Expert: Jayendra Upadhye - 10/22/2004

Question
Regulus, is a class B star in Leo and has an aboslute magnitude of 0, The sun has an absolute magnitude of +5.  Which is truely brighter, and by how many times?

Answer
Hi Mary,
1 - Regulus has absolute magnitude 0 (actually -0.6: ref link6), is 77 light years away.
2 - The sun has absolute magnitude +5 and is eight light-minutes away. (visual magnitude -26.3 ref: link 3)

Regulus or alpha Leo visual magnitude 1.35 and it is a main sequence Blue Giant. The sun has visual magnitude -26.3.
That makes the sun APPARENTLY brighter!
However
absolute brightness of Regulus can be calculated thus:-
factor of increase using the inverse square law =
(77 ly/32.6 standard ly) squared = 5.58
abs.Br = 1.35 br. at 77 ly - [log(5.58)/log(2.512)]
= 1.35 - 1.8665 = - 0.5
we use the minus sign as our change is towards the brighter side as we move from 77 to 32.6 light years standard distance.
our value compares favourably with -0.6 as given in link 6.

For the sun, the absolute brightness is 4.83 (ref link 5)

thus between regulus and sun, the diff is 4.83 + 0.5 = 5.33 this corresponds to a change in brightness of 2.512 raised to 5.33 = 135.5 [ref link 5]
this tallies also with 130 times as bright as the sun.(ref:- link 4)

USE LINK 5 IT HAS EXCELLENT EXAMPLES.
Note on method:- [from http://www.physics.gmu.edu/classinfo/astr103/CourseNotes/ECText/ch13_txt.htm#13....]
Comparing stars' brightnesses at the same distance is, of course, a comparison of their luminosities. The reference distance selected by astronomers is 32.6 ly or 10 pc (parsec). The magnitude a star would have if it were 32.6 ly from us is called the star's absolute magnitude.
Why the distance 32.6 ly? Such a distance makes the range from brightest to faintest for absolute magnitude (roughly -7 to +17 for stars in our Galaxy) approximately the same as the range for apparent magnitude (roughly -1 to +17); a convenience that aids in memorization.

The difference between a star's apparent and absolute magnitude, that is m-M, is called its distance modulus, because it is related to the ratio of the star's distance to 32.6 ly. From its numerical value we can determine how many times brighter or fainter the object appears compared with its brightness at the reference distance of 32.6 ly.


links:-
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find a regulus fact-sheet (magnitudes of its components etc) here:- (link 1)
http://www.alcyone.de/SIT/mainstars/SIT000542.htm
and here:-  (link 2)
http://depts.clackamas.edu/haggart/WhatsUp/BrightStars.htm
and here:- (link 3)
http://www.brainyencyclopedia.com/encyclopedia/l/li/list_of_brightest_stars.html
and here:- (link 4)
http://www.carbonar.es/s33/Leo/alfa_leonis.html
and here:- (link 5)
http://www.google.ae/search?q=cache:UpepRIpxVeQJ:www.abdn.ac.uk/physics/astro/as...
and here (link 6)
http://www.naasbeginners.freeuk.com/AbsoluteBeginners/Magnitudes.htm
Jayen