Open any book or magazine about astronomy and you’re likely to encounter the word “magnitude.”
Much as 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 brightness on a scale of 1 to 6. He designated the brightest stars as “1st magnitude” and the dimmest as “6th magnitude.”
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, 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 that now appears high overhead at dusk, is a good case in point. Stars found to be brighter than those were assigned negative magnitudes, and those stars fainter than naked-eye visibility were assigned larger magnitudes.
Remember that number line you learned in school? Thought you’d never see it again, didn’t you?
The range of celestial brightness is 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: 2nd magnitude. Above Kochab lies Pherkad, a 3rd-magnitude star. To the right of Kochab lies Zeta Ursae Minoris, a 4th-magnitude star. And finally, above Zeta we find Eta Ursae 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.