Solar mass ($M_{\odot }$)

The solar mass, written $M_{\odot }$, is a mass unit based on the Sun, approximately $1.99\times 10^{30}\mathrm{kg}$. Expressing stellar masses in this unit makes comparisons immediate: a $2M_{\odot }$ star has roughly twice the Sun's mass. It does not follow that it has twice the solar luminosity, because the mass-luminosity relation is nonlinear.

Orbital observations constrain the gravitational parameter $GM$ more directly than mass in kilograms. The nominal solar mass parameter supplies an exact conventional conversion for such comparisons; converting it to kilograms additionally requires the measured gravitational constant. A nominal constant is therefore distinct from claiming that the physical Sun's changing mass is known exactly.^[ref]

The exact conversion factor $1.3271244\times 10^{20}{\mathrm{m}}^3{\mathrm{s}}^{-2}$. Dividing a measured gravitational parameter by this factor expresses a mass in nominal solar units, avoiding a separate conversion through the measured gravitational constant.^[ref]