The brightest of a star measurable on a scale is called its magnitude.

Here's a bit more detail:

* Apparent Magnitude: This is the brightness of a star as seen from Earth. It's measured on a scale where smaller numbers represent brighter stars. The scale is logarithmic, so a difference of 5 magnitudes corresponds to a 100-fold difference in brightness.

* Absolute Magnitude: This is the brightness of a star if it were located at a standard distance of 10 parsecs (about 32.6 light-years) from Earth. This value allows astronomers to compare the intrinsic brightness of stars regardless of their actual distance.

So, when we talk about the "brightest" of a star, we're usually referring to its apparent magnitude.