By Julio Chanamé, Young Researcher of MAS and member of the UC Institute of Astrophysics
Binary stars are two-star system that are “linked” and orbit around each other thanks to the mutual force of gravity. This is exactly the same as the Earth-Moon system or the Sun-Jupiter system.
In general, binary stars have been an important gold mine for modern astronomy. Thanks to simple observations of its components’ movement, and given that we know how gravity works, binary stars have been almost the only method available to accurately and reliably measure individual stars’ masses, which is the most important property of these and the only that determines its internal structure, evolution and final destination. At the same time, certain type of binary stars, whose components eclipse one another, remains to this day one of the most accurate methods for measuring the distances that separate us from the galaxies around us. Finally, binary systems containing white dwarfs (very compact remnants of stars that have already completed their cycle of life) are those that are involved in supernovae explosions that produce many of the chemical elements present in the universe, therefore being responsible for chemical evolution that ended up with the presence of humans on the third planet of the solar system. Like these examples, there are many more that can be listed.
However, binary stars used to study these areas are only a fraction of the complete population of existing binary systems in a galaxy like ours. In particular, the binaries normally used are those in which the distance or separation between their components is less than 100 UA (an AU is the average distance between the Sun and the Earth, approximately 150 million km.) The time needed for stars of a binary system to complete an entire circle around each other –which is known as the orbital period– depends on the separation and the masses. If for a binary with a separation of 1 UA and a mass similar to the Sun, this period is approximately of 1 year, but for a one of 100 UA of separation and the same mass, this period will be of about 300 years.
Surprisingly, there are binary systems that we call “wide” in the Milky Way, whose stars are separated by distances hundreds of times larger than these. The widest binaries found in our galaxy have a separation of 200,000 UA and more. Since their orbital periods are extremely long, these binaries cannot be used for the purposes previously mentioned.
But wide binaries have, of course, advantages that those of small separation do not. There is one, quite interesting, and that is based on something almost intuitive: the slower the movement of stars around each other in a binary system, the easier it will be for the binary to “break”. In another words and in the opposite sense: The closer the stars in a binary system, the stronger the gravitational attraction between them, the higher the orbital speed and therefore, the more difficult it will be to make the two stars separate and go their own way. In astronomical jargon, the smallest the separation between components, the stronger the stars in the binary are linked. Extremely wide binaries are weakly linked and therefore susceptible to break, consequently their components become isolated stars, far from the other.
Due to that susceptibility to be broken by the environment where they live, the study of the properties of a population of wide binaries reports about the gravitational characteristics of the aforementioned environment. In the Milky Way’s case, gravitation is controlled by the effect of dark matter, of which we know very little. Wide binaries, then, can be used to study this dark matter’s distribution and nature, which for several decades has evaded all the science’s efforts to understand it.
Recently, using the first catalog of observations of the Gaia Space Mission, we have been able to identify the largest and clearest sample of very wide binary stars in the solar neighborhood [1,2]. Our first characterization of this local population of wide binaries give us great hope that the next Gaia catalogs will allow us to learn more about still unknown properties of how dark matter is accumulated and distributed in the Milky Way, this is what holds the key to achieve a better understanding of what this is, which is, for sure, one of the most urgent problems of contemporary science.