The brightness of a celestial object as measured from Earth is referred to as Apparent magnitude.
Let's break down the options to clarify why:
1. Absolute brightness: This term refers to the intrinsic brightness of a celestial object, often measured as if the object were a standard distance away from Earth (typically 10 parsecs). It does not account for the object's actual distance from the observer on Earth.
2. Apparent magnitude: This is the correct answer. Apparent magnitude measures how bright a celestial object appears from Earth, taking into account both its intrinsic brightness and its distance from us. The lower the apparent magnitude, the brighter the object appears to us.
3. Luminosity: This is the total amount of energy emitted by a celestial object per unit of time. Luminosity is an intrinsic property of the object itself and is not affected by the object's distance from Earth.
4. Proto-star: This term refers to an early stage in the formation of a star. A proto-star is a mass of gas that represents an early phase before nuclear fusion has started in its core. It is not a measure of brightness.
Therefore, the term that best represents the brightness of a celestial object as seen from Earth is Apparent magnitude.
