What concept is Absolute Magnitude ($M$) used to compare directly between stars?
Answer
The inherent luminosity (true power output) of stars
Absolute magnitude removes the variable of distance, allowing an astronomer to compare the true energy output of stars regardless of their current location in the galaxy.

Related Questions
What does a lower stellar magnitude number signify?Which object is assigned a negative magnitude value in this system?Which ancient astronomer established the initial framework for classifying stars into visual magnitude classes?According to the scale's logarithmic nature, how much brighter is an object that is five magnitudes brighter than another?Approximately how much brighter is a star of magnitude +4.0 compared to a star of magnitude +5.0?What is the primary factor that Apparent Magnitude ($m$) depends on besides the star's intrinsic light output?At what standard, fixed distance is a star's Absolute Magnitude ($M$) defined as its apparent magnitude?What concept is Absolute Magnitude ($M$) used to compare directly between stars?What do filter magnitudes, such as $B$ magnitude and $V$ magnitude, allow astronomers to determine about a star?If a star has a negative $B-V$ value, what does this imply about its color and temperature?