A number of the stars that are visible in the night sky are not in fact single stars, but rather what is known as double stars. When these stars are viewed through a telescope or even a pair of binoculars, a second companion star is found to be right next to it. Some cases resolve three stars in the grouping. Most stars are part of a system of stars of some sort. Binarys are stars that are very close to one another.
Binary stars have unique evolutionary properties. When one dies, the other is typically close behind. The remanat of the first will often feed off of the other shortening its lifespan. In the case of the star Sirius, it has a dark companion, often known as the pup, which is a remnant of a former partner.
For the most part, this should be self explanitory. A write up should include disscussion of the evolution of binary systems and of the different types of binaries.