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The astronomers' magnitude scale Night sky columnist Joe Rao discusses how the concept of magnitude excludes one star.
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Often in these weekly Night Sky columns I refer to the magnitude of a star or planet. The concept of magnitude is relatively easy to grasp. Put simply, magnitude indicates an object’s degree of brightness. The lower the figure of magnitude, the brighter the object.

As you'll learn below, the scale narrowly and rather harshly excludes one star -- which you can find in tonight's sky -- from a lofty first-magnitude rating.

This system had its origins in the classification by brightness of about a thousand stars carried out by Hipparchus in the second century B.C. The mean of the twenty-one brightest stars in the sky is defined as first magnitude, while the faintest stars normally visible to the unaided eye on a clear, dark night are said to be of sixth magnitude.

Sixth magnitude stars shine 1/100 as bright as first magnitude stars -- a difference of five magnitudes corresponds to a difference in brightness of a factor of 100. The scale is logarithmic. A difference of one magnitude corresponds to a difference in brightness of 2.512. Thus second magnitude stars are said to be 2.512 times brighter than third magnitude stars and 2.512 x 2.512 = 6.31 times brighter than fourth magnitude stars, and so on.

In many astronomy textbooks magnitude "bins" usually categorize star brightness. Any star that, for example, falls between magnitude 1.50 to 2.49 would be designated as "second magnitude."

So in the sky, were we to find two stars placed at the extremes of that particular range, they would appear to our eyes to significantly differ in apparent brightness by a factor of 2½. Yet, both stars would be categorized as second magnitude stars.

Conversely, if we have two stars in the sky, one of magnitude 2.49 and the other of magnitude 2.50, both will appear equally bright to us. Yet, thanks to binning, the star of magnitude 2.49 will be classified as a second magnitude star while the star of magnitude 2.50 will be recognized as a third magnitude star!

If you think that’s a bit unfair, recall that there are 21 stars in the sky that are categorized as first magnitude. Interestingly, however, were it not for the miniscule difference of just one-hundredth of a magnitude, that listing of first magnitude twinklers could be increased by one. If that were so, then one constellation could then have the unusual distinction of having within its boundaries the two first magnitude stars that were placed at the very top and bottom of that list.

That constellation is reaching its highest point in the southern part of the sky at around 8:30 p.m. local time this week: Canis Major, the Big Dog.

Within this constellation we find the brightest star in the sky: Sirius, the Dog Star. It shines at a dazzling magnitude –1.44. Its blinding presence overwhelms a neighboring star, Adhara, which shines at magnitude +1.50 and ranks as the twenty-second brightest overall in the sky. Yet by virtue of binning, poor Adhara misses -- by just 0.01 of a magnitude -- the cutoff for official first-magnitude classification. As a result is it recognized as just another second magnitude star.

And here is yet another dramatic example of how appearances can be deceiving: Adhara is actually 175 times more luminous that Sirius. But because it’s 431 light years away as compared to just 9 light-years for Sirius, Adhara appears only 1/15 as bright from Earth.

So while all the stargazing accolades go to Sirius, one could argue Adhara, the Rodney Dangerfield of the cosmos, should be recognized as the true dazzler.