Some binary systems are sufficiently close to Earth and the stars are well enough
separated that we can see the two stars individually in a telescope and track
their motion over a period of time.
We term such systems visual binaries.
The Castor binary system
Example: The Castor System
The adjacent image shows one
example, the star Castor, which is actually a visual binary as observed
through a telescope
(it was the first binary discovered, by Herschell in 1790). In this case the plot
is of the orbit of one star relative to the other and the year of the
observation is given at the corresponding point on the orbit.
Orbits for Binary Systems
In reality, binary star systems are governed by Kepler's laws, as
modified by Newton
to account for the effect of the center of mass. Then each star executes an
elliptical orbit such that at any instant the two stars are on opposite sides of
the center of mass. The orbits generally are
as depicted in the following figure.
Orbits for binary star systems
Sirius and Companion
The Sirius Binary System
An example is shown in the image to the left of the Sirius binary star system,
which consists of a more normal
main sequence star (Sirius A) and a
companion (Sirius B).
The image adjacent right shows a telescopic view of the Sirius system. The faint
point of light indicated by the arrow is Sirius B, almost lost in the glare of the
much brighter Sirius A.
The orbits are drawn to scale, but the sizes of the two
stars are not to scale in this image. Both stars would be but points of light at
this scale, and Sirius A is considerably larger than the Sun while the white dwarf
Sirius B is about the size of the Earth.
Interactive Animations for Binary Systems
Here are two interactive animations that allow you to explore the motion of the
stars in a binary star system:
Tilt of Binary Orbits
One important consideration for visual binary orbitals is that the plane of
for such systems is not usually perpendicular to our line of sight.
In general, there is some tilt angle i, as illustrated in the
Thus, when we see the orbit of a visual binary we do not see
the actual orbit but only the projection of that orbit on the celestial
For example, if the orbit looks like an ellipse, that could be
because the orbit actually is elliptical, or because the true orbit is a
circle but we are seeing it from an angle that makes the circle look
flattened and therefore elliptical.
In many cases it is possible to determine the angle i by
careful measurement in order to deduce the true orbits of the binary system.
In other cases we cannot and the angle i remains uncertain.
List of Multiple Star Systems
Here is a list of
600 Multiple Star Systems.