Solar mass

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The solar mass is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, about two nonillion kilograms or about 332,950 times the mass of the Earth. Its conventional symbol and value are:

<math>M_{\bigodot}=1.9891\times10^{30}\hbox{ kg}</math>

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

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

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 of a 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 AU is extremely well measured using interplanetary radar and G is well measured, but the solar mass persists as one of astronomy's arcane historical conventions.