Luminosity is a fundamental astrophysical parameter defined as the total amount of energy emitted by a celestial body per unit time. It measures the intrinsic brightness of objects such as stars, galaxies, and other astronomical sources, independent of their distance from the observer. Luminosity is typically expressed in watts ($\mathrm{W}$) or in terms of solar luminosities ($L_\odot$), where one solar luminosity is approximately $3.828 \times 10^{26}$ watts.

Luminosity ($L$) is mathematically defined by integrating the emitted radiation over all wavelengths and directions:

$ L = \int_0^\infty 4\pi R^2 \sigma T^4 d\lambda $

For a star approximated as a blackbody, luminosity is expressed using the Stefan-Boltzmann Law:

$ L = 4\pi R^2 \sigma T^4 $

where:

• $R$ = radius of the star
• $T$ = effective surface temperature
• $\sigma$ = Stefan-Boltzmann constant ($5.670374419 \times 10^{-8} \,\mathrm{W\,m^{-2}\,K^{-4}}$)

Stars are commonly classified by their luminosities, alongside their spectral types, into distinct luminosity classes:

• Class 0: Hypergiants (extremely luminous stars)
• Class I: Supergiants (very luminous giants)
• Class II: Bright giants
• Class III: Giants
• Class IV: Subgiants
• Class V: Main sequence stars (dwarfs)
• Class VI: Subdwarfs
• Class VII: White dwarfs