Expert: Tom Whiting - 8/6/2009

Question
Explain the difference between Absolute Brightness and Apparent Brightness

Is the answer:
Absolute is a measure of how bright the star REALLY is, and this number is fixed, it doesn't matter how far away a star is, the absolute magnitude will never change. Apparent magnitude is how bright the star appears to be

Answer
Hi Monica,
Yes, you are discussing apparent magnitude (m) versus Absolute Magnitude (M).
And yes, apparent magnitude is what the star (or ANY object) just appears to be in our sky
irrespective of intrinsic luminosity (real brightness as you say) OR distance too.

Absolute Magnitude eliminates the distance factor.  We "place" the object at a pre-determined
distance (10 parsecs or 32.6 lightyears) and determine what it's apparent magnitude WOULD BE
at that distance (using the Inverse Square Law). This then becomes the bodies Absolute Magnitude.  So all Absolute Magnitudes are what the apparent magnitude would be IF the body was exactly 10 parsecs distant.
Realize any standard distance could have been chosen, but whoever designed the system
originally, picked the value of 10 parsecs.
And yes, basically since all objects are calculated to a standard distance, Absolute Magnitude
defines the body's intrinsic luminosity, or real brightness as you say.

But it's NOT true that (M) never changes over time. For instance, as the sun brightens over
millions of years, its Absolute Magnitude will get brighter.  Nova and supernova, the Absolute
Magnitude changes drastically in a few hours. And most variable stars, except for Cepheid
Variables (which are our standard candles for distance determination), the (M) also changes over a period of weeks or months, varying up and down from a mean value.
Never say, "never".
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA

FOLLOW UP:
But yes, for typical main sequence stars, over our short lifetimes, the Absolute Magnitude
remains relatively fixed and stable...of course, so does the apparent magnitude too!
Clear Skies,
Tom