system known as a compact binary.

Computer simulations made by Taam reenacted the conditions in which the common envelope of the two stars suffers an ejection and revealed that this common envelope can have diameters of a few astronomical units across, surrounding a stellar core as small as the Earth. During material sharing the diameters of the orbits of the two stars can vary violently, thus the simulation needs to be tracked in very small increments.

Due to the fact that the simulation placed the stars in close orbits of each other, the area containing the common envelope, which presented most interest, received limited movement inside the nested grid of the program, generating situations in which the ratio between masses of the two stars was increased. To resolve this problem Taam, augmented the program by using a FLASH code that allowed the placement of the nested-grid in areas where density varied violently.

So far, current simulations have only been able to reproduce binary systems, enclosed in a volume of only one astronomical unit across, showing that the gas spinning around both of the stellar cores during material sharing exceed critical speed that enables mass ejection in all directions, but mostly around the areas in the equatorial plane of the system, while ejection in the poles were extremely low.

The team that produced these simulations argue that future high resolution models will reveal the result of these interactions for the creation of binary systems in the universe, including models of stellar evolution, and the combinations of initial parameters of star evolution, to estimate the total number of types of stars that populate the universe. Currently available models only include one-dimensional stellar representations and extremely simple interactions, and only study the impact these binary systems produce instead of modeling them.

However, ultimately they will unveil the particular set of conditions in which the common envelope phase takes place and the process of compact binary systems formation, as well as how the universe in general evolved.