What is the solar constant?

About 1361 W/m², the flux from the Sun at 1 AU. It varies slightly with solar activity.

How does flux relate to apparent magnitude?

Magnitude is a logarithmic scale of flux. A difference of 5 magnitudes corresponds to a factor of 100 in flux.

In astronomy, flux and irradiance are often used interchangeably for power per unit area received from a distant source.

Calculate the observed flux from a star of known luminosity at a given distance: F = L / (4 pi d^2).

Luminosity

Distance

Observed Flux

1,361.1277 W/m²

Observed Flux vs Luminosity

Flux is the power received per unit area at a given distance from a luminous source.

F = L / (4π d²)

As distance doubles, flux drops to one-quarter.

Solar flux at 1 AU.

Calculate luminosity from flux and distance.

See brightness change with distance.

Find apparent magnitude from absolute magnitude.