The Sun has us spoiled. In our solar system, a single star occupies the central position and dominates the movement of the rest of the objects that form it. But this is not the case for most stars in the universe . At least of the observed systems, which are millions.
Most stars are part of a multiple solar system. Without literally going any further, the three closest stars are part of the triple system of Alpha Centauri. This system is made up of two very Sun-like stars , Alpha Centauri A and Alpha Centauri B, orbiting relatively close to each other, and another star, a red dwarf called Proxima Centauri (or Alpha Centauri C) , orbiting the Sun. main pair from a distance. Alpha Centauri A and B describe an elliptical orbit, coming closer to each other to 11 astronomical units, slightly more than the distance between the Sun and Saturn, and moving apart to about 36 astronomical units, farther than the orbit of Neptune. An astronomical unit is nothing more than the average distance between the Sun and the Earth and is equivalent to approximately 150 million kilometers. Around this pair of stars with a relatively compact orbit, Proxima Centauri orbits at about 4300 astronomical units, taking more than half a million years to complete an orbit. Proxima Centauri is so named because it is the closest star to the Sun.
Going further, but not much further, we have several examples of known stars that are truly multiple systems . Sirius, the brightest star in the sky, located near the constellation Orion , is actually a double system. The largest and brightest star , the one we can see with the naked eye, would be Sirius A , a star twice the mass of the Sun and 70% larger in diameter. Its companion, Sirius B , would be a white dwarf with impressive features. Its mass is slightly greater than that of our own Sun and yet it is about the size of the Earth. This makes it one of the most massive and compact white dwarfs known.
Another known star that would be part of a multiple system would be Polaris, the Pole Star. Similar to the Alpha Centauri system, Polaris consists of two closely orbiting stars , at distances less than the distance between Jupiter and the Sun, and a third distant companion at a distance of more than 2000 astronomical units. .
Another interesting case is that of Mizar, a star that is and is not multiple at the same time. Let me explain: Mizar, the penultimate star on the handle of the pan made up of the brightest stars in the Ursa Major constellation, is usually considered a double star, along with Alcor. The interest of this pair of stars lies in the fact that their separation can be observed with the naked eye. However, these stars are more likely to form a visual double star , rather than a physical one. That is, an apparently double star, but which is not gravitationally bound. Because of the position of these stars relative to Earth, they appear very close together in the sky. It is true that these stars are especially close to each other, we believe that they are around 1 light year apart, but not enough to consider that they form a binary system.
What are multiple systems are each one of the stars separately . Alcor is actually a dual system, while Mizar is a quadruple system . The 4 stars of Mizar are distributed in two pairs of stars, each one of which orbits each other very closely, while the two pairs reach a minimum distance of 380 astronomical units from each other, about 3 times more than the distance that separates the Earth. Land of the Voyager probes. If it is confirmed that Mizar and Alcor are closer than we currently believe , this system could become a sextuple star system. However, it would not be the system with the largest number of stars. That credit goes to Gamma Cassiopeiae , the central star of the W marked by the five brightest stars in the Cassiopeia constellation.
The main star, responsible for the brightness that we can perceive from Earth with the naked eye, is a blue giant, with a mass 17 times that of our Sun. This star is incredibly young, having formed just 8 million years ago and is likely to end exploding as a supernova within a relatively short time, leaving behind a neutron star. Very close to it orbits another star with, we believe, a mass similar to the Sun. On the other hand , and at a greater distance, orbit the so-called Gamma Cassiopeiae B, C, D. The first two are simple stars, which orbit the main giant from the distance. However, Gamma Cassiopeiae D is itself a quadruple star system, orbiting each other and whose center of mass orbits the parent star.
Around these multiple systems, the appearance of stable planetary orbits is more complicated, especially for planets orbiting close to only one of the stars. However planets can be found orbiting several stars with compact orbits. For example, the Kepler-64 quadruple system, configured similarly to Mizar, with two compact pairs of stars orbiting each other at great distances, has a confirmed exoplanet orbiting one of those pairs. The planet is about the size of Neptune and orbits at a distance similar to that between Venus and our Sun, with the two stars even closer together.
Around the star Kepler-444 we have the other possibility. The central star , with 75% of the mass of the Sun, has on the one hand 5 planets similar in size to Mars orbiting considerably closer than Mercury to the Sun and on the other hand a pair of red dwarfs, with 25% of the mass from the Sun, each orbiting at a greater distance.
All these examples show that our solar system is quite simple and even boring, although this simplicity could have been key when a planet appeared on which life could develop.
REFERENCES:
D.J. Hutter et al, 2021, Surveying the Bright Stars by Optical Interferometry III: A Magnitude-Limited Multiplicity Survey of Classical Be-Stars, Astrophysical Journal Supplement Series, DOI: 10.48550/arXiv.2109.06839
