How Many Stars Can A Planet Have?
In this video we will talk about multiple star systems.
A multiple star system is a group of two or more gravitationally bound stars.
Let's start with the simplest type: the double star system,
also designated as a double star, physical double star or binary system,
in which the two components orbit around the common center of mass.
If the two stars have a similar mass, to an external observer who observes them from a certain
distance, the two stars will seem to orbit each other. If, on the other hand, the two stars have
very different masses, the less massive star will seem to orbit around the more massive one, because
the common center of mass falls within the latter. If there are no tidal effects, no perturbation
from other forces, and no mass transfer from one star to another, a binary system is stable and
both stars trace an elliptical orbit around the center of gravity of the system indefinitely.
If, on the other hand, the components of a binary system are close enough (narrow binaries), they
can reciprocally distort their atmospheres and, in some cases, they can even exchange material
so as to modify their normal evolution. A variety of narrow binaries are so-called contact binaries,
which are so close to each other that they share a considerable percentage of matter.
In a binary system, the brightest component is called primary, the less bright secondary.
One or both stars may be accompanied by planets. It is important to point out that, in addition to
the double physics linked gravitationally, there are also double visual stars,
which are not physically linked, but are located close together only for a perspective effect,
and which we will not deal with in the video. Examples of binary systems are:
- Sirius, a binary consisting of a type A star with main sequence and a white dwarf
- Procyon, who is similar to Sirius - Mira, a variable consisting
of a red giant and a white dwarf - Delta Cephei, a Cepheid variable
- Epsilon Aurigae, an eclipsing binary - Spica
Let us now speak of multiple systems, formed by three or more gravitationally bound stars:
systems of three stars are called triple or ternary systems;
quadruple systems consist of 4 stars, quintuple systems of 5 stars and sextuple systems of
6 stars. There are also systems made up of 7 or more stars, but they tend to be unstable due to
the complex gravitational interactions between the components. These multiple star systems
are much smaller than open star clusters, which typically contain 100 to 1000 stars.
Multiple star systems can be divided into two main dynamic classes:
- hierarchical systems, which are stable,
characterized by nested orbits that do not interact much, and thus each level of the
hierarchy can be treated as a two-body problem; - “trapezia”, so called by the name
of the Trapezium in Orion, and characterized by highly interacting unstable orbits. Modeled
as an n-body problem, they exhibit chaotic behavior. They can have 2, 3 or 4 stars.
We now deal with hierarchical systems. The first type of hierarchical system
is the triple star system, formed by three gravitationally linked stars.
The three stars travel in an elliptical orbit around the common center of mass,
and are generally arranged so that two of the stars form a narrow binary star,
while the third star orbits a great distance from the binary, on a very wide orbit.
This configuration, called "hierarchical", is very stable because the narrow binary star can
be considered as a single star in the orbit calculation, and therefore the motion of the
stars can be described by the two-body problem, which concerns the motion of two point-like bodies
under the action of Newton's law of universal gravitation and which are therefore in orbit
around the common center of gravity. However, if the orbits of the binary and the single star were
comparable, the system could become unstable, and this would lead to the expulsion of one
of the three stars from the system. Some examples of triple stars are
- Polaris: the system is composed of a yellow supergiant star of spectral type F7 (Polaris)
and two white-yellow dwarf companions of spectral type F (Polaris Ab and B).
- The Rigel Kentaurus (α Centauri) system consists of a main pair,
consisting of a yellow dwarf (Rigel Kentaurus A) and an orange dwarf (Rigel Kentaurus B),
around which a red dwarf (Proxima Centauri) orbits at a great distance.
- HD 188753 is a triple star system located about 149 light years from Earth,
in the constellation of Cygnus. The main star, HD 188753A, is a G-type (yellow dwarf) star
with a mass of 1.06 times the solar one. At a distance of about 12 AU there is another
pair of stars that make up component B which is a spectroscopic binary whose components are called
HD 188753 Ba and 188753 Bb, respectively an orange dwarf and a red dwarf. These two
stars have an overall mass equal to 1.6 times that of the sun, and orbit around HD 188753A,
or rather around the common center of mass, over a period of 25.6 years. Ba and Bb,
on the other hand, orbit each other in a period of 154.45 days, while their semi-major axes are
respectively, for Ba and Bb, of 0.28 and 0.37 AU. - A special mention deserves the star system of GW
Orionis. Until recently it was believed that there were no planets orbiting three stars
at the same time, but a recent study published in Monthly Notices of the Royal Astronomical Society
has overturned this idea. The researchers started from the study of the triple star system
GW Orionis, a very young star system, formed less than 10 million years ago. GW Orionis consists of
a narrow binary whose two stars are just one astronomical unit apart, and a third companion
8 astronomical units from the binary. The observations revealed that around the three stars
of GW Ori there is a disk of dust that surrounds them all, a circumtriple disk. About a year ago,
the telescopes of the Alma network were able to distinguish three dust rings:
the first with a radius of 46 astronomical units, the second of 188 astronomical units and the
third of 366 astronomical units. To get an idea, Neptune's orbit is ten times closer to the Sun
(30 astronomical units) than the last ring of GW Ori's system. But the oddity is another: the disks
are not aligned with each other, and there is an empty region within one of them, about 100 au from
the center of mass of the stars. The strangeness of this system actually lies in the presence of a
deformed, misaligned protoplanetary disk literally torn apart by the stars around which it orbits,
identified by a team of researchers who used the Very Large Telescope of the European Southern
Observatory (VLT of ESO ) and the Atacama Large Millimeter / submillimeter Array (ALMA).
