Week of July 20-26, 2014

Open any book or magazine about astronomy and you're likely to encounter the word "magnitude." With natural phenomena, such as earthquakes or tornadoes, the term refers to intensity. In astronomy, magnitude measures a star's brightness, but this is a concept that mystifies many beginning stargazers.

It was the ancient Greek astronomer Hipparchus who compiled the first catalogue of star brightnesses some 21 centuries ago. In it, he listed the stars of each constellation and rated their brightnesses on a scale of 1 to 6. He designated the brightest stars as "1st magnitude", the dimmest as "6th."

When astronomers began measuring stellar magnitudes with modern technology, they learned two interesting things. First, they found that each of Hipparchus' magnitudes is about 2.5 times brighter than the next greater magnitude. In other words, a 2nd magnitude star is about 2.5 times brighter than one of 3rd magnitude. And, secondly, that there were objects brighter than 1st magnitude, as well as countless objects fainter than 6th. So they expanded the scale to accommodate these.

For example, stars 2.5 times brighter than 1st magnitude were termed "zero magnitude" Vega, the brightest star now appearing high overhead at dusk, is a good case in point. Those found to be brighter than those stars were assigned negative magnitudes, and those fainter than naked-eye visibility were given larger magnitudes.

Remember that "number line" you learned in school? Thought you'd never see it again, didn't you!

The range of celestial brightnesses is pretty impressive. The sun shines at a stunning magnitude -26 and the full moon appears at -12.5. The faintest objects seen by professional telescopes have been measured to be well below 30th magnitude.

Now, throwing around numbers like this can be interesting, but to see how this brightness scale actually works, go outdoors after dark and look midway up in the northern sky. There, Northern Hemisphere stargazers should spot Polaris, the North Star. It's not the brightest in the heavens as many beginners think; it's rather faint — only a 2nd magnitude star.

Streaming westward from Polaris after dark this week are the stars of the Little Dipper. This star grouping is tough to see from anywhere near the bright lights of a city. From there you'd be lucky to find Polaris and the two "guardian" stars Kochab and Pherkad — so called because they seem to circle Polaris continuously as if guarding this important celestial object. But drive to a dark site in the wilderness, and you'll have much less trouble locating all seven stars of the Little Dipper.

Once you find it, you'll discover that each of the Little Dipper's "bowl" stars shines with a different magnitude. The brightest is known as Kochab, and appears much the same as Polaris: second magnitude. Above Kochab lies Pherkad, a 3rd magnitude star. To the right of Kochab lies Zeta Ursa Minoris — a 4th magnitude star. And finally, above Zeta we find Eta Ursa Minoris — a 5th magnitude star.

This handy region of the sky provides us with a good sense of how star brightnesses are classified, and serves as a great comparison for measuring those of other stars around the heavens.

Visit Dennis Mammana at www.dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.