A star’s apparent brightness is not the same as its absolute brightness.
The following definitions were provided by the Chandra X-ray Observatory’s glossary that was published by Harvard.
- A star’s apparent brightness is the star’s brightness as it naturally appears in the sky, at its natural location from Earth.
- On the other hand, a star’s absolute brightness is the apparent brightness that the star would have if it was placed at a standard distance of 10 parsecs from Earth.
A parsec is a standard unit that is used in astronomy. A parsec is equivalent to 3.28 light years. This is equal to about 3.086 x 10^13 kilometers.
Therefore, a star's apparent brightness depends on its distance from Earth. If a star’s actual location is greater than 10 parsecs from Earth, then the star’s apparent brightness will appear dimmer than the star’s absolute brightness. Light appears dimmer as the distance from the light source increases because, as the light travels, it spreads out.