WHAT'S UP: Stars and fireflies decorate the summer sky

Aug 5, 2011

Cooler bodies give off mostly infrared light, and light with even longer wavelengths that our eyes cannot see. Hotter bodies give off mostly visible light, and light with even shorter wavelengths that also our eyes cannot see.

Our eyes see green light best. And not coincidentally, the sun — our star — glows most brightly in green light. The sun appears white, not green, however — because the mixture red, orange, yellow, green, blue, indigo and violet light the sun gives off is just right, perfectly tuned with the sensitivity of our eyes to give the most pleasant, mellow white.

The flash of a firefly is green. But fireflies shine with light produced by chemical reactions. Fireflies make light without heat; very much unlike stars.

How far away could you see the flash of a firefly?

Brightness and distance are intimately tangled. The farther a luminous object is — like a star or a firefly — the fainter it appears.

Isaac Newton and other scientists way back when noted that brightness is inversely proportional to distance squared.

Which is to say, that if you take a light source and move it twice as far away, it will appear four-times times fainter. If you move the light source three times farther away, it will appear nine-times fainter.

The faintest stars you can see with your eye alone are about 1,000-times fainter than the brightest stars in the sky.

To appear as dim as the dimmest stars in the sky — 1,000-times dimmer than the brightest stars in the sky — a firefly would have to skitter and flit to a distance 30-times greater than across the lawn (because 30 x 30 is about equal to 1,000).

If across the lawn is 50 feet, then a firefly at 30 times this distance — or 1,500 feet — would be just barely visible as a faint point of light (1,500 feet is about a quarter of a mile).

What about if you had a nice pair of binoculars or a small telescope? How far then could you see the flash of a firefly?

What if you used the Hubble Space Telescope? How far away could the HST detect the flash of a firefly?

Astronomers measure the brightness of stellar objects in magnitudes. The magnitude scale is a bit awkward for everyday use.

Brighter stars have smaller magnitudes than dimmer stars. For example, the brightest stars in the sky are about magnitude minus 1. The faintest visible stars are about magnitude 6.

And the magnitude scale is not linear. A magnitude 5 star, for example, is 100-times fainter than a magnitude 0 star.

Because of this, a light source appears five magnitudes fainter than an identical second light source, if it is 10-times farther away (this may take some contemplating).

The HST can detect stars as faint as about magnitude 30.

For a firefly to appear this faint, it would have to flitter and skit to a distance 1 million-times greater than across the lawn. If across the lawn is 50 feet, then a firefly at 50 million feet would be just barely visible by the HST. The HST could detect the flash of a firefly 10,000 miles away.

— By Doug Furton, a member of the physics faculty at GVSU. Send questions and suggestions to dgf@inbox.com.

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