Final answer:
The luminosity of a star is the total energy that it emits per second as electromagnetic radiation. It's a key characteristic of a star and is expressed relative to the Sun's luminosity (Lsun). Understanding luminosity is crucial to distinguishing it from apparent brightness and to relate it to a star's temperature as per the Stefan-Boltzmann law.
Step-by-step explanation:
The luminosity of a star is the amount of energy that the star generates and releases as electromagnetic radiation. It is the total amount of energy at all wavelengths emitted by the star per second. When comparing stars, astronomers often express their luminosity in terms of the Sun's luminosity, using the symbol Lsun. Therefore, if the star Sirius has a luminosity expressed as 25 Lsun, it means that Sirius is approximately 25 times more luminous than the Sun.
In addition to luminosity, it's also important to understand the concept of apparent brightness, which is the amount of a star's energy that reaches a given area on Earth each second. However, apparent brightness depends both on the star's luminosity and its distance from Earth, and not all the energy emitted by a star will reach an observer on Earth due to the vast distances involved.
The relationship between a star's temperature and its energy output is described by the Stefan-Boltzmann law, stating that the energy flux from a blackbody is proportional to the fourth power of its absolute temperature. This relationship showcases how a hotter star will emit significantly more energy, making its luminosity much greater.