On August 21, 2017, millions of Americans will witness the first total solar eclipse to cross the continental United States in 99 years. While hordes of citizens prepare to flock to the eclipse’s path of totality, scientists, too, are staking out spots for a very different reason: to investigate the secrets of the sun’s elusive atmosphere.

It contains 99.9 percent of all the matter in our solar system and sheds hot plasma at nearly a million miles an hour. The temperature at its core is a staggering 27 million degrees Fahrenheit. It convulses, it blazes, it sings. You know it as the sun. Scientists know it as one of the most amazing physics laboratories in the universe.