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Astronomy: Understanding the Magnitude of it All

2-4-11 orion Phil Massey, Lowell Obs.NOAOAURANSF
The stars of Orion are part of the winter hexagon. Credit: Phil Massey, Lowell Observatory, NOAO/AURA/NSF

Each season holds different meaning for stargazers. Spring is notable for the wide expanse of galaxies that can be viewed in the evening sky. Summer is popular for viewing the Milky Way. Fall brings famous star clusters into sight. And winter is known for having the largest collection of bright stars.

Stars are assigned a magnitude based on how bright they are. This classification system has been around for thousands of years. In ancient Greece, stargazers identified the brightest stars they saw and labeled them as 1st magnitude. Stars that were two-and-a-half times dimmer were considered 2nd magnitude, and stars two-and-a-half times dimmer than that were 3rd magnitude, so on.

The winter hexagon and winter triangle are two targets to view that will help you compare some of the brighter stars. Sirius, the brightest star in the heavens, lies at the bottom of the winter hexagon and shines at magnitude -1.4. Sirius is a first magnitude star because it has a magnitude of one or brighter. Follow the link above to learn the other stars in the winter hexagon: All of them are first magnitude stars.

An example of a 2nd magnitude star would be Polaris, the North Star. Compare Polaris with the stars of the winter hexagon to see how the brightness drops off. Polaris is the last star in the handle of the Little Dipper. Look to the opposite side of the Little Dipper, to find the star in the bowl that is farthest from Polaris. This star is 3rd magnitude Pherkad. (The other star on the far side of the bowl of the Little Dipper is 2nd magnitude Kochab.) The two other stars in the handle of the Little Dipper and the first star that connects the handle to the bowl of the Little Dipper are all 4th magnitude stars. In light-polluted locations, these stars may be difficult to see. The remaining star that marks the other corner of the bowl of the Little Dipper is just at the threshold of 5th magnitude.

-- Kelly Kizer Whitt loves clean, clear, and dark skies. Kelly studied English and Astronomy at the University of Wisconsin-Madison and worked for Astronomy magazine. She is currently the Feature Writer for Astronomy and Space at Suite101.com. You can follow her on Twitter at twitter.com/Astronomommy.

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