In this blog and my next, I'm going to try to demystify the magnitude system — the scale astronomers use to measure the brightness of a celestial object. Let me start with a little history.
The first known observer to describe and catalog differences in star brightnesses was Greek astronomer Hipparchus (ca. 190–120 B.C.). He divided his catalog of roughly 850 visible stars into six brightness ranges, or magnitudes. He called the brightest 1st magnitude and the faintest 6th magnitude. Observers used this system, almost unchanged, for more than 18 centuries.
But then came Galileo. In addition to discovering the phases of Venus, Jupiter's large moon, and lots more, he noted his telescope did not simply magnify — it revealed the invisible. In 1610, Galileo coined a term that had not been used before. He called the brightest of the stars below naked-eye visibility "7th magnitude."
The telescope, therefore, demanded an expansion of Hipparchus' magnitude system. But not only on the faint end. Astronomers noted stars of the 1st magnitude varied greatly in brightness. Around the time of English astronomer Sir William Herschel (1738–1822), observers adopted a loose system that defined a difference of one magnitude as being a brightness difference of 2.5 times.
In 1856, English astronomer Norman R. Pogson (1829–1891) suggested astronomers calibrate all magnitudes such that a difference of 5 magnitudes would equal a brightness difference of 100. We still use Pogson's formula today. Rather than 2.5, however, two stars one magnitude apart differ in brightness by exactly 2.5118865.
Astronomers routinely use two main divisions of magnitudes to describe the same object. Apparent magnitude describes how bright an object looks. Back in the day, observers measured apparent magnitudes by eye. Not anymore. Ultra-sensitive CCD cameras now provide measurements with accuracies of 0.01 magnitude.
With the other magnitude — absolute magnitude — astronomers indicate how bright an object really is, compared to all other celestial objects. Two things determine absolute magnitude (also called luminosity): apparent magnitude and distance. Absolute magnitude defines an object's brightness if it were exactly 32.6 light-years from Earth. As you might guess, absolute magnitude tells astronomers a lot more about an object than apparent magnitude.
It's fun to compare magnitudes. Let's take the Sun and Orion's brightest star, Rigel. The Sun, because it lies so close, has an apparent magnitude of –26.7. That makes it the brightest object in the sky by far. But take the Sun on a trip out to 32.6 light-years, and you'd find its absolute magnitude drops to 4.6, just slightly brighter than the faintest star in the Little Dipper.
Rigel, on the other hand, is truly brilliant. Its apparent magnitude of 0.14 makes it the seventh-brightest star in our sky. But its absolute magnitude, –7, indicates it outshines the Sun by 50,000 times.
Within the two main divisions, other magnitudes are possible. If an observer measures a star's light through a red filter, the result is the star's red magnitude. A blue filter yields blue magnitudes, and so on. By comparing two different color magnitudes, astronomers can obtain a color index. The most widely used color index is B-V. In this comparison, a star's visual magnitude gets subtracted from its blue magnitude. The more positive the result, the bluer the star. The more negative the difference, the redder the star.
Next week, I'll chat about making the magnitude system work for you.