Re: [Phys-L] absolute magnitude

[Dan Schroeder <dschroeder@weber.edu>]

In other words, if the sun were 10 pc away it would be invisible from  
Pomona and barely visible from (parts of) Ogden.  If it were 100 pc  
away, it wouldn't even be close to naked-eye visibility.  And yet  
nearly half of the naked-eye stars, including plenty of the bright  
named stars, are more than 100 pc away!  As it turns out, only a small  
handful of the naked-eye stars are less luminous than the sun.  On the  
other hand, about 90% of all stars are less luminous than the sun.   
Depending on the sample that you want to compare to, I think you could  
defend a standard anywhere from 1 pc to 1000 pc.  But yeah, it makes  
sense to choose something in that range, and of course, powers of 10  
are convenient.

> The sky's a lot darker in Ogden than in Southern California.  I  
> suspect it's a pretty rare night when we see many more than 400 stars!
>
> Seriously, though, I still suspect that the choice has to do with  
> wanting the range of absolute magnitudes to line up in some  
> reasonable way with the range of visual magnitudes.  Note for  
> instance that the absolute magnitude of the Sun--a pretty average  
> star--is 4.8, which also happens to be a pretty average magnitude  
> for so-called "visible stars."  If one were to use, say, 100 parsecs  
> as the standard distance for evaluating absolute magnitudes, then  
> the Sun would (if I've done the calculation correctly) come in at  
> something closer to a not-even-close-to-visible 10.
>
> John Mallinckrodt
> Cal Poly Pomona