Editing Star (section)

== Units of measurement ==

Although stellar parameters can be expressed in [[International System of Units|SI units]] or [[Gaussian units]], it is often most convenient to express [[mass]], [[luminosity]], and [[radius|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:

:{|
| [[solar luminosity|nominal solar luminosity]]
| ''{{solar luminosity}}'' = {{val|3.828|e=26|u=W}}<ref name="Prsa16">
  {{cite journal
   | last1=Prsa | first1=A. | last2=Harmanec | first2=P. | last3=Torres | first3=G. | last4=Mamajek | first4=E.
   | display-authors=etal
   | title=Nominal values for selected solar and planetary quantities: IAU 2015 Resolution B3| journal=Astronomical Journal
   | date=2016 | volume=152 | issue=2 | pages=41 | bibcode=2016AJ....152...41P
   | doi=10.3847/0004-6256/152/2/41 |arxiv=1605.09788| s2cid=55319250
  | doi-access=free }}</ref>
|-
| [[solar radius|nominal solar radius]]
| ''{{Solar radius}} = {{val|6.957|e=8|u=m}}<ref name="Prsa16"/>
|}

The [[solar mass]] {{solar mass}} was not explicitly defined by the IAU due to the large relative uncertainty ({{val|e=−4}}) of the [[Newtonian constant of gravitation]] ''G''. Since the product of the Newtonian constant of gravitation and solar mass
together (''G''{{solar mass}}) has been determined to much greater precision, the IAU defined the ''nominal'' solar mass parameter to be:

:{|
| nominal solar mass parameter:
| ''G{{solar mass}}'' = {{val|1.3271244|e=20|u=m3}}/s<sup>2</sup><ref name="Prsa16"/>
|}

The nominal solar mass parameter can be combined with the most recent (2014) CODATA estimate of the Newtonian constant of gravitation ''G'' to derive the solar mass to be approximately {{val|1.9885|e=30|u=kg}}. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.

Large lengths, such as the radius of a giant star or the [[Semi-major and semi-minor axes|semi-major axis]] of a binary star system, are often expressed in terms of the [[astronomical unit]]—approximately equal to the mean distance between the Earth and the Sun (150&nbsp;million km or approximately 93&nbsp;million miles). In 2012, the IAU defined the [[astronomical constant]] to be an exact length in meters: 149,597,870,700 m.<ref name="Prsa16"/>