= Solar mass
{title2=$M_\odot$}
{wiki}
The <solar mass>, written $M_\odot$, is a <mass> unit based on the <Sun>, approximately $1.99\times10^{30}\,\mathrm{kg}$. 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.<Resolution B3 on recommended nominal conversion constants for selected solar and planetary properties by International Astronomical Union>[Resolution B3: nominal solar mass parameter as an exact SI conversion constant]{ref}
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