More than two-thirds of stars are not solitary like our Sun.  They are binary systems, meaning they contain two stars that orbit each other about their common centre of gravity.  Stars like our Sun are much more rare, and we are not sure what the difference in formation is between binary and solitary systems.  Binary systems are much more useful from a scientific perspective, as we can study their orbital period and separation to infer a wide range of properties such as masses and distances.  A special class of stars, called RR Lyrae variable stars, have puzzled astronomers for years...