Editing Luminosity (section)

== Relationship to magnitude ==
{{Main|Bolometric magnitude}}
Luminosity is an intrinsic measurable property of a star independent of distance. The concept of magnitude, on the other hand, incorporates distance. The [[apparent magnitude]] is a measure of the diminishing flux of light as a result of distance according to the [[inverse-square law]].<ref name="HAWAII2003">{{cite web |url=http://www.ifa.hawaii.edu/~barnes/ASTR110L_S03/inversesquare.html|title=The Inverse-Square Law| author=Joshua E. Barnes| date=18 February 2003 |publisher=Institute for Astronomy - University of Hawaii |access-date=26 September 2012}}</ref> The Pogson logarithmic scale is used to measure both apparent and absolute magnitudes, the latter corresponding to the brightness of a star or other [[celestial body]] as seen if it would be located at an interstellar distance of {{convert|10|parsec|m|abbr=off|lk=on}}. In addition to this brightness decrease from increased distance, there is an extra decrease of brightness due to extinction from intervening interstellar dust.<ref name="ASTRONOTES1">{{cite web|url=http://www.astronomynotes.com/starprop/s4.htm|title=Magnitude System|date=2 November 2010|publisher=Astronomy Notes|access-date=2 July 2012}}</ref>

By measuring the width of certain absorption lines in the [[stellar classification|stellar spectrum]], it is often possible to assign a certain luminosity class to a star without knowing its distance. Thus a fair measure of its absolute magnitude can be determined without knowing its distance nor the interstellar extinction.

In measuring star brightnesses, absolute magnitude, apparent magnitude, and distance are interrelated parameters—if two are known, the third can be determined. Since the Sun's luminosity is the standard, comparing these parameters with the Sun's apparent magnitude and distance is the easiest way to remember how to convert between them, although officially, zero point values are defined by the IAU.

The magnitude of a star, a [[unitless]] measure, is a logarithmic scale of observed visible brightness.  The apparent magnitude is the observed visible brightness from [[Earth]] which depends on the distance of the object.  The absolute magnitude is the apparent magnitude at a distance of {{convert|10|parsec|m|abbr=on|lk=on}}, therefore the bolometric absolute magnitude is a logarithmic measure of the bolometric luminosity.

The difference in bolometric magnitude between two objects is related to their luminosity ratio according to:<ref>{{Cite web| url=http://csep10.phys.utk.edu/OJTA2dev/ojta/c2c/ordinary_stars/magnitudes/absolute_tl.html| title=Absolute Magnitude| website=csep10.phys.utk.edu|access-date=2 February 2019}}</ref>
<math display="block">M_\text{bol1} - M_\text{bol2} = -2.5 \log_{10}\frac{L_\text{1}}{L_\text{2}}</math>

where:
*<math>M_{\text{bol1}}</math> is the bolometric magnitude of the first object
*<math>M_\text{bol2}</math> is the bolometric magnitude of the second object.
*<math>L_\text{1}</math> is the first object's bolometric luminosity
*<math>L_\text{2}</math> is the second object's bolometric luminosity

The zero point of the absolute magnitude scale is actually defined as a fixed luminosity of {{val|3.0128|e=28|u=W}}. Therefore, the absolute magnitude can be calculated from a luminosity in watts:
<math display="block">M_\mathrm{bol} = -2.5 \log_{10} \frac{L_{*}}{L_0} \approx -2.5 \log_{10} L_{*} + 71.1974</math>
where {{math|''L''<sub>0</sub>}} is the zero point luminosity {{val|3.0128|e=28|u=W}}

and the luminosity in watts can be calculated from an absolute magnitude (although absolute magnitudes are often not measured relative to an absolute flux):
<math display="block">L_{*} = L_0 \times 10^{-0.4 M_\mathrm{bol}}</math>