Lsun

Lsun, often written L⊙ in formal notation and sometimes rendered as Lsun in plain text, denotes the solar luminosity, a standard unit of luminosity in astronomy. It represents the total energy emitted by the Sun per unit time across all wavelengths (the bolometric luminosity). In astronomy, Lsun is used as a reference scale to express the luminosities of stars and other objects relative to the Sun.

The commonly adopted value of the solar luminosity is about 3.828 × 10^26 watts. This figure comes from measurements of the solar radiation at Earth’s orbit (the solar constant) and the relation L⊙ = 4π(1 AU)^2 × F⊙, though slight variations exist among sources depending on the adopted constants and wavelength coverage. The unit is widely used because many astronomical phenomena are compared to the Sun’s energy output.

Luminosities expressed in Lsun allow quick comparison: a star with L = 10 Lsun is ten times as luminous as the Sun. The symbol is not a physical constant but a defined reference that simplifies scaling in stellar astrophysics, exoplanet studies, and galactic energetics. As a practical unit, Lsun is used to express a wide range of astronomical luminosities, from small stars to luminous galaxies, facilitating communication and interpretation across disciplines.