(CNN) — It started as a hobby. It's evolved into a cause.

Filmmakers Gavin Heffernan and Harun Mehmedinovic got into astrophotography as a way to escape the urban grind and try new photography techniques. Now the two have become advocates against light pollution.

"There's no way to get around that issue when it comes to astrophotography because you're always having to run away from cities to get to dark skies," Mehmedinovic said, adding that many photographers have to drive for hours to get away from city lights.

They stitch together hundreds of long-exposure images to create their timelapse videos. The result is a swirl of stars punctuated by satellites, meteorites and aircraft streaking across the sky.

"The 25-30 second exposures reveal more stars and planets up above, they also magnify the lights on the ground, so even cities 100 miles away were blasting out our photos with floods of light," Heffernan said.

That got them thinking about the environmental effects of all that light.

The International Dark-Sky Association says that light pollution can disrupt wildlife, mess with humans' cicadian rhythms and increase energy consumption.

A study last year found that about one-third of people around the world -- including 80% of Americans and 60% of Europeans -- can't see the Milky Way because the haze of light from street lights, buildings and other sources drowns out the starlight.

The pair decided to write a book and, after raising money through Kickstarter, spent three years criss-crossing the country to tell the story of the dark skies.

"By the end of it all, I think we visited about 45 of the 50 states," Mehmedinovic said.

The whole adventure is a bit of a blur. They would go out night after night for weeks at a time. They ended up shooting in hundreds of locations. They generally only spent only one night at each location, so they used as many as six cameras to catch different angles.

"Some of my favorite spots were the Eureka Dunes at Death Valley, Mauna Kea Observatory in Hawaii, and the Enchanted Highway in North Dakota," Heffernan said. "There's lots of gems out there but my favorite spots are where you can imagine you're on another planet."

Finding "dark skies" is harder on the East Coast, because the population is so dense. But Mehmedinovic says that Fort Jefferson, in the Dry Tortugas National Park off Key West, and western West Virginia offer incredible views.

"There are many beautiful places still in the country, I don't know how long they will last," Mehmedinovic said.