On Aug. 18, 1868, the French astronomer Pierre Jules César Janssen observed a total solar eclipse in India. He aimed a spectroscope at the Sun's bright prominences and broke their light into component wavelengths. In that spectrum he found a bright yellow line that matched no element then known on Earth. It sat near the familiar yellow sodium lines called D1 and D2 but did not quite align with them, and its exact position was not established at the time.
Months later the English astronomer Norman Lockyer independently observed the same line and concluded it came from an unknown element. He designated it D3, distinguishing it from the sodium doublet, and noted it had a shorter wavelength. Janssen later confirmed the line lay very close to the sodium lines but was slightly more refrangible, bending a little more as it passed through the instrument.
Lockyer named the proposed element helium, after Helios, the Greek god of the sun. His paper arrived at the French Academy of Sciences on the same day as Janssen's, so both men received credit for the discovery.
That credit was initially a dubious honor. Many colleagues doubted that a new element could be inferred from a distant spectrum, and some ridiculed the conclusion.
The historical record complicates the familiar story. Janssen saw the mysterious line during the eclipse but did not identify helium from it, and the substance remained a solar mystery for decades. For years the only evidence for helium was the solar line itself. Later observations confirmed the line was not simply an unusual form of sodium, and its persistence in solar spectra eventually led to recognition of a new element.
The detection was the first time spectroscopy revealed an element in an astronomical object that had not yet been discovered on Earth. The method rests on a principle: the lines a source's light carries, or the bands missing from it, act like unique fingerprints for the chemical elements it contains. A hot, glowing gas emits bright lines at its own set of wavelengths, as the solar prominences did in 1868, while a cooler gas in front of a hotter source absorbs light at those same wavelengths and removes them from the light passing through it. Either pattern maps the elements present, even at enormous distance.
The groundwork came from Gustav Kirchhoff, who in 1859 established that the chemical composition of the Sun and other stars could be deduced from the spectra of the light they emit. Kirchhoff's work covered both cases: the bright emission lines of a glowing gas and the dark absorption lines it produces against a hotter background. The approach turned an unreachable object into a sample that could be analyzed at a distance. Helium's case shows how an astronomical measurement can precede and shape a chemical discovery: chemistry's list of elements had to accommodate a substance the Sun revealed first. Before helium, every newly identified element had first turned up in terrestrial samples, a pattern the solar detection broke.
Helium's path to the ground took years. In 1882 the Italian physicist Luigi Palmieri found the first evidence of helium on Earth, in the lava of Mount Vesuvius. Helium was later isolated on Earth through the work of Ramsay and others, and the interval between the 1868 eclipse and that isolation ran to roughly 27 years.
Helium is relatively rare on Earth, a product of the radioactive decay of elements like uranium, yet it is the second-most abundant element in the observable universe after hydrogen. That contrast follows from where each element forms. Stars convert hydrogen into helium in their cores through fusion, the process that powers the Sun, which is why helium is common across the cosmos and scarce on a rocky planet.
Helium's properties keep it in demand. Gas helium is commonly used to pressurize and purge rocket propellant systems. Liquid helium holds the lowest boiling point of any element, so it cools sensitive scientific instruments on spacecraft.
