On August 18, 1868, astronomers turned their telescopes toward a solar eclipse, not knowing they were about to expand the periodic table.
Their goal was to discern the ingredients of solar prominences, the giant loops of glowing, electrified gas the sun occasionally belted out. When astronomers split the prominences' light into its components, they detected some colors they'd never seen before—the sign of an undiscovered element.
This mysterious gas was helium, a substance so inert and whisper-light that it'd take scientists another 27 years to isolate it on terra firma. You may think of helium as the stuff in party balloons, or the gas that can make your voice squeaky. But helium has revealed itself to have an