Expert: Philip Stahl - 11/30/2009

Question
Good afternoon Sir

I am attempting to decipher the following:

Discuss stellar magnitude.  What are the differences between absolute and apparent magnitudes?

Magnitudes represent the relative brightness of stars with smaller magnitudes corresponding to brighter stars.     
Absolute magnitude (M) defines the body’s intrinsic luminosity or relative brightness of a sky object as seen from a standard distance of 10 parsec (~32.6 light-years).  Astronomers consider it the “true” magnitude of the object. Absolute magnitude is a measure of true stellar luminosity.

Apparent magnitude (m) of a celestial body refers to how bright stars appear to humans observing from the Earth, which is usually different from its absolute magnitude (M); depending on how far away from the Earth the sky object is located.  A star closer to Earth may appear brighter than one farther away, even though its absolute magnitude is fainter.  Apparent magnitude (m) ignores the effect of distance and tells you only how bright the star looks as seen from the Earth. Any type of apparent magnitude (m) can be converted to an absolute magnitude (M).

Am I on the right track?

Thank you in advance.

Answer
Hello,

Basically you are "on the right track" but I'd quantify the magnitude definition a bit more. For example, what is the brightness *ratio* for two stars that differ by ONE magnitude? What you want to do here is to give the ratio of brightness associated with the one magnitude (arithmetic) difference. Knowing this, one can then generalize to the brightness ratio corresponding to *any difference* in magnitude. (Hint, if star A is 5 *magnitudes* lower than star B, then star A is 100x brighter than star B)

The apparent and absolute magnitude difference is ok, but would grab even more attention by referencing a particular example. So why not use the Sun? You should easily be able to find its apparent magnitude, and absolute magnitude.

I would also not say or write the apparent magnitude is "usually different from its absolute magnitude (M)". I'd say it is *generally* different, simply because nearly all astronomical objects are greater than 10 pc distance!

For your last sentence- claim, I would also show exactly HOW it could be done. How can the apparent magnitude (m) be converted to an absolute magnitude (M).  Since you'd likely already have used the Sun (in a revised basis) this would be a great chance to show how the Sun's apparent magnitude can be converted to its absolute magnitude.

Hopefully, this will help you produce better revised answers.