Stars give out varying degrees of brightness. Larger stars generally are much more luminous than smaller ones. To measure the brightness or luminosity (also called as radiant power) of a star, astronomers use the Sun’s own level of luminosity as their point of comparison. One solar luminosity unit, therefore, is equal to about 3.839×1026 W, or 3.839×1033 erg/s, or the Sun’s commonly accepted luminosity. However, the Sun’s luminosity does not always stay the same; it frequently fluctuates. The solar cycle (sunspot cycle), which lasts for eleven years, causes a variation of about ±0.1%.

The amount of energy transported from the Sun equals the solar luminosity. Picture the Sun enveloped by a series of imaginary, transparent spheres. Energy from the Sun which goes through these spheres stays roughly the same, though the surface area of the spheres becomes larger. This is the reason why distance is a factor in the solar luminosity of a star: the closer a star is to you, the brighter it seems; while it will be less bright if it is farther away from you. Astronomers devised a formula that can calculate the luminosities of all stars called the inverse square law. This can be used to measure not only the solar luminosity but also the total energy the Sun gives out and the amount of energy that the Earth receives from the Sun. By factoring in the distance, the inverse square law can also be used to measure the original luminosity of any star.

As mentioned earlier, the Sun’s luminosity usually fluctuates. Its solar luminosity experiences variations especially during its eleven year solar cycle, caused by sunspots- cooler, darker regions that appear on the surface of the Sun on a periodic basis. This fluctuation in the luminosity of the Sun often arouses speculation, with some attributing to it the acceleration of global warming on Earth, and abnormal weather conditions.