Total amount of energy radiated per second. It has units of energy per second (e.g. ergs per second). Since many astronomical objects radiate away energy this is an important characteristic. We compare luminosity of an object to the solar luminosity, the total energy given off per second by the sun. One solar luminosity is 4 × 1033 ergs per second. Luminosity has the same units as Power, e.g. energy per second. The Watt is the familiar unit of power. For comparison, a 400 Watt light bulb is 10-24 solar luminosities.