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Space Facts

Star Magnitude

How is the brightness of stars measured?

Magnitude is a measure of the brightness of a star. "Apparent magnitude" or "visual magnitude" refers to the brightness of a star as seen from Earth. "Absolute magnitude" refers to the intrinsic brightness of any star, or brightness from a standard distance of 10 parsecs (approximately 32.6 light years). Naturally, an intrinsically very bright star can seem very dim if seen from a great distance. A very weak star seen from close up can seem very bright. Absolute magnitude removes distance as a variable.

"Bolometric magnitude" refers to the total amount of energy from the star, including all non-visible wavelengths. "Photographic magnitude" is similar to "apparent magnitude," except that it depends on the sensitivity of the photographic material — typically biased toward the blue end of the spectrum.

The scale of magnitudes is borne out of antiquity, where each magnitude is roughly a two-fold increase in brightness. In recent times, the scale was streamlined so that a difference in 5 magnitudes represented a hundred-fold increase from dimmest to brightest. This resulted in a brightness ratio for one magnitude of approximately 2.512 (or the 5th root of 100). Those new to the terms of astronomy may take awhile to get used to stellar magnitudes, where the brighter the star, the smaller the number. Unlike a scale of, say, 1 to 10, where 10 is best, in astronomy the smallest number is "best" or "brightest." The concept is simple, though. The brightest stars were thought to be the most important, or "first" magnitude stars, and the second, third, and fourth magnitude stars were of decreasing importance.

Some objects have magnitudes that are brighter than first magnitude, and use negative numbers. In fact, our sun is said to be a magnitude -26.7, the full moon -12.5, Venus at brightest -4.3, and Sirius -1.42.

Naked eye visibility is typically limited to about magnitude +6.0, though persons with exceptional eyesight can see fainter objects under excellent seeing conditions.

Astronomy Data Book, by J.H. Robinson & J. Muirden — John Wiley & Sons, New York
A Field Guide to the Stars and Planets, by D.H. Menzel — 1964, Houghton Mifflin Company, Boston