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Dennis Mammana


Planetary Dance at Dawn Week of August 17-23, 2014 Early morning risers who have been paying attention to the dawn sky will most have likely spotted the two bright "stars" low in the east; perhaps you've even noticed that they appear to be approaching one another as time …Read more. Seeing Double in the Summer Sky Week of August 10-16, 2014 At first glance, every star looks pretty much the same. I know this because stargazers tell me all the time. But if you take a few minutes to look more closely with binoculars or a small telescope, you'll discover that …Read more. Will the 'Super' Moon Be Super? Week of August 3-9, 2014 If the full moon appears especially large when it rises over the horizon around sunset next weekend, Saturday and Sunday, August 9 and 10, that's because, well, it is. Regular readers of this column know about the optical …Read more. Winter Sky Preview Week of July 27 — August 2, 2014 As much of our planet's Northern Hemisphere swelters under the oppressive heat of late July and early August, it's nice to know that the seasons will soon be changing. And before we know it, many of us will be …Read more.
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Seeing a Cosmic Yardstick


Week of Aug. 24-30, 2014

Can you tell how far the stars are just by looking up at the night sky? You could if all stars were identical. Then, if star A appeared four times dimmer than star B, it would have to be twice as distant; if it appeared nine times fainter it would lie three times farther, and so on.

But stars are not alike, and therein lies the rub. Each star pumps out vastly different amounts of radiation, and we can't assume that a star is farther just because it appears fainter. In fact, the opposite is often true — that the brightest-appearing stars are farthest away.

This occurs because stars radiating the least can be seen only if they're relatively nearby; those that are most luminous are much less common, but we can see them at greater distances. So it's not just a matter of judging star distances based simply on their brightness.

One way we can measure stellar distances directly is by triangulation, a technique we all learned in elementary school. Hold your thumb out at arm's length and alternately blink your eyes. Your thumb will appear to shift its position relative to background objects; move your thumb closer to your eyes and the shift you see will become larger. Your brain knows how far apart your eyes are and, from the thumb's shifting position it can calculate pretty accurately how far away it must be.

We can do the same with stars — not by blinking our eyes, of course, but by measuring their exact positions at six month intervals, when the Earth lies on opposite sides of its orbit around the sun, say once in summer and again in winter.

If the star appears to shift its position between these two times, we can calculate how far it must be.

Unfortunately, this works only for the nearest of stars. For more distant stars, astronomers must be cleverer.

And, indeed, they are. They've found an entire class of stars that serves as an accurate cosmic yardstick. The first of these stars to be discovered is known as Delta Cephei, and we can find it easily with the naked eye in the constellation Cepheus, the king, in the northern sky after dark this week. This star oscillates in size and temperature, and appears to brighten and fade in our sky every 5.3 days.

Not until 1912 did Harvard College astronomer Henrietta Leavitt notice that the average brightness of such "Cepheids" was related to their periods of pulsation. From this, astronomers soon developed a period-luminosity law, which states that the longer it takes a Cepheid to undergo one cycle, the more intrinsically luminous the star must be.

Armed with this new law, astronomers can measure the period of a Cepheid anywhere in our galaxy (or beyond) and can easily infer its actual luminosity. And, by comparing how bright it appears with the light it's actually emitting, they can calculate its distance.

In this way, Cepheid variable stars have become one of astronomers' most valuable cosmic yardsticks, and help them to measure the size of the entire universe.

And there's one right over your rooftop after dark tonight.

Visit Dennis Mammana at To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at



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