Everything about Star System totally explained
A
star system or
stellar system is a small number of
stars which orbit each other, bound by
gravitational attraction. A large number of stars bound by gravitation is generally called a
star cluster or
galaxy, although, broadly speaking, they're also star systems.
Star system is occasionally also used to refer to a system of a single star together with a
planetary system of orbiting smaller bodies.
Binary star systems
A stellar system of two stars is known as a
binary star,
binary star system or
physical double star. If there are no
tidal effects, no perturbation from other forces, and no transfer of
mass from one star to the other, such a system is stable, and both stars will trace out an
elliptical orbit around the
center of mass of the system indefinitely. See
Two-body problem.
Examples of binary systems are
Sirius,
Procyon and
Cygnus X-1, the last of which probably consists of a star and a
black hole.
Multiple star systems
Multiple star systems or
physical multiple stars are systems of more than two stars. Multiple star systems are called
triple,
trinary or
ternary if they contain three stars;
quadruple or
quaternary if they contain four stars;
quintuple with five stars;
sextuple with six stars;
septuple with seven stars; and so on.
Dynamics
Theoretically, modelling a multiple star system is more difficult than modelling a binary star, as the
dynamical system involved, the
n-body problem, may exhibit
chaotic behavior.
Many configurations of small groups of stars are found to be unstable, as eventually one star will approach another closely and be accelerated so much that it'll escape from the system. This instability can be avoided if the system is what Evans has called
hierarchical. In a hierarchical system, the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system's
center of mass. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on. In this case, the stars' motion will continue to approximate stable
Keplerian orbits around the system's center of mass, unlike the more complex
dynamics of the large number of
stars in
star clusters and
galaxies.
Observation
Most multiple star systems known are triple; for higher multiplicities, the number of known systems with a given multiplicity decreases exponentially with multiplicity. For example, in the 1999 revision of
Tokovinin's catalog of physical multiple stars,
551 out of the 728 systems described are triple. However, because of
selection effects, our knowledge of these statistics is very incomplete.
, §2.
Because of the dynamical instabilities mentioned earlier, triple systems are generally hierarchical: they contain a close
binary pair which has a more distant companion. Systems with higher multiplicities are also generally hierarchical. Another system known with six stars is
ADS 9731, which consists of a pair of two triple systems, each of which is a
spectroscopic binary in
orbit together with a single star.
Examples
Binary
Triple
Polaris, the north star, is a triple star system in which the closer companion star is extremely close to the main star—so close that it was only known from its gravitational tug on Polaris A until it was photographed by the Hubble Space Telescope in 2006.
Alpha Centauri is a triple star composed of a main binary yellow dwarf pair (Alpha Centauri A and Alpha Centauri B), and an outlying red dwarf, Proxima Centauri. A and B are a physical binary star, with an eccentric orbit in which A and B can be as close as 11 AU or as far away as 36 AU. Proxima is much further away (~15,000 AU) from A and B than they're to each other. Although this distance is still small compared to other interstellar distances, it's debatable whether Proxima is gravitationally bound to A and B.
HD 188753 is a triple star system located approximately 149 light-years away from Earth in the constellation Cygnus. The system is composed of HD 188753A, a yellow dwarf; HD 188753B, an orange dwarf; and HD 188753C, a red dwarf. B and C orbit each other every 156 days, and, as a group, orbit A every 25.7 years.
Quadruple
4 Centauri
Mizar is often said to have been the first binary star discovered when it was observed in 1650 by Giovanni Battista Riccioli, p. 1; , but it was probably observed earlier, by Benedetto Castelli and Galileo. Later, spectroscopy of its components Mizar A and B revealed that they were both binary stars themselves.
HD 98800
Quintuple
Nu Scorpii
Sextuple
Castor
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