You might remember from your high school or college 101-level Astronomy class the concept of Spectral analysis, that different chemical substances produce different spectra of light at high temperatures The phenomenon was uncovered in the 19th century and put to use in studying not only the composition of earthbound substances, but heavenly ones as well. One such investigation of the sun yielded a curious result — which would have profound implications for science later on.

On this day, August 18, in 1868, French astronomer Pierre Jules Janssen noticed in his spectral analysis of the sun a line that could not be replicated in the laboratories, suggesting the presence of an entirely new gas. It was named helium.

Although the second most-abundant gas in the Universe, making up something like 10% of all matter, it is relatively rare on Earth. As one of the “noble gases” it does not typically form compounds, and its light weight allows it to escape earth’s gravity. Yet the gas is important for a lot more than dental surgery and floating balloons: supercooled helium makes a great refrigerant, used to keep the temperature stable on everything from superconductors in MRI machines to rocket fuel on the Apollo missions.