Solar mass

The "solar mass" is the standard way to express mass in astronomy. It is used to describe the masses of other stars, and other galaxies. It is equal to the mass of the Sun, about two nonillion kilograms. Its value and conventional symbol are:

M_{\bigodot}=1.9891\times10^{30}\hbox{ kg}

The solar mass is 333,000 times the mass of the Earth.

The solar mass can be determined from the length of the year, the distance of the earth to the sun (the astronomical unit) (AU), and the Gravitational Constant (G) as

M_\bigodot=\frac{4 \pi^2 \times (1{\rm AU})^3}{G\times(1{\rm yr})^2}

but until recently, neither the AU nor the gravitational constant were precisely known. However, a determination of the relative mass of another planet in the solar system or binary star in units of solar masses does not depend on these poorly known constants. So it was useful to express these masses in units of solar masses (see Gaussian gravitational constant). Today, the Astronomical Unit is extremely well measured using interplanetary radar, and the gravitational constant is well measured, but the solar mass persists in astronomy as one of that field's many arcane historical conventions.

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See also