Binary stars are star systems consisting of two stars orbiting around their common center which is called the bary center.

Scientists officialy classify the brighter star as the primary star. While the dimmer of the two is the secondary (classified as A and B respectively).

Lots of the stars we observe in the night sky are actually two or more stars orbiting together. The most common of the multiple star systems are binary stars, systems of only two stars together.

Thanks to observatories such as NASA’s Kepler space telescope, we know that two-star systems can indeed support planets, although planets discovered so far around double-star systems are large and gaseous. But an interesting question is: If an Earth-size planet were orbiting two suns, could it support life?

Such a planet could indeed be quite hospitable if located at the right distance from its two stars. Also, it wouldn’t necessarily even have deserts. So, In a particular range of distances from two sun-like host stars, a planet covered in water would remain habitable and retain its water for a long time, according to a new study in the journal Nature Communications.

“This means that double-star systems of the type studied here are excellent candidates to host habitable planets, despite the large variations in the amount of starlight hypothetical planets in such a system would receive,” said Max Popp, associate research scholar at Princeton University in New Jersey, and the Max Planck Institute of Meteorology in Hamburg, Germany.

The binary stars classifications.

Visual Binaries
A visual binary is a binary system in which the component stars of the system can be individually resolved through a telescope. Therefore, long-term observations can then be made to plot the relative positions of the members of the system. So, over time this data is accumulated and used to calculate the orbits of the stars.
Spectroscopic Binaries
Some stars orbit too close to each other. So, how can they be detected as binaries? The majority of binary systems have been detected by Doppler shifts in their spectral lines. Such systems are called spectroscopic binaries, a system where stars are so close together that they appear as a single star even in a telescope.
Eclipsing Binaries
These are the kind of binary stars whose brightness varies periodically as the two components pass one in front of the other. This feature is based on the line of sight rather than any particular feature of the pair.
Astrometric Binaries
Some stars, when being observed repeatedly over time, they show a perturbation or “wobble” in their proper motion. So, if this is a periodic occurrence we can infer that the perturbation occurs due to the gravitational influence of an unseen companion. We have a system in which a visible star and a dimmer companion orbit a common centre of mass. Therefore, binary systems detected by such astrometric means are called astrometric binaries.

Center of mass animations

Orbit1.gif
(a.) Two bodies of similar mass orbiting around a common center of mass, or barycenter.
Orbit2.gif
(b.) Two bodies with a difference in mass orbiting around a common barycenter, like the Charon-Pluto system
Orbit3.gif
(c.) Two bodies with a major difference in mass orbiting around a common barycenter (similar to the Earth–Moon system)
Orbit4.gif
(d.) Two bodies with an extreme difference in mass orbiting around a common barycenter (similar to the Sun–Earth system)
Orbit5.gif
(e.) Two bodies with similar mass orbiting in an ellipse around a common barycenter.