Astro Bob: Fireflies flash, stars twinkle, we connect

If you live in the eastern half of the country, fireflies are a familiar sight on June nights. Like stars, they create their own light but use an entirely different process.

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Fireflies glide, dip and rise in a field along a forest edge in Lakewood Township, Minn., around midnight June 17, 2021. Details: ISO 400, 150mm lens at f/3.2, 150-second exposure. (Bob King)

Warming nighttime temperatures this month mean a return of the fireflies. I can't get enough of them. As the stars slowly wheel overhead, hundreds of the flying beetles flash along the forest edges and within the trees, each with a light pattern unique to its kind. I've counted at least 10 different species.

One of the most familiar is the Big Dipper, or J-Stroke firefly, the kind many of us caught as kids. They fly near waist-level and flash for just under a second while dipping and rising in a glowing "J."

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Fireflies peak in June and July in northern Minnesota. They like warm nights but shy away from light-polluted areas, preferring open fields, lake shores and forest edges. To encourage fireflies where you live, turn off outdoor lighting. Because this was a time-exposure photo, stars made short trails. (Bob King)

There are 1,900 species worldwide, with 170 species in North America. And they don't all flash the same color. That revelation hit me the night I identified my first candle firefly, with its sputtering, amber flash. Other species, like the variable triple flash, flare three times in succession while darting through the air. Flashbulbs emit a single, quick, spark-like flash every three seconds as they search the trees for mates.


Of course, that's what all the flashing and flourishing is about: courtship. Males light their lanterns hoping a female firefly somewhere low in the weeds will be impressed enough to send a signal in return — often just a single flash — to let him know the door is open. He descends, joins the female and if all goes well, we'll see fireflies again next season.

Sometimes it doesn't go well. Female photuris fireflies mimic the flash patterns of several photinus species to lure them into their ken. A hapless male who accepts the invitation is promptly eaten for dinner. Less you balk at the horror of it all, they do this to procure defensive chemicals possessed by photinus that are used to repel predators like spiders. Just nature's give and take.

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Fireflies cook up their special light by combining oxygen, enzymes and energy in a process called bioluminescence. (Bob King)

Fireflies produce their own light through bioluminescence. The light-emitting organ is often located in the lower abdomen. When a flash is desired, the insect combines oxygen from the air with the chemical luciferin, an enzyme called luciferase and ATP, a compound that stores and transfers energy in cells. This insect alchemy creates a flash of "cold" light that varies from orange to yellow to green.

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The same basic chemistry the firefly uses makes mycena chlorophos, the night-light mushroom, glow at night. (Lalalfdfa / CC BY-SA 3.0)

In an incandescent light bulb, only about 10%-20% of the energy is converted to light with the rest lost as heat. LEDs do much better with an efficiency around 80 percent. But fireflies put both to shame with a "bulb" that's 96% efficient. Bioluminescence based on luciferin and luciferase is also found in certain mushrooms, jellyfish, marine plankton and bacteria.


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Massive stars create more complex elements from simpler ones through nuclear fusion in their cores. When a star runs out of fusible material it star collapses and explodes as a supernova, right, releasing the freshly forged elements into space. (Left: NASA, Right: ESO)

On a recent night, I took a few photos of the busy firefly scene in a neighbor's field. A sweet breeze fluttered the leaves of nearby trees, while a half-moon provided just enough light to find my way about without taking away from the flickers and flashes. Stars looked down over the scene as if all this was their creation. And in a funny way it is.

Chemical elements and compounds unleashed into space as a star ages or explodes are rich in complex elements like nitrogen, oxygen, carbon and iron essential to life. Stars cook these elements up in their hot, high-pressure-cooker interiors.

When you make a pizza you take separate items like cheese, tomato sauce, onions and sausage and arrange them into something more interesting. Then you heat the works in the oven to blend the ingredients into something truly delicious. Similarly, a star takes simple ingredients like hydrogen and helium and "cooks" or fuses them in its interior into more complicated elements. These can even combine in the star's atmosphere into familiar substances like water and silicate dust (the stuff of rocks).

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Fireflies line both sides of this country road under a starry sky in July 2019. (Bob King)

Later, that material gets incorporated into new stars and planets, including this gem of a world called Earth. Oxygen is crucial to bioluminescence. Every molecule of it that lights a firefly's abdomen arose in the bellies of stars more than 10 billion years ago. Each flash is a reminder of that connection, one we also share.


"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at .

"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
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