Final answer:
The mass of a distant star can be most easily determined if it is a binary star, as their orbits allow for calculation of their individual masses. Sirius and its white-dwarf companion is an example of such a system where initial masses and evolution can be studied.
Step-by-step explanation:
The mass of a distant star can be determined most easily if it is a binary star. In the case of binary stars, by analyzing their orbit around their common center of mass, it is possible to calculate the mass of each star. For example, Sirius, the brightest star in our northern skies, has a white-dwarf companion. Although the white dwarf has a smaller mass, less than the 1.4 Msun (Chandrasekhar limit), it originally could have been more massive than Sirius and evolved faster to become a white dwarf. Visual binaries allow the two stars to be resolved telescopically. Spectroscopic binaries reveal the presence of two stars through their combined spectrum. The mass-luminosity relation is another aspect linking the mass of a star to its brightness—more massive stars tend to be more luminous.