Binary Stars
Consist of 2 Stars orbiting around their center of mass
Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as spectroscopy. If binaries happen to orbit in a plane containing our line of sight, they will eclipse each other; these are called eclipsing binaries.
Binary stars are classified into four types according to their observable properties. Any binary star can belong to several of these classes; for example, several spectroscopic binaries are also eclipsing binaries.
1) Visual Binary Stars:
A visual binary star is a binary star for which the angular separation between the two components is great enough to permit them to be observed as a double star in a telescope.
The brightest visual binary called primary star and the others called secondary star.
2) Spectroskopic binaries:
is a binary star in which the separation between the stars is usually very small, and the orbital velocity very high.
Unless the plane of the orbit happens to be perpendicular to the line of sight, the orbital velocities will have components in the line of sight and the observed radial velocity of the system will vary periodically.
Since radial velocity can be measured with a spectrometer by observing the Doppler shift of the stars' spectral lines, the binaries detected in this manner are known as spectroscopic binaries.
Most of these cannot be resolved as a visual binary, even with telescopes of the highest existing resolving power.
3) Eclipsing binaries:
is a binary star in which the orbit plane of the two stars lies so nearly in the line of sight of the observer that the components undergo mutual eclipses.
In the case where the binary is also a spectroscopic binary and the parallax of the system is known, the binary is quite valuable for stellar analysis
Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity. If one of the stars is larger than the other, one will be obscured by a total eclipse while the other will be obscured by an annular eclipse.
4) Astrometric binaries:
is a binary star for which only one of the component stars can be visually observed.
The visible star's position is carefully measured and detected to have a wobble, due to the gravitational influence from its counterpart.
The position of the star is repeatedly measured relative to more distant stars, and then checked for periodic shifts in position.
Typically this type of measurement can only be performed on nearby stars, such as those within 10 parsecs. Nearby stars often have a relatively high proper motion, so astrometric binaries will appear to follow a sinusoidal path across the sky.
Source: Wikipedia
