The luminosity of an object as discussed in Section 8.2 is the energy it emits per unit time, e.g., joules per second (J s-1) or the equivalent unit, watt (W). It can refer to the energy emitted over the entire electromagnetic spectrum, but it can mean the energy emitted in a more restricted frequency band, e.g., the visible band. Luminosities in optical astronomy usually mean the total energy radiated in the visible and

Table 9.1. Mass, luminosity, and energy examples


Mass (kg)

Luminosity (j/s = W)

Total energy radiated" (J)

Energy used by all




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