A pair of gravitationally-bound A pair of stars that appear close stars that can both be seen with to one another on the sky but which may or may not be a bound system A pair of gravitationally- bound A pair of gravitationally-bound stars that can only be identified stars that can only be identified as a binary by using careful as a binary by using careful study of the brightness study of the spectrumSpectral Class Most massive Least massive main sequence Smallest Most like our Largest Sun main sequence star radius.