Stargazer: Determining just how bright stars are
It was the ancient Greek astronomer Hipparchus who compiled the first catalogue of stars' 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 and the dimmest as 6th magnitude.
When astronomers began measuring stellar magnitudes with modern technology, they learned two interesting things. First, 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. Second, there were objects brighter than 1st magnitude, as well as countless objects fainter than 6th magnitude. They expanded the scale to accommodate.
For example, stars 2.5 times brighter than 1st magnitude were termed zero magnitude. Stars found to be brighter than those were assigned negative magnitudes, and those fainter than naked-eye visibility were given larger magnitudes.
Remember that number line you learned in school? You thought you'd never see it again, didn't you?
The range of celestial brightness 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.
Throwing around numbers like this can be interesting, but to see how this brightness scale actually works, one needs only to go outdoors after sunset and look skyward.
Right now, the brilliant planet Venus, which is high in the west at dusk, shines at magnitude -4.3, while the planet Mars — the reddish star just above it — appears at magnitude +0.8. That's a difference of 5 magnitudes. In other words, Venus appears 100 times brighter than Mars!
To see even fainter examples, look midway up in the northern sky after dark. There, Northern Hemisphere stargazers should spot Polaris, the North Star. It's not the brightest in the heavens, as many beginners think. Actually, it's rather faint — only a 2nd-magnitude star.
Streaming downward from Polaris after dark this week are the stars of the Little Dipper. This grouping is tough to see from anywhere near bright city lights. From there you'd be lucky to find Polaris and the two guardian stars, Kochab and Pherkad — so named 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 the Dipper, you'll discover that each of the stars in its bowl shine with a different magnitude. The brightest is Kochab — it appears much the same as Polaris, as 2nd 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 it serves as a great comparison for measuring those of other stars around the heavens.
And now, as our tiny blue world completes yet another journey around its life-giving star, I'd like to wish each of my readers a happy, healthy and safe New Year. I truly hope your stars shine ever more brightly in 2017 and that our paths cross many times!
Visit Dennis Mammana at www.dennismammana.com.
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