There are at least three radically different sources of light in the above photograph of Texas' major cities, shot on June 17 by a crew member aboard the International Space Station. Take a second to orient yourself – the large yellow blotch at the bottom is Houston, and the almost equally sized glow-blob north of that is Dallas/Fort Worth – and see if you can detect them.
It might be hard to distinguish with the colossal distances involved (Houston to Dallas is about 230 miles as the buzzard flies). The most obvious light producers in this photo are the cities, including the four largest in the state; follow the yellow highway west from Dallas/Fort Worth, and it cuts through the glimmering photon-oases of Austin and then San Antonio. Street lamps and building lights make these burgs flare like campfires in the middle of a black desert, with the slight fuzziness above Dallas/Fort Worth coming from a layer of migrant clouds.
Next, in the lands south of San Antonio you'll see a sea of golden pinpricks like a small galaxy of townships. These are actually fires from petroleum and gas apparatuses mining the Eagle Ford Shale, a massive underground repository of hydrocarbons about 50 miles wide and 400 miles long. The formation is a hotbed of fracking and economic speculation, with one study estimating it could grow about 5,000 new wells in the next 7 years to produce as much as $21.5 billion in revenue.
Here's a version NASA made with a key that highlights the gas sites (and also, ahem, the need for a copy editor in the Fort Worth region):
Another natural source of illumination is easy to spot if you let your eyes wander way northwest. Flickering in the distance like a floating disco jam is the blue-white-purple bursts of thunderstorms over Oklahoma. On this particular night, a rowdy bunch of storms lashed out at the Sooner State with a gust front, battering the roofs off of a few unlucky buildings. Collect Three Astro Nerd Points if you got all three correct!
Images courtesy of NASA/SS Crew Earth Observations