how are they measured?
It is often most convenient to express mass, luminosity, and radii in solar units
Star
Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun.
characteristics of stars based on the characteristics of the sun A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. have these been observed since ancient times? UNANSWERABLE
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Star [SEP] Units of measurement
Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of "nominal" solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters: The solar mass M was not explicitly defined by the IAU due to the large relative uncertainty (10) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass together (GM) has been determined to much greater precision, the IAU defined the "nominal" solar mass parameter to be: However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 10 kg.