A binary star system is two stars that are stuck in each others
gravity. Astronomers believe that about half of all stars in the sky
are binary stars. There are four ways of seeing binary stars, and they
are called Visual binary stars, Spectroscopic binary stars, Eclipsing
binary stars and Astrometric binary stars.
The first type Visual binaries means that because the stars are so far
apart from each other they look like two separate stars in the night
sky. Visual binaries takes tens or hundreds of years to orbit each
other.
Spectroscopic binaries are binary stars that are so close together that
you need a special tool to detect them as separate stars called a
spectroscope.
Eclipsing binaries are binary stars that are closer together than
visual binaries, they orbit each other within a couple or few days.
Take a red dwarf and a white dwarf for example, since the red dwarf is
bigger, when viewed from Earth it will eclipse the white dwarf and look
a lot dimmer for a few hours and then once they
are side by side the light from both can be seen and is brighter. When
the white dwarf eclipses the red dwarf the light dims a tiny bit from
the last stage and finally the stars will be seen on opposite sides and
light from both of the stars will be shown again. This keeps happening
and from Earth it looks like the star is changing from dimmer to
brighter.
Astrometric binaries are two binary stars and one is big and one is
small. Astronomers can detect tugs on the larger star from the gravity
of the smaller but still big star. For example, they can detect that
other stars have large planets like Jupiter from a small wobble on the
star, its so small it is really hard to detect but I think the smaller
planets and other gas giants could help it be detected too a bit.