question about absolute magnitude scale

[baric]

I realize that 5 points in the absolute magnitude scale represents a 100x change in luminosity. I also realize that the reference distance for a stellar object is 10 parsecs.

But why is the Sun's magnitude set at 4.85? Sol is often used as a reference point for stars, whether it be stellar radii or mass (and why not temperature?).

It just seems like having Sol's magnitude benchmarked at 5 instead of 4.83 would be so much simpler, especially since the scale is based on 5-point intervals.

So how did the 4.85 come about?

[Champion_Munch]

But why is the Sun's magnitude set at 4.85? Sol is often used as a reference point for stars, whether it be stellar radii or mass (and why not temperature?).

Is that it's actual luminosity, rather than visual? I don't think the greeks had ways of determining those kinds of things thousands of years ago (I think they were the ones who devised a magnitude of 1 - 6 for visible stars). :D

with regards

[The_Radiation_Specialist]

isnt the suns magnitude -26?

[Fr. Wayne]

The Sun's luminosity is 3.90 x 10 (exp33) ergs/sec.
The Sun's absolute magnitude is 4.74.
a difference of 1 magnitude = a brightness ratio of 100(exp1/5) or 2.512

[Romanus]

The Sun's absolute visual magnitude is about 4.83-5 (I've seen it vary along those lines). It's absolute bolometric magnitude--its brightness if we could see all of its radiant energy--is 4.74.

[Thanatos]

The absolute magnitude of the sun is a referemce brightness number given by this:
http://astrosun2.astro.cornell.edu/academics/courses/astro201/mag_absolute.htm

[tony873004]

You can compute a star's absolute magnitude with the following formula:
http://www.orbitsimulator.com/BA/absmag.GIF
A slightly larger number could have been chosen instead of 10pc to make the Sun exactly 5, but that would be sloppier than leaving it at 10pc and having the Sun just under mag 5.

[grant hutchison]

So how did the 4.85 come about?Apparent magnitudes came first, before we knew how far away the stars were. The apparent magnitude scale was tied to various reference stars, firstly just "of first magnitude" of "second magnitude", and so on. Once photographic techniques were introduced, the apparent magnitude of these reference stars was refined to fit in with Pogson's formalization of the magnitude scale (5 grades = 100-fold change). Meanwhile, we were also sorting out the distances to stars, and coming up with the idea of absolute magnitude. By that time, with the apparent magnitude scale already well established, I guess the choice would have been either to choose a non-integral standard distance, so that the Sun came out with an integral magnitude, or to choose an integral standard distance, in which case the Sun came out with a non-integral standard magnitude.
Since we're generally converting between absolute and apparent magnitude for a given star, the integral-standard-distance route had a marginal advantage, because it slightly simplified the sums. I guess there is also the possibility that no-one had nailed down the apparent magnitude of the sun accurately enough at the time the decision was made ... it's a bit of a stretch comparing the daytime sun to a standard reference star.

Grant Hutchison

[baric]

Apparent magnitudes came first, before we knew how far away the stars were. The apparent magnitude scale was tied to various reference stars, firstly just "of first magnitude" of "second magnitude", and so on. Once photographic techniques were introduced, the apparent magnitude of these reference stars was refined to fit in with Pogson's formalization of the magnitude scale (5 grades = 100-fold change). Meanwhile, we were also sorting out the distances to stars, and coming up with the idea of absolute magnitude. By that time, with the apparent magnitude scale already well established, I guess the choice would have been either to choose a non-integral standard distance, so that the Sun came out with an integral magnitude, or to choose an integral standard distance, in which case the Sun came out with a non-integral standard magnitude.

Thanks for the great answer! It makes a lot more sense now :)

[rnt20]

Apparent magnitudes came first, before we knew how far away the stars were. The apparent magnitude scale was tied to various reference stars, firstly just "of first magnitude" of "second magnitude", and so on. Once photographic techniques were introduced, the apparent magnitude of these reference stars was refined to fit in with Pogson's formalization of the magnitude scale (5 grades = 100-fold change). Meanwhile, we were also sorting out the distances to stars, and coming up with the idea of absolute magnitude. By that time, with the apparent magnitude scale already well established, I guess the choice would have been either to choose a non-integral standard distance, so that the Sun came out with an integral magnitude, or to choose an integral standard distance, in which case the Sun came out with a non-integral standard magnitude.
Since we're generally converting between absolute and apparent magnitude for a given star, the integral-standard-distance route had a marginal advantage, because it slightly simplified the sums. I guess there is also the possibility that no-one had nailed down the apparent magnitude of the sun accurately enough at the time the decision was made ... it's a bit of a stretch comparing the daytime sun to a standard reference star.

Grant Hutchison

Indeed. Magnitudes are generally used for nighttime astronomy, so it was more important to get a scale which was useful for nighttime astronomy than one which was useful for solar observations.

Note that the current apparent magnitude system is no longer determined by reference stars, but rather as a table of luminosity measurements at http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1982lbor.book.....A&db_key=AST

[ngc3314]

Indeed. Magnitudes are generally used for nighttime astronomy, so it was more important to get a scale which was useful for nighttime astronomy than one which was useful for solar observations.

Note that the current apparent magnitude system is no longer determined by reference stars, but rather as a table of luminosity measurements at http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1982lbor.book.....A&db_key=AST

Sort of. The fundamental definition of each magnitude system is still tied to a particular set of stars, since that can be more accurate than we know the energy zero points. However, people are getting better at the tedious tie-ins between laboratory standards and stellar magnitudes - but some magnitude systems are reproducible at the 0.001-magnitude (0.1% in flux) level, which is better than we know the absolute calibrations.