 Using the stellar spectrum , astronomers can also determine the surface temperature , surface gravity , metallicity and rotational velocity of a star . If the distance of the star is found , such as by measuring the parallax , then the luminosity of the star can be derived . The mass , radius , surface gravity , and rotation period can then be estimated based on stellar models . ( Mass can be calculated for stars in binary systems by measuring their orbital <unk> and distances . <unk> <unk> has been used to measure the mass of a single star . ) With these parameters , astronomers can also estimate the age of the star . 
