Read · 2 min
A multiple star system is a group of stars connected by a relatively strong gravitational force.
Binary Star Systems
In a binary star, two things can happen - either one star is sufficiently more massive, resulting in the second star orbiting it, or they are approximately the same mass, in which case the stars are both in orbit around a common point (though their orbits do not necessarily coincide - only one focus of the ellipses, which is the center of mass of the system, is always common). The star Sirius, the brightest as seen from Earth, is a binary, in which Sirius B is nearly 10,000 times less bright. Orbital periods can be under an hour, in stars classified as "AM Canum Venaticorum stars", or hundreds of thousands of years (see Alpha Centauri below).
Types of binaries
Visual - Stars can be separated with a telescope or even with binoculars.
Spectroscopic - These cannot be resolved into visual with current technology. However, they are known to be binary due to the Doppler effect - their emitted light changes from blue to red and back as they orbit a center.
Eclipsing - In these systems, the two stars line up such that one either partially or totally eclipses the other. Such stars become fainter when they line up, which happens twice during an orbit.
Astrometric - Some stars seem to orbit an empty space when observed from the Earth. In these systems, the object being orbited might be a neutron star or even a black hole, so it is not necessarily visible.
Cataclysmic - these stars irregularly have huge increases in brightness and then drop back down. In these systems, the primary is a white dwarf, and the secondary star is so close that it gets distorted and transfers its mass into the primary (such stars are nicknamed "donor stars"). The donor star can either be a smaller white dwarf, or as large as a red giant. If the white dwarf accretes sufficient mass, this can re-trigger fusion of carbon and trigger a supernova, completely destroying the white dwarf (hence cataclysmic).
Systems with More than Two Stars
Some star systems have more than two stars - Alpha Centauri, the nearest star system to our Solar System (at 4.3 light years away) has two bright stars, with a third, called Proxima Centauri, that is 0.24 light years away (about 5% of the distance from the Sun to the system), and with a calculated orbital period of possibly 500,000 years (it has been conjectured that the orbit of Proxima Centauri is hyperbolic, but is currently accepted that the orbit is elliptical). The star Castor, or Alpha Geminorum, was discovered to be binary in 1678; however, each component is a binary star, and there is a third eclipsing binary in the system, so Castor is actually a six-star system.