Malmquist bias
The Malmquist bias is an effect in observational astronomy which leads to the preferential detection of intrinsically bright objects. It was first described in 1922 by Swedish astronomer Gunnar Malmquist (1893–1982), who then greatly elaborated upon this work in 1925. In statistics, this bias is referred to as a selection bias and affects the survey results in a brightness limited survey, where stars below a certain apparent brightness are not included. Since observed stars and galaxies appear dimmer when farther away, the brightness that is measured will fall off with distance until their brightness falls below the observational threshold. Objects which are more luminous, or intrinsically brighter, can be observed at a greater distance, creating a false trend of increasing intrinsic brightness, and other related quantities, with distance. This effect has led to many spurious claims in the field of astronomy. Properly correcting for these effects has become an area of great focus.
In everyday life it is easy to see that light dims as it gets farther away. This can be seen with car headlights, candles, flashlights, and many other lit objects. This dimming follows the inverse square law, which states that the brightness of an object decreases as 1/d2, where d is the distance between the observer and the object.
Starlight also follows the inverse square law. Light rays leave the star in equal amounts in all directions. The light rays create a sphere of light surrounding the star. As time progresses, the sphere grows as the light rays travel through space away from the star. While the sphere of light grows, the number of light rays stays the same. So, the amount of light per unit of surface area of the sphere (called flux in astronomy) decreases with distance and therefore time. When observing a star, only the light rays that are in the given area being viewed can be detected. This is why a star appears dimmer the farther away it is.
If there are two stars with the same intrinsic brightness (called luminosity in astronomy), each at a different distance, the closer star will appear brighter while the further will appear dimmer. In astronomy, the apparent brightness of a star, or any other luminous object, is called the apparent magnitude. The apparent magnitude depends on the intrinsic brightness (also called absolute magnitude) of the object and its distance.
...
Wikipedia