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This is an important discovery because it shows that those "exotic" planets that form around two
or more stars (in this case we always refer to the planet Tatooine from Star Wars to immediately
understand what we are talking about) can in fact be born in inclined rings in bent discs precisely
because of the strong gravity that the stars can imprint. Unlike our solar system, where the
planets are more or less all on the same plane of inclination, in other star systems this may
not be the case and this can happen especially in those systems where there are two or more stars.
Another peculiarity is that the three stars of GW Orionis do not orbit on the same plane
but are misaligned with each other and also with respect to the disk. Precisely
this misalignment would have caused the "rupture" of the protoplanetary disk.
Multiple star systems with four or more stars can give rise to very complex configurations.
These configurations were studied in 1968 by David S. Evans using the pendant diagrams,
so called because they recall certain decorations that hang from the ceiling. Each level of the
diagram shows the decomposition of the system into one or more smaller stellar subsystems.
Each pendant in the diagram represents a subsystem,
and, in turn, can be formed by two stars, or by a star and a smaller subsystem.
Each of these subsystems can in turn have a hierarchical structure, which means that they in
turn must be divided into even smaller subgroups and so on. Each level of the hierarchy can be
treated as a two-body problem, considering the stars of a binary pair as if they were a single
star. In these systems, stars follow stable Keplerian orbits around the center of mass.
Some examples of multiple star systems with four or more stars are:
- Capella: is a multiple system consisting of four components, grouped into two binary stars. The
first pair, which constitutes the predominant element of the system, is formed by two giant
stars of spectral class G, both having a mass of about 2.5 solar masses and a radius that is
around 10 times that of our star. The two stars, which orbit around the common center of gravity
following a rather narrow trajectory, are in a rather advanced phase of their evolution:
the secondary is preparing to become a red giant, while the primary is in the phase of
helium fusion. The other couple is away 10000 astronomical units from the first
and it is composed of two faint main sequence red dwarf stars.
- The star V1054 Ophiuchi / Gliese 643 is a system made up of five stars divided as follows. The
narrow binary formed by the two red dwarf stars V1054 Ophiuchi Ba and V1054 Ophiuchi Bb and the
red dwarf star V1054 Ophiuchi A orbit around the common center of gravity, thus forming a triple
star system. Also present in the system are the star Gliese 643 and the star V 1054 Ophiuchi C.
- Castor or Alpha Geminorum, formed by six components, arranged in three different pairs.
The first two, Castor A and Castor B, are two spectroscopic binaries belonging to the first
subclasses of spectral class A, separated from each other by 4.8 seconds of arc. Castor
A and Castor B orbit around the common center of gravity. The third pair, Castor C or YY Geminorum,
is formed by two red dwarfs and is separated from the first two by about 70 arc seconds.
Trapezia systems are usually very young and unstable star systems.
It is believed that they form in star-forming regions within the nebulae,
and that they tend to fragment into stable systems, expelling some of the components.
These systems take their name from the Trapezium cluster in the heart of the Orion nebula,
and represent a fairly widespread type present near or within nebulae.
Finally, let us mention the designation of multiple star systems. The components
of multiple stars can be named by adding the suffixes A, B, C, etc., to the system name.
Suffixes such as AB can be used to denote the pair formed by A and B. The sequence of letters B,
C, etc. it is usually assigned based on the separation from the component. Components
discovered near components already observed may receive the suffixes Aa, Ba, and so on.
The Multiple Star Catalog of A. A. Tokovinin uses a notation system in
which each subsystem in the pendant diagram is designated by a sequence of digits.
For example, in diagram of the figure below the larger system receives the number 1, while the
subsystem containing the primary receives the number 11 and the one containing the secondary
receives the number 12. The subsystems contained in the two largest receive numbers of three,
four or more digits. When this method of notation is used to describe non-hierarchical systems,
the same number is used to designate multiple subsystems. For example,
a system with three visual components, A, B and C, neither of which can be grouped into subsystems,
will have two subsystems designated by the number 1: the binaries AB and AC. If components B
and C were to turn out to be binaries, they would receive the numbers 12 and 13.
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