The solar mass, M ☉ short, is an astronomical unit, which is defined by the mass of the sun. It amounts to
This corresponds to 332 946 Earth masses.
The unit is used to specify the mass of astronomical objects that are larger and more massive than planets. These mostly stars and star clusters, massive gas clouds and dark nebulae, galactic nuclei, black holes and whole galaxies. More rarely, clusters of galaxies in solar masses can be estimated, and in the future probably the entire universe.
Most of the other fixed stars ( "suns") have masses that range from 0.1 to 10 solar masses. Our central star is therefore " good average " of these main-sequence stars and normal giant stars. Mostly there are hot supergiants with 50 to 100 solar masses.
Determination of the mass of the Sun
With the help of Kepler's third law, the solar mass is obtained (for the approximation M ☉ >> M ⊕ ) from the large semi-major axis a of the earth ( mean distance Sun-Earth, about 1 astronomical unit ), the orbital period P of the Earth ( years long) and the gravitational constant (G = 6.6738 · 10-11 m³ / kg s ²):
Of course, neither the astronomical unit nor the gravitational constant was about 100 years ago, known more precisely than about 1 %, and therefore an accurate determination of the Sun's mass in kilograms impossible. In contrast, the relative mass determination was possible of each planet in our solar system in units of the solar mass, if this planet is orbited by a moon (which in all except Mercury and Venus the case). For the planet mass could set up an equation with distance and round trip time of its moon and the gravitational constant, in turn, was the absolute mass determination difficult. However, since the orbital periods and the axis ratios are known by all planetary orbits long, the masses were from the two equations are compared and the planet mass are given with good accuracy in units of the solar mass.
The same was true for double stars whose distance on the cosmic distance determination in astronomical units but could not be measured in absolute terms. Therefore, it prevailed, that these masses were summarily given in solar masses (see also: Gaussian gravitational constant ). Today, the distance between the Sun and the Earth is measured very accurately, because you can fall back on radar measurements of interplanetary space probes. The gravitational constant G is now known to 4-5 decimal places. Nevertheless, remained the solar mass - like many others - as obtained in astronomy unit of measurement: on the one hand as historically established convention, and on the other hand because of the manageable numbers and as an idea help with such a huge "astronomical numbers ".
Solar mass and related units
A solar mass equal to:
- 27,068,510 lunar masses M ☽
- 332 946 Earth masses M ♁
- 1047.56 Jupiter masses M ♃.
Our Milky Way is about 180 billion solar masses, which corresponds to about 3.6 × 1041 kg. In other galaxies, such information is still very uncertain and is currently only useful for close, well -measured systems.
In the general theory of relativity, it is sometimes customary to quote the mass in units of length. Where ( the gravitational constant G and the velocity of light c ):