[/caption]Pierre Janssen was a French astronomer who discovered helium in 1868. He was observing a solar eclipse in India when he noticed the yellow spectral emission lines of the element. An English astronomer by the name of Norman Lockyer observed the same spectra and proposed the name helium after the Greek name for the sun, Helios. Helium can be observed at 587.49 nanometres in the spectrum of the chromosphere of the Sun.
It was first thought that helium could only exist in/on the Sun because the spectral results could not be produced in the lab. That did not stop researchers form looking for it. In 1895, Sir William Ramsay discovered helium after treating cleveite, a uranium mineral, with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, noticed a bright-yellow line that matched the spectral line observed in the the Sun. Ramsey sent samples of the gas to Sir William Crookes and Sir Norman Lockyer who verified that it was helium. It was independently isolated from cleveite the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who were able to accurately determine its atomic weight. In a bit of irony or opportunity lost, American geochemist William Francis Hillebrand found the element prior to Ramsay’s discovery while testing a sample of the mineral uraninite. He attributed the lines to nitrogen and lost the claim to the discovery in the process.
Several interesting properties of helium have been discovered in the ensuing years. In 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha particle is actually a helium nucleus. In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than one kelvin. He tried to solidify it by reducing the temperature more but failed because helium does not have a triple point temperature where the solid, liquid, and gas phases are at equilibrium. The element was eventually solidified in 1926 by his student Willem Hendrik Keesom. He managed to do so by subjecting helium to 25 atmospheres of pressure. Helium was one of the first elements to be found to have superfluidity. In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero(superfluidity). In 1972, the same phenomenon was observed in helium-3 by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson.
Here on Universe Today we have a couple of great articles related to helium. One is about the possibility that white dwarfs can merge and form helium stars and the other is about liquid metal helium. Astronomy Cast offers a good episode about the energy spectra that we have been talking about in this article.