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Photograph by NASA
Can you count the stars in this picture? This dazzling infrared image from NASA’s Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. In visible-light pictures, this region cannot be seen at all because dust lying between Earth and the galactic center blocks our view.

Counting the stars

August 07, 2009 at 01:53 PM


How many stars can your count in the sky? Maybe this seems like a childish notion, but aren’t we all ever learning and amazed at the wonder about us? To listen to some, one would think mankind has it in full control and is master of his own destiny with optimistic assurance for tomorrow. Yet on this tiny – minuscule—speck of wonderful dust we call Earth orbiting a fairly average star among billions in the Milky Way, in one of practically countless other systems in the universe as we know it, we at any age have a lot of cause for awe.

With that, how many stars can you count? In the daytime of course that’s easy! If you answer “zero,” then we should devote another column to the sun, which indeed is a star. If you answer “one,” then you are right, unless you get too picky. With a telescope, under excellent conditions and if you know just where to point, it is possible to find the brightest stars even in a blue daytime sky. The hardest part is having no references in the blue sky to guide you—unless the bright star happens to be next to the moon.

The rare times of total solar eclipse bring out many stars in the daytime, as the sun is covered by the moon.

At night, if the sky is very dark and clear and you have a wide-open view, it would seem there are millions of stars spread across the celestial dome above you, in view without optical aid. Actually, the number of stars you can see at any one time is about 7,000. That of course is plenty to dazzle your eyes under ideal conditions, adding with them the gray, billowing Milky Way Band, the bright planets and occasional meteor.

Star brightness is measured on a scale of magnitude, with the higher the number being the fainter the star. One would think then that a zero-magnitude star would be the brightest in the night, but oddly, there are two so bright they are listed with negative (-) magnitudes. Sirius, the brightest in the night sky, is -2.

There are 10 stars brighter than +1st magnitude and 40 brighter than +2. There are 140 brighter than +3. There are 530 more brilliant than +4 and 1,620 brighter than +5. There are 4,850 stars brighter than +6, and 14,300 of greater magnitude than +7.

The unaided human eye normally can see as faint as +6th magnitude, so in the entire celestial sphere, there are about 14,300 stars you can see. Naturally we only see half the celestial sphere at any one time, so about 7,000 stars will be above your horizon if it is very flat (as you see on the ocean from a ship). There’s a planet eclipsing the other 7,000 stars—Earth! We are in a state of constant TOTAL STELLAR ECLIPSE.

Of course the number of stars you actually do see will likely be much less, given hills, trees, passing clouds, moon light, light pollution from manmade sources, time given to allow your eyes to adjust to the darkness and one’s eyesight.

Excellent eyesight and unusually good conditions will allow you to see to +7th magnitude. Binoculars will deepen your plunge into the vast cosmic ocean and reveal stars to perhaps +9th magnitude. A small telescope, such as a department-store refractor with a front “objective” lens of 2-inch “aperture” (width) will show you stars of +11th magnitude (if you kept the lens cap on before observing to keep dust out, and if you take the lens cap off before using the scope!). A 6-inch telescope show stars fainter than +13th magnitude. A 10-inch telescope almost reaches +14.5.

The number of stars seen increases exponentially with deepening magnitudes as well as telescope aperture. There are 41,000 stars brighter than +8; 117,000 brighter than +9; 324,000 brighter than +10; 870,000 brighter than +11; 2,270,000 brighter than +12; and so on. Stars brighter than +20 number 1 billion!

Many amateur astronomers enjoy mounting a camera to a motor-driven telescope, which can track the stars. A few minutes exposure will bring out fainter stars than you would see with the eye alone at the eyepiece. The camera is also superior in bringing out color of stars, galaxies and nebulae; the eye is much less sensitive to color in dim light; that is why only the brighter stars show any color, if at all.

Interestingly, even under pristine dark skies, the starry sky “background” is never completely black. There is a natural phenomena in the upper atmosphere called “airglow.” This is a photochemical process whereby in the daylight, ultraviolet radiation from the sun bombards atoms in the atmosphere. At night this energy is given off as infrared and a dim but visible light. There is also a soft and very faint glow from “Zodiacal Light” caused by sunlight reflecting off dust particles in the solar system, in the same plane as the planets. Then there is the occasional Aurora or Northern Lights, an electrical disturbance in the ionosphere caused by particles from the sun. Distant lightning flashes (“heat lightning”) may also add to the natural nighttime light show. These phenomena, however, are all natural and given to us as part of the sky to behold. What is disturbing is the spoiling effects of mankind’s so-called progress.

Ideal conditions are rare to find, and increasingly so with society’s ever-expanding demand to light up the night. Air pollution also takes a toll. Natural conditions of summer often bring high humidity that translates into haze, which scatters the light. Light pollution from towns or the mall is even more disturbing then, and few stars may be seen. We can all do our part by turning off unnecessary lights or at least installing a motion detector light rather than one that is on all the time. Night lights can also be capped to reflect the light to the ground rather than wasting it on the sky.

It is especially precious when we do have an excellent night of stargazing that is well worth staying up a little longer to take it in.

Your reports, questions and comments are welcome; e-mail .(JavaScript must be enabled to view this email address).

First-quarter moon was on July 28, and full moon is on Aug. 5, leaving fewer visible stars to count this week. Keep counting ... and keep looking up!

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