How Bright is that Star? Part 1 Magnitude

Magnitude The Greek astronomer Hipparchus devised the first system for rating the brightness of Stars. The brightest stars like Betelgeuse, Rigel, & Deneb he called 1st magnitude. Not quite so bright, like Mizar in the Big Dipper’s handle, were 2nd magnitude, and so on…down to the dimmest, 6th magnitude. (Notice that the lower the number is the brighter the star is.) Modern astronomy still uses this system, but has refined it.

Decimal and Negative Magnitudes Using more accurate measures of brightness stars are now rated more precisely. A 1st magnitude star is 100x brighter than a “6th ” Each order of magnitude is therefore 2.15 times brighter than the one below it. Magnitude is now given in decimal form. Deneb now rates a 1.26, and Betelgeuse rates .87. Hipparchus underestimated how bright the brightest were, so now we even use negative numbers for the very brightest, Sirius rates -1.44. This is the modern apparent magnitude system

Absolute and Apparent Magnitude Hipparchus thought all stars were at the same distance, on the celestial sphere, they are not. Thus how bright a star appears is affected by the inverse square law and depends on distance. The Magnitude a star appears to have is now called its “Apparent Magnitude” “Absolute Magnitude” is a truer measure of how bright the star really is. It is what magnitude the star would have if it was placed 10 Parsecs away. Sirius rates 1.4…. Deneb rates –8.7!