Final answer:
The luminosity of a star is its total energy output into all space per second, not just its brightness as seen from Earth or a specific distance. It is measured in terms of the Sun's output.
Step-by-step explanation:
Luminosity of a Star
The luminosity of a star refers to the total energy output a star emits into all space in one second, covering all wavelengths of light. This concept is distinct from a star’s apparent brightness, which is how bright the star appears from Earth and is affected by the star’s actual luminosity and its distance from Earth. Apparent brightness decreases with the square of the distance to the star. In practical terms, astronomers use the Sun’s luminosity as a reference point (denoted as Lsun) to express the luminosity of other stars. Thus, if you were asked, ‘The luminosity of a star is,’ the correct answer would be C) its total energy output into all space